solar panels

[cazzascotta]

hello,
I am thinking of a solar panel to top-up my batteries when I am not present for a certain time.
My question: what power (Watt) must the panel have to maintain the maximum of charge in TWO batteries every of which is 74 Ah and are mounted in parallel? Which type of controller do you suggest to avoid overcharge?
Thanks, also a scheme will be very welcome...

 

[sarabande]

to answer that, we really need to know what power you are taking out of the batteries over what time, so that you can calculate the required replenishment.

IMHO it is not worth buying cheap panels, so stick with BP or Kyocera which have a guaranteed life of 25 years (I think).
These people have been very helpful to me in the past, and will send you diagrams and details of the controller / panel(s) you need, and talk you through the specification, installation, and commissioning. They also have frame clips etc.

http://www.aireandsun.co.uk/

(Customer only connection)

 

[DavidTav]

I think you will receive a lot of good, and in some cases conflicting, advice on this. This seems to be an area where you can either spend a relatively large amount or buy a much cheaper solution. For what it is worth, I have two 85 amp batteries on my boat and to help maintain the charge I bought one of the Maplin 5 watt panels. At less than £20 this is definitely the cheap option! I have fitted it with bungee cords over the forehatch. The boat is on a swinging mooring. I have only had it fitted for a few weeks, but it definitely seems to be helping maintain the battery charge. I don't think Maplins are selling the 5watt panel at the moment, but there are a few others available on their site. I guess if you pay considerably more then you will get a more effective solution. A lot depends, as another poster stated, what sort of usage you demand of the batteries.

 

[VicS]

If both are fully charged when you leave and you only want to maintain them in a fully charged state a relatively small panel will do. 5 watts may be sufficient if the batteries are in good condition. 10 watts should be more than adequate and will still be small enough not to need a regulator but it will be big enough to top the batteries up if they are not quite fully chaged

However if you are leaving the batteries half discharged and want to recharge them in say a week you will proabably need about 80 watts, at least. You will also then need a regulator.

You can happily go up to 10watts on a 70Ah battery without needing a regulator so 20 watts for the two in parallel will also not need a regulator.

(It's worth perhaps mentioning that BP Solarex now only do the framed panels. A bummer because I wanted one of the semiflexible 10 watt ones!)

 

[MarkJohnson12345]

Get as big a one as you can afford.

I bought a 20 watter off Ebay from France.

It produces about 1/2 amp when pointing directly at the sun. But just resting in the cockpit it produces .1 or .2 amp most of the sunny time.

All adds up over 5 or 6 days.

It does keep the batteries topped up during the week, but I doubt it would actually charge the battery from a state of discharge in our latitude and miserable weather. Certainly not very quickly.

The ozzies will tell you they are wonderful, but then their climate is also extremely sunny.

Certainly better than nothing, but don't expect miracles.

MJ

 

[VicS]

[ QUOTE ]
I don't think Maplins are selling the 5watt panel at the moment, but there are a few others available on their site

[/ QUOTE ] They were a couple of weeks ago. A framed panel and therefore not what I wanted.
I was actually more interested in the 10watt version but did not buy it because it was framed and I did not like the quality.

 

[gandy]

[ QUOTE ]
I bought a 20 watter off Ebay from France.
It produces about 1/2 amp when pointing directly at the sun.
...

[/ QUOTE ]
That sounds more like 6 watts, rather than 20. Or 7 if you give them the benefit of the doubt and say its charging at 14V.

 

[simonfraser]

------------------------
I bought a 20 watter off Ebay from France.

It produces about 1/2 amp when pointing directly at the sun. But just resting in the cockpit it produces .1 or .2 amp most of the sunny time.

All adds up over 5 or 6 days.
-----------------------------------------

yep, i agree, mount them facing more sideways than upwards

 

[cazzascotta]

Thanks to all. The panel I need is only to maintain the charge during the week, max for 1 month, when I'm away. Usually I leave the batteries charged, but the bilge pump is in automatic, so it is better have the batteries charged then not.
What I don't understand is why a low power don't need a regulator, because I don't want the batteries overcharged nor the charge go back to the panel...
Thanks again.

 

[VicS]

[ QUOTE ]
What I don't understand is why a low power don't need a regulator, because I don't want the batteries overcharged nor the charge go back to the panel...

[/ QUOTE ]

The small current that continues to flow from a small panel into a charged battery will do no harm and the amount of water lost (by electrolysis) will be insignificant, at least as far as flooded acid batteries are concerned.

The back flow of current from the battery through the panel, during the night for example, is prevented by a diode. You should check that the panel you choose includes one. My old one had it in the lead but the new one has it incorporated into the panel. If a panel is sold specifically for battery charging it will almost certainly include a diode but it is worth checking the specification nevertheless.