Solar battery chargers

I am looking to install solar panels and a charger on my caravan. It already has 2 x 100AH batteries.

Using a single 200W panel as an example, it has a Vmp 18V and Imp 11A (equals 198W, close enough).

I see all sorts of chargers, ranging from 10Amp up to 40Amp, and this is where I am confused.

If I assume that the battery needs 13.8V to charge, then surely the maximum current the charger can output is 14.4A (ie assuming 100% efficiency - unlikely, more like 85 - 90%).

So how can a 40A charger deliver 40A?

Does it actually require something in the order of 560W of solar panels to deliver 40A at 14V?

If I have a single 200W panel, am I wasting money buying a charger greater than 15A?

Bob Small
Reply to
Robert Small
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solar comes and goes with the weather and the time of day.

40A is the maximum the charger can do. If the panels or illumination isn't sufficient the charger will charge at a slower rate,

yeah, somewhere round there, the charge regulator won't be 100% efficient

if you have to spend more money to get it then you probably are.

?? 100% natural 

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Reply to
Jasen Betts

The 40amp charger can handle loads up to 40amp, it doesn't produce power but controls it. If you never increase your panel number then you don't need more than

15amp as you say. In practice I would say you will add more panels for marginal situations so the 40amp controller will cover that.

Cheers ............... R.P

Reply to
Rheilly Phoull

If they are in parallel, then the suggested maximum charge/discharge rate is 20Amps (C/10). This will give you the longest life.

What brand? I thought the maximm 12V panel was 120Watts and any higher was actually 24V.

See comment above.

you need a supply of 40amps, or more likely with a good solar controller/attery charger, at 12V, you need 40Ax12V= 480Watts+. Say 500Watts as there would be losses in the controller/charger.

Yes,but see first comment on charge/discharge rate.

Are you likely to add a second panel? If yes, then no. If no, then yes.

You also need to keep in mind that that "200W" is the peak output, which is probably only going to happen for a short period each day. Probably a cool summers day with good direct sunshine about noon, say 11am to 1pm.

Reply to

Good point. So a 20A charger would be optimal.

200W 12V panels are now quite common, although the majority are 24V. The 200W panels are about 15 - 20% cheaper (per Watt) than the 120W panels.

My rough calculations show that I use around 50AH per 24 hours (mostly at night).

And to get a full 20A charge I need around 300W (assuming 90% efficiency)

At this stage I am thinking 2 X 200W panels with a 20A charger. Hopefully the panels will deliver 300W over a wider time frame, maybe an extra hour, which would then give me around 60AH daily charge. If I went with 2 x 150W panels, then I would be back to around a 40AH daily charge.

Bob Small
Reply to
Robert Small

This site

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has been extremely good for concrete information and it is from someone who isn't trying to sell you batteries.

Pooey. OTOH, I paid less that $300 each for my 120watt, which I'm happy with for a first project.

I'm still lacking concrete figures for my consumption, so you are way ahead of me there

My apologies for being vague here, but I can not remeber whee I've seen this. It was from a thread on the house mains solar mobs. Maybe off the green tech forum on

Somewhere there is a site that lets you play with orientation(direction and panel angle) and it shows you how this affects your theorectical daily maximum output over the seasons. I think it does summer & winter at the same time.

One trick was to mount them for (close to) a winter maximum which improves the winter production but reduced the summer production to match. Caveat, if your major requirement is cooling, you may not want this, but for other purposes it is probably a good idea.

If I remember corrrectly, it flattened but broadened the production.

Also keep in mind that in summer, panels are derated because of the heat they experience.

OTOH, building a mount that can be seasonally ajusted for two panels may be a simple job for you.

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