Charging two batteries using 1 mppt charger

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ranger56 asked Jan 17, '24

Charging two batteries using 1 mppt charger

I have two batteries that are isolated and my intention is to charge the primary then the secondary using a MPPT charger. I’m finding out that this is not as straightforward as I thought. There’s MPPT chargers out there that can do this but not victron. I’m for the most part decided on using victron products as I feel they provide a quality product and their software is well supported.

One of my dilemmas is that my secondary battery will have twice the capacity of the primary. With that said, charging the primary battery is priority #1. I feel charging them in parallel would hold back the primary as the secondary would soak up amperage.

My second option is to charge one battery via the MPPT and the other via a DC to DC charger. I’ve been reading the manual for the solar smart chargers but one thing I’m not quite understanding is the limitations of the load output. Messing around in the app I can see that you can set certain criteria before the controller enables power to the load circuit. My question is, does the solar charger have the limitation of either charging the battery or supplying power to the load, or can it keep a float charge on the battery while supplying excess energy to the load circuit, which in this situation would be a DC - DC charger that’ll charge a secondary battery.

Alternatively, and this is a third option that I’ve seen get recommended on here, is to charge one battery using a DC-DC charger powered by the second battery, and to keep the second one charged up using the MPPT. I’m not a huge fan of this option personally.

In my opinion option 2 is probably the best option, if it can be made to work.

smart solar set-up help orion dc-dc Help

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klim8skeptic ♦ commented · Jan 17 at 06:34 AM

@Ranger56 Victron make the PWM Dou that will do what you want.

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ranger56 klim8skeptic ♦ commented · Jan 17 at 03:18 PM

My panel puts out 90W so I’d rather use an MPPT to get everything I can out of the panel

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Alexandra ♦ commented · Jan 17 at 06:00 AM

The real issues is the totally isolated part.

I would love to know what mppt you are referring to that can track two separate banks.

You would either have to have a bank selector. Isolate PV and switch banks.

The load output of most mppts is the same as the max battery charging but it won't have three stage charging. You don't mention your model but tech specs are available for all.

What battery type do you have? The differing capacity of the batteries is often not an issue but different depth of discharge would be.

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ranger56 Alexandra ♦ commented · Jan 17 at 03:16 PM

There are a handful of them on the market but they are from Chinese companies so for all intents and purposes it’s irrelevant since I don’t consider them an option.

I’m looking at the smart solar 100/20. Definitely way over what I need but the lower models have sand inside them for cooling and I need the ruggedness of the metal fins, or at least would like them. I’ve read the specs sheet but like I said it’s still unclear to me if once the battery that is connected to the solar charger is full, if the smart solar is able to maintain a trick charge while then sending excess amperage to the load side.

I do expect the batteries to be at different depths of charge hence I want to do it this way. They are going to be an AGM 65Ah and an AGM 100Ah.

I’m keen on using an MPPT charger since I only have a 90W panel so any extra juice I can pull out of it is going to be quintessential.

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ahtrimble commented · Jan 17 at 03:34 PM

It will take the same number of hours to charge both batteries no matter how you do it.

# of hours in (charging) = # of hours out (inverting).

I run 3 batteries (2 x 205ah + 1 x 230ah) I charge them as a bank with dual MPPTs since I have two arrays. The MPPTs are connected to a Victron PowerIn as are my batteries. Are they always perfectly balanced? Nope. But they do the job just fine...about 4 days with no sun and I am still pushing power just fine. Day #5 I fire up the genset through one of the two Quattros.

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