Range problem on LifePo4 48V10Ah battery

[ultraboy]

I bought a LifePo4 48V10Ah battery 3 mounts ago from a reseller. Similar to this:

I had a great range in the beginning but now it has dropped every single time i've ridden.

The strange thing is that it's on 56.6V when fully loaded and when the battery is "out of juice" it's stil on 52.3-52.6 volt!

Isn't LifePo4 cells supposed to go down to 44-46V before safety cutof? I've read on this forum some people taking "48V" lihium batterier down to 35,2 V!

What can be wrong? Is the BMS acting up? It the BMS programmable or do i need a replacement "chip" of some kind?

Can anything else bee wrong?

PS: also when i turn the battery ignition OFF, i still get about 20-40V of output from the battery, is that normal?

 

[wesnewell]

Pretty much all commercial battery packs come with a bms. It monitors each cell group and shuts off output if one goes below a certain voltage. When you recharge the pack, the bms is supposed to balance out the voltage of all the cell groups so they will all start out at the same voltage. Try leaving the charger hooked up over night for this process to happen. It takes time. If that doesn't fix the problem, you'll have to start troubleshooting the pack, which will require taking it apart and measuring voltages. With the battery turned off, there should be no voltage from it. If still connected to the controller, you may see residual voltage from the caps for a short time.

 

[ultraboy]

i can't "Try leaving the charger hooked up over night for this process to happen" since the charger (and fan) turn of when it's done. And since the voltage is 56V it's pretty full, i guess? Thats 3,5V per cell (assuming its 16 cells).

Could something have happened to the BMS? can i remove the chinese BMS and install a good one or what should i do?

Is it safe to open up the battery pack?

 

[wesnewell]

The charger will turn off, but that doesn't mean the pack is balanced. It just means its reached a certain voltage. Some cell groups may be at 3.9V with other at 3.3V. The bms will bleed the higher cell groups down and the charger will turn on again. This process will continue until the cell groups balance out. On a pack way out of balance, this process could take days.

 

[DAND214]

As WES ssaid, "leave it on the charger".
Charger will coninue to charge as the cells are drained so other can be charged.

LET IT CHARGE!

Dan

 

[dnmun]

i can't "Try leaving the charger hooked up over night for this process to happen" since the charger (and fan) turn of when it's done. And since the voltage is 56V it's pretty full, i guess? Thats 3,5V per cell (assuming its 16 cells).

Could something have happened to the BMS? can i remove the chinese BMS and install a good one or what should i do?

Is it safe to open up the battery pack?

why do you assume the 'chinese BMS' is defective and you need a 'good one'? do you realize the chinese make all of these products? i also wondered why you don't say which battery you have or show a picture of your own instead of the picture of a GM battery.

you can open the battery case and pull out the BMS and measure the individual cell voltages at the BMS sense wire plug while it is charging to find the cell that is the high one that turns off the charging. you can then use a power resistor across that cell to drain the charge off and the charger will continue charging until the high cell climbs back up to 3.9V again. but this will increase the speed you can force the pack to balance.

 

[ultraboy]

Thanks for the tips! I always disconnect the charger since i'm affraid of overcharging, fires or destroying the charger (i heard lead acid chargers should never be left more than 24 hours). I will try this!

Can anyone tell me what voltage a used and and fully charged "48V" Lifepo4 battery should be at (with an automatic charger))? 46 discharged and 56 charged?

>>nmun
i dont know what my supplier ordered, so i don't know the brand.
And i made a Casing of sheet metal which is hard to take of so i used a picture from the internet similar my battery.

 

[dnmun]

your charger should have about 57.6-58.4V on the output depending on the BMS used. lifepo4 is not subject to fires from overcharging. it will not overcharge in any case if it has a BMS which it does appear to be the case.

you are gonna have to get the battery apart in order to work on it anyway so you should go ahead and get started on it now so you can get the battery working. it seems like it has serious problems if it has no range at all now so i expect you will find a defective cell eventually. but you have to be able to work on it and be able to discharge it through your watt meter into a dummy load to measure the capacity.

 

[d8veh]

You can only find out what's wrong by checking the cell voltages. The total voltage doesn't help very much. If you have pouch cells, I bet one's swollen. If they're small cyclindrical cells, it's probably a broken weld or you've squished a corner cell. You have to find the multi-pin connector to the BMS and measure the voltages there when it's charged and when it cuts out. Then you will know what you have to do.

 

[ultraboy]

The battery, i was now told, was from bms battery. It's probably the "HIGH C-RATE 48V10AH LIFEPO4 ALLOY 07-CASE BATTERY PACK":
http://www.bmsbattery.com/48v/623-48v-15ah-lithium-ion-alloy-shell-ebike-battery-pack.html

I've opened it up and was surprised to se there where battery pouches and not cylindrical cells. They where 16 puches marked 3.2V 10000mAh. Thats 51,2V 10 Ah.

When the battery is fully loaded it says about 55-56V and when depleted about 52V.

The range has dropped drastically to less than 20% of the original capacity in the last 2 months! It only last about 10-15 minutes at lowest power! Only a few km of range!

As d8veh told i should "find the multi-pin connector to the BMS and measure the voltages there when it's charged and when it cuts out".

I'm assuming it the 2 small white "houses" with 8 cables in each. I measured them from left to right, starting with the bottom row getting:
1. 3.4V (3.4V)
2. 6.9V (3.5V)
3. 10.3V (3.4V)
4. 13.8V (3.5V)
5. 17.2V (3.4V)
6. 20.7V (3.5V)
7. 24.1V (3.4V)
8. 27.5V (3.4V)

9. 31.0 (3.5V)
10. 34.4 (3.4V)
11. 37.8 (3.4V)
12. 41.3 (3.5V)
13. 44.8 (3.5V)
14. 48.2 (3.4V)
15. 51.7 (3.5V)
16. 54.9 (3.2V)

I made several test and this is the most accurate. Does it matter where i connect the negative? i tried 3 spots (black cables) and it yielded the same result. Can i measure the cells individually or only like i did, calculating voltage difference? Is any the cells broken? Do i need to test it when the battery is depleted as well?

Battery pack without the casing and blue plastic wrap.

BMS.

BMS.

BMS

BMS; multi-pin connector?

 

[Racer_X]

The battery, i was now told, was from bms battery. It's probably the "HIGH C-RATE 48V10AH LIFEPO4 ALLOY 07-CASE BATTERY PACK":
http://www.bmsbattery.com/48v/623-48v-15ah-lithium-ion-alloy-shell-ebike-battery-pack.html
16. 54.9 (3.2V)

I think cell 16 is low compared to the rest. I don't know if that would be enough to trigger the bms to turn off and kill power to the battery. I could be a bad BMS.

 

[Schlafmutze]

Lifepo4 has a very flat discharge curve, when at 3.3 it might be at 40% dod or 80% dod.

The 3.2 cell is likely a lot lower than the rest, and I mean a lot.

 

[r3volved]

Do you disconnect from charger as soon as battery is charged?
When was the last time that you left it on the charger for a whole weekend to balance?

 

[Tench]

I had exactly the same symptoms of reduced range from an over the counter bike I bought for the wife a few years ago, opening the very similar looking battery case revealed pretty much what you have found. I brought all the cells back into line with a single cell phone charger (took a few days) then it was fine again. The low cell is hitting LVC and the BMS is cutting the output off to protect it while the rest of the pack is still largely charged.
Basically a dumb bms that does everything except keep the cells in balance!!

Leave it on the charger for a few hours after the light has gone out each time you charge it.

 

[ultraboy]

Thanks for all the input!

I took if for a ride last night, lasted only 2.5 km at max throttle and i measured the battery again, cell by cell:
1. 3.3V
2. 3.4V
3. 3.3V
4. 3.3V
5. 3.4V
6. 3.3V
7. 3.3V
8. 3.4V
9. 3.3V
10. 3.4V
11. 3.3V
12. 3.4V
13. 3.3V
14. 3.3V
15. 3.4V
16. 2.7V

The last cell seems problematic. I will try to have it raised. My importer has good chargers so i hope he can charge only the "lazy" block.

 

[mclark999]

This is not to troubleshoot, just a question because I have the same battery. What was your range when you first used it? How much do you weigh, pedal, etc?

Thanks,

Michael

 

[ultraboy]

Its not a traditional ebike but a Mushing kick scooter so it has no pedals, and i seldom kick. Toprange was 18 km with a load of 160 kg incl scooter and 2 passagers:

 

[wesnewell]

The bms constantly draws power from the battery. It has to to run. Most will draw power from 1 cell, or cell group if it uses paralleled cells. It appears that your pack uses cell 16 for power. If you disconnect the charger as soon as the charger turns off, that cell will continue to lag behind the other cells a little more each time you charge until you get to the point you're at now, where it is so under charged, that it hits lvc almost immediately after charging. The solution for this is simple. Don't disconnect the charger as soon as it turns off. The bms will bleed down the over charged cells and turn back on and start charging all the cells again. After enough cycles, all cells should be balanced. But since you've been disconnecting the charger so early, cell 16 has been losing capacity with every charge. It's now way out of balance with the other cells, and may take days to balance out if left on the charger. Now you can manually charge that cell up and get your capacity back, but you will have the same problem later if you continue to not leave the charger hooked up for longer periods of time. it would be nice if the on/off switch on the pack also disconnected the bms so it doesn't continue to draw power, but I don't know of any that do this. Of coursae since I don't use a bms, that's not saying much.