Pack discharger

Pack discharger
final discharger accessory for charger

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Notice
I had a guy in Arizona stick the discharger in a closed hatch at >100F, and when the discharge ended, the discharger stayed on, and the charger started charging, the discharge is 2A, the charger is only 1A, so the pack saw a 1A discharge, and the only way to stop it was to turn off the pack, or unplug the discharger from the car harness while current was flowing. Not a good thing.

The Power mosfet failed shorted due to overheating.

A shorted mosfet is out of control of the charger, so it would have totally discharged the pack had he not been checking it when it started charging.

We can't stop the discharger if the mosfet won't shut off, but we can make the charger make some very noticeable noises to get your attention, and we can advise that all cycling is done in a car that is well ventilated, and below 100 degrees., preferabally much lower than that.

We will have new code hopefully early next week, that we will install in all the new batch of chargers.
This code detects the shorted discharger and makes a lot of noise to let you know.
It also gives full discharge control to the user when they are in the password protected tech edit Discharger stop bypass mode.

This was an extreme temperature condition, and under normal temperatures, the heat sink is hardly warm.
The Max load current is 2A, the mosfet is rated for 8A, so we are well into the conservative area, and should not have thermal problems unless the ambient is too high.
I Also advise that the rug and IMA switch cover is exposed so the main breaker can be turned off to end a shorted discharge safely. Pulling the charge connector may arc and damage the connector pin.
Glad we heard about this before I shipped a bunch of the chargers.
For the people that already have upgraded code, and a discharger, contact me and we will get your chip upgraded, just ask that you send the old one back.

Discharger assembly and test video

So be careful, Best leave the discharging for the moderate temperatures below 90F
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The charger can do a much better job of balancing a battery if the pack can be discharged deeply, but not so deep that any cells reverse.
The Dis-charger accessory for the grid charger can do that function.
The discharger is pretty simple, just an optically isolated 250V Power MOSFET that acts as a computer controlled HV switch to connect and disconnect the discharger to the pack.
The schematic for the basic circuit is here:
Discharger rev 2 shoplight

Another function that can be performed with the charger and discharger is subpack internal resistance comparisons.
Every battery has a characteristic called Internal resistance (IR). This is a very important number, as it will determine how well the cell can output current. A cell with high IR will drop in voltage much more than a cell with low IR when under a load,and will heat up much more as well.
The same cell and will rise in voltage when charged more than a low IR cell.

The actual voltage drop with knowledge of the current flowing can give us the IR, which for NIMH batteries, can be a pretty small value.
So how do we reliably measure this small value with the charger/discharger?
The difficulty in measuring IR's small voltage, is that the voltage is riding on the 7-8V DC that the subpack is at, so the meters ability to see voltage of as little as 0.002V when on a scale that can read 8V requires a special differential meter. A second problem is that as the load is draining current from the pack, the overall voltage is also dropping as the SOC drops, so you have a moving target.
The discharger and charger have a pulse mode.
This mode is protected by the tech edit password, as it must be used correctly or over discharging of the pack can occur.Since we are trying to compare the IR of all the sticks to determine if any are high in IR, the pack would be disassembled on the bench to allow access to the end points of the sticks.

The pulse mode turns the discharger on, and then off at 33HZ, so effectively current is flowing for half the time, and not for the other half. The frequency is nearly 50HZ, so it falls in the range of the regular 50-60HX AC line.
The test mode is turned on, and while the pack is slowly discharging.
Since the series string of cells is all being pulsed at the same time, the same current will be drawn on all cells, so all cells have the same pulsing current.
The AC meter setting will block the DC of the cell, and only show the AC voltage produced when the current goes on and off, and the stick voltage varies at 48HZ. The voltages are still small, but most DVMs can easily and reliably read the AC value.
Run quickly down the string of subpacks, and record the voltages, and if any subpack has a noticeably higher AC voltage, it will likely be the one causing your problems. The resistance of the harness, Harness fuse, main contactor, and main 100A fuse are all adding to the total pack IR, so the best place to get a true pack IR is right at the charger tie in points, but be aware that the main fuse, main switch, and all the internal bus bars are being included in the test.
On the subpack level test, we are right on the subpack terminals, so the IR value will be quite accurate. I will be doing a whole series of You tube videos on how to use the charger and how to do this test, and how to interpret the results.
A graph of one of the subpacks that was pulsed with a 30A charge, clearly shows that the blue trace (cell4) has a much higher IR than the rest of the cells in the stick.
Cell #4 with High IR

Discharger current at different voltages
170V = 1.701A
158 = 1.689A
150 = 1.643A
140 = 1.580A
125 = 1.482A
120 = 1.450A
119 = 1.382A
100 = 1.309A
90 = 1.23A
80 = 1.162A
70 = 1.087A
60 = .997A
50 = .910A

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Be aware that until you have Charger code V2.0, or higher the discharger will not work.
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The cost are:
Shoplight $24
HV Mosfet $9
Aluminum heatsink/box cover, materials and labor $ 20
Extension harness with HV tap attached to shop light cord $50
Aux control cable, with Aux connector attached $7
Labor to rewire shop light and attach cables $25
Total $135, plus actual shipping.
Shipping:
Zone 8 --$37, Zone 7 --$32, Zone 6 --$29, Zone 5 --$26, Zone 4 --
$20, Zone 3 --$17, Zone 1-2 --$15

The discharger will come with the required in charger connector.

A special 8" Aux harness with 3 foot cable for tie in to your DIY discharger would be $50
Contact me directly if you are interested.
contact us

FAQ
My charger is working great. Now, I've been following your work on the discharger and am trying to determine whether I really need one. I stumbled across this a post of yours from back in July of 2011:

thread

In this post you state:

"A smart discharger/cycler/tester, for deeper diagnostics of a pack that is not able to be brought back with the charging/balancing." This sentence gives me the impression that the discharger is perhaps not necessary for everybody and that the charger, alone, may be sufficient for some.

As I'm sure you can understand, I'm just trying to control my spending if I don't really need the discharger. For somebody who isn't very knowledgeable in battery chemistry, can you help me understand how I can determine if I can skip the discharger?

When I wrote that, I was expecting that we would have no problem doing a good discharge by driving, but that seems to be more difficult than I anticipated.
From my experiments here, the charging balances all cells at 100% SOC, but it looks like unless we discharge the pack to as low a point as possible, the cells capacity will not recover as well as it could. When discharging with driving, we are both discharging and charging, and the car fights us when we try to deplete the pack, and finally the pack will never fully discharge below 20%.
This cycling from full to empty is the basis of many RC battery chargers/conditioners, and is the main technique that is being used by battery rebuilders, as the first step in rejuvenating an unbalanced pack.
The silver Insight that has been sitting for 2.5 years in my driveway, has an old pack that sat idle except for maybe 4 charges over that period.
I pulled it out as a test pack for charger code testing.
It was first soak charged for 10 hours, and then discharged.It took 30 minutes to discharge on the first cycle. I ran the discharge/charge cycle 3 times. On the last cycle, the pack ran the discharger for 2.5 hours. a huge improvement in capacity.
The discharger also gives us a repeatable way to safely discharge the pack to a safe minimum value, so on the charge half we get a real number for mAh to fill the pack. So not only is it good for the cells to cycle deeply, it is also a good way to determine the condition of the pack, and see if further charging will bring further improvements.
SO if your pack is still in decent shape, and you discharge while driving when ever you can, you may not need the discharger.
But it opens up a whole new level of cycling and capacity measurements that are not possible without it.

Cycling is probably not something we need to do frequently, so possibly we can set up a loaner program, since it just plugs in, and involves no wiring?


Mike