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post #7 of Old 04-18-2013
Join Date: Sep 2008
Location: Vancouver, BC
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Re: Basic marine deep cycle battery questions

Googling a few things it looks like your alternator is rated for 5 amps output, a Trojan Group24 deep cycle is rated at 85 AH, an ICOM m504 VHF is rated at 5.5 amps (high power transmit) and 1.5 amps (receiving at max audio), an incandescent anchor light is probably 20W - I am lazy and this is not accurate so call it 2 amps.

To your questions:
1) I'll assume you are not a chatterbox and will only be monitoring the VHF, but you will be doing it loud enough for the whole anchorage to hear. 1.5 amps + 2 amps = 3.5amps. One hour of usage will consume 3.5 amp-hours. You have available 85 amp-hours before your battery is dead, but 42.5 if you want to treat your battery properly.
85/3.5 = 24 hours.
42.5/3.5 = 12 hours.

2) Same assumptions as in 1) and your alternator puts out 5 amps so you can assume (for the sake of this analysis) that one hour of charging will give one hour of usage.

3) As stated above, there are battery monitors that tally the current in and out of the battery and estimate the available battery capacity but I suspect for your purposes a voltmeter is sufficient IMHO.

I hope this helps,
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