BMS Help

columbiajack

10 mW
Joined
Mar 19, 2013
Messages
20
Got an issue with my battery. It is a LifePO4 36v 20ah. Purchased last year and sat over the winter, Gave it a charge and it didn't take much to top it off. I went for a ride today not even a mile and it went dead on me. I looked at my volt meter and sure enough dead nothing. Rode home hooked it up to the charger and the charger started to cycle like it was getting full. Checked the output from the battery and it is 27.4v. When I check across the string I get 3.3, 6.8, 10.1, 13.9, 17.6, 21.2, 24.7, 28.2, 32.0, 35.5, 39.2, 42.8. So in my limited mind the battery is full and there is a problem with the output of the bms.
Now I am at a loss the part numbers on the bms have faded and I cant read them. How would I go about getting a replacement bms? Yes it is a China based battery. Only info I have is
36 v Lipo 36120 cells
12 cell
Capacity 20 amp hr
Charging voltage 45-46
Max continuous disch. 40 amps
Rated disch. amps 20
Disch. cutoff protection 60 amps
3c discharge rate
1c charge rate
I would really appreciate any input Jack
 
columbiajack said:
... I get 3.3, 6.8, 10.1, 13.9, 17.6, 21.2, 24.7, 28.2, 32.0, 35.5, 39.2, 42.8. So in my limited mind the battery is full and there is a problem with the output of the bms.
Your pack is not balanced.
I guess you unplug the charger as soon as the light turns green, not letting the bms do is job.
Let it charge until the light REMAINS green for an hour.
All your cells should be 3.6v. to be full.
 
Thanks for the replies. I guess I should have looked after the battery over the winter. I will put it on charge and see what happens..........jack
 
you need to measure the cell voltages while it is on the charger when it reaches full charge and the light turns green. measure each cell to .01V with a good voltmeter and post the numbers here. you can measure the adjacent cells on the solder pads on the BMS where the sense wire plug is soldered to the pcb.
 
I disconnected the bms to get these readings
1 3.2
2 3.5
3 3.2
4 3.5
5 3.2
6 3.6
7 3.3
8 3.4
8 3.5
9 3.2
10 3.2
11 3.5
I will try with the charger connected next when ever and if it fully charges. Right now you can hear the charger power up for about 45 sec when it is first connected.Then it goes silent with both green and red light up. It is the smart charger..............jack
 
First, be sure all your connections are in good working order. I have, in the past had a loose battery connector, in the middle of the pack, that was just loose enough to cause the bms to trigger as if I had a low voltage. It was an intermittent cutoff only when the connector wiggled just loose enough to cut the voltage to the bms. So check your connections.

Once you have done as DNMUN suggested. Leave your battery pack on the charger over night to give enough time for your bms to bleed down the hot cells and balance them to what ever extent the bms is capable.

Again measure and post the results. If you can, post to the .01 volt

Then go for a ride. Keep it close to home but long enough for the the cells to be well drained. Take your DMM and what ever tools you need in case your bike cuts out prematurely. If it does cut out, measure the cells on the spot to see if one of the cells is much lower then the others thereby causing a low voltage cutoff.

The point is to isolate the problem.

:D
 
It should have 12 cells as a 36v lifepo4 battery. Just looks out of whack as all 11 cells above 3.0v good. long time on the charger to start.
 
Happy to report my battery came back to life after a long time on the charger 26 hrs. I won't forget about the battery next winter for sure. Thank you to everyone...........jack
 
Ok short ran recharge for many hours after green light comes on. Then short ride and charge overnight. I would Not test it's limit or test up a hill. Then charge over and use.
Please do not ride to find the range of your battery. If you have to use 80% . It will wake up after it's long sleep. Good luck.
 
This is a common problem with storing BMS equipped battery packs. If you have no elegant way to disconnect the BMS/PCM when stored, a person should use calendar reminder to insure a good charge every month or two.

Kinda shame since good Lithium tends to exhibit very little self-discharge and would likely store unattended for months, if not years. Not with a BMS/PCM though as it needs to get it's power from someplace.
 
Ykick said:
This is a common problem with storing BMS equipped battery packs. If you have no elegant way to disconnect the BMS/PCM when stored, a person should use calendar reminder to insure a good charge every month or two.......

How about a timer switch that activates the ac for say, 6 hours per week?

:D
 
e-beach said:
Ykick said:
This is a common problem with storing BMS equipped battery packs. If you have no elegant way to disconnect the BMS/PCM when stored, a person should use calendar reminder to insure a good charge every month or two.......

How about a timer switch that activates the ac for say, 6 hours per week?

:D

Dunno how that interval would work out? I have an old alloy LifePo4 pack which sits unused for very long periods of time. I just use an iCal reminder to run the charger once per month. If I'm not home to take care of it I set another reminder to go off when I'll be home again. I charge until its finished balancing and IIRC that's usually much more than 6 hours? Which depends on the balance of the pack, of course.

Simplest and most effective thing would be to just open the unit and disconnect BMS but this pack is sealed pretty good. Oh well, figure it out as we go, I suppose???
 
I can easily unplug the bms, but I will monitor it more closely next winter. I took it out for a good run with no issues.
 
I had problem like this - never knew that the BMS was consuming power over the winter. When I checked it cells were very low below 2v. My battery charger would not start but I was able to single charge enough cells to get charger on. Everything seemed okay until my charger failed a month later - not sure if it was related to this. Wish I had disconnected the BMS !
 
the problem of having the BMS consume large amounts of charge over time to provide the circuit current has been limited to the ping signalab BMS and other designs that use the optotransistor to provide the voltage to the output and charging mosfets..

with no knowledge, you cannot just ascribe the imbalance in the battery to leaving the BMS powered up.

if you wanna analyze why your battery is not balanced then you need to follow a systematic analysis and get over the need to have the answer before the analysis.

it is worse than just being lazy to jump to conclusions that are promulgated by the guys here who you are a fan of because their ebike goes fastest. when you never analyze the problem and just jump to a conclusion then you never learn anything about how your battery and BMS work.
 
Ykick said:
e-beach said:
Ykick said:
This is a common problem with storing BMS equipped battery packs. If you have no elegant way to disconnect the BMS/PCM when stored, a person should use calendar reminder to insure a good charge every month or two.......

How about a timer switch that activates the ac for say, 6 hours per week?

:D

Dunno how that interval would work out? I have an old alloy LifePo4 pack which sits unused for very long periods of time. I just use an iCal reminder to run the charger once per month. If I'm not home to take care of it I set another reminder to go off when I'll be home again. I charge until its finished balancing and IIRC that's usually much more than 6 hours? Which depends on the balance of the pack, of course.

Simplest and most effective thing would be to just open the unit and disconnect BMS but this pack is sealed pretty good. Oh well, figure it out as we go, I suppose???

I would not leave the charger hooked up and run it on a timer to charge periodically. Depending on how the charger works, it may drain the pack down through the charging leads worse than the BMS does.
 
dmwahl said:
I would not leave the charger hooked up and run it on a timer to charge periodically. Depending on how the charger works, it may drain the pack down through the charging leads worse than the BMS does.

Good point. The bleed down resistors in the charger could drain the battery. But an inexpensive diode inline would stop that.

:D
 
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