Charging battery with tow vehicle, expected amps?

[tbrady]

I've noticed that my TV is barely charging our battery when traveling.  My battery monitor is showing less than 1ah going in to the battery when I hook it up...

I expected this to be a lot higher.  Has anyone measured this on their vehicle?

[DavidM]

Well battery charging is proportional to how discharged the batteries are. With half full batteries I get 10-15 amps when hooked up to the TV. It will be just a few amps with full batteries.

How discharged were yours when you measured?

But charging is also very sensitive to voltage drop. If you have a bad connection somewhere, either your TV or between the camper's brake connector and the battery then that will also limit charging current. Also check your TV's alternator output voltage at its battery terminals. Should be in the high 13s.

David

[tbrady]

I think you may have hit on the issue David...  I have a lithium iron battery now and the voltage doesn't drop until there is a significant drop in state of charge.  I can't find the chart right now but basically until the battery is something like 70% depleted you do not see much voltage drop.

[DavidM]

Yes, charging Li batteries is entirely different from lead acid ones. You probably have a battery management system which controls charging and discharging. It probably needs more than 14 volts to begin to operate, so at the typical 13.8 volts of most TV alternators you get little charge current.

There are devices that might help, but I am loath to recommend them. The Sterling regulator/charger presents a low impedance to the engine alternator which lets the alternator put out significant amps from a mid 13s voltage. Then it converts the voltage to something higher to supply the battery. But I don't think they have a Li setting.

David

[ADR]

Most tow vehicle haves inadequately sized wiring to supply a lot of charge current to trailer batteries.

re Sterling-  I have the BB1230 in my Transit to charge my house battery.
It does do LiFePO4 if needed.