LiFePo4 BMS

[Tanya]

Could anybody explain to me what a battery BMS does when discharging a battery?
Does the BMS allow the battery to drain each cell until the combined voltage of the battery drops to the safe discharge level?
Should all cells be charged to the same level? What voltage should the cell be charged to? My battery charger cuts off when
it reaches 52v. Some cells are 3.8v and some are 3.3v. Will the BMS draw from the 3.8v first until all cells are the same
voltage like 2.8volts? If you don't use a BMS on a LiFePo4 battery will some of the cells drain faster then the other cells?

Tanya

 

[dnmun]

when discharging, the BMS is monitoring the current flowing out of the battery. if the current exceeds a fixed high level, the logic in those analog parts causes the output FETs to be shut off, this blocks current flowing out of the battery.

also the BMS monitors the cell voltage and if one cell drops below the low voltage cutoff point,the LVC, then the logic tells the output FETs to shut down.

when charging, the BMS controls the charger current going through the BMS to the battery. if the voltage on any cell reaches the high cutoff point, then the charging FET turns off and no more current flows into the battery, preventing it from overcharging.

most BMS like the signalab board, or gary's 4-24 cell BMS thread, use a bypass or shunt resistor to allow current to flow around a cell when it reaches the final, fully charged voltage for that cell, and the current that bypasses the cell creates a voltage drop (V=IR), which is equal to the voltage across the cell it is bypassing in parallel with it. that current is called the shunt current and determines how fast the cells farthest out of balance keep charging since the shunt current is the total charge current then.

the sum of all the voltages equals the charger voltage, so the battery charges up until that voltage peaks when all the bypass or shunt transistors are on, and no more current flows into the cells, or only a tinytiny amount. that's end of charge, and the BMS will shut down the charging current when one of the cells climbs up to the high setpoint, and triggers the charging FET to turn off.

most of the problems people are having appear to be problems with the charger never having a sufficient voltage for the BMS to allow the cells to all charge up fully so that the BMS will shut off before the lowest cells get fully charged, and repeated cycles like this on the new batteries is causing them to go far out of balance so that one cell ends up in LVC in the first few discharges, if not on the first.

hope this helps, maybe someone else can fill it in better for you. or ask more, it's always great to see girls taking an interest in tinkering with stuff. most don't wanna do it because of the cultural oppression but in general i think they are more thoughtful in fixing stuff, and often can do better work. keep it up.

 

[Tanya]

Thank you dnmun,
I'm lost when I have electrical problems with my bike. I live in Santa Cruz ca and I ride my electric bike about 20 miles a day most every day. Without a electric motor I can only ride half as much and half the fun. My first electric bike was a eZee Sprint but the batteries kept wearing out so I decided to make a new bike. I mounted a crystalyte 5304 motor on the back of an electra cruiser. I bought a Yesa 48v 20ah battery and put it in a military ammo can that just fits between the frame. For about a year I have had no problems until now. My battery shuts down when I only reach the half way point of my ride. Someone told me that the cells might be out of balance so I checked them and found that some cells are 3.3v and other cells are 3.8v. I also read that if I leave the battery on the charger eventually the cells will balance out. It hasn't happened yet.
Is there anything that I can do to bring my battery back to normal?
Maybe a new charger with higher amps?

Sincerely
Tanya

 

[dnmun]

wow, an electra glyde of ebikes, cal here in portland has a cruiser like that with the 5304 in the rear hub and has yesa cells too. he rides a long steep hill besides a long commute to the far west side of town.

i think he had the same size pack too, and he had 2 bad cells last year. it sounds like you may have battery problems and not just BMS or maybe the BMS coulda cause them to go bad.

go hang "Yesa" on the front of your thread with edit, and the yesa guy will see it and come help. but 3.33V is not good if that is charged. do you know charger voltage, you measured cell voltages where?

if the cell drops to that point immediately after it is charged to 3.8V i think that is a sign of damage,but don't know like the RC guys do about batteries.

but if you can get to the BMS and expose the plug, try to measure the voltages on each shunt resistor from one end to the other, while the battery is charging and then measure it again when the charger is off. maybe by then the yesa guy will come around and help. so hanging Yesa on front will get his attention.

watch for lose wires on the sense leads too, but measure all the cells and we can start there, i'll ask cal if i can find him.

 

[dogman dan]

It's possible that your battery is a wee bit undersize for the 5304 motor. For sure it is if you have 1c cells. I am not sure what the yesa you have is capable of. If you haven't had a lot of bms cutouts, you may not have been abusing the thing that much, but if you have some good size hills on your route, your amp draw on the hills may have been high enough to put that little extra strain on the battery and now the weakest cells are showing themselves. If you have the 20 amp controller, of course that would help, but I think you may need to use a smaller motor for the remainder of this batteries life. The quick and dirty solution would be to get some lead batteries and paralell them, to take the strain off the yesa cells. Adding a 10 ah lifepo4 in paralell would also solve the problem, assuming that the yesa will keep going for awhile with some help from another battery.