solar panels again

[cazzascotta]

so, to summarize:
my boat batteries are 74Ah each and are always connected in parallel;
my problem is to maintain them full charged when I am not using the boat (about 1 week) without any consumption excelpt for the bilge automatic pump;
My questions:
if as suggested in this forum I will buy a circa 10 watt flexible panel without a regulator, will I connect to which battery?
if I leave the panel connected when I start the engine, will this be dangerous? or will I disconnect the solar panel when the alternator starts?
is there any scheme so I can study it before buying anything?
Thanks very much.

 

[DRANNIE]

Buy a regulator for the panel and although others might correct me you probably need more than 10a. 10a will not put a lot back into the battery. I have a simple set up with one 75a battery and not other form of charging. My 20w panel goes into one side of the regulator, the battery into the other. The live boat feed then also comes out of the regulator and off to the main power switch. In theory this stops the panel overcharging the battery and stops power draining back into the panel. The regulator also cuts power when the battery gets to a low charge level preventing battery damage.

It wasn't very expensive, mailspeed in Burnham were helpfull in sourcing all the bits.

 

[bedouin]

If the objective is just to maintain them in charged condition then a 10W panel will be fine, and a regulator will not be needed. If you want to put much charge in then you will need larger panels, and a regulator.

In this circumstance simply connect the panel across the batteries, positive to positive and neg to neg. It would probably be sensible to put a fuse in one of the leads close to the connection to the battery.

You should check the spec of the panels to ensure that they have a blocking diode to prevent the batteries discharging through them at night (most do). If not one should be fitted. Keeping it connected with the engine running should not cause any problems

 

[DRANNIE]

I think he wants to run a bilge pump when away from the boat. (depends on how leaky his boat is I suppose). The regulator was about £30 so in the scheme of things?

 

[forrestmichael]

I'm also interested in the same thing, my cockpit drains into the bilge and although i cover the cockpit while away something to keep the battery going for 2-3 weeks maximum would be good, would 10W be enough for this, sorry to hijack the post a bit, but i think we want to know the same thing.

Mike

 

[VicS]

As said and explained before 10 watts will be adequate to keep two 74Ah batteries fully charged and will not need a regulator although there is nothing to stop you fitting one if you want to.

However i missed the bit about the bilge pump.

You must make some estimate of the power the bilge pump is using. You need to know how many amps it takes and make some estimate of its average total running time per day, in hours. Multiply the two together to get the average daily requirement in Ah.

Next you need to look at typical daily output figures for solar panels and choose a size that will exceed the pumps requirements. If you now find that a larger panel is necessary, you probably will, you will also almost certainly need to fit a regulator.

It would be better to oversize the panel and fit a regulator than to risk under-sizing it to avoid the need for one.

A fuse in the connection to the solar panel has been suggested as "sensible". IMHO it is essential!

Michael2432

The answer to your question is that you too need to estimate the power required by your pump and choose a panel that will exceed that by an acceptable margin. Again the size of panel will probably mean that a regulator will be required.
There is no hard and fast figure at which a regulator is necessary but a guide you might find is that 10 watts into 70Ah is about the sensible limit for operating with no regulator.
If you are not running a pump then generally experience is that 5 watts is quite adequate to keep a 60 or 70 Ah battery topped up and even replace a little bit of power used as well.

 

[forrestmichael]

Ok, thanks a lot for the response to my question, i'll do some sums and then probably opt for a 20W panel, the sun hardly shines here anyway!

Thanks again

Mike

 

[bedouin]

Given that he only has 10W of solar panel the regulator is not necessary - if he ups the output of the panel then he may need to consider a regulator.

As to the drain from the bilge pump, a typical bilge pump will draw less than 5 amps. Even if you reckon on it running for an hour a week, than is less than 5Ah/week, which is probably less than the self-discharge rate of the battery bank. Anyway it is well within the capability of the solar panel.

If anyone has a boat so leaky that it requires more than one hour pumping a week then they have a serious problem!

 

[lw395]

Bilge pump:
Say 4A for 500gph type 50W
Guess your cockpit area let's say 10sqm for a big RIB
How much rainfall? 50mm yesterday?
Thats 10x100x.5=500dm3 (litres) or 110 gallons,
So (assuming low back pressure?) your 500gph pump runs for
0.22hours / day. Average this over 12 hours is about 2%
So 1 Watt average!
I would want some margin, but this gives a handle I hope.
Leaky boats are a different matter of course.
And anyway, you should put a cover on it to keep the sun off!
Cheers,