Help with 24v/12v system

[Rybrx7]

Hi,

Hoping to get some advice. I've fitted a 100w solar panel to the roof of pur horsebox. This system is allegedly 12v, however in low light the panel outputs around 26v


The charge controller I have is rated for either 12 or 24 volts.

Now this is where it gets complicated, the truck has two 12v batteries in series for the 24v truck wiring.

I have been advised that

1) I cannot mix voltages on the charge controller (it can charge 2 batteries)
2) the panel is "12v" so won't charge the 24v truck battery.


So I'm stumped, the panel outputs more than 24v so how is the seller claiming this is a 12v system? Will this work with my current set up? I have it wired so it is connected to the negative of battery 1 and the positive of battery 2 (they're linked in series)

If this wont work, my priority is maintaining the truck battery. With it being in series if I charge just battery 1, will that trickle charge battery 2 in series? If no, as the controller can charge 2 batteries simultaneously, can I go charge controller port 1 to battery 1 in series, then charge controller port 2 to the 2nd battery in series? I'm unsure if I could charge it like that.

Thanks

Ryan

[littleharbor]

Brand and specs of your panel? That is an odd voltage. Most 12 volt panels are in the 18 volt Vmp range and 21 volt Voc. range. 24 volt panels are more like 36 and 45volts respectively.

What truck do you have that has a 24 volt system? My Dodge 3/4 ton with a Cummins diesel has dual batteries but is still a 12 volt system.

[NCmountainsOffgrid]

agree, I doubt your 'truck' is 24v, not anywhere near any common voltage for ANY vehicle - 12v is the standard. And, yes, if the panel is a 12v panel, it is. Your solar controller will automatically sense the battery voltage and set itself accordingly, to maintain and mange the solar output to charge them.

[Rybrx7]

It's a 10 tonne Eurocargo, it is 24v. Two 140ah/800amp batteries wired in series for 24 volts.

They're what I'm primarily trying to keep topped up with Solar while not being driven.

I will dig out the specs for the Solar panel, it isn't an expensive one, just from an ebay kit that got good reviews. It's a 100 amp panel.

Assuming that my multimeter is dodgy and it is in fact outputting a lower voltage, could I

A) charge just the first 12v battery of the 2 in series
b) buy a second solar panel and wire in series to produce 24v+ solar.

If B is the only option to getting 24v, bearing in mind I'm onky intending them to trickle charge the main batteries over a period of 3-4 days between uses, would I need another 100w panel, or could I get away with a small panel (I.e. cheaper) and still get the 24 volts required?

Thanks

[SunEagle]

It's a 10 tonne Eurocargo, it is 24v. Two 140ah/800amp batteries wired in series for 24 volts.

They're what I'm primarily trying to keep topped up with Solar while not being driven.

I will dig out the specs for the Solar panel, it isn't an expensive one, just from an ebay kit that got good reviews. It's a 100 amp panel.

Assuming that my multimeter is dodgy and it is in fact outputting a lower voltage, could I

A) charge just the first 12v battery of the 2 in series
b) buy a second solar panel and wire in series to produce 24v+ solar.

If B is the only option to getting 24v, bearing in mind I'm onky intending them to trickle charge the main batteries over a period of 3-4 days between uses, would I need another 100w panel, or could I get away with a small panel (I.e. cheaper) and still get the 24 volts required?

Thanks

To charge a 24v battery system you will need to wire 2 12v panels in series to the charge controller. You will also need to match up the panels as best as possible or the charging amps will be reduced to the rating of the smaller wattage panel.

[Mike90250]

To charge a battery, you must supply voltage higher than the battery voltage, to push charge into the battery. For a 24V battery, you need about 35V for the charge controller to be able to work properly.

Read the instructions carefully for the controller, some are automatic sensing of the battery voltage, and some need manual configuration.

[Rybrx7]

Thanks for the responses all, I suspected as much.

I was hoping not to have to fit another, but may be my only option. Thankfully it's over 22ft long so more than enough roof space

I'm assuming I cant connect the solar to just the first truck battery in series, therefore only charging one 12v battery not the full 24v? If that's possible, would battery 1 "flow" into battery 2 as its charging, therefore charging both batteries?

I may be talking nonsense but no idea how charging series batteries would work

Thanks

Ryan

[Ampster]

Your first assumption was correct. Charging one battery in a group of two or more in series will leave that battery with more charge than the others.

[SunEagle]

Thanks for the responses all, I suspected as much.

I was hoping not to have to fit another, but may be my only option. Thankfully it's over 22ft long so more than enough roof space

I'm assuming I cant connect the solar to just the first truck battery in series, therefore only charging one 12v battery not the full 24v? If that's possible, would battery 1 "flow" into battery 2 as its charging, therefore charging both batteries?

I may be talking nonsense but no idea how charging series batteries would work

Thanks

Ryan

While some of the energy may "flow" from one battery to another you will still end up with unequally charged batteries. Unfortunately you need to provide at least 35V to charge a 24V battery system.