Question about DC to DC charging

Hey all, you folks were so helpful with my last snafu, I thought you might be able to help me with another question.

Today I installed a 40a DC to DC charger for my solar battery bank. I tested it out with the batteries down about 10 amps, it worked flawlessly gently stepping down the charge level as the batteries were topped up. For context: I have a 500ah lithium battery bank wired in parallel fed by two 40 amp charge controllers and (now) a DC to DC charger. These batteries are separate from the "house batteries" which are two 6v 200ah regular AGM batteries wired in series. I use the 500ah battery bank for powering the 110 in the coach with a 3000w inverter. I assume the 110v system when connected to shore power (or my inverter) also charges the "house batteries", but I have not confirmed this.

My question is this: Assuming the 110v system charges the house's 12v system, wouldn't that create an infinite power loop where the 110v system is charging the house's 12v system which the DC to DC charger picks up and feeds back into the 500ah battery bank, then again back out to the 110v on and on? Would that even matter if it did?

It will, it matters. Simple answer is to turn off the converter, easy way is to flip the CB for the converter.

Yes what he said

Would that even matter if it did?

Click to expand...

It wouldn't matter if everything was 100% efficient but since it's not this arrangement will eventually drain your batteries

Well a few interesting developments, as I wanted to test this theory further. I decided to use the circuit breaker I installed on the B+ wire from the alternator to the charger as a switch to do some testing.

First, this particular dc-to-dc charger is only powered on when it receives a 12v signal from the ignition being on, at which point it will come alive in order to use the alternator to charge the battery bank. Therefore, the charger is inert when the ignition is off, so I'm not too worried about it draining the batteries with the engine off. I can't speak to the other type of these chargers out there that Do Not require an ignition signal, but my assumption would be that the logic in those controllers only allows charging to take place at a set voltage, likely 13 or 14+. In any case, this finding shows that the charger does not need to be disconnected from its power source when activating the 110v system via the inverter and no power loop will take place here.

But that leads me to my next big question, can the 110v system be kept on while the engine is running and the dc-to-dc charger is filling that battery bank, or will this cause some kind of damaging circular power loop? Well, what appears to happen here is, with the engine running, the converter is rendered inert. Since there is an ignition signal and charging voltage coming from the alternator, the converter does not activate to trickle charge the house batteries. I'll need to do some more testing on this to be sure (run the house batteries down and check for converter activation), but so far it appears that the inverter just acts normally powering the loads on the 110v system while the alternator works to charge the house batteries and the battery bank simultaneously.

So far, it appears that the inverter can be powered on 100% of the time and the dc-to-dc charger can also be left connected to it's power source 100% of the time and everything should play nice together. One thing I have not tested yet, however, is what happens in a simple key-on-engine-off scenario where the dc-to-dc charger would receive a 12v signal to come alive but have no alternator charging voltage to use. It could be that nothing happens as the converter tends to trickle charge at a very low voltage and thus would not meet the threshold for dc-to-d charging to take place. Or, it could be that that scenario leads us to a loop of battery bank>inverter>110v>converter>12v>dc-to-dc>battery bank... but again I question whether there would be anything wrong with that scenario.

Thoughts?

Some DC-DC charger are have "isolated grounds". Most inverters also have "isolated grounds" (neutral NOT connected to ground). You need to validate this.

If you keep the grounds "isolated" from each other AND from the 120VAC neutral, you will not have a problem.

For extra safety, I would replace all 120VAC receptacle with GFCI receptacles, NOT CONNECTING THE GROUND WIRE ! The GFCI circuitry is designed to work this way. GFCI measures the current going in to the appliance and the current coming out of the appliance. If they are different, it trips !