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Go Back   SailNet Community > On Board > Gear & Maintenance > Electronics
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  #1  
Old 04-22-2010
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solar panel to keep battery topped up?

Hello all

Mu current set-up: I have one deep cycle battery in my boat. The battery is labelled as;
marine crank: 730 amps
reserve capacity 160min/100Ah

The battery is used to start my single cylinder 7.5 hp diesel engine and almost nothing less. I generally sail on the weekends both Saturday/Sunday and one day during teh week when work allows. The engine will run approx 15 minutes to leave the harbour and 15 minutes to return to my slip.
My concern is that the battery is not getting enough charging time as I have read it is not good for a deep cycle battery to sit unattended for 5-7 days. Is this true?
Does it make sense to add a small solar panel to provide a trickle charge to extend the life of my battery? If I add one how many watts?
I am not interested at this time to add a charging system for multiple battaries as the way in which I use the boat will not change in the short term I just want to ensure I can provide the battery with all of the charging it needs to keep it "healthy"

Any thoughts?

John
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Old 04-22-2010
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Hello,

IMHO, running the engine for 30 minutes a week will be more than enough to keep the battery charged. This assumes your battery is not running lights and things when you are off the boat.

5-7 days is nothing for a battery. If it were 5-7 weeks unattended, then I would look for a solar panel.

Barry
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Old 04-22-2010
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You're ignoring the fact that wet cells do self discharge, and the fact that their current acceptance rate is VERY LOW when they are above 80% SOC or so. If the batteries are above 80% SOC, it is very unlikely that 30 minutes would restore very much at all, especially since he probably has a fairly small alternator on his boat, due to the small engine size.

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Originally Posted by BarryL View Post
Hello,

IMHO, running the engine for 30 minutes a week will be more than enough to keep the battery charged. This assumes your battery is not running lights and things when you are off the boat.

5-7 days is nothing for a battery. If it were 5-7 weeks unattended, then I would look for a solar panel.

Barry
Johhny—

It would probably make a lot of sense, for the good of your batteries if you got a small 20 watt or so solar panel. This would probably be able to keep up with the self discharge of the batteries and top the battery off.

Something like this would do the job nicely and could also be used to keep the batteries topped off during winter storage...
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Old 04-22-2010
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My thoughts are while it cant hurt i have a mooring in and area with about 1000 boats and < 2% have a panel and seem to do fine

ON my my J24 i never did more than carry the battery home twice a season with out issue

On my current project Cal 29 i plan on using more power and may mount and 80 watt panel
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Last edited by tommays; 04-22-2010 at 07:41 PM.
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Old 04-22-2010
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But how many of them are relying on running the engine just 30 minutes to keep the batteries topped up???

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Originally Posted by tommays View Post
My thoughts are while it cant hurt i have a mooring in and area with about 1000 boats and < 2% have a panel and seem to do fine

ON my my J24 i never did more than carry the battery home twice a season with out issue

On my current project Cal 29 i plan on using more power and may mount and 80 watt panel
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Old 04-22-2010
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I'm with BarryL and tommays on this. Twenty years on a mooring when i usually ran the 11 hp engine far less than 30 minutes each time I sailed and never ran the batteries dead. A solar panel would be good insurance but not necessary.
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A little PV unit would not hurt and might get it topped up, which I also doubt happens in 30 mins unless starting is the only thing it is asked to do.

As for size, well lets say the bat is losing 1% a day (that’s high, it is likely less). One percent is 7.3amps over 24hr or about 0.3amp/hrs. In other words if a panel was to supply about 0.3 amps continuously it would overcome the self discharge. Of course a panel only works 12hrs a day (max) so you would want a panel that can supply at least 0.6 amps. It needs to supply that at a higher than fully charged battery voltage, say 17volts so a 17x0.6= 10watts, a 10 watt panel would be needed.

But there is no way a 10watt panel will actually give you a regular 10watts for 12hrs a day, day after day. It is very safe to double that to 20 watts and not have to worry about a regulator. Just check water regularly.

In other words and fewer words, yeah IMO that 20watt panel would do the trick, and on sunny weeks might even fully charge the battery.
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Old 04-22-2010
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Hey all

Thanks for all the replies.
I am at a dis-advantage here as my boat has only been mine since last fall, previous to this it was owned by my father, and while I sailed it regularly I did not pay enough attention to maintenance. With that said I did notice last season that the battery would start strong at the beginning of the season and slow degrade over time. I did attach it to a battery charger several times last season to bring it back up to 100%. My primary concern is two-fold
1) Always have enough juice to turn over the engine. Practically speaking it is the only reason I have a battery on-board.
2) Keeping the battery strong so as to make it last as long as possible



I am sure I could continue to use the battery as I have, hooking it up to a battery charger as needed, but I do like the idea of keeping topped up via a solar panel.

SD thanks for the link.

John


edit: my engine is a farymann diesel with no external alternator, currently I do not know what kind of charging output it produces, it is on my list of things to check once the boat is in the water.

Last edited by johnnyandjebus; 04-22-2010 at 08:53 PM.
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Old 04-22-2010
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J&J,
We've been using an older model solar panel like this: SE 500 Panel and Mount Solution
which has always helped to keep one of our two batteries at a higher state of charge.
With a 7.5 watt rating I suppose it is ok for keeping 1 battery topped off and it comes with the mounting hardware for your pushpit or stern rail.
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Old 04-22-2010
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I'd vote for leaving it be. A wet battery won't take any damage from sitting alone for one week or even two. The sulphites that precipitate out and solidify, won't be a problem until the battery has been sitting for 30+ days.

And finding a WATERPROOF solar panel, that won't rot out on you, in something small and affordable like a 20W size...won't be easy or affordable.

Sounds like the battery is a Group31 that costs something like $125-150, hardly worth buying a solar panel for it.
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