Stupid question about charging car batteries.

[MysterG]

I mentioned elsewhere that I have in my garage a 100ah battery being charged by 200W of solar panels on the roof through a cheap PWM charge controller.

Currently I have my radio and an Echo dot connected and at some point in the future I plan on trying a diesel heater install and would use this battery for that, plus some 12v lighting.

Most of the time though there is no load on it, but I also have 2 trickle chargers maintaining the batteries of 2 cars. Both 40-50ah IIRC.

Could I attach an inverter to my big battery and plug one (or both?, probably not) trickle chargers into that and indirectly charge the car battery "off grid"?

 

[jerrytug]

You can find 12vDC to 12vDC battery chargers, quite common on yachts etc. Might be pricey. But more efficient.
Inverting 12vDC to 240vAC, then converting it back down with a mains battery charger, does not seem ideal.
Also, invertors have a high current draw, your solar panel might not keep up with the discharge.
It would work, but if you didn't check it daily especially in dull weather, you might flatten and knacker your 100 Ah battery.
(Diesel heaters have a high start-up load, at least 10 amps, so use nice thick wires. )

 

[CompoSimmonite]

I got a 12v to 14v inverter off eBay cheaply

 

[hotponyshoes]

Why not just add another twin output (or replace the existing controller with a 3-out) and just charge all 3 straight from the solar?

 

[MysterG]

Why not just add another twin output (or replace the existing controller with a 3-out) and just charge all 3 straight from the solar?

I did consider that, but was a bit confused. The current controller specifies that the batteries are connected to it before the panels, and removed in the reverse sequence.
Was not sure how it would work/cope with a couple of batteries that would need to effectively be removed from the system every time I went out in a car? Plus the fact they are not all identical batteries.

 

[qwakers]

it wont care, as long as some batteries are connected, it will have somewhere to send the power.

 

[hotponyshoes]

They don't need to be identical, that's the point of the multi-output controllers.

 

[optima21]

you can get buck boost converters to do that, either basic ones like this

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I got a digital one though

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and fitted it into a 13.8V power supply and now use that for charging car batteries, (since my intellegent charger died), I set it at 14.5 V at up to 4A but they are adjustable for both current and voltage

 

[Prajna]

or grab a couple of Imax clone LiPo chargers. They do lead acid as well and run off 12v. Or a split charge relay, which will only charge the car batteries while solar is coming in.

 

[GRW]

200w of Solar sounds good... but in our northern climate its probably averaging 15, maybe 20%, tops. So think more in terms of your Solar being 40w, for the average 10-ish hours of daylight.

I'd strongly suspect that you dont have enough power to properly mantain charge a 100AH battery at all...and adding two trickle chargers as a constant draw - well it ain't gonna help the situation.

 

[hotponyshoes]

200w of Solar sounds good... but in our northern climate its probably averaging 15, maybe 20%, tops. So think more in terms of your Solar being 40w, for the average 10-ish hours of daylight.

I'd strongly suspect that you dont have enough power to properly mantain charge a 100AH battery at all...and adding two trickle chargers as a constant draw - well it ain't gonna help the situation.

Even only 40w would be over 3 amps which would be fine to maintain 3x charged batterys.

Set the controller up properly and you can use it to send all of the power onto the garage battery to recharge that fully before it tops up the cars.

I think the pair of panels I am running onare only 100w or so and they recharge one battery that powers stuff one battery used for engine starting and one on trickly at the same time.
They also keep all 3 batteries topped right up over the winter when the panels are in a barn so not even in direct sunlight (although there is no draw on them over the winter)