solar panel advice please.

[nobananas]

I've just come back from a very hot and sweaty week camping in the lakes.

I needed a way to charge the 4 smart phones and 1 tablet that I took with me so before we went I bought a small 7ah 12v sealed lead acid battery and built it into an enclosure with ciggy lighter sockets built in so I could plug the various charger adapters into it.

I also took a small 1.2w solar panel ( the sort designed to trickle charge your car battery whilst in storage) which I had kicking around hoping it would keep the battery topped up in between charging stuff. It worked OK for about 4 days but then the batteries ability to charge dropped off.

I'm assuming that the solar panel was probably doing very little and I had just depleted the battery over a few days.

Now without getting too technical (I've just been looking on a site that got waaay too technical !) can anyone suggest what output solar panel I should be using ?.

The battery isn't being used during daylight so the solar panel doesn't have to 'keep up' with the discharge rate but it does need to top the battery up during the day so that it can be used in the evening or overnight to charge the various devices.

I would rather not have to use a solar charge rate controller if possible.

I was looking at a suitcase type panel rated at 4w with built in discharge protection diode would this work ?

[woz]

Yes - its like a car - you need more power.
Your Solar panel will give you a max of 1.2W obviously the daily total varies on time of year and weather - even assuming you have it in the optimal location.

You correctly assumed the panel is not providing enough total energy per day to keep up with the demand on the battery. So you need another charging source or more capacity to last until you get back in the car or get home.

To give an example of the amount of daily energy you can harvest using the small panel, I have a 1.5kW PV system on a south facing roof. It generates up to 10 kWh of energy a day max. - average is about 8 at present and 1-4 in the winter.

Applying the same assumptions (optimal location, on all day etc.) your panel could give about 7-8 Wh on a good day.
Your battery - a 7.2Ah 12v job, will store about 84Wh (7Ax12v) assuming perfect efficiency - in practice you could probably use about 40Wh of this regularly without killing the thing unless its a deep cycle (leisure) battery.

So its going to take about 4-5 days to charge your battery from half empty to fully full, in July. (40Wh total at 8Wh per day)
On the other side of the equation, how long does it take for the chargers to use the energy you have put in ? Say start with a ful battery and dont charge it - how long would it last?

Obviously a 4W panel is going to give you 3 times better performance. - I would still recommend having an alternative source for poor weather/nights - how about a wind up charger? Serioulsy !! LOL Something the kids could do while you are lighting the fire??

Hope its not too techie - apologise I studied engineering and have sold PV systems for a living.

If it doesnt make sense, - just ask. PM me or something. Is there a Maplins near you - the guys in there can generally give good gobbldegook.

If you want to get techie, then you prob will need a charge controller - the 4w is its peak output, but you need to check the voltage too. Off peak conditions, if the voltage drops below 12v then it wont charge the battery at all even if it could in theory generate "some" power. a decent controller will keep the panel working at its optimum so you dont waste what energy you do have.

Woz
p.s. its temptingto say you wouldnt be so sweaty if you only carried ONE smartphone around with you LOL

[nobananas]

Thanks woz, sorry didn't see your reply until now. Have had a 4w panel delivered today and have got a charge controller on it's way, I did a bit more digging about after I posted and found a site that said pretty much what you have confirmed. The instructions that came with the panel also confirmed that a charge controller would be required if the battery is under 30ah. Not sure about a wind up charger, haven't got any kids to use as slave labour and can't see me winding it myself whilst the missus makes a call !