charging batteries in parallel using external & internal chargers

[KGs2108DS]

I'm adding a 2nd 12V battery for longer runtime (more amp hours) while boondocking. Also planning to purchase a 25amp smart charger for quicker charging. Micro Minnie 2108DS built-in charger is 6amp per owners manual (and Victron battery monitor confirms this). I use a Honda EU2200i to run the trailer and charge the battery (takes quite a while). Question: can I add the 25amp smart charger using the generator 110VAC outlet to charge the two batteries WHILE the trailer is plugged into the generator, and the internal 6amp charger is running as well. both chargers would be running simultaneously. Also, how is the best way to attach the 25amp smart charger? Both leads to one battery, or + lead to one battery, and - lead to the other? thanks!

[creativepart]

Yes, both chargers can run at the same time and yes, you stagger the charging cables with a plus on one battery and the negative on the other battery.

One thing to note, some smart chargers are smart enough to sense a high voltage at the battery, caused by the converter doing it's charging, so they can erroneously assume the battery doesn't need much charging and dial back the charge current.

It's more work and more money, but replacing the charger in the OEM converter with a higher amperage, more modern and smarter charger might work better in the long run.

I picked up a Victron AC to DC 14-amp charger for a specific charging task last year and I love that it's super smart and has Bluetooth that works with the Victron smartphone app. They do make higher amperage AC to DC chargers, too.

https://www.amazon.com/gp/product/B07TJK3X71

[KGs2108DS]

I'd might be a bit nervous about switching out the current 6amp charger with the Victron, though I really like to idea! I'd be hesitant to dig into the electrical panel and what's behind it (thinking this is the location of the OEM battery charger). Could I do this? Or need to take it to a dealer? Could I find instructions/schematics somewhere? would I need to run any new wiring? Or, is it as easy and removing the panel, d/c the current 6amp charger and replace w/the victron (guessing it's not haha!). thanks!

[creativepart]

Sorry, no, I didn't mean switch out the converter's charger with the Victron. I'm sorry I wasn't clear.'

I only mentioned the Victron if you were buying an additional charger.

I think you'd need to buy a new charger with the same brand as your OEM converter. The charger is usually separate from the rest of the converter and the company that makes your converter will also make higher amperage chargers and chargers with more features.

While it is scary with all those wires. The charger is usually on the floor behind the "power center". There are plenty of videos on YouTube of folks doing this change and it shouldn't be too big a deal.

Sometimes you can get into the side of the space where your converter is and access the charger from the side without removing the power center.

I should add, that not all converters have the charger separate. So, that's definitely something you'd need to double check.

[Ray,IN]

Keep in mind, 12V batteries have a specified maximum charge rate which varies with battery capacity. This is to prevent overheating and warping the plates.

[powercat_ras]

If you charge flooded lead acid or AGM batteries with two or more chargers in parallel with chargers designed / set up to charge them, you will fall out of absorbtion charge mode into trickle charge mode due to the interaction of chargers sensing the a higher voltage than just the battery alone due to the other charger's output. This happens once the batteries get somewhere at or above 80 % charged.

So once the batteries reach somewhere above 80% you would get best results turning off one or the other of the chargers. You could see exactly when to do this if you install a battery state of charge monitor with shunt to see the % state of charge and the amps the chargers are pushing into the batteries. When you saw the amps drop to 1 or 2 you would know it's time to turn off one of the chargers.

There are now non Victron monitors sold on Amazon that are quite affordable.

Something like this: Voltmeter + Ammeter + State of Charge w 350 Amp Shunt $ 44