Solar Panel Charging capacity - Advice needed please

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Sandgrounder

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I want to install an automatic electric bilge pump to a keelboat I recently acquired. I will fit a 120 amp/Hr batttery. I have the choice of either having two batteries and changing them every week, which would be a nuisance, or fitting a solar panel to keep the battery charged; the boat has no motor.
Can someone advise me what capacity solar panel I would need to operate such a system assuming that the pump runs for a few minutes most days?

Thanks in anticipation
 
Depends on the power requirements of the pump and how log you expect it to run each day. It will also depend where you plan to fit the panel, if it's in a spot that suffers from shading yield will be seriously affected.
 
It depends on how big the pump is and how many a few minutes a day is.

A small pump like the Rule 500 draws 2 amps when running.

Assuming it runs for less than 30 minutes a day then you need to replace 1 Ah every day.

ON an average summer day on the south coast a small 10W panel will cover that with ease. In the middle of winter in the north then even 30W might struggle.

You also need to bear in mind that if you go above 10W panel size on 120Ah Battery you will definitely need a charge controller.

If it were me I would put in a small panel = about 10W, monitor the battery for a while and then lift over the winter.

Not sure though why you would need to swap the batteries weekly.

Again assuming 1Ah a day draw on the pump then you could do it monthly. You should never let the battery discharge to below 50% and it is a struggle to get batteries above 80% of nominal charge without lengthy float charge period so if you plan on a capacity of about 40% of the nominal then you will be about right. With 1 Ah per day and 120Ah nominal that would give you about 6 weeks before the battery got too low.
 
PaulRainbow said:
Depends on the power requirements of the pump and how log you expect it to run each day. It will also depend where you plan to fit the panel, if it's in a spot that suffers from shading yield will be seriously affected.
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If you want to keep cost to a minimum and keep the installation simple, you could fit a 10W panel without a controller. This will probably be fine for limited pump usage and will also be fine if the pump barely runs at all. Once you get larger than 10W you would be well advised to include a controller as well so that's another £20 or £30.

As Paul says, whether the 10W panel will suffice depends on how often the pump runs. Can you borrow one and try it for a few weeks. If the battery is still fully charged after the trial then you know it's enough.

Richard
 
Solar panels are relatively cheap so you should buy the largest panel you can physically mount on the boat conveniently.
The controller can be quite cheap from China. The battery is the expensive part especially if your bilge pump ends up flattening the battery. That depends on the amount of water ingress.
An alternative might be a 40watt panel wired directly through a voltage (buck) regulator to the pump. No battery and of course the pump will only operate in day light hours but can run continuously. (perhaps not the recommended way but worth a thought) olewill
 
If you want to check how much energy you'll get throughout the year this piece is still available: http://www.pbo.co.uk/expert-advice/solar-panels-everything-you-need-to-know-24455. I like olewill's idea though, on reflection, I'd still add a small battery - are you using such a large battery though choice or just because you have one available? PWM regulators are almost as cheap as buck convertors so I'd stick with one of those.
 
ghostlymoron said:
Additionally, I'd try and find where the leaks are - you shouldn't need to pump every day.
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There is that!
I know people with leaky wooden racing boats, many use a Rule 500 pump. A leak that flattens a 40Ah battery in a week is regarded as serious.
I also know people who use similar pumps in RIBs. Flattening the battery in a fortnight would be regarded as an awful lot of rain.
 
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