Solar Panels

[Andrewells]

I am considering a solar panel to maintain/trickle charge my engine battery.(110 Amp) I am away from the boat for 2/3 weeks at a time.
Is it possible to know how much charge a battery loses on average over a week of inactivity?
Is it posible to estimate an average percentage of a given rating (given mainly cloudy days and not always correct sight/alignment of panels) that a solar panel can acheive?
I am trying to work out whether I need a 5W or 10W or 15W panel for this or am I way out?
Many thanks

 

[Ecosse120]

PBO this month had an articel in it including working out what size/type solar panel you may need.

 

[steve_cronin]

If all you want to do is avoid the "natural losses" of a stored battery in this country then you need very little. However, the 5watt will only give you around 0.35amps(since watts = amps times volts & your charging voltage will be around 15 volts to be effective) which could easily get lost in the wiring and connectors and since the 15 watt doesn't cost much more than the 10, I'd go for that but with a controller /reverse flow blocking diode to avoid overcharging/reverse flow at night - yes these panels actually absorb current at night!

Since you appear to be building a Pedro, have you thought of SERIOUS solar power as no doubt your new vessel might just find it's way to the European Inland Waterways in the future and having enough power to run a fridge without relying on the engine or generator will be a great advantage. In that case you will need at least 110watts.

Steve Cronin

 

[peasea]

I have two 110 amp. Delphi leisure batteries installed and these are charged from the 50 amp alternator of the Bukh 24 engine . These batteries run eveything including autopilot etc.
They will each start the engine after standing unused for three months.In the winter they are topped up during layup of five months, by a solar panel aprox. 18" square I bought from Maplins on offer for around £35.It is just left horizontal on the coach roof and can give 5amps. Even on a dull day it puts out around 20volts. Hope this is helpful .

 

[bedouin]

Lead Acid batteries tend to self-discharge by about 1-2% per week so in order to maintain the charge you only need about 1-2 Ah per week

 

[William_H]

The whole question is full of variables such that you might as well buy what you fancy and more importantly what you can reasonbably and unobtrusively fit. Forget ever running a refridgerator unless you go to serious solar on a rear gantry.
The 5 watt panel will certainly help your battery combat natural discharge. expect max .25 Amp for perhaps 9 hours per day in summer. This panel will be fine without a regulator.
A 10 watt panel will give max .5 amp so about 4 amp hours per day sunny summer and will easily keep up with natural discharge even perhaps in winter. It will be OK without ideal orientation. If you leave it on without a regulator then you should monitor fluid levels carefully as it could cause battery to use too much water. (over charge).
Any higher power would be safer with a regulator and would aid in recovery if you discharge your battery but may be a waste for your requirements.
I keep a small panel attached to the boom cover by 2 bungee cords and plug it in when I leave the boat but remove it for sailing. it is only 2 watt but then used in Oz summer.
good luck..... olewill

 

[nmelton]

I can confirm that a 10 watt panel will be OK for your needs if you simply want to to keep the batteries charged while you are away. My two 110A batteries are more than adequately topped up (boat on a swinging mooring) by a Unisolar 10 watt flexible panel and regulator.

During the Summer hols when we are living aboard the panel is obviously not up to supplying the loads imposed on the battery but the relatively cheap cost of a low output panel will be repaid in the long term by the prolonged battery life.

Rgds, Nigel

 

[VicS]

I'll go along with birdsailor as i use a 5watt panel to keep a small, 45Ah, leisure battery topped up. In fact since my only other charging source is the negligible output from an outboard it is my main charging source.

 

[Andrewells]

Just a quick note to thank you for your thoughts and contribution. They are appreciated.