Voltage during balancing

[mechanix]

I have a 10 cell prismatic lifepo4 pack that I just put together. The cells are 20ah and the bms is a ping v2. Last night during its first balancing phase, I checked the pack voltage and it was 40V, thats 4V per cell assuming there arent any low ones! Is this normal? I though lifepo4 wasn't supposed to get over 3.6 or so. Anybody have any idea whats going on here?

Joel

 

[olaf-lampe]

Did you use the right connector for charging? There is a extra wire for charging that's controlled via the BMS and cuts off the charger. It should be labelled " charger-" or the like.

-Olaf

 

[cell_man]

What charger are you using. It can only go to 40V if that's what your charger is outputting. A 10 series pack should be charged to 36.5V. What is typically called a 36V charger is intended for 12 series lifepo4 and will have a final charge voltage of approximately 43.8V. You might have got away with damaging the cells, maybe not. Hopefully not.

If the BMS was connected properly it should have disconnected the charger from the cells once any of them reached the HVC typically 3.9V.

 

[dogman dan]

Yep, my new ping 48v battery charges to 3.75 per cell at the end of balancing, but then remove surface charge and if falls to about 3.625 v per cell. So what cellman advises is right on. For sure you don't want to charge a 10 cell lifepo4 with a normal 36v charger. 4 volts is too high and could damage the cells.

 

[Gregory]

What did you use to check the cells? Cheap multimeters and/or near flat multimeter batteries can be a cause.

 

[mechanix]

Thanks for all the replies, I looked a little closer and it appears one of the cells is low. From what I understand, this bms "bleeds" off the excess voltage from the higher cells, then charges again and repeats until all the cells are balanced. Should I just let it do it's thing, or tear into the pack so I can give the runt cell a little boost?

Joel

 

[olaf-lampe]

When one cell is low, that makes it even worse! Unplug the charger immediately and try to discharge the pack a little to get all cells down to 3.3V
When your voltage reading are correct, its all to late
Olaf

 

[mechanix]

The charger is for a 10s setup, and I measured its output at 40.2V. I also double checked the connections at the bms and they are correct. The pack voltage was actually at 39.5V when I measured it during it's first balancing cycle. I'm thinking it's that one low cell. I'll spot charge it later today and see it that will make a difference. Thanks guys.

Joel

 

[dnmun]

if it really is 10S, it is already cooked. if you keep charging a 10S lifepo4 at 40V, it doesn't put any more charge in it than the normal 3.65V/cell .

why would you use 10S anyway? the BMS is made to be 12S so you coulda just had a normal pack and charger.

 

[mechanix]

yeah, it is 10s, I had 10 lifepo4 cells, so I ordered a 10s bms and charger from ping. Is it possible that when taking the voltage reading the bms had a voltage rise across it making the voltage of the pack appear higher than it really is? The reason I'm asking this is because I took the voltage of each cell and they were all between 3.4V and 3.45V, while one was sitting at 2.8V.

Joel

 

[mechanix]

Problem solved! It was the low cell. I took it out for a ride and when I got home it had zero voltage and was warm, but the other cells were still perfectly balanced and cool to the touch. I switched it out with a new cell. Thanks for all your help.

Joel

 

[dnmun]

something is wrong here. if you had a ping BMS on your pack, it would not have allowed the cell to go to zero. it woulda shut down to protect that cell, none of this stuff jibes. where did you get the new cell? why wouldn't you have used that in the pack originally? to make 12S so you could charge and run normally.

 

[dogman dan]

I'd say problem not solved if you are still overcharging your cells. Did that get fixed? 40v is still pretty high for a 10s pack.

 

[mechanix]

oh yeah, I failed to mention that I switched volt meters and the chargers output was reading just under 39V, which I think is about right, I'd guess. I then rechecked all the cells and none of them were overcharged, thank God. I just now need to get the new cell to balance with the old ones. Ha ha on a side note, I used the old cell as a resistive load to remove some of the charge from the new cell to get it even with the old ones.

Joel

 

[dogman dan]

I still think 39 volts is a tad high for a 10 s pack. Mabye theres an adjustable pot in the charger you could turn down to 36-37v?

 

[mechanix]

dnmun,

I only use the bms for charging. I have two packs wired in series (a 12s and a 10s). Both packs are 20 ah. I can switch from charging mode to riding mode with a toggle switch so I don't have to worry about using diodes for the bms's. I have a 35a controller which keeps the amperage in check, and I monitor the ah's used with a CA. So that's why the cell crapped out, I figure if the cell is lower than the others, I need to replace it anyways

Dogman,

I already opened the charger up, but there is no toggle switch in this one, which is funny because my other ping charger has one... I guess the charger has to be slightly higher than the max voltage of the pack, or else there would be no current to top off the pack (i=V/R)

Joel