Why not change the second battery to Lithium?

[Kruegerruda]

Is it not possible to replace the second battery with a lithium battery and use it as a supply battery? The second battery is charged via the alternator and disconnected from the starter battery by the Smartpas. Or am I seeing this wrong?

 

[anand]

Lithium batteries utilize a different charging profile than the EFB batteries (both starting and second).

If you replaced the SmartPass with a DC-DC charge controller, with lithium specific output voltages, then you could absolutely use lithium for the second battery; this is my plan.

 

[nuclearmonkey]

My understanding is that a potential downside is during heavy load where two same type batteries can be linked together, the lithium can not. Example would be winching.

 

[anand]

My understanding is that a potential downside is during heavy load where two same type batteries can be linked together, the lithium can not. Example would be winching.

This would be accurate for some battery isolators, however, that is not exactly how the SmartPass120 operates.

The SmartPass120 sends power from the aux battery to the starting battery in one of two instances:

1. The aux battery can send a low amperage charging "pulse" to attempt to continuously recharge/top off the starting battery, from previous threads I think I remember the charge rate to be ~5A
2. If the starting battery voltage reaches 9.5 volts, it will connect both batteries for a period of 10 seconds. This is specifically intended to sense a dead main battery that is attempting to start the vehicle, and the 10 seconds is with the theory that will be long enough for the engine to start and the alternator to start supplying power.

Some other isolators will simply direct connect the batteries when the voltage on either side is high enough (for instance, the Blue Sea ML-ACR). The ML-ACR also has a switch to force the connection (or disconnection), and thus you could force connect both if you were doing a very extended winching operation; but in almost all cases, the battery drain from the periodic nature of winch pulling/reseting is kept up with by the alternator output.

 

[nuclearmonkey]

Appreciate the reply. I wasn’t referring to the smartpass per se… It’s been about 8 months since I spoke to them, but I had a call to Lifeline about replacing my house battery in my defender with a lithium. They advised against for any of the times the batteries could possibly be joined due to chemistry and voltage differences. The lithium IIRC, couldn’t handle the sudden high amperage draw (say winching). I believe they also advised against using it to jump the starter for similar reasons. The battery would shut itself off as it sensed the massive current draw - before the starter battery would have enough time to jump. I believe this is why a lot of the Aussie trucks are running dual agm’s with a lithium as well.

If kept effectively separated… then obviously totally different conversation