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Catalina/Capri 25/250 Sailor's Forums

General Sailing Forum

Battery Capacity and Charging - Check my figures

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kevinmac
Admiral

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USA
732 Posts

Initially Posted - 02/26/2006 : 13:15:25

I have been doing some research on battery capacity, to see what I need to do for batteries for cruising.

My boat currently has just the stock Group 24 battery. Although I don't know what battery Catalina put in it, and I have not had time to look, I assume that it is about a 100 amp-hour battery.

From West Marine's spreadsheet, I find that I should plan on getting about 35% of this out of the battery in actual use, because most folks don't discharge below 50%, and they don't keep charging above 85%, ergo, 35% = 35 amp hours.

If I am very frugal with electricity during sailing, I will use:

Raymarine ST60 Tridata = 45ma x 8 hours sailing = .36 amp hours
Garmin GPS 192C = 750ma x 8 hours sailing = 6 amp hours
Icom handheld, monitoring = 40ma * 8 hours = .32 amp hours

Anchoring overnight uses:

Stock anchor light = 1 amp x 8 hours = 8 amp hours
Interior lights = 3 amps x 2 hours = 6 amp hours

Total needed per day if anchoring = 20.68, call it 21

So, although I would like more juice, my stock battery should hack one of use including anchoring.

But recharging seems to be the interesting problem.

My Yamaha 8HP outboard alternator is rated at 6A at wide open throttle. Since I never use wide open throttle, I am guess that I will get 4A out of it.

According to the West Marine worksheet, I should plan on 85% charging efficiency, so it will take 6.2 hours of engine operation to recharge it.

Hmmm... not what I had in mind.

Do these figures seem sortof right to everyone? If so, it seems that the reason folks put big batteries on board (like Arlyn's golf cart batteries) is to survive several days (with more electrical load perhaps) anchoring each night, and then recharge at a dock somewhere.

So I could put two Group 24 batteries in, and get a second night at anchor before having to spend the night at a marina.

Or I could buy a Honda generator, put it on a platform behind the transom, and use that for charging. But the 2000 model only puts out 8A, so that would still require 3 hours of runtime to recharge.

I feel like I must be missing something here...please check my figures and educate me if you are so inclined.

Kevin Mackenzie
Former Association Secretary and Commodore
"Dogs Allowed"
'06 C250WK #881
and
"Jasmine"
'01 Maine Cat 30 #34

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britinusa
Web Editor

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USA
5404 Posts

Response Posted - 02/26/2006 : 14:46:37

Kevin,
I think you are about right, although in practice there are additional variables to consider (line losses, demand when voltage drops, etc.) but the assumptions seem pretty safe.
We have been doing the same thing here. The cost of switching over to LEDs is not cheap, but the longer term value is probably worth it.

We're looking at the Subaru Robin 1700 generator as that will run the new ac we'll be installing before the summer. Had not considered to use it to charge the system, but would seem sensible and not difficult.

Nice to know that others are spending more than 1 night about their boat.. Jim B has done some awesome trips in his boat, check out his website.

Paul

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jmoritz
Deckhand

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21 Posts

Response Posted - 02/28/2006 : 11:40:10

"Or I could buy a Honda generator, put it on a platform behind the transom, and use that for charging. But the 2000 model only puts out 8A, so that would still require 3 hours of runtime to recharge."

For the Honda is that 8A at 120 volts A/C? That would be quite different than 12 volts DC. This is the first thing that popped out at me. Maybe someone else can confirm. Everything else seemed to jive to me.

Thanks,

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kevinmac
Admiral

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USA
732 Posts

Response Posted - 02/28/2006 : 13:45:51

Thanks for looking it over. The Honda 2000 is 8A at 12V.

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