Recharging with external charger

If you have two chargers hooked up (Which reminds me... I do just now) then one of 3 things will happen.

1: one will charge the other will relax
2: Both will charge
3: Both will charge causing the batteries to be damaged by the too fast charge

odds are both will charge, but at a reduced rate. so as to not damage the batteries.

DEPENDING on what converter you have in the RV and where it's located.. You might be better off with just one of them running.. But that said.

your plan should work.

This is where the Harbor Fright 10-amp charger shines. When the smart chargers are trying to find their --- with both hands at 14.4 volts the transformer stupid charger is going to keep charging at around 8 to 10 amps with a decent generator. At 13.8 volts? The HF will most likely be at 10-14 amperes.

My previous HTT had a "dumb" Elixir converter that wasn't good at battery charging. I bought an deck-mount Iota-55, and on a generator I put the IOTA right on the battery, opened the disconnect switch, and ran the trailer on the on-board Elixir converter while the IOTA charged the battery. 13.5V on the Elixir, 14.8 volts on the battery.

Fast forward to the new TT and WFCO converter. WFCO's aren't known for going into bulk mode easily. I mounted the IOTA to the ceiling of the pass through, tapped off a front bed 115V outlet, and ran a switched outlet into the pass through. The 12V side is a 3' run to the batteries. I also put the factory WFCO on it's own breaker (C-H, found one in the parts bucket!) The plan is to run the IOTA, open the disconnect switch, and let the WFCO power the trailer while the IOTA charges the battery. For camping with hookups I can shut down the IOTA and just run on the factory WFCO.

From the OP: To clarify, my converter is a WFCO which never seems to go above 13.8 volts, so the purpose of the external charger is to get one that will charge faster. Perhaps it is better to get it started first. Maybe I can just start the charger before I plug the TT into the generator.

I would use the cut off switch to get started. Once all is running a few minutes I would close the switch. No they will not fight each other. The only issue is to get the portable started without getting confused with the converter increasing the voltage before the portable starts.

Answer is: It depends.

If the built-in charger is multi-stage (can reach 14.4+ volts for high amp-rate charging), allowing it to go alone at first, per Mex, is most likely best. When the first charger falls back to 13.7 volts, then add the portable charger. Both will most likely keep charging at 13.7 volts (absorption mode) until one senses "full" charge ahead of the other.

If you have a "clamp-on" multi-meter, you could easily check the AMPs being output by each charger and add the total. Use the clamp on the POS(+) cable off each charger. If the amps total more than the best charger alone you are gaining ground.

On the other hand if you start with both chargers, the multi-stage charge may either drop out of "bulk mode" early, or never get into it. This way you may fall behind.

Tug a war rope, batteries on one side, charger or chargers on the other. Little chargers pull like youngsters, 40 amp chargers pull like regular men, 100 amp smart chargers pull like lumberjacks.

No worries. Don't let the voltage go much above 14.8 before you back off to the smaller charger.