Solar battery chargers

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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

Re: Solar battery chargers

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

Re: Solar battery chargers
On 4/10/2012 3:33 PM, Robert Small wrote:

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

Re: Solar battery chargers
On 04/10/12 17:33, Robert Small wrote:

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.

Re: Solar battery chargers

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

Re: Solar battery chargers
On 05/10/12 08:49, Robert Small wrote:

This site http://www.batteryfaq.org/ 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

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 whirlpool.net.au

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.