Battery load handling at low charge level

[pekka8]

My 12V 200Ah lifepo4 battery shuts down at 12.75V (~15% charge) when trying to draw 2000W. I guess that is normal? Lower the voltage/charge level, less amps you can draw from a battery?

(sorry should be at battery discussion)

 

[sunshine_eggo]

My 12V 200Ah lifepo4 battery shuts down at 12.75V (~15% charge) when trying to draw 2000W. I guess that is normal? Lower the voltage/charge level, less amps you can draw from a battery?

(sorry should be at battery discussion)

At low charge levels, a single cell may hit lower limit and shut it down.

The heavy load may pull the DC voltage below the inverter limit.

 

[pekka8]

At low charge levels, a single cell may hit lower limit and shut it down.

The heavy load may pull the DC voltage below the inverter limit.

Didn't remember that Inverter has safety limit also.

 

[JeepHammer]

My 12V 200Ah lifepo4 battery shuts down at 12.75V (~15% charge) when trying to draw 2000W. I guess that is normal? Lower the voltage/charge level, less amps you can draw from a battery?

(sorry should be at battery discussion)

You are having an issue I ran into 10 years back when I started learning Lithium batteries...

With LiFePO4 batteries, VOLTAGE isn't a good way to do things...

The reason is, the voltage of LiFePO4 batteries stays flat so long, you can't tell the SOC (State Of Charge) by voltage.

Your battery BMS is set to shut down at 12.75v, that *Could* be one cell, damaged or out of balance...
Or it could be a very conservative low voltage shut down to protect the cells.

200 Ah is pretty small for current LiFePO4 batteries. Is this an older battery? Can you access the cells without splitting a case?

Putting a $200-$300 Display BMS on cells this small isn't financially feasible. Most display BMS units allow you to top charge and bottom charge memory, then use Watts in/out to tell you exactly what available power you have, but they also tell you what all the cells are doing so you know when one is out of balance. (Easy fix)

On the small batteries I assemble, I put an active charge balancer on them, one big enough to get a real job done. It's cheap insurance to reduce maintiance... 5 Watts (instead of mW) for about $15-$20.

I do it with the big cells too. Again, a 100-250 mW balancer built into BMS units is worthless if the battery is used daily, doesn't have months to balance cells.

Of course none of this is going to help you if you have a BMS you can't access, change settings or see what's going on with the cells/BMS.

 

[pekka8]

You are having an issue I ran into 10 years back when I started learning Lithium batteries...

With LiFePO4 batteries, VOLTAGE isn't a good way to do things...

The reason is, the voltage of LiFePO4 batteries stays flat so long, you can't tell the SOC (State Of Charge) by voltage.

Your battery BMS is set to shut down at 12.75v, that *Could* be one cell, damaged or out of balance...
Or it could be a very conservative low voltage shut down to protect the cells.

200 Ah is pretty small for current LiFePO4 batteries. Is this an older battery? Can you access the cells without splitting a case?

Putting a $200-$300 Display BMS on cells this small isn't financially feasible. Most display BMS units allow you to top charge and bottom charge memory, then use Watts in/out to tell you exactly what available power you have, but they also tell you what all the cells are doing so you know when one is out of balance. (Easy fix)

On the small batteries I assemble, I put an active charge balancer on them, one big enough to get a real job done. It's cheap insurance to reduce maintiance... 5 Watts (instead of mW) for about $15-$20.

I do it with the big cells too. Again, a 100-250 mW balancer built into BMS units is worthless if the battery is used daily, doesn't have months to balance cells.

Of course none of this is going to help you if you have a BMS you can't access, change settings or see what's going on with the cells/BMS.

Battery is new and I don't have access to internal parts. Manual says:

Over-discharge protection each cell 2.50±0.05V (4 cells).

I have used it with small loads down to 11.5V and not sure if it's BMS or inverter's battery safety limit that kicks in at that point (don't have access to inverter settings for now).