RC LiPO imbalanced discharge question

[theloadedquestion]

Hey all,
So I just got back from a ride where my LVC kicked in only about halfway through my trip and I had to pedal all the way home. This seemed weird, but I was also doing some controller tuning and therefore lots of stop, WOT, stop, readjust, WOT, etc before even getting to the trail. And it's a power hungry trail, so I wasn't yet suspicious. However, when I got home and started using my cell checker I got a bit of a shock when the first 4a pack I tested was still showing around 3.9v/cell, then the next 5a showed arpund 4.1v/cell! WTF. But then the answer revealed itself with the next pack, a 5s which was warm to the touch when I picked it up, and tested at just 3.2-3.3v/cell. Ouch! I proceeded to check the rest of the packs and all were similar to that first one at around 3.8-3.9v/cell. So it was basically one 4a pack that for some reason took all the abuse, and one 5a pack that contributed basically nothing, with the rest sharing the remaining load evenly.

What the ever loving f-bomb happened here? As I said I was doing some tuning and did wildly vary battery and phase amps, and their ratio throughout that testing, and then proceed to abuse the setup on some steep desert mountain trails. But none of that is unique to this outing and I've certainly never seen anything like this before. Usually everything is almost perfectly balanced and I only occasionally even have to balance charge. Thanks god for my LVC kicking in.

Any ideas what happened here? Are these batteries screwed?

 

[e-beach]

We will probably need more info, like photos of your wiring. If you want to post pictures make sure they are under 512k each for easy loading.

However my first question is this: Are you mixing old and new packs?

 

[theloadedquestion]

We will probably need more info, like photos of your wiring. If you want to post pictures make sure they are under 512k each for easy loading.

However my first question is this: Are you mixing old and new packs?

Nope, all packs bought as part of the same purchase, same brand, capacity, etc. They probably have eqivalent of about 5 full cycles on them over the course of maybe 15-20 partial cycles. Never been discharged below 3.5v/cell (until that one 5s pack today), and no apparent damage or puffing. I feel like it could have been a wiring issue but I don't know how, no changes have been made and no damage is apparent. Will inspect all my xt90 plugs more closely. Maybe I didn't seat a connection all the way when I plugged it all back in after charging.

 

[e-beach]

Could also be a bad cell in the low pack. However, tell me about your 4a pack and 5a pack wiring.

 

[theloadedquestion]

Could also be a bad cell in the low pack. However, tell me about your 4a pack and 5a pack wiring.

I don't know why that autocorrected to a from s. I have 6 5000mAh Turnigy 20c LiPO packs, 4 x 5s and 2 x 4s, each pair of 5s is paralleled as well as the pair of 4s first, for 1 x 4s 10Ah and 2 x 5s 10Ah, which are then wired in series for 52v 10Ah. All uses 10ga wire and xt90 plugs.

Balance charging everything now and we'll see what happens on the next ride after I inspect everything and pay extra close attention to my hookups.

 

[e-beach]

5s and 4s fully charged would be different voltages....5 x 4.2 = 21v and 4 x 4.2 = 16.8v. Sounds to me like the 4s packs are being stressed by the setup you are running.

 

[j bjork]

There must be a problem in your wiring. I guess the low and high packs were paralell?

 

[theloadedquestion]

5s and 4s fully charged would be different voltages....5 x 4.2 = 21v and 4 x 4.2 = 16.8v. Sounds to me like the 4s packs are being stressed by the setup you are running.

No parralelling between any 4s and 5s, only same voltage, same capacity paralleled. It's just a series connection, 2 x 5s for 36v, plus 1 x 4s for 52v. All cells are same voltage when connected in series and that's all that should matter for series connections.

 

[theloadedquestion]

There must be a problem in your wiring. I guess the low and high packs were paralell?

I wish I had paid closer attention, but yes I believe so. I believe it was the 5s pack testing at @ 3.2v/cell parallel with the 5s pack that tested at 4.1v/cell. If so, that would indeed suggest to me that I had an improperly seated connection or something similar. Thanks.

 

[theloadedquestion]

So after further testing and another ride I believe one of my 5s packs has been damaged, possibly a damaged cell, possibly multiple cells. One pair of 5s that are paralleled, with one 5s draining fast and appearing to take more of the load, and one 5s draining slow and appearing to take minimal load. After this second ride (before which I made darn certain there were no wiring/connection issues first) the cellog was showing around 3.6v/cell on the suspect pack and 3.9v/cell on the other 5s it is parallel with. Damn. I tried to test internal resistence on my cheap ass hobby king charger and while I don't at all trust it, it is showing a value of 3 for one cell, and it usually shows a value of 0 for any pack I test, and I've never seen it be above 1, so I suspect I have damaged cell there? Any other way to test this without taking the pack apart? Is it dangerous or damaging to continue to run this pack parallel with the other 5s?
Thanks!