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How to Charge the 12v House Battery From the 48v Solar Setup?

Jeff H

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Sep 9, 2021
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I'm not sure how to descrbe this problem but here's my best shot...

I've had a 48v solar setup for a year and a half running the ac side of my 5th wheel with good success. My only problem is I don't understand how to tie it in to the RV's 12v system. I need to replace the factory12v battery and would prefer to tie it in to my existing 48v lithium battery bank, but I don't know how to go about doing it. I've looked at this stepdown converter on Amazon (https://amzn.to/3UTVOT7 - not an affiliate link) but I don't know enough about how it all works to be sure it is the way to go. What is the right/best way to do this?
 
Is the 12v system only for electronics and stuff, or is it also the starter battery?

You'll want something like a Victron 48v to 12v dc to dc charger, in whatever amperage amount you think necessary. You'll need a functioning 12v battery still if it's a starting battery system.. otherwise a high enough amperage dc to dc charger can handle it for you and you could possibly just toss the 12v batteries depending on peak amperage requirements.

The Victron chargers can be either a straight 48v to 12v supply (customizable voltage) or a charger for certain 12v battery chemistries.
 
There are DC to DC step down converters like the link you posted the problem is they regulate the output to 12V and you can't charge a 12V battery with only 12V. You actually need a DC to DC Charger. Victron and Renogy are 2 companies that come to mind. In fact, there was a post earlier today on this forum regarding a Renogy device to do just what you need, although I believe it was a 12V to 12V system.
 
Is the 12v system only for electronics and stuff, or is it also the starter battery?

You'll want something like a Victron 48v to 12v dc to dc charger, in whatever amperage amount you think necessary. You'll need a functioning 12v battery still if it's a starting battery system.. otherwise a high enough amperage dc to dc charger can handle it for you and you could possibly just toss the 12v batteries depending on peak amperage requirements.

The Victron chargers can be either a straight 48v to 12v supply (customizable voltage) or a charger for certain 12v battery chemistries.
Thanks for the reply. The biggest draw is the 12v refrigerator, no need to start the 5th wheel. I’ll look into that charger you suggest.
 
The step down converter you linked to will work. Since you already have a 48v system running an AC unit, you have an inverter.. you could also just plug or hardwire a small battery charger into the AC circuit from that inverter somewhere and leave it hooked to the 12v battery or the 12v distribution center/fuse panel permanently. Many battery chargers have nice little touches like multi-stage charging, some form of temp compensation, reverse polarity protection, etc that the small converter will not have. The biggest downside of that approach is if there are a lot of times you ARE running the 12v system (fridge) but have the 48v inverter turned off.
 
The step down converter you linked to will work. Since you already have a 48v system running an AC unit, you have an inverter.. you could also just plug or hardwire a small battery charger into the AC circuit from that inverter somewhere and leave it hooked to the 12v battery or the 12v distribution center/fuse panel permanently. Many battery chargers have nice little touches like multi-stage charging, some form of temp compensation, reverse polarity protection, etc that the small converter will not have. The biggest downside of that approach is if there are a lot of times you ARE running the 12v system (fridge) but have the 48v inverter turned off.
To get by while hooked up to shore power I've been leaving a battery charger hooked up to the battery. The problem with that is when the battery gets fully charged the charger shuts off and I need to manually turn it back on which means I have to monitor the state of charge... or when the freezer starts thawing, whichever comes first. I think I'll try that converter first to see if it solves the problem. I can always switch to the more costly victron option later. I'm pretty sure the wattage/amps output will cover everything with plenty to spare. Thanks for the input.
 
rookie here, but what about using a solar controller such as the victron MPPT 100/20 - 48v? it will auto track to 48v input, output is selectable, charge up to 20 amps, has about 20mA of parasite, and can be custom set to match his 12v battery.. that could also be used to start the generator, getting the system down to 2 batt banks.? Is that possible or smart? kind of acting as a DC to DC charger with step down built in.. and it would show up in the victron app.. thoughts?
 
I doubt you're going to be able to directly charge a 12 volt battery from a 48 volt source.

You either need to power a 12v charger with a step down converter, or run an 48 volt inverter and use a normal 120v charger.

If you have 48 volts DC, then you have a normal 120v ac inverter for that battery right? Why not just get a NOCO charger and plug it in?
 
I doubt you're going to be able to directly charge a 12 volt battery from a 48 volt source.

You either need to power a 12v charger with a step down converter, or run an 48 volt inverter and use a normal 120v charger.

If you have 48 volts DC, then you have a normal 120v ac inverter for that battery right? Why not just get a NOCO charger and plug it in?
That's what I'm doing now but it's not ideal... and a pita.
 
That's what I'm doing now but it's not ideal... and a pita.
What is the problem?

I'm not aware of any battery charger products designed to go from 48vdc to a 12v lead acid cell.. although come to think of it, there might be some kind of demand for such a thing.

Tell me exactly what you're doing and what problems you're having and I might be able to come up with something for you.
 
It should work fine, it’s just not the cheapest way to do it since bottom dollar on an MPPT is like $60 and most are over $100. By comparison a 48v to 13.8v step down converter, while less configurable, can be had <$40 and a 120v to 12vdc psu or charger can be had ~$20.

As far as the existing 120vac battery charger turning off and needing to be manually turned on, that just means you got one that’s too smart. ?

A lot of the cheapie ‘battery maintainers’ stay on no matter what and if battery voltage happens to rise above its charging voltage it will just sit there and do ‘idle consumption’ and nothing else. Ive used cheap meanwell knockoff PSUs like this.. say i set it to 12.7v. If battery voltage is maybe a volt below that it will max out current. As battery voltage rises closer to 12.7 current tapers off, if battery goes past 12.7 the psu does nothing except sit there eating 4-6w of ‘idle draw’. If voltage drops below 12.7 it picks up again. It’s not really a ‘battery charger’ but it’s a ‘voltage backstop’ that keeps the 12 volt lead acid system from dropping to a voltage where it will suffer sulfation.
 
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