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Christopher A avatar image
Christopher A asked

12V House power and 24V PV Charger and Inverter.

I'm in process of scaling up my Motorhome power solution. Right now, it is 1200 watts solar to four 6V agm batteries in series/parallel 12V configuration.

My Idea is to rewire the battery to series 24V and charge from MPPT from solar. I would like to purchase a 24V Phoenix inverter, but at the same time keep the 12V House DC and Alternator 12V charging.

I'm about maxed out at 12V on solar and see the benefits of going to 24V, as I have room to add 600 more watts. My controller will handle 1800 Watts at 24V output with no issue.

Is it practical to tap off two 6V batteries to run the coach 12V DC portion (not much current is used, lights and water pump, etc) and keep the alternator charging @12V but use solar to series charge the bank @ 24V from the MPPT controller and pull 24V to the inverter? I know several suggest DC24V-DC12V converters and I have looked at Equalizers too. My concern is the alternator charging just half the bank and what implications that, if any, will have to the whole system. Perhaps some isolators of some kind would be needed here? The reason for all this mess is I'm sizing all this for the future when I upgrade to expensive lithium. I want it all to be at 24V for the inverter portion but don't want to trash the new AGM batteries at this point.

Actually, when charging with the alternator @12V to half the bank, Solar will also be charging @24V to the whole bank via the MPPT... If that is even possible without causing Cherrnobyl.

Educated thoughts on this are welcome. I have exhausted all my searches and asked The Google for optons.

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JohnC avatar image
JohnC answered ·

Ha, Chernobyl comes to mind. Please don't tap into a string of batts for either charging or loads. It's *very* bad practice and never recommended. Pb's are hard enough to keep in balance at the best of times.

The easiest way to deal with 12V loads is just a Victron Orion dc > dc converter, which will actually hold a more stable 12V than you'll have now.

Or you could use a separate 12V battery(s) fed with just little workshop charger from ac/inverter. I've some spare batts I'm doing that with right now, on a $5 power point timer set for sun times. I have the Orion too, but it's redundant for now.

You could also consider 48V, if you might like to go packaged Li (say BYD or Pylontech) in the future. But won't suit your existing batts.

What to do with your alternator, maybe someone else can chime in? The Orion also comes in a 12/24 version, but I'm not sure if it would work off an alternator? The Buck-Boost converter could do this, but it's a specialty item and v expensive.

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Christopher A avatar image Christopher A commented ·

there alternator has some sort of voltage regulator built in. So, I either scrap it and take it out of the loop, so to speak, or figure out a way to incorporate it to top off batteries while driving. It's a power source, so I'm inclined to try and incorporate it into the scheme if it's practical to do that.

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Pat Davitt avatar image Pat Davitt Christopher A commented ·

With 1200, or more, watts of solar on the roof; I don't think you will miss the alternator input. The solar works when you are driving down the road, as well as when you are parked. It also simplifies your system.

Using a 24/12V converter is a good thing since it provides clean, consistent voltage to your 12V DC loads. The Victron units accept 18-35 volts on the input side and outputs a factory default 13.2 volts.

Pat

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st3v3 avatar image
st3v3 answered ·

Why can you not stay with a 12V system?

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Christopher A avatar image Christopher A commented ·

I'm at 1200 watts of solar. From what I have learned it's not practical to have much more than that @12V. 24V is more efficient with terms of wiring, fusing, battery size, etc.

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a-v avatar image
a-v answered ·

You should also consider the batteries nax. charge current when adding solar panels, if you do not want chernobil... (at least inflated battery cells). Lead acid batteries generally does not like to be charged more than max. 20% of their battery capacity, but for the right figure you have to check the exact value with the make and modell of your batteries.

If you want to charge 24V battery bank from 12V alternator, use the Buck-boost DC-DC converter. I know it is not cheap but it will be useful also when you switch to Lithium to limit the current drawn from the alternator.

There are also 12,8V Lithium batteries and using two MPPT chargers on 12V in parallel is maybe cheaper or the same price as a DC-DC converter before the loads, of course depending on the powers. Also, you can add the second MPPT when you have enough battery capacity (room in charge current).

I would never use 12V loads from a 24V battery without a DC-DC converter. The converter is relatively cheap respect to the much more expensive batteries that would die soon without. Using separete batteries makes the system more complicated and you split up your available power on board.

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JohnC avatar image
JohnC answered ·

This thread prompted me to take another look at the Orion (cos I have one, and for you too Dishusa). There's quite a range, but I looked at/bought the Orion-Tr (isolated). https://www.victronenergy.com/upload/documents/Datasheet-Orion-Tr-DC-DC-converters-isolated-100-250-400W-EN.pdf

The manual's pretty basic: https://www.victronenergy.com/upload/documents/Manual-Orion-Tr-isolated-DC-DC-converters-EN-FR-NL-ES-IT-DE.pdf

Both links suggest the remote terminals (yes, onboard) can be operated with a "engine run/stop switch", ie an ignition switch, and infers that it would accept from a standard alternator. The specs suggest so too. So if it were wired from the 12V engine batt it would cease activity when the engine is turned off.

Output V can be adjusted via "potentiometer", which must be that little hole on the lower rhs (tiny screwdriver needed).

It doesn't have full batt charge profiles onboard, but set to a typical Float of (say) 27.6V, should work fine to keep the batts up whilst travelling.

It does seem silly to have 2x Orions, one bucking & one boosting, but I'd rather do that than staying with 12V at your size system.

More for you to chew over. The big swallow is irreversible once you choose that inverter.. :)

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Christopher A avatar image Christopher A commented ·

Yes, the inverter is forcing all these decisions. Plus the batteries to support the inverter. At this time, I can moc up the 4, 6V batteries to be 24V. My intention is when I switch to LifePo4 across the system, as I cannot find any way on earth, heaven, hell, purgatory or South Dakota to have lithium and lead acid co-exist in some form in the same system. I have looked everywhere sans having two different charge systems feeding two separate inverters combining into my power panel.

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