LFP voltage drop under load

[roaringgirl]

Hi, I have a 400Ah, 12V LFP battery. At 75% charge state (according to the shunt) the voltage is 13.3. When drawing a little over 100A, the voltage dips to 12.5 and recovers immediately that the load is removed. Is this normal/reasonable? What is the relationship between voltage drop and load? Is it linear?

Thanks

 

[William_H]

I don't know if this is reasonable for that battery. 100Amps will test connections and wiring. So is this a measurement of battery voltage at the terminals? If so then you might like to check actual voltage of individual cells under load. If one tends to drop more than the others then you may have a faulty cell. If they all seem to drop similar amounts then yes that is the best your battery can do.
If you are reporting what your control panel tells you then it may be wiring drop. Connect a multimeter between positive terminal of battery and any positive point at panel. this may indicate a volt drop in wiring including any isolation switch etc. Do same with negative line battery to panel. ol'will

 

[roaringgirl]

The voltmeter is reading from the terminals

 

[Slipstream 34]

Just looked at our 4x100ah LFP setup. A draw of 195 amps drops the voltage from 13.5 to 13.0. That's at 87% charge.

 

[geem]

Hi, I have a 400Ah, 12V LFP battery. At 75% charge state (according to the shunt) the voltage is 13.3. When drawing a little over 100A, the voltage dips to 12.5 and recovers immediately that the load is removed. Is this normal/reasonable? What is the relationship between voltage drop and load? Is it linear?

Thanks

That does seem a little low. Our 280Ah 24v bank with 100 amp load drops to +26v when at circa 80%. What kind of lithium? Drop in or custom build? What kind of bms? Active balance or passive?

 

[Poey50]

Hi, I have a 400Ah, 12V LFP battery. At 75% charge state (according to the shunt) the voltage is 13.3. When drawing a little over 100A, the voltage dips to 12.5 and recovers immediately that the load is removed. Is this normal/reasonable? What is the relationship between voltage drop and load? Is it linear?

Thanks

My hunch is that your state of charge is off and is much lower in reality. Because of the flat discharge curve 13.3 volts may apply to a wide range of states of charge.

 

[roaringgirl]

That does seem a little low. Our 280Ah 24v bank with 100 amp load drops to +26v when at circa 80%. What kind of lithium? Drop in or custom build? What kind of bms? Active balance or passive?

It's 2 Victron 200Ah batteries. They each have their own internal BMS and one external sitting over them both. The cells get balanced by the BMS on a full charge.

 

[[70521]]

I'd be interested to know what you are using onboard that draws 100 amps. I struggle to get up to 10 amps.

 

[geem]

I'd be interested to know what you are using onboard that draws 100 amps. I struggle to get up to 10 amps.

For us it's the immersion heater (50A), watermaker (77A), kettle (105A), induction hob (83A) all at 24v so double it for 12v system. 280Ah lithium battery at 24v

 

[vas]

a watermaker is good at drawing 80A@24V...

edit: geem beat me

 

[geem]

It's 2 Victron 200Ah batteries. They each have their own internal BMS and one external sitting over them both. The cells get balanced by the BMS on a full charge.

How does that work? Two BMS's?

 

[roaringgirl]

How does that work? Two BMS's?

According to Victron:

Victron LiFePO4 batteries have integrated Balancing, Temperature and Voltage control (acronym: BTV) and connect to the VE.Bus BMS with two M8 circular connector cord sets.
The BTVs of several batteries can be daisy chained. Please see our LiFePO4 battery documentation for details.

The BMS will:

Shut down or disconnect loads in case of imminent cell under voltage
Reduce charge current in case of imminent cell overvoltage or over temperature (VE.Bus products only, see below)
Shut down or disconnect battery chargers in case of imminent cell overvoltage or over temperature

 

[roaringgirl]

I'd be interested to know what you are using onboard that draws 100 amps. I struggle to get up to 10 amps.

Espresso machine is 1.6kW.
A single induction ring is 800W and we have 2.

1600W/12V = 133A