Battery packs exposed

The construction of the two packs is examined and compared, as well as some tips on keeping the pack in good condition.
WARNING, you can be killed or hurt fooling with the battery pack. Don't even think about working on or removing the pack if you are not fully confident in your knowledge of electronics. Get the service manual and electrical manual, and follow the procedures for safely removing the pack from the car.
The HV switch needs to be off whenever the pack is being worked on. The development of tools and hardware to replace defective cells will also be discussed.

Lead acid battery failure examined


Looking into a sulphated lead acid battery

Had some sulphated lead acid batteries that I was going to trade in, so I figured that I may as well get my moneys worth and see how they are made, as well as what was going on in side.
I got out my saws all and carefully cut off the top of two different brands.
It looks like the + terminal is susceptible to the plates corroding right through, and the sulphation can be so severe that it warps the plates until they short, and can lift the big buss bars to the point where they break off.
Not something that will recover very well, so we gave up on the rest and junked them all.

Cleaning the corroded buss bars from a Prius battery pack


Cleaning the Prius hybrid battery buss bars

The prius hybrid batteries have been having cronic issues with the copper buss bars between modules getting corroded by Potasium Hydroxide weeping through the terminal seals. The effect is a crust of hard material that is non conductive, eventually causing battery failure.
I want to build up more of the 48V modules for my EV's, and wanted to find a way to clean the strips more efficiently than sanding and using steel wool, which can take as much as several minutes of hard work each.
I remembered my Dad showing me how to make old copper pennies shine buy dunking them in muriatic acid (Hydrochloric acid). I still had a container of the stuff which can be bought at most hardware stores, so I gave it a try. It immediately causes the crust to dissolve in a flory of bubbles, and then it stops, and the strip is mostly shiny copper. A thorough wash in running water finishes the job by washing off all the acid. Time will tell as to how it look a year from now, but it looks good now.

Getting the lead out


culling out the good from the bad

I needed the big Trojan T105 batteries for the solar battery bank, so I needed to make a replacement.
Most of the yard buggy use is to move something from one part of the yard to the other, or to collect wood, so the buggy only needs enough battery to make a couple of trips around the yard, instead of the giant heavy lead batteries which pack 220AH, I decided to make up some prius packs equivalent to the 48V lead pack, but with only 15-20AH. They charge faster, and are much lighter.
Started by reconditioning the old gen one (288V) modules. These modules came from several sources, including two packs that were in tough shape, so it was not surprising to find very poor performance from more than half of the modules. The grid charger is a nice tool for this, and to prevent the distortion of the modules durng the gassing stage of the charge, I made some end plates with 3/4" plywood, and some long 1/4-20 threaded rods to compress the modules.
I made a special paralleling cable where I can plug the 48V modules into a common buss, and then plug both ends of the buss cables into the 50A Anderson connectors already on the buggy.
WOW! the NIMH internal resistance is much lower than the lead acid, so the voltage droop under full load is almost non existent, compared to the lead batteries. Can scrub out at will on any surface, so now the issue will be to get enough weight over the rear wheels, so I can pull my trailers. Will rig up a solar quick charger for the packs, but until then, my grid charger will do the job but more slowly.

2006 and up civic packs


civic dual subpack construction

many people have asked if the MaxIMA and other replacement sticks can be used in the later civic packs. The answer is maybe.
The sticks them self are actually 2 of the 6 cell sticks, but instead of the big screw connections that we see on the replacement sticks, we see a welded buss bar making the far side connections. The sticks also do not have the PTC sensors. I would expect that a copper buss bar could be used, but the end cap would not work as designed, so the stacking of the sticks could have difficulties. If I ever get a pack to play with, I will design a solution.

MaxIMA stick full discharge


MaxIMA full discharge

A typical discharge graph for one of the MaxIma sticks. The little wiggle in the trace is caused when labview stops recording the signal for a few seconds while writing a PDF file, and is not a problem in the battery. We see nearly an additional AH of capacity, a higher discharge voltage (less Internal resistance), and less heating even with the additional AH. All good.

Testing the sticks at the cell level


testing to the cell level at currents similar to the car

I revived my labview based stick tester and will be using it to test the new MaxIma sticks.
Here is a trace of a stock stick at the 75-80A load. Note that we see 5.8Ah and a temperature rise to over 120 F

Bumblebee battery replacements for the stock subpacks


Max IMA batteries show improved internal construction

Several people have picked up the ball that was dropped by Hybrid battery repair, and are offering new sticks for the civic and the insights, so it is time that we see just how well the sticks are built and how well they perform.
I got 4 sets of 20 subpacks from Bumblebee, and am in the process of evaluating the performance, but we also know that the construction of the cells is a big factor in how well they can handle the currents involved with use in a Hybrid car. The first photo shows the inner cell construction of the MaxIMA cell
more and longer weld points on this cell allow the current to flow with lower internal resistance, so in theory that should translate into less voltage drop under the same load as well as less heating.

How it is wired


Insight pack internal connections

Wanted to do some charger/discharger test on a pack that was pretty bad, so I took off the covers, and made this chart, which should be a nice road map for any battery explorers.
********Danger*********
Adding the jumper across where the big fuse and switch was located as shown here removes any protection from shocks or short circuits, so proceed only if you know what you are doing. May be better to use a DC rated fuse in the jumper to maintain short circuit protection.

**************************

Converting a telephone truck to electric


Ful electric telephone truck

Old telephone trucks make a great tool for cleaning gutters, painting, siding and many other uses. I converter my 40 year old truck to full electric, this blog which starts at the page bottom follows the conversion process.
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Time to get serious about determining what actually fails in the sticks


Getting ready to start testing

Since Hybrid battery repair has abandoned the pursuit of cell level rebuilding of the subpacks, and I have already built the welder, and other fixtures and test systems,that he does not want, I will use the equipment to dig deeper into determining how the subpacks look at the cell level.

A look at the traces below will show that many subpacks only have one cell that is a bad actor, others show overall deterioration that may or may not respond to charging.
Time to pin down some of the failure modes, and with the new smart chargers, we should be able to troubleshoot a problem pack and determine which sticks are acting up, and then which cells within the sticks require replacement to get the pack back into a useable condition for the least cost.
I grabbed all of the packs and sticks I have, and have assembled them so I can begin the testing.
The automatic whole pack discharger is working well with the 6 100W bulbs, so I will run capacity recovery test on all the packs I have available to see what we see.

What actually goes wrong with the batteries????


What is wrong with these things

Hybrid battery packs develop several problems as they age, which are aggravated based on temperature history, and length of time spent inactive.
Inactivity allows the cells to self discharge, and since all cells are going to do this to one degree or another, and the rate of self discharge can be quite different from one cell to another, a typical pack that is setting codes like the 1449,1447,1433, will have cells that have fallen way behind others , and the IMA control system which works to keep the cells in the 20% to 80%, has no mechanism to rebalance them.
All Nickle based batteries develop this self discharge because they can spend a lot of time sitting at a middle SOC, where Nickle dendrites (little whiskers of nickle) will grow and gradually provide a leakage path between the + and - plates, eventually leading to a shorted cell.

Storage and self discharge
Another thing that happens when a cell drops down in the SOC relative to the other cells is it develops an effectively reduced capacity that is recoverable called Memory effect.
memory effect
Memory effect capacity loss is reversible by fully cycling the cell from 100% to less than 1V/cell.

So the code setting pack has this unmatched set of cells with widely different SOC, and wide spectrum of effective AH capacities, which reduces the effective capacity of the pack as a unit to a value that sets the code.

When the car charges the pack to full, (80% SOC) it stops when it thinks the pack is full, with the assumption that all the cells are the same, where in fact some cells are still only partial charged.
The discharge limits are also skewed so the pack stops allowing assist long before the highest capacity cells are empty because one of the low capacity cells has dropped out.
When that happens the rest of the pack discharges through the empty cell effectively reverse charging the cell.
Cell reversal
The IMA safeties kick in causing the SOC guage to drop to the bottom, and aggressive charging begins to pull the reversed cell back from sure destruction.

The longer the pack is used like this, the worse the pack gets.
Another factor in this picture is heat.
Once a cell gets to 140F the internal pressure can get so high that it vents potassium hydroxide and the cell will permanently loose AH capacity. No amount of charging can bring back capacity lost due to venting.

The charger and discharger system will first fully charge all cells to 100% SOC, for the first time since the pack was installed. This can take a long time, as we have to limit the charge current so the cells that fill up first do not overheat.
When a cell is fully charged, all the charge energy becomes heat, which is why the cooling fan should always be used during charging.

Getting the whole pack to 100% will fix some packs, especially ones that have become unbalanced due to just sitting and self discharging, and have not developed any memory effect.
Hypermileing where you do not use the pack much can cause the pack to develop the same self discharge unbalance.

Most packs have both memory effects and real capacity loss, as well as differing degrees of self discharge, so the discharger aspect of the system becomes a great tool to extend capacity on the memory effected cells by cycling the pack between full and the detection of the dropout of the weakest cells in the pack, The idea being that the weakest capacity cells are the ones causing the problems, so since they will be the cells that get the deepest discharge, if the issue is memory effect those cells will recover capacity with each cycle, and the minimum voltage reached during the discharge should get lower and closer to the 1V/c level which would be 120V on a 120 cell Insight or first gen civic. A well balanced pack even if at low capacity will discharge uniformly so the discharge would stop at near 1V/cell.
In the real world that is seldom is seen, since all packs will have several of these issues happening at the same time, so there will be a point where the discharge will repeatedly stop at the same elevated voltage, like 138V. At this point the cell that is dropping out is likely one that has lost capacity due to venting or simply age, and to get that pack back in balance, we look at the other cells when that cel drops out to see if they are also ready to drop, or if the majority of them are still quite good.
At this point, if the car is still setting codes, we need to take the process from the car to the bench.
Using a procedure that we are still tweaking, we can use the charger/discharger, and datalogger to discharge the pack with the ends open so we have access to the ends of each stick, to make voltage measurements.
If the other cells are still well over 7V replacing the stick that dropped out with one of higher capacity, would then allow the cycles to continue to reduce memory effect and increase capacity of the rest of the pack.

The photos and graphs below were done with my cell level test fixture and Labview graphing program for my pc.
This is a graphical analysis of my Insights Subpacks after I got an IMA and Check engine light. The pack never did recalibrations, and it set the IMA code several days before setting the check engine code.
P1447 set the IMA, P1449 set the check engine. A system reset would not clear the codes. Each subpack was charged most of the way, topped off, then discharged while watching the temperature and voltage across each cell.
I looks to me like subpack 10 and 12 are the guys that set the code. I hope to be able to replace the cells that are bad, cycle and rebalance the rest and try the pack back in the car.
Please feel free to comment on the graphs with your opinions about what they show, my comments are very preliminary since I have only put them through a single cycle so far.
I charge @ 5-6A, and can do a high rate charge of 30A. The discharge is at 30A, and can be increased to ~70A.
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Seprating the cells for reuse


Deconstructing the subpack

Ok I can test the subpacks down to the cell level, so the bad cells can be identified, now how do we get the cell replaced. Grabbing the stick with plastic strips in the milling machine vice, I milled off the welds being careful to not machine into the stainless steel case. Once the welds were removed, the cup just slid off the underlying case with hardly any force required. The case only needed a light sanding to make it like new. The two end caps also were quite easy to remove and they were left in a reusable condition.
All of this is good news. With a proper fixture to hold the sticks, I could separate the end caps and cells on a whole stick with everything ready to become part of another subpack.
No new parts required.
Time to get on the welding machine.

Forever solar desk lamp


Recycled Insight battery cells

We lost our power a week or so ago, and the usual search for a flashlight with good batteries was a pain in the A**.
In researching the Insight subpacks, I had to remove a weak cell from the stick to examine it. That left 5 pretty good D cells, the only problem being that they are welded together. I carefully used a dremmel grinder to grind off the welds, and in the end had 5 nice high rate NIMH cells. A few short pieces of heat shrink, and they were ready to be put to work. I installed them in a cheap LED emergency lamp, and made a small 4 cell solar charger.The solar panel current was passed through a diode so it would not discharge the cells at night. Now I am ready for the next power outage

Inside the cells


Insight 6.5AH cell inside

Ok now that I have a good electrical test fixture and have identified a cell that was shorted, recovered, but not back to full capacity, it is time to take a look at the insides of the cell.
The first thing that jumped out at me was the really nice welds. they are deep, show little discoloration in the heat effected zone,with no surface oxides, so they were probably done in an inert gas. The welds were so strong that the sheet metal tore rather than the weld separating.
I made a test fixture to see how much force it would take to "crack" the stick at the cell to cell boundary. Amazingly I was not able to break the welds, even up the the 300 lb max that the scale would take. I had to bend it back and forth 4 or 5 times to get the welds to break out of the metal.
I sanded of the weld stubs, and put the cell in a vice to drain out all the energy.
I turned the cell in my lathe and cut off the top.
The top inner welds were also solid, but I was a bit surprised to find that the top terminal and bottom terminals were simply pressed into the spiral wound plate.The cell must have been assembled in a press to give adiquate contact pressure. I carefully unwrapped the perforated nickle strips which measure about 24 inches. I looked at the plates and separator blotter material, and saw no signs of holes or burned areas(Capacitor Zapp), so this somewhat supports that the capacitor discharge did not blow any holes.Very interesting.

PTC strips exposed


Funny how we assume what is inside

Finally got a look under the black and clear covers of the PTC strip.
It turns out it is really 6 discrete PTC devices that are connected by stainless steel strips, so the active temperature sensitive area is located right in the center of the cells.Very interesting, I will run a resistance to temperature profile on one, and see what the actual characteristics are.

The PTC thermistors


Protecting against a short like a self resetting fuse

Several years ago I did some experiments to figure out what the red thingies on the battery packs were, and what they are for.
I hit one with a heat gun, and saw the resistance shoot up from ~ 160 ohms to over 600 with a quick blast. I explained my findings on Insight central, but someone else tried the test in an oven, and was not able to duplicate what I found.
I finally found myself with a loose Insight side terminal plate in my hand, and decided to put my original theory to a different type of test.
I set up my lab supply with 30V, and the current adjustment was set to the 1 A maximum for the supply, so no power supply current limiting would be involved.
I clipped two probes to the thermistor leads, and used one of the banana jacks as an on off switch. On first application of the voltage, the current pulsed up to just about the expected 185mA, but rapidly over only a few seconds the current dropped to only 25mA and the thermistor was at 250 degrees F. I held it for a minute, and the thermistor stayed at the 250 f.
If the battery tap cable was crushed and all wires shorted, all that would happen would be for the battery end plate to get warm. Nice safety. Swap crushed cable, and the battery pack is good to go.
A self resetting fuse.

Electric rake #1


Electric rake #1

Raking leaves when your property has a lot of oak and Maple trees can be a big chore each year. Several years ago, I made my first attempt at an electric raking machine.
Yes, yard blowers and vacuums are available, but most of them are gas powered, and they do not really rake the lawn which clears thatch.
I built this contraption from a surplus gearmotor, a drawer slide, a screen door spring, and some rope.
The rake is lifted when the motor crank starts moving up, as the drawer slide has an adjustable rope that limits the handle up travel. Once the gear motor crank starts the down stroke, the rake hits the ground, and the rear of the handle responds by stretching the adjustable screen door spring, which is adjusted for the correct raking down force.The gearmotor arm sweeps rearward for the raking stroke, then lifts the rake at the end of the rearward stroke.
It works pretty well, but is a bit slow and has a fairley short stroke, so while it is a cool electric raking machine concept, It needed a lot of work to make it be a time and labor saver.
Back to the drawing board.

Making a Cheap battery State Of Charge gauge more accurate


Making the SOC meter more useable

I had to buy some speed controllers and other EV parts for my yard machine, so I took a chance and got what was supposed to be a 36 Volt system battery SOC gauge.
kelly controller SOC gauge

I took out a variable voltage power supply, and checked the led display's SOC against a Trojan battery company equivalent voltage chart, and found that it was not even close. I could not use it as it was, and the thing was glued together with no way to get inside.I took my band saw and cut the outer tube so the back could be removed. The circuit looks like a series of resistors that make up a voltage divider and some op amp comparators to turn on the leds when the voltage rises over the voltage divider taps. This type of comparator can be quite accurate, but it depends on voltage references and precision resistors,which this unit does not use. What was most interesting was that there is a potentiometer on the circuit that can adjust the divider current to make the whole scale move up or down. I set the power supply to the 100% SOC voltage according to the trojan chart, and then adjusted the pot so the 100% led just came on. I then adjusted the power supply down until each led went out, and noted where the 50% SOC was so I knew when to get to a charger. While this setting may be a bit temperature sensitive, it at least made the gauge usable.
To gain access to the pot, without cutting open the whole case, drill a hole in the indicated place on the back cover and you should be able to adjust the gauge without cutting it open. Of course one could calculate the precision resistor values required for accurate SOC throughout the range, and replace the 10% resistors that they used, but relative SOC is all I need so simply getting the 100% and 50% points will be sufficient accuracy for me.

Solar headlight


carbon free light for years

Headlights are a great way to save energy, by taking your light with you rather than turning on room lights. I modified this headlight by punching a small hole in the side, and carefully soldered to the battery end terminals. The wires are run to a two terminal piece of some breakaway .1" on center headers. The solar panels are in series, and a diode is in series on the + lead, to prevent discharge of the cells at night. with 750MHA NIMH AAA batteries, a full day of sun will give several hours of light per evening. The batteries should last for years.

Battery current sensor


How it works inside?

If a larger capacity battery pack is used in the Insight, we must "fool" the BCM into allowing the full AH capacity of the new pack to be used.
I setup a BCM and junction board with current sensor, to determine the best way to fool the current sensor into thinking that only 1/10 of the actual current is passing through it. After a thorough analysis of the system, I determined that the Insight battery current sensor was outputting a current loop signal with ~.20mA/A, and since it is a current loop, a simple current dump resistor to BCM ground should do it.

Inside the 100A semiconductor battery pack fuse


The insides of a high power fuse

The battery packs of all hybrid cars have a very special fuse to protect the circuits and the pack from short circuit and over current conditions.An Insight owner came over so that I could help him diagnose his IMA problem, and we found tht the main 100A fuse was blown. He had already replaced the inverter stage, which must have been the cause of the fuse failure.
Opening a high voltage high current DC circuit requires a special type of fuse. It is called a semiconductor fuse, but this name is misleading. There are no semiconductors in the fuse, the name describes it's ability to blow extremely quickly so any semiconductor devices it is powering will be protected, and this means rapidly quenching the hot arc that forms when a high power DC circuit is opened. The metal element is similar to a regular fuse, but the fuse is filled with an insulating glass like powder that rapidly melts and quenches the arc.I opened the blown fuse so we could see how it is constructed.

Did it again for the last time


More cooked prius subpacks

Well I cooked some more prius subpacks. This time was 10A for several hours. I am not going to do this again. I have a lot of microswitches in my surplus drawers, so I will add a switch in series with the pack charging path that will sense the first sign of internal pressure build up (adjustable), and kill the charge when the first swelling is detected.This seems to be around the 100% SOC point.

A bad Prius subpack explored and fixed????


blasting a prius subpack

Carolyn Coquillette from Luscious Garage in San Fransisco,
( http://www.lusciousgarage.com/ )was kind enough to send me a first gen Prius battery pack. The pack was bad and the codes indicated two bad subpacks.
Carolyn attended one of the Hybrid classes that I was involved with, and is a highly qualified hybrid auto technician.If you live in the bay area, and have a hybrid, you should consider Luscious Garage for all of your service.
I have not had the opportunity until now to examine a Prius subpack that was actually bad, so I immediately labeled the subpacks, and removed them from the enclosure. A quick voltage test showed two subpacks that were a full 1.2V or more lower than the rest, and one that was about .7V lower.
I put the two bad subpacks on a charger to see if the subpacks would recover, but shortly after the charge seemed to be complete, the subpack dropped back to the same condition, indicating that the subpack likely had a shorted cell.
I used my milling machine to expose the cell to cell connections, and indeed, one cell was at 0 volts while the rest were fully charged.
One must be very careful playing with charged NIMH batteries in a milling machine.
My end mill managed to hit the plates of one of the good cells, and the current lit up the shorted area cherry red.
In my RC modeling days, when a NICAD cell would short, it usually was a symptom of a plate to plate short caused by a nickel whisker.The trick we used to do was to blast away the short by discharging a big capacitor into the cell. Sure enough, I took the shorted cell, and blasted it with 250V from a 6100uf cap, and the cell was able to be charged. Next I took the other subpack with the same problem, and blasted the whole subpack with the same capacitor.
While I do not expect the blasted subpack to be equal to a good subpack, it was nice to see that the initial response was to fix the shorted cell. If ever there were an imbalance condition, this is it. one cell started at zero charge while the rest were fully charged. I will give the subpack some time to equilibrate, and then compare it under load with a good subpack in a series configuration.
While blowing out some plastic chips from the machined subpack, some of the potassium hydroxide electrolyte sprayed past my glasses into my left eye. It burned a bit, and I immediately flushed my eye with a lot of water, and my eye is fine. Batterys are dangerous in many ways.

Rebalancing the Insight battery?


Long term storage warning

A common problem with the MT Honda Insight, and to a lesser degree the Civic hybrids is a condition called recalibration. The most common version of this condition shows up as a rapid drop in the battery SOC gauge from some intermediate value to empty. The car goes into a forced charge to bring the SOC back up, which if it happens frequently, can take its toll on the MPG.
One of the possible reasons for this condition is an unbalanced SOC between the different cells in the pack.
The warning label from a civic battery pack warns to run the car for 30 minutes a month. It is likely that the self discharge characteristics of the cells is not predictable, so some subpacks will discharge to a lower level than others, and there does not seem to be a rebalancing mechanism in place to correct the imbalance once developed?
The civic hybrid battery packs made 2005 or before are a good source for replacement 6 cell subpacks with the PTC temperature strips attached.

What happens to a Prius subpack when you overcharge it?


Opps! Cooked some prius subpacks

Well it happened again. I put my dual prius subpack on my 6A 75V solar panels for a quick recharge between the solar dish runs, and forgot it for several hours. When I finally remembered it, the black tape holding the subpacks together had nearly ripped apart and the cells swelled up with a lot of internal hydrogen pressure. Usually the subpacks are clamped tightly so they cannot swell, so they would vent the gas instead. I measured 140F between the packs, and submerged them in water to cool them.
Even after an overnight cool down, the subpacks were still swollen. I drilled six holes in two boards, to clear the locating tabs, so the clamping pressure would be uniform, and squeezed the subpacks together. Several hours of clamping, and the cells were back to normal shape, with no signs of any fluid venting.
I used the same packs to run my next canning run, and also to run a 30A 12V heater, and they seem to

Have been using the same dual pack in my lawn tractor for the last 2 years and they are working fine.
Some tough batteries for sure.

Insight Battery pack turbo cooler


Insight Battery Turbo Cooler

As any owner will tell you, when you are pumping charge in and out of the pack with MIMA, it can get hot, especially in summer. The battery is cooled with a two speed fan that draws air into the battery pack from behind the passenger seat inlet vent.MIMA will force the two fans on at full speed when the temperature of the battery gets over 95F.
I made a Turbo Cooler for my pack by ducting the passenger foot AC outlet to the battery pack input duct. I set the fan on high, and the duct to foot only. The driver side vent is ducttaped over, so that all the air comes out of the passenger side.
The increased air flow helps a lot, with mild MIMA use, but if you are really pushing the hybrid ratio to the electric side, the pack can still get into the 100 F range. Because of the large thermal mass of the heavy batteries, it takes a lot to cool them even a few degrees. To Turbo Cool the pack, I wait till I am going down hill, so the power is nearly free, and turn on the AC to full, set to 60 F. A long hill and you can cool the pack several degrees.

Those useful Prius subpacks

Prius NIMH batteries are called on to produce over 100A, and can be charged at 100A as well. I have been collecting the batteries and have quite a few, and have been finding some nice uses for them. I have several vehicles that I only use occasionally, and keeping a good 12V battery in each is wasteful, and is not that good for the batteries. I tend to carry a battery to the vehicle whenever I use it, and remove it when finished. Lugging a 50-75 lb battery over 100 feet is a lot of work. Just to see how it would work, I connected two 7.2V prius subpacks in series, and using a 100A Anderson connector, plugged them into a dodge caravan. The car cranked just like a new 12V battery. I put a connector on my 1965 John Deer tractor, and bang, it cranks effortlessly.
My lawn tractor uses the same configuration. The batteries do not need charge regulation, as the alternator or vehicle charging system will regulate to 13.6V , about 80% SOC on the NIMH.
A 5 lb starting battery compared to a 50 lb one. What a labor saver.


prius subpacks start van

State Of Charge (SOC) determination with NIMH batteries


IMA system SOC determination

The terminal voltage of a lead acid or Lithium battery can be used to determine the SOC, since it drops as the battery is discharged.
The discharge voltage of a NIMH pack is very flat, compared with other batteries, so the best way to determine the SOC is to measure the current into and out of the pack. The minimum SOC where one of the subpacks reaches the drop out area of the curve is used to start a forced recharge called a recalibration. The full SOC condition can de measured by watching for the voltage change at the full charge point, as well as the rapid rise in temperature when at that point.

Battery pack monitoring


Battery pack monitoring system

Back in 2001 when I first started being active in the Insight on line community, I was concerned about the life of the battery pack, especially the issue of recalibrations. Our understanding of the reasons for the "recals" have not advanced much since then.
I designed a battery monitoring system that used relays for isolation to read each of the 14.4v monitoring points in the pack. The connection to the pack was accomplished with a special spring loaded back probe that just clipped onto the battery. A Labview based data acquisition card and software scanned the taps and recorded the voltage graphically.
I fully expected that my pack would eventually start having the recal issue and wanted to be ready. I am now nearly at 100K on my car, and still have not experienced a single recal. Honda says that gradually accumulating errors in the coulomb counting SOC software is the reason that recal's happen, but I ask what causes the errors? My intuition says that one cell in a subpack is of lower capacity, and since the pack is one long 120 cell series string, that one cell will cause the whole pack to have less capacity than the system expects. When the BCM senses the weak cell dropping voltage as it becomes depleted, the system stops assisting and starts charging to prevent the cell from becoming reverse charged which is instant death. A smart system would have the ability to balance the pack by charging that cell more than the others. Unfortunately since the cell is in a 6 cell subpack, that is not possible with either the Honda or Toyota systems.

The battery controller

The Prius and Insight BCM modules are quite different. The Insight one is not as complex. The main controller for both is painted over so the part numbers cannot be read.

Prius battery pack

The Prius battery system is made up of 6 cell subpacks that are cooled and mounted in a much more practical fashion. The subpacks each have a thermal well, and high current connections. The bumps and sockets on the subpack sides allow a small air gap for the cooling air to pass through. Because the internal pressure in the cells would deform the plastic sides of the subpack, the whole 40 subpack assembly is compressed between two metal plates, so the pressure cannot deform the subpacks, and each subpack has a pressure relief valve. A cleaner easier to service pack for sure.

Insight Battery switching/current monitoring

The Insight pack is protected by a 100A fuse, and high voltage DC switching relays. The current into and out of the battery is measured by a hall effect current sensor, and the Current into the MPI is measured by a second current sensor.

Insight battery

The insight battery pack is made up of 120 D cell NIMH batteries, grouped into packs of 6 cells. You can see that the air path for cooling the pack is not very open, therefor the cooling of the pack is not very agressive. The pack has a strip of thermal sensing material that is in intimate contact with each cell in the pack. The D cell subpacks are terminated with a large stainless steel nut that can handle the 100A output. This is the special part of the pack, as we have not found a D cell that has similar high current contacts, so cell replacement with new subpacks is not an option at this time. The civic packs up to 2005 are drop in replacements, so you could get one of those much more readily available used packs and rebuild your Insight pack with the civic subpacks which are direct replacements with a more robust cell structure. The 2006 and up civic packs are built differently and are not direct replacements.