48V LifePO4: Run with one dead cell?

[frogblender]

I have a 48v 10AHr pouch LifePo4 chinese ducttape battery, which was lasted me about 3000km. Alas, the 10Ahr started cutting out around 5Ahr. Took it apart, made sure all cells were balanced, and found one very puffy and low-capacity cell.

Can anyone comment on the wisdom of removing the offending cell, and running a 15cell pack instead of 16?

This would require a BMS mod, which I think I've figgered out: each cell has two resistors which drive the overvoltage and undervoltage cutoff bus lines. If I desolder the two resistors for the offending cell, and remove the cell, and jumper across where the cell used to be, I should be golden. I'd also have to put in a cell-shaped piece of wood or something where the cell used to be, in order to maintain mechanical compression of the pack?

 

[dnmun]

how did you determine what is balanced? how were you able to determine the lower capacity of the cell your refer to? since we have no pictures and no way to know what brand it is kinda impossible to guess if it is 5Ah pouches or something else.

 

[frogblender]

how did you determine what is balanced? how were you able to determine the lower capacity of the cell your refer to? since we have no pictures and no way to know what brand it is kinda impossible to guess if it is 5Ah pouches or something else.

I determined the state of balance using a voltmeter on each cell.
I charged the bad cell individually using a bench power supply.
I measured the capacity of the bad cell by discharging with a bench programmable load.
The pack is 16 10Ahr pouches. No-name chinese cells.

 

[dogman dan]

I don't know how to modify a bms to 15s, so my approach would be to chuck the bms, and run 15s naked.

A pain, but better than running out at 5 ah all the time. Meanwhile, investigate the bms mod, or get a bms that can run any cell count.

You will have to have a charger for 15s too, so if you can't adjust your charger down 3.5v that could be another problem.

 

[frogblender]

I don't know how to modify a bms to 15s, so my approach would be to chuck the bms, and run 15s naked.
A pain, but better than running out at 5 ah all the time. Meanwhile, investigate the bms mod, or get a bms that can run any cell count.
You will have to have a charger for 15s too, so if you can't adjust your charger down 3.5v that could be another problem.

I figure the existing 16s charger will work fine with 15s - the BMS will cut out and prevent any overcharging. This, of course, requires the BMS to be present.

 

[dnmun]

how do you figure that? if the charger is set to charge to 60V as it is for the ping batteries then each cell would go to 4V each if it is 15S. no way to know if the BMS will function if you do your hack the way i imagine it must be in your mind. no picture of the BMS either.

 

[999zip999]

15s x 3.6v. 54 volts. So charger must be set to 54v. I would put a set of sense wire for cell meter.
Are you running a 25 amp controller on a 10ah pack ?

 

[frogblender]

how do you figure that? if the charger is set to charge to 60V as it is for the ping batteries then each cell would go to 4V each if it is 15S. no way to know if the BMS will function if you do your hack the way i imagine it must be in your mind. no picture of the BMS either.

One of the functions of a BMS is to monitor EACH cell voltage, and shut off the charging fets when ANY cell exceeds the max voltage (3.75v or whatever). So yes, my charger is 60V max, but no, no cell should ever reach 4V, cuz the charge current'll cut out as soon as any cell hits 3.75. That's how I figure I can use a 16s charger on a 15s pack, as-is.
My BMS is a chinese no-name, but the per-cell over- and under-voltage monitoring look like they use a wired-or configuration. And a quick google of BMS schematics, and common sense, indicates the individual cell monitoring is wire-or'd to the main fets.

I think it is a bad idea to run an Lifepo4 pack without a BMS - the BMS does alot of useful functions... namely, it prevents overvoltage and undervoltage on a per-cell basis, prevents overcurrent, and does balancing. And if you accidentally short your pack, it'll prevent the pack from blowing itself bonkers delivering 200amps? 500amps? I dunno what the short-circuit current of a 10AHr naked pack is, but it might be enough to jump-start a locomotive, and certainly enough to drastically reduce its lifespan.
What's not to like about a BMS? If a BMS is cutting out, there's a good reason.


999zip999: My controller's max draw on the 10Ahr pack is between 30 and 35 amps. When hot, it'll be closer to 35. When cold, closer to 30.

 

[dogman dan]

Well, that might work, but it won't work well. I think it will tend to charge too fast as the end of the charge nears.

 

[999zip999]

Unless you have some great cell you are cooking your 10ah battery at 30 amps. The battery should run a 15-18amp. Controller maybe that's high.

 

[frogblender]

Unless you have some great cell you are cooking your 10ah battery at 30 amps. The battery should run a 15-18amp. Controller maybe that's high.

15 amps? Where's the fun in that? That'd be less than half the power I'm used to. Anyhoo, fun as it may be, you are right in that 35A is probably rough on a 10Ahr pack. Although, the 35A is not continuous... as long as you consider less than 10 seconds at 35A not continuous...

 

[999zip999]

I liked 80volts 40amps 3200watts on a Muxus slow wind but it melted after 25min. with hills. I almost got away with it if not for the last hill. So the same with low c-rate battery.
Now I will only use 5C or better cell.

 

[dnmun]

Unless you have some great cell you are cooking your 10ah battery at 30 amps. The battery should run a 15-18amp. Controller maybe that's high.

15 amps? Where's the fun in that? That'd be less than half the power I'm used to. Anyhoo, fun as it may be, you are right in that 35A is probably rough on a 10Ahr pack. Although, the 35A is not continuous... as long as you consider less than 10 seconds at 35A not continuous...

how is it that the BMS allows you to draw 35-40A if it is a 10Ah pouch pack? it sounds like the BMS mosfets are shorted on the output. i realize you don't know how a BMS works but you would be better off dropping the charger voltage so the pack can balance.

you will also have to solve the problem of hacking the BMS so it will function with the one channel missing. i doubt if it will work like you imagined but those of us who do this kinda repairs put the missing channel on top.

 

[frogblender]

how is it that the BMS allows you to draw 35-40A if it is a 10Ah pouch pack?

I believe the BMS' overcurrent cutoff is 40A. And my max controller draw is 35A, not 40.

it sounds like the BMS mosfets are shorted on the output.

The BMS cuts out just fine. If the fets were stuck-on, then the BMS wouldn't cut out.

i realize you don't know how a BMS works but you would be better off dropping the charger voltage so the pack can balance.

From what's been said so far, I'm fairly sure I know how a BMS works better than anyone who's so far replied to this thread. Dropping the charger voltage is a large undertaking. Once the main charge current shuts off, I see no reason why a 15s pack won't balance with a 16s charger: each channel will either bypass or trickle charge, same as usual. Those big SMT power resistors you see on a BMS will dissipate an extra (16/15) 6.7%, which is no big deal. Why do you think the balancing won't work?

you will also have to solve the problem of hacking the BMS so it will function with the one channel missing.

I outlined how to do this above. The over and undervoltage monitoring appear to use a wired-or configuration, with each channel driving the wired-or bus through (on my BMS) a 1k resistor . Remove a channel's resistor, and that channel will be ignored. Granted, I haven't yet tried, but the BMS is a very simple circuit, and I see no reason why it shouldn't work.

i doubt if it will work like you imagined

Any basis for saying this? Have you tried? I outline above quite clearly how the BMS works.

but those of us who do this kinda repairs put the missing channel on top.

Putting the missing channel on top would require unsoldering/resoldering/shuffling around cells - which is alot of work, and the solder heat is not too good for the cells. It appears to me to be alot easier to just use some minor brainpower and desolder the correct two SMT resistors on the BMS. What's the advantage of moving the hole to the top? Have you done this?

 

[frogblender]

I liked 80volts 40amps 3200watts on a Muxus slow wind but it melted after 25min. with hills. I almost got away with it if not for the last hill. So the same with low c-rate battery.
Now I will only use 5C or better cell.

It's always that last hill...

 

[dnmun]

of course i have done it and restructured several BMSs. i have done it all with BMSs.

we have no idea which BMS you are using so there is no way to tell you how to do it exactly but your statement about how you have to unsolder the pouches indicates you don't understand what i was talking about. you only have to resolder the sense wires above the cell that is replaced by a shorting jumper.

your statement about how you could just use the same high voltage on your charger for 15S also is a clear indication you don't understand how the BMS functions to balance the battery pack.

 

[frogblender]

your statement about how you could just use the same high voltage on your charger for 15S also is a clear indication you don't understand how the BMS functions to balance the battery pack.

Please explain if you can.

 

[frogblender]

you only have to resolder the sense wires above the cell that is replaced by a shorting jumper.

Shuffling between 1 and 15 sense wires (depending on where the offending cell happens to be) sound likes alot of work too. And what do you do with the 16th now-unused (cuz there's now only 15 cells) sense wire?

 

[dogman dan]

Well, do tell us how it goes. If you can see, you can makes some interesting observations.

But since 90% of the people out there are only capable of looking, I have a tendency to assume they cannot see.

Since I never tried this, my automatic reaction was to say no, don't try it. Prove us wrong, I love it to learn from others experiment.

 

[ecycler]

I am curious about this experiment as well. Can you please take some pictures?

 

[dnmun]

hwy89 rewired a bestechpower BMS in order to skip channel #1 after he accidentally shorted out the balancing shunt mosfet on channel #1.

even though it only has 2 channels of voltage to drive the output mosfet gates, instead of three normally, it still works.

i use a 24S D131 lipo BMS with a 21S lipo pack and it doesn't require any modifications since all the channels are filled in order from the bottom up to 21S.

hacking the ping signalab requires more effort to convert to lower serial counts. there you have use jumpers across the optotransistor outputs on the channels that are missing a cell.