LiFePo 48v pack with failed cell?

[Geoff V]

Gentlemen
I recently aquired a very secondhand but quality bike from a well known online aution with a 1000w hub conversion advertised as ' motor and battery in working order'. After collection I find the BMS has been shorted, destroying some of the suface mount components and two of the monitoring leads had been damaged and had been joined by twisting together and covered by gaffa tape! If placed on charge, the charger cuts out within a few mins. and the capacity of the pack appears to be non existant with voltage dropping to 6v when loaded with a small light bulb.
After remonstrating with the seller, who sent 3 old SLA batteries as compensation! I have ordered a new 48v 15Ah pack from HK and whilst waiting for delivery I will continue with the refubishment program.

However it occured to me that some/most of the pack may be recoverable and as it is reported to be 14months old may well be worth the effort. I spent several hours reading the relevent sections of this forum until what's left of my memory cried enough and forced me to ask the questions in a new thread.
For safety reasons I disconnected the BMS and measured the cell voltages individualy and in sequence. The first cell appears to have failed, reading 0v but the rest are all between 3.4v and 3.6v which has left me wondering if I could:-

1. Recover this cell.
2. Isolate this cell and run at a slightly lower overall voltage.
3. Source a replacement BMS that would operate with one cell removed.

Your words of wisdom would be much appreciated and any thought on a source for another suitable BMS.

 

[amberwolf]

For safety reasons I disconnected the BMS and measured the cell voltages individualy and in sequence. The first cell appears to have failed, reading 0v but the rest are all between 3.4v and 3.6v which has left me wondering if I could:-

1. Recover this cell.

http://www.endless-sphere.com/forums/viewtopic.php?f=14&t=44625

2. Isolate this cell and run at a slightly lower overall voltage.
3. Source a replacement BMS that would operate with one cell removed.

Sure, you can always do that. Most BMSs probably have a way to bypass unused channels. Some with a marked jumper or pads on teh board, some by figuring out what pins on optocouplers or sense lines to bypass or cut. No idea what would be a "good BMS" though, as pretty much every one of the ones I've seen on Chinese packs either in person or in threads on ES has had at least one reported problem with it, and some of them many--but it's not known if those were caused by design, manufacturing, handling, usage, bad luck, or other problems.

 

[dnmun]

i think you can hack that Vpower BMS down to 15S. i can do it with both of the ping signalabs and i have figured out how to do it on the headways except for the newest digital 16S BMS.

i looked at the Vpower BMS but most of the spots needed to access the resistor on the output of the transistor that turns on the signal to the LVC signal line is covered by the black goop. you need to unsolder the resistor to disconnect from the signal line.

so if you wanna do it with that BMS we can try, or you can get a ping BMS to hack up.

or you can try to get back to 16S by finding another cell to replace the dead one.

 

[dogman dan]

Good example of why I always say, on a used ebike, value the battery at $0. Unless capacity tested and ok.

You mentioned ordering some Hk packs. If you have an RC charger of 8s, you could just break the pack into two sections and charge on that. Two 6s sections would make a 36v pack. Then just practice the same monitoring you'd do when running your lipo with a "humam bms" .

Or, sell off cells to others who want to repair a pack with a few dead cells.

 

[Geoff V]

Gentlemen

Many thanks for taking the trouble to help a newbie climb the step part of yet another learning curve.

The advice you have offered is steering me towards stripping the old pack and removing the dead cells such that I have a useable lower voltage pack, this will be valuable experience and will I'm sure help me to operate these batteries correctly in the future. Hopefully I have gained enough knowledge from this Forum to at least be an apprentice to a 'Human BMS', time will tell!

GeoffV

 

[dnmun]

no, you do not wanna experiment with being a human BMS. that is just bogus stuff.

you have not told us what kinda pack. that looks like a Vpower BMS so you may have a Vpower pack. so already two strikes against you and that BMS may be ruined too.

 

[Geoff V]

dnmun

I'm sorry, but I don't understand yout comments about 'two strikes against you'?

The old pack with the burnt out BMS may well be a Vpower as there were remnants of their sticker still attached to the battery.

I was simply saying, that with the advice received, it would be a valuable exercise for me, to remove the failed cells and monitor the remaining cells manually as I no longer have a serviceable BMS.

If you consider this to be a foolish plan, please advise.

GeoffV

 

[dnmun]

have you opened the pack to see how bad it is?

how did the BMS fail? or did it fail? it is just a dead row? shorted where the metal strap on the positive end of a row shorted to the case where it was dropped on the corner?

 

[Geoff V]

I've not yet stripped the pack and it has no visible damage, I have no idea how the previous owner was careless enough to short the BMS such that some components have be obliterated (please see picture above).
He did say that he had continued to use the bike, presumably having bypassed the BMS, which leads me to beleive this is the reason for one or more cells going out of balance, one down to 0vs.
It seems to me to make sense to try to recover the good cells even if it is necessary for me to become the BMS.

GeoffV

 

[dnmun]

you gotta take the pack apart and find out what is wrong. i cannot tell much from your picture.

 

[Geoff V]

dnmun et al.

Thanks for all the advise which has resulted in a succesfull recovery.

The battery was a Vpower 48v 10 Ah 16S 11P with 18650 cells, two groups had failed, probably due to damaged sense leads and to the obliterated BMS.

I have removed the two groups of 11P and now have a very useable 42V pack and a much better understanding of the cell arrangement and the wiring.

One observation, is the poor quality of the spot welding, fortunately these joints were easily soft soldered.

Final task, find a suitable charger.

GeoffV

 

[dnmun]

do you have the entire pack apart now? go through and put insulating spacers under the straps at the ends where it goes past the anode on each end of the strap. you have to do this or the pack will short out again and you will lose more.

you can convert it to a 12S pack and use a 36V charger. i have a 12S Vpower BMS you could use on it then. i have hacked a 16S to a 14S v1 signalab too that i used on a pack i built up from ping pouches, but then you need the charger too. lemme know.

what is your location? put it in your personal stuff on the side there.

 

[arkmundi]

I was simply saying, that with the advice received, it would be a valuable exercise for me, to remove the failed cells and monitor the remaining cells manually as I no longer have a serviceable BMS.
If you consider this to be a foolish plan, please advise. GeoffV

No, not foolish at all. Automating battery health often carries the liability of putting the human brain second, like we do with our computers & memory. If I lost my laptop, I'd be disabled, so I get the digital enhancement. By temporarily setting your BMS aside, you'll put yourself as the primary, learn more about your pack and make your own decisions. Make sure you got a Watt's Up, Cycle Analyst or Celllogs for data collection. Disregard the long standing debate on the ES forum on to BMS or not to BMS - form your own opinion, which means experimenting with both approaches.

It seems to me to make sense to try to recover the good cells even if it is necessary for me to become the BMS.

Exactly. Battery management, keeping it in good health, the cells balanced and so forth can be automated or manual. You can always add in a BMS latter if you get tired of it.

Final task, find a suitable charger.

Be sure to visit http://batteryspace.com and look at their selection of chargers. I'm very happy with my 36V smart charger from them.

 

[dnmun]

no, you have to use the BMS to charge the pack in order to make the cells balance. you just cannot do it by some good effort. either a balancing charger or shunt type balancing BMS with bulk charging. especially with the Vpower pack. if you use a Vpower BMS and keep it charged and insure against shorting again then the Vpower pack will last a long time. you need the BMS to protect it from accidental total discharge when left on.

it just has the weight and low C effectively negating the cheapness. but you gotta go with what you brought. i have some of those Vpower cells too, just do not have a 16S Vpower BMS.

 

[Geoff V]

Gentlemen please.

This discussion could rage on without conclusion.

If I have understood matters correctly, both BMS and non BMS routes have merits and dangers, on the one hand Chinese BMS have an unfavourable reputation regarding reliability. On the other hand failure to monitor the pack voltage in use or failure to switch it off can result in the loss of more cells or the complete pack.

My current thoughts are to charge via the sense leads with a RC multiple charger to ensure balance and monitor the pack in use with a Cycle Analyst, I spent too many years in aviation to feel comfortable operating without instrumentation.

When the replacement pack arrives I will operate it with the BMS as supplied and decide as the months pass which system I prefer.

dnmun
I have insulated the tails, must have read it somewhere on the Forum and I live in Cambridge UK, I also envy the range of products you have on your side of the Pond.

Thanks once again

GeoffV

 

[dnmun]

if you wanna try to hack that BMS to 14S, we could try that. you would need some good pictures of the side where the comparators and transistors are. then unsolder a little resistor on the pcb. you could then adjust the charger voltage down on your charger to match the 14S.