Help with a mixed voltage battery set up

[Rybrx7]

Hi all,

I have a horsebox here in the UK, I have 2 solar panels on the roof to charge the truck batteries (24v system) with one cheapy solar controller that came with the panels. This works as expected. I however also have a 12v 110amp leisure battery that all the lights etc run off that is always draining, that i would like to keep topped up from the solar panels.

I currently have a split charge relay from the 2nd battery of the 24v truck batteries (they're 12v each in series) - this used to trigger when the truck was running to charge the leisure batteries. For some reason that has stopped working and i'm tempted to rip it out as it's only useful when i'm out and about driving the horsebox (once every 2 weeks at most for short journeys)

What I'm hoping is that there is a way of using existing 2 solar panel circuit to also charge the 12v system. I know that I can't used mixed voltages on one controller, so what are my options? I don't want to spend a fortune on voltage converters etc, but some form of intelligent battery charging would be nice.

I have another spare charge controller, i'm assuming I cannot use the 2nd output from the 1st controller (the one that charges the 24v system) and connect to the input of the 2nd controller (as if it's the solar panels) and then output to the 12v battery?

Thanks

Ryan

[Mike90250]

I think if you want to charge another battery, you will need another panel & controller.
If you add another 12V controller to your existing panels, you will steal charging power from the 24V battery, and then it won't stay as well charged.

[Rybrx7]

I think if you want to charge another battery, you will need another panel & controller.
If you add another 12V controller to your existing panels, you will steal charging power from the 24V battery, and then it won't stay as well charged.

Ahh that's what I was hoping to avoid, roof space is limited with the 2 panels already up there.

I only really need to trickle charge the 24v truck batteries, so if that's the case then even if it's not as efficient it sounds like linking 2 charge controllers together could work?

Thanks

Ryan

[Mike90250]

Yes, you would need to use a MPPT controller, expensive, but it will not limit the PV panels to 16V, which would then never charge the 24v battery.

A PWM controller will pull the panel voltage down to the battery (12V) voltage, until it thinks the battery is full. That might work, with the 12V battery and controller functional in the morning, and maybe by noon, the 24v battery may start charging

it will be a learning experience !

[bcroe]

Seems like your original setup worked, but was a problem to keep switching to the
best performance. If you just want something of a trickle charge to your 24V batteries,
I amprroposing a scheme.

First put your panels in series to have enough voltage to charge 12V or 24V, and use an
MPPT charger to efficiently charge the 12V battery. With the negative terminals of both
batteries and the panel string connected together, connect a small 24V or 48V bulb to
the positive of the panel string (the MPPT is already connected there). The bulb should
be rated about your desired trickle charge current, like 50 or 100 ma. An incandescent
bulb has a characteristic of drawing a near constant current no matter how low the voltage
across it is, but its voltage rating must be above the open circuit panel voltage minus the
24V battery min voltage. Same type bulbs in series work for more voltage.

Connect the other side of the bulb(s) to the anode of a 1A diode (1N4004 etc), and connect
the cathode (band) end of the diode to the 24V battery +. This will provide a trickle near
the bulb rating, with a comforting glow, very cheaply but not very efficiently. It will not cut
off if the 24V bat is full (that is a MAINTAINER), that feature requires a small voltage regulator
set to that point plus a diode drop, located between the bulb and the diode.
good luck, Bruce Roe

[bcroe]

As usual I must wait till tomorrow to correct spelling, or be subject to moderation. B

[Rybrx7]

Could I use something like the below? Only about £20

The input would be Output 2 of my solar controller

ORI241205200_1080x.jpg

[inetdog]

My only concern would be just what the Output 2 of your solar controller is designed for.

[chrisski]

Could I use something like the below? Only about £20

The input would be Output 2 of my solar controller

ORI241205200_1080x.jpg

I don’t know if that will work as a charger.

I’ve put some thought into getting a 24 Volt system in my RV to power a bigger inverter, but my RV would still be wired for 12 Volts for things like the lamps. I’m sure that would work fine for that, but I have a 12 volt brake system that needs a 12 volt battery to operate. I doubt that 12 volt converter would charge the battery. I don’t think it would work with that 24 VDC to 12 VDC converter alone.

Also, I have a PWM charge controller with a second power out labeled load. That load is not very strong, and the only documentation I have on it is “Not to plug things like inverters to the load,” there’s no amp limit, just basically don’t put something big.

The RV converter I use that takes shore power and turns it to 12 Volts has built in logic that will adjust voltage based off the charging cycle, and if that does have a battery charge mode, perhaps.

So, I’m trying to figure out how to solve a similar problem for a 24 volt system to charge a second 12 volt battery efficiently, but I’m more concerned about building my first 12 volt system for my RV.

The same company makes a 24 VDC to 12 VDC charger, https://www.victronenergy.com/dc-dc-...t-non-isolated, which I may look at to do what I want, but it is more than 10 times more expensive than the money you mentioned.

[bcroe]

Could I use something like the below? Only about £20

The input would be Output 2 of my solar controller.

DC to DC buck converters such as that are similar in construction to an MPPT controller, but
cannot do an MPPT function because the feedback function is entirely different. Or, I just do
not understand what you are trying to do. Bruce Roe