Question about DC to DC charger output

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Vannautical engineer

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I have a DC to DC charger, specifically the Renogy DCC50S that has a solar controller in the same unit.

My question is, if I have the vehicle running, and therefore power from the alternator is going into the charger, and I have something drawing power out on the house battery side of the charger, will the charger provide power directly to the device that is drawing power, or will it be drawing power out of the battery itself, and then the battery will get topped up by the charger once it's drawn down a little bit?

Now that I type this out and think about it, I think I might have answered my own question, but I'm still not 100% sure. I would think that the charger is just supplying out whatever current is demanded on the house battery circuit, and the charger really has no knowledge or ability to provide power specifically to either the battery or a device that is using power. So I guess the device would just be powered directly off of the charger in this case, and the battery is just not really part of this whole exchange of power at all since the charger is probably keeping the voltage of the whole system slightly higher than the battery would on its own anyway?

Only reason I'm asking is that I'm wondering if I install accessories on the house battery circuit that will primarily be used when the vehicle is running, does that mean my house battery is constantly getting a little bit of a cycle and then replenished all the time when I'm driving? But I'm thinking that no, it wouldn't be.
 

tx2sturgis

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You are kinda 'overthinking' this. 

Sorta like eating a hot dog and then wondering which arm or leg got more of the nutrition out of it.

Think of the house battery and the loads attached to it as one single load when there is incoming charge current, as far as the source of that charge current is concerned.

Yes, the house battery will accept the current needed to keep its charge voltage at about the same level as the incoming charge voltage. Any remaining available or excess current is available for the other loads. A lead acid battery receiving a constant voltage charge (from the DC/DC charger or any charger) is not using any of it's 'cycles' unless: the load placed across it's terminals is consuming MORE than the incoming available charge current.
 

PlethoraOfGuns

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Don't see why anyone would think about this, it really shouldn't matter. But if your mind wants to tickle it, I would say go off the theory that electricity takes the path of least resistance. Electrons may very well possibly bypass the battery and go right to a load in your scenario.
 

Vannautical engineer

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Yeah, probably overthinking it. I guess my only thought is that I don't want to be constantly cycling my house battery up and down by a little bit the whole time I'm driving if some of the accessories that I mainly use while driving are connected to the house circuit as opposed to the main vehicle electrical. But like I said, after thinking about it, I don't think that would happen anyway. The charger is just putting power out onto the house battery circuit, and it will output as much current as is being demanded, whether that's from the battery pulling in a charge, or from accessories consuming power. Since the charger is probably always going to put out a slightly higher voltage than the battery (otherwise how could it push current into the battery?), the ultimate source of power for anything on the house circuit when the vehicle is running would be the charger.

I guess another way I could really put the question to rest is to just run the vehicle, turn on a bunch of stuff on the house battery circuit, and watch my battery monitor amperage, because that's a direct measurement of how much current is flowing in or out of the battery. If the battery monitor shows little to no current ever coming out of the battery even with a bunch of stuff turned on, then the charger is supplying the current to everything directly.
 
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