LiFePO4 with BMS using a RV lead acid charging system. What is the capacity loss loss?

[JohnIy]

The RV voltage output from the alternator is 13.8V when the engine is running.
Assuming I don't install a DC-DC converter, the LiFePO4 battery would only be partially charged when I'm driving.
What would be the approximate capacity loss I would see with at the lower charging voltage?
I do have solar on the RV, so when I'm parked I could completely charge the batteries if the sun is shining.

 

[MisterSandals]

What would be the approximate capacity loss I would see with at the lower charging voltage?

Negligible if held at 13.8V for a long time.

 

[hwy17]

Batteries should be able to get to 100% and balance on 13.8V. The risk is if there's only a lifepo4 on the circuit and the BMS cuts off the charge the sudden voltage spike can damage the alternator.

If the lifepo4 is paralleled with a lead acid during charging that's safer for the alternator.

 

[BentleyJ]

13.8V is 3.45V per cell. That will produce at least 95% if the time is long enough as MisterSandals mentioned. At a given charge voltage, current will tail off as the battery reaches full charge. If there was a smart shunt in the system you could monitor current and see when it starts tapering off.

 

[MisterSandals]

during charging that's safer for the alternator.

Hmmm, i was thinking that these would be charged by an existing RV converter? After all, an alternator would be producing well over 14V.

But yah, LiFePO4 pulling directly from alternator could overheat an alternator without a (recommended!) DC-DC charger.

 

[JohnIy]

From Will's testing it's clear that the BMS limits the discharge current. I was under the impression that the internal battery BMS limits both the charging and discharging current.

 

[sunshine_eggo]

From Will's testing it's clear that the BMS limits the discharge current. I was under the impression that the internal battery BMS limits both the charging and discharging current.

BMSs don't regulate current. They are an on/off switch. If current limits are exceeded, the BMS cuts the battery out of the circuit. If the limit is 80A, the BMS will not keep it at 80A. Once more than 80A is incoming for a given time period, the BMS opens the circuit allowing 0A.

The issue is that LFP takes on more charge at lower voltage, so they will draw the max the alternator can deliver for the giving system wiring resistance. An alternator that can't protect itself by reducing its output can be damaged/destroyed by being run at max output for extended period.

In an RV, there's often a substantial voltage drop due to wire length between battery and alternator. This can serve to help regulate the current as the Alternator will "see" a higher voltage than the battery due to wiring resistance. Once the alternator hits its absorption voltage (14.4V?), it naturally tapers current as it holds that voltage.

If your alternator actually maintains 13.8V, you will likely be fine as there will be significant voltage drop, and current will start tapering sooner.

 

[JohnIy]

In an RV, there's often a substantial voltage drop due to wire length between battery and alternator. This can serve to help regulate the current as the Alternator will "see" a higher voltage than the battery due to wiring resistance. Once the alternator hits its absorption voltage (14.4V?), it naturally tapers current as it holds that voltage.

If your alternator actually maintains 13.8V, you will likely be fine as there will be significant voltage drop, and current will start tapering sooner.

A case where Ohm's Law is your friend.