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Old 07-17-2011, 01:05 AM   #1 (permalink)
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Question Balancing theory - learning & experiment

I've seen lots of battery balancing theories some of them based on a single (mentioned or not) chemistry. Different types vary quite a bit in what is required for their prolonged health!

I've got some old lithium polymer batteries for my RC helicopter which I believe will work for experiments. This is going to be very boring and I'll setup a PIC to cycle them with my charger/discharger.

Now while they fail differently then Li-Ion they still fail in more or less the same ways. (They tend to catch fire and burn when overcharged or punctured, or burst if abused) Draining one to nothing isn't harmful as far as I can tell, provided you don't try to recharge it

Here are a few things evtv got me thinking about (only seen the episode on BMS's) I'll look for a follow up when I have time but I have about an hr or two at best to do research (or anything else ( )

If I understand correctly (I may not) Balancing is more or less about getting your weakest battery in the midpoint between the other batteries when it comes to li-ion batteries. This is because they don't need to be topped off, and they don't need need to be fully cycled. This means in practice we don't need them top or bottom balanced we ideally want them all to reach about 50% at roughly the same time. Because of the discharge/charge curve this is a very small change in voltage so difficult to measure accurately as best as I can work out.

So what I still need to learn is a few things and this leads to my experiment:

Heres what I'm going to do I have a (very) old 3 cell lipoly that when new was 2.5Ah I think. This is small enough for getting results quickly vs a 'small' 60Ah lifepo4 battery, not to mention MUCH cheaper. I won't fly with this pack because its so worn down its not a very long flight time. Actual capacity is most likely in the 1.2Ah range. Actual flight time on this pack used to be 12-15 minutes now its maybe 5-6. :/

What I'm going to do is quite simple. IIRC batteries when hooked up in a string will self balance while current is flowing to some extent. They won't balance perfectly but will be close depending on the load your drawing. This doesn't apply with high power discharges though because of internal resistance and limitations. My theory is that if you slowly discharge ALL batteries once a month or two on a common load they will self balance close enough to not be an issue provided A) You don't over charge any single cell, and B) Don't over discharge any single cell.

I'm going to take that battery I have that is not flight worthy and cycle each cell one by one, determine which is the lowest capacity battery. I'm then going to charge the weakest one to between 40-60% of current capacity, then I'm going to charge the other 2 like normal (most likely to 98-99.5% capacity)

I'm then going to draw 1.5C from the pack until the lowest hits the LVC. Then I'm going to charge until the highest hits the HVC. Then check the voltages, write them down and do it again. If I'm right I should slowly see the lowest cell get 'close' to the strongest one in terms of SOC. Remember its starting the discharge cycle at 40-60% SOC. I should see it get to 90% before the highest hits its HVC. If it does self balance I'll make a heatsink to take up to 20C and I'll repeat at 2.5C, 5C, 10C, 15C, and 20C, provided the batteries are still alive and kicking under such EXTREME ABSUE. (They are rated at somewhere between 5-10C intermittent-discharge MAX IIRC)... The battery is missing its label though.. due to swelling.

This will all take time because A) Its going to be done outside, B) Its going to be done in a fire suppresion bag (I need 2 more, so each cell can fail on its own) and I need to get my PIC chip to physcially remove the charger from the batteries. It will also check temp. I need it to alert which cell tripped an event (eg HVC, LVC, and thermal cutoff)

My question to everybody is this: If anybody knows of somebody doing an experiment like this then let me know =) I don't want to redo it... Second, am I correct about chemistry? I may be able to find some small lifepo4 batteries and will experiment with lowest capacity cells if I can find if its under $50 for the batteries -- assuming this test shows an improvement in the balance.

This would mean that a good BMS solution is really none, but rather a LVC/HVC on a per cell alarm. Also to prevent a discharge all 'nodes' would be powered from a bus and power would be supplied from the master. This means completly reworking my BMS concept which is why I'm posting a new thread...

Any input or information is appreciated. NOTE: This does not apply to SLA's because as far as I can tell from (not reading about them but rather about lithium batteries) is that they need to be top balanced to be best maintained. They may self balance though and I may look into testing that if the results are promising. I only need one battery... dissasembled, and preferably nearly end of life... to bad I recycled my last trucks SLA last month

sorry to write a book... sorta...

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Old 07-17-2011, 04:55 AM   #2 (permalink)
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Update

I just found a 3s 1500Mah LiFePo4 pack for TX's 5C discharge and 2C charge rated, I'll use them for the same experiment, unless I'm not setup to do it until they get here in which case I'll test them. =) Only $9.99 + S&H but I think its going to ship from China so it may be awhile. Under $20 total for an experiment I can't complain S&H is a killer though...
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Old 08-06-2011, 03:04 AM   #3 (permalink)
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I don't think the batteries self balance. That's something that Jack (evtv) I think is a believer in, but I am not familiar with any evidence of it.

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