What size solar panel do I need?

Hi

I have a:

- 39l Engel fridge/freezer that I believe uses 5-2.5 amps per hr (though apparently only when the compressor is on which depending on the ambient temp varies); and

- 40 AH secondary battery that is currently trickle charged when the engine is running.

and am looking to get a solar panel charging kit. But I am having great difficulty in understanding:

1) How long my battery should support the fridge for from being fully charged. Currently it does not seem to last very long and I am not sure if this is because of the size of the battery or because the battery is loosing charge?

2) What size solar panel I would need to enable me to replenish the battery if I were to run the fridge for 24hrs?

3) Whether the two system i.e. tirckle charge and solar would work in parallel when driving or whether I would need to switch to 'solar' once stopped?

Having checked a number of websites & forum discussions it seems to me to be something of a 'black art'?

Many thanks

Nick

in my caravan I have a 110ah battery and it only keeps the fridge going for about an hour.. admittedly it's probably a bigger fridge than yours.. so when you say trickle charging you would need to be supplying more power than the fridge is using otherwise it isn't charging at all.. how long is it lasting for you at the moment?

Caravan fridges are not as economical as dedicated 12v systems. Engel's draw a lower ampage than most and i think your ampage figures are still a little high.

I would scrap what your thinking about solar for a start and get your charging system sorted through the alternator 1st. 40AH is very low for a car battery in a 4x4. Is this your only battery or are you running 2? split charge or parallel? which split charge are you using, how is it set up and what rating is it? what rating is your alternator?

A standard 80ah car battery should run an engel for 3 days before becoming to discharges to start the engine. Thats why I think you should get it sorted before even thinking of solar.

With the right system you can camp for 3-4 days without turning the car over or needing solar power. how long do you see yourself being out in the sticks for? If your not venturing to far and your still in a populated area a CTEK car charger hooked to mains for 24hrs will recharge the battery more effectively and quicker than solar.

test your battery to see if you are losing charge somewhere before upgrading it. Constant full charge/discharge does no good to batteries after a while. You need to keep these in tip top condition. Get that sorted 1st.

hope this helps.

G

I'd replace your 40ah secondary battery with as big as you can. For a 12v compressor fridge, i'd be going for at least a 110ah deep cycle leisure battery - a good quality one, not cheap caravan battery!

Solar panel wise, depends on how much space you have. It may be worth looking at flexible panels that you could bonnet mount for example - prob is with them, they only go up to about 43w and are a lot more expensive than normal framed panels - upside is they are a damn sight more durable (you can walk on them!) and can be installed flat to the body. You could mount a framed one up on roof rack, but they are quite fragile if your doing serious offroading.

It may also be worth looking at a better battery to battery charge system for your truck, that will make better use of your alternator whilst the truck is running - charging the batteries much faster and more intellegently than a standard split charge relay.