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post #45 of Old 10-09-2012
Maine Sail
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Re: AGM Battery Upgrade and Charger

Originally Posted by chucklesR View Post
For 5 hours, then anchoring over night -

Take out the AP, GPS Speed and Depth, then add in more lighting and fans plus a anchor light (some are 1amp per hour, some are less).
Add back in the AH you get WHILE you are sailing and sitting (I got 30-35 a day with my 80w panel).
For multi-day anchoring out figure 40-50 ah a day on the hook. If you have a standard 50amp alt you really need to know that 1 hour at idle is REALLY only 20-25 amp.

Now figure your panel(s) for that.
The op said he was on a mooring:

Originally Posted by dvuyxx View Post
I'm usually on a mooring
Most alternators, with a bank like the OP described, can get the bank back to about 80% state of charge without too much trouble, if you run the engine long enough. If he can get back to 80% then a solar panel sized for a mooring top up will get him the charge in the bank he needs for longevity. Sizing for cruising loads will obviously require a larger panel.

This is why a good battery monitor should often be the first cash outlay in a system. The OP has no idea where his SOC will end up, or how to figure his loads, so he ideally needs to see it, touch it and feel it in order to size a solar panel for an intended purpose. A battery monitor for $144.00 can do this for him and allow him to "see" what is going on..

Any solar is better than none but if he was always getting back to 80% and only needed to replace 25 Ah's then a 140W array could be an over kill expense when a 50W panel would have him topped up by the time he came back to use the boat....

BTW when I got back from the Annapolis show the battery on my car was FLAT DEAD. I had left the dome light on at 3:00 am in the lit parking garage so never knew it.

It is a BCI Group 35 battery, pretty small. I threw it on my bench charger and it took over 12 hours to charge it to full. When full it was taking about 0.3A at 14.4V. The charger has double the capacity the battery will "accept" and yet to get to "full" it took over 12 hours..

No sailboat runs the motor that long unless doing the ICW so this is why solar can be sooooooo useful to mooring sailed boats because it has the time required to top the bank up, which an alternator just does not, due to acceptance and the bank limiting current once in absorption..

With two group 31's figuring 105Ah's each he has a 210 Ah bank. At 80% SOC he will have a deficit of 42Ah's.

42 Ah's X 1.15 (charge inefficiency) leaves him with a real world deficit of 48.3Ah's needing to be returned to the batteries.

If he wants to recharge in:

2 Days = 24 Ah's per day
3 Days = 16 Ah's per day
4 Days = 12 Ah's per day
5 Days = 9.5 Ah's per day
6 Days = 8 Ah's per day

If we figure 4.5 hours per day at "rated" (Imp) output he'd need..

2 Days = 5.3A rated panel (roughly a 100W panel)
3 Days = 3.5A rated panel (roughly a 65W panel)
4 Days = 2.6A rated panel (roughly a 45W panel)
5 Days = 2.1A rated panel (roughly a 40W panel)
6 Days = 1.3A rated panel (roughly a 25W panel)

The above is "best case" scenarios. I see the averages run between 3 hours per day to 4.5 hours and sometimes 5 hours per day at rated panel (Imp) where isolation numbers are high like Maine is in June.....

-Maine Sail / CS-36T

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Last edited by Maine Sail; 10-09-2012 at 10:42 AM.
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