Voltage on starter battery in parallel with LifePO4

[LifesBetterWhenYoureBeating]

I have a LifePO4 battery that can be put in parallel with my starter battery via a 1-2-Both switch. I don't intend to ever use them in parallel, but I'm surprised that the voltage of the starter (approx 12.7v) doesn't change when I put them in parallel, even though the LifePO4 voltage is 13.4v. is that normal? If so, why?

Thanks in advance!

[goboatingnow]

Originally Posted by gsalgado

I have a LifePO4 battery that can be put in parallel with my starter battery via a 1-2-Both switch. I don't intend to ever use them in parallel, but I'm surprised that the voltage of the starter (approx 12.7v) doesn't change when I put them in parallel, even though the LifePO4 voltage is 13.4v. is that normal? If so, why?

Thanks in advance!

Well a voltage drop over a paralleling wire of 0.7v would suggest a shed load of current is flowing.

It’s altogether a really bad idea.

[YANDINA]

The BMS in the LiFePO4 is probably shutting down to protect it.

[LifesBetterWhenYoureBeating]

Originally Posted by goboatingnow

Well a voltage drop over a paralleling wire of 0.7v would suggest a shed load of current is flowing.

It’s altogether a really bad idea.

I'm not seeing a voltage drop. The starter is at 12.7 and LFP at 13.4 before I parallel them. When I parallel them, the starters voltage (shown on my victron BMV 712) actually increases by 0.06 for a sec or two, and the same for the amp draw (+2A).

I understand it's a bad idea an don't intend to keep them like that but couldn't find a way to physically prevent that from happening with my current switch so I'd like to have a better understanding in order to decide how important it is to replace the switch

[LifesBetterWhenYoureBeating]

Originally Posted by YANDINA

The BMS in the LiFePO4 is probably shutting down to protect it.

I thought about that, but if that was the case wouldn't my other loads stop? That's not what I'm seeing (my fridge compressor/fan doesn't stop even for a sec when I parallel the batteries), and the historical amp draw graph on my victron BMV 712 monitor only shows a 2A spike for a sec

[goboatingnow]

Originally Posted by gsalgado

I'm not seeing a voltage drop. The starter is at 12.7 and LFP at 13.4 before I parallel them. When I parallel them, the starters voltage (shown on my victron BMV 712) actually increases by 0.06 for a sec or two, and the same for the amp draw (+2A).

I understand it's a bad idea an don't intend to keep them like that but couldn't find a way to physically prevent that from happening with my current switch so I'd like to have a better understanding in order to decide how important it is to replace the switch

Unless something is amis, measure using a decent voltmeter both voltages before and after. The Li should quickly drag up the lead acid to its operating point

[LifesBetterWhenYoureBeating]

Originally Posted by goboatingnow

Unless something is amis, measure using a decent voltmeter both voltages before and after. The Li should quickly drag up the lead acid to its operating point

I did that too. The readings from my voltmeter match those of the monitor.

I don't understand what you mean by the LFP dragging up the LA to its operating point though

[goboatingnow]

Originally Posted by gsalgado

I did that too. The readings from my voltmeter match those of the monitor.

I don't understand what you mean by the LFP dragging up the LA to its operating point though

Both batteries have to be at basically the same voltage or massive current is flowing and the drop is I2R heating in the wire.

If the SLA is loading the Li then the Lithium will settle to the operating voltage based on its chemistry and the discharge amps.

There simply cannot be unexplained voltage difference over a wire

[YANDINA]

Originally Posted by gsalgado

I thought about that, but if that was the case wouldn't my other loads stop? That's not what I'm seeing (my fridge compressor/fan doesn't stop even for a sec when I parallel the batteries), and the historical amp draw graph on my victron BMV 712 monitor only shows a 2A spike for a sec

If the BMS shuts off due to excessive current when the batteries are in parallel, the lead-acid battery will continue to provide power to the other accessories. When you turn the switch off and the BMS turns back on because it saw the current drop to zero, it will look like accessories on the lithium battery were running all the time.

[LifesBetterWhenYoureBeating]

Originally Posted by YANDINA

If the BMS shuts off due to excessive current when the batteries are in parallel, the lead-acid battery will continue to provide power to the other accessories. When you turn the switch off and the BMS turns back on because it saw the current drop to zero, it will look like accessories on the lithium battery were running all the time.

Of course! That's most likely what's going on, then