Headway battery advice

[atatistcheff]

I recently noticed that a 4s battery I built with 8 Headway cells was not performing near capacity. I disassembled and found that one cell was about a volt low after the pack had fully charged. I built it using a Daly 60A BMS from eBay. I suspect that the answer is replacing the bad cell but I'm wondering if there is any way to test the BMS balance function? I know that if the cell is bad the balance function can't fix it but I'd like to have some confidence that the balancing is working.

 

[Ampster]

I am surprised that if the pack was 2p4S that one cell would be lower than the rest. I would have espected that each buddy pair would be the same voltage. The exception would be self discharge after disconnnecting. That would be easy to test, just charge that cell and see how long it holds its voltage after normal settling.
As to your orignial question about the balancing circuit it seems unlikely that the balancing was off if one cell was okay. I would be more suspect about a bad connection between those two cells.

I am nostalgic about Headways. I got my start with some Headways on an ebike conversion. That led to an EV conversion and then to several iterations of hybid inverters with DIY battery packs.

 

[atatistcheff]

Ya, that's a good point. The buddy cell was low too but I don't think it was as low. I probably should have been more scientific in my measurements. Top balanced them and the first capacity test is a lot closer to what I would expect. I'll charge them up and then do some more cell testing. Thanks!

 

[atatistcheff]

After the top balance then reassembling the pack and doing a capacity test, upon charging the pack again I found that 3 out of 4 pairs were at 3.33V and one pair is at 3.48V. Leaving the pack sitting overnight the voltages are the same in the morning. Is it too much to expect the BMS to bring down that high pair of cells into the 3.3V range? It doesn't seem to be doing any kind of balancing.

 

[Ampster]

Is it too much to expect the BMS to bring down that high pair of cells into the 3.3V range? It doesn't seem to be doing any kind of balancing.

Typically many BMSs start balancing at 3.4 Volts and some only do it when charging. What are those settings on your BMS?

 

[TomC4306]

After the top balance then reassembling the pack and doing a capacity test, upon charging the pack again I found that 3 out of 4 pairs were at 3.33V and one pair is at 3.48V. Leaving the pack sitting overnight the voltages are the same in the morning. Is it too much to expect the BMS to bring down that high pair of cells into the 3.3V range? It doesn't seem to be doing any kind of balancing.

You say after top balancing however those eight cells are definitely not top balanced if six are at 3.33 and two are at 3.48.
How exactly did you top balance?

 

[atatistcheff]

BMS is from Gridvision.net which is apparently defunct so I don't have any specs on the particular model. It looks like a Daly BMS and they say they don't start balancing until one cell gets to 3.6V. This one is probably similar which means I am probably expecting too much from this BMS. I'll put it on a charger for a while and see what happens. These are used headways from batteryhookup so they are anything but matched cells. I suppose I just need to lower my expectations a bit. Thanks!

 

[SparkyJJO]

Probably need a better BMS too. Those Daly BMS don't have a good reputation. And their balancing current (bleed off) is something like a paltry 30ma or something nearly useless.

Those cells aren't fully charged at only 3.33v though, at all.

 

[Ampster]

Those cells aren't fully charged at only 3.33v though, at all.

If that is the resting voltage they may be close to full. He did say he charged the pack and one pair is at 3.48 volts

 

[SparkyJJO]

If that is the resting voltage they may be close to full. He did say he charged the pack and one pair is at 3.48 volts

But that's only one pair. The rest aren't there.

 

[Ampster]

But that's only one pair. The rest aren't there.

Six cells (3 pairs) are at resting voltage after being charged and two cells seem to be stuck at 3.48 which is in excess of resting voltage so he probably has an issue with those two. From the beginning his issue has been about those two cells. I agree it might be useful to know what voltage charging terminated at so rule out that the six are undercharged.