How do I test the cells in a battery to see if any have failed?

[GM Canada]

Hello everyone

Here is my question of the day. As it says in the title. How do I test a battery to see if any cells have failed? I want to check a 36v16ah lithium and a 48v12ah lithIium. I am sure this was discussed somewhere but I can seem to come accross it. Maybe someone can help?

Gary

[GM Canada]

Just got this picture from china, Looks pretty simple...

Gary

[MonkeyMagic]

I will try and locate the post I wrote a fair bit of instructions....

[Bikemad]

I am sure this was discussed somewhere but I can seem to come accross it. Maybe someone can help?

Gary,

The easiest way to check the voltage of the individual cell groups is to measure the voltage at the balance lead connection.
Clip your black meter lead on the battery -ve connection and then make a note of the readings on each of the balance leads, and also check the reading on the battery +ve lead to ensure all groups of cells are measured.

Some typical test results can be seen here.

Unfortunately, a faulty cell group may still give a normal unloaded voltage reading, but if you can find a way of loading the battery and make the cells work harder, the voltage will normally fall noticeably lower on a faulty cell group.
My 240V 3kW fan heater will draw approx 2.4Amps (130Watts) from a 48V pack, but I have to make sure it doesn't get too hot because the fan doesn't work on a low voltage DC supply. Two or more similar heaters in parallel will obviously produce more load.
A kettle should also work nicely, but you may dehydrate while waiting for it to boil.

If you use a four way extension lead, you can plug in the kettle, an iron and a couple of heaters too and probably draw 9 or 10Amps if you're lucky.



Check out this post for more information.

Alan
 

[Phil]

Hi Gary,
Monkey sent me this info a while ago (thanks Monkey)
Thread here. http://goldenmotor.com/SMF/index.php?topic=2816.0
Not sure if I've posted this correctly  
There's a photo in the original thread.

MonkeyMagic
Cheeky Chimp
PhD. Magic

Posts: 761


STILL Rebuilding the dual drive!!!!!!!!!!!!!!!!!!!


    Re: Another Battery problem
« Reply #1 on: January 19, 2011, 03:34:48 AM » Quote  

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G'day Phil

Having already opened your battery - this test you have completed really outlines the main sections of the battery. The total should be your readings + ~4.1V for the cell block connected directly to the BMS.

That's why you get the reading of 41.9V however,

Sounds to me that cell block #1 could be faulty/failed. What happens here is your other cells will overcharge and your total reading will and could be within range, however once you draw substantial current through the cell, the voltage lowers to the point of your BMS cutting out.

If you find your battery is charging quickly, or quicker than normal then your BMS could be cutting out due to the cells overvoltage - your charger then thinks "oh my job is done, I'm switching off now" and you "think" you have a fully charged battery - when really it's just a matter of discharging the over charged cells before the BMS cuts out again.

Considering your readings, it would pay to inspect each cell block individually.
Here's how to do this: [scroll to the bottom to get to the point lol]

The biggest concern here is putting to much heat when soldering. You will need some locking pliers or something to 'hold' onto the nickel battery tangs so you avoid putting any heat into the cells. The nickel heats up extremely fast - and is overlooked alot when soldering.

1. Have a real good look at your battery, take a picture from the top and bottom - so you can re-attach all of your BMS wiring when you put it back together. Wear some protective glasses, as there is quite a bit of solder, and you may fling some around doing this

2. At the rear end of the pack (non-BMS side) you will see a big clump of solder. Since there is so much, you have to be really concious of the heat - or you could easily damage your other cells while doing this.
Use locking pliers, or 'helping hands' or a bunch of aligator clips EITHER side of the nickel battery tang. If you have an adjustable heat soldering iron great - otherwise use a 40-60W iron and prepare your solder tip so it will draw solder away quickly as possible. Touching the battery tang while using solder braid or a solder sucker will also be a good idea. With gentle force, the battery tang will come apart.

3. De-solder all the BMS wiring from the top of the pack. Remember to mark the wires or positions, the cable will sit back the same way anyway, but at least you will be clear on what goes where.

4. Remove all the red taped protective cardboard, take the sticky tape of both sides of the pack

5. Remove the BMS - be very careful again with the direct GND connection (BMS-[D]) as this is another big battery tang with heaps of solder on there. Try drawing the heat away from the cells using your tools like above.

6. BMS removed and everything else, you should be left with your pack, and a wire that was soldered to BMS+ you have removed. Starting from the rear (where there was no BMS) gently pull the pack apart. Again its protected in the middle with the red adhedsive cardboard. You can split the pack enough to test each individual cell block 1 by one. If you get to the front, and you are sure the failed cell is there - then you can decide to de-solder the pack completely. The solder point inside, up the front is the most difficult because the wire is pushed through the battery tangs, then heavily soldered. If you try to heat that part up while attempting to pull it apart - you can easily add too much heat into this and damage the cells.

You should have some work to do with the above.

Once you get the readings of each cell (using your multimeter set to 20V range) post them up here
Hopefully it's just 1 block or just a few cells and you can fix it before any further damage.

Peace out
MM







 

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[GM Canada]

Thank you for your assistance guys.

Here are a few pictures of the test. How much variance should there be? Or should they all be exactly the same?

Gary

[Cornelius]

Preferrably, they should be at equal voltage, but the spec says max 0.2V difference...Your 2 first pictures shows a difference of 0.37V, and that will lead to reduced performance over time, if it have not already happened.

How do you charge and discharge it?
Most people recommend to let it sit on the charger at least 1/2 hr after charge have finished to get the cells to equalize... But i've seen several PCM's that states that equalizing between cells only happens if one let the battery rest at least 1/2 hr after charge (with charger and load disconnected.)
Maybe you could find out what is the case with our GM batteries?!

[GM Canada]

Thanks for your input. These are actually a customers pictures. He has allowed me to use them, Here are the voltages. What do you think?

<quote>
Hi Gary,
I have test the cells of the battery and the result is :
4,17 V        -        4,05 V
4,18 V        -        4,18 V
3,80 V        -        4,15 V
4,06 V        -        3,93 V
4,18 V        -        4,18 V
</quote>

So it looks like 2 bad cells possibly 4 I guess.

He only sent me ten numbers though..

Gary

[Bikemad]

He only sent me ten numbers though..

If it's a 36V pack there will only be 10 groups of cells to test (10 x 3.7 = 37V, 13 x 3.7 = 48.1V). If it's a 12Ah pack there will be three paralleled cells per group, and four per group if it's a 16Ah pack.

So it looks like 3 bad cells possibly 6 (12Ah) or 4 bad cells possibly 8 (16Ah).

Hope this helps.

Alan