Basic marine deep cycle battery questions - SailNet Community
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post #1 of 16 Old 04-17-2013 Thread Starter
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Basic marine deep cycle battery questions

Would like ballpark estimates:

1. How long will a typical battery run an anchoring masthead light and a VHF radio without being recharged?

2. How long would it take a 4hp 4-stroke 2012 Evinrude outboard with alternator feature to recharge a typical battery enough provide at least an hour or so of VHF radio usage?

3. Is there a device that can tell you how much power (or "juice") is left in your battery - like a gas tank full/empty indicator?

Thanks once again for being such a great resource.
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post #2 of 16 Old 04-17-2013
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Re: Basic marine deep cycle battery questions

1. How 'big' (ie amphour rating) is the battery? What does your anchor light draw? LED?

2. Again, how big a battery and how many amps can your 'alternator' put out?

3. Xantrex and Victron both make good battery monitors that will do that for you.

Obviously the bigger the 'tank' the longer it will last, but it will also take longer to 'fill up'. Generally you should not run batteries down beyond 50% of their capacity in amps.. eg if you have a 120 A battery, you shouldn't use more than 60 before recharging it some. Typical outboard alternators are marginal outputs. A small solar panel may well keep up with your needs, esp in summer. Getting LED lights, esp for the masthead, will vastly improve your times between charges.
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post #3 of 16 Old 04-18-2013
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Re: Basic marine deep cycle battery questions

I just replace a running light with an LED since it was out. We've got very similar boats, and probably similar battery uses. I'm interested to hear responses. I have yet to get any way to charge my battery. I'm hoping it will just last all summer, but it won't. I'll have to find a solution. Let me know how your outboard alternator works. I'll probably go with a solar panel on mine though.
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post #4 of 16 Old 04-18-2013
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Re: Basic marine deep cycle battery questions

Matthew,

Take the amp draw of the light and VHF, which should be written on the packaging somewhere, or stamped on the fitting. It will certainly be on the VHF on the data plate. Add those numbers together.

Then take the Amp hour rating of the battery, divide that by 2. Then divide that number by the total amp rating of the two devices. That will give you a best guess (in hours) of how long the battery will last. Actual number get more complicated since this ignores line losses, and in efficiencies, but will get you close.

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Re: Basic marine deep cycle battery questions

Why divide the amps on the battery by two? Is it because as the poster above said, you really don't want to drain more than half your battery?
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Re: Basic marine deep cycle battery questions

Yes, if you regularly drain more than 50% then you will greatly reduce the life of the battery.

On my Catalina 25 I found that a small 20w solar panel and the 6 amp alternator in my motor was more than enough to keep the batteries charged. I could only measure charge state via a voltage meter (the depth finder had this feature), but I never saw it drop below 12 volts and only once saw it go below 12.5 volts. I did not have a shore power charger on that boat.

That boat had all LED lighting. The greatest power draw was from the autopilot.

Your VHF radio will certainly use much less power in receive mode than when transmitting (especially if transmissing at 25 watts). The manual might provide better guidance on power consumption than the back of the device.

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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,
Ken
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post #8 of 16 Old 04-18-2013
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Re: Basic marine deep cycle battery questions

Didn't as the question, but this is helpful as I would like to do a night sail across Michigan if it ever stops raining and warms up enought to work on the boat. I am thinking I should have a third deep cycly in "reserve" just for added security.
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Re: Basic marine deep cycle battery questions

Quote:
Originally Posted by Mark1948 View Post
Didn't as the question, but this is helpful as I would like to do a night sail across Michigan if it ever stops raining and warms up enought to work on the boat. I am thinking I should have a third deep cycly in "reserve" just for added security.
Should all be part of one large bank vs separate banks

Long detailed explaination. One part of it is battery life. Here as some numbers not exact but should be close enough to explain.

Suffice it to say this- 2 battery bank
1 Battery separate from 2nd battery each 105 ah .... or
2 batteries in one bank at 210 ah

Suppose you run 50 ah in one day. Its 50% of one of the one battery banks or 25% of a battery bank with two batteries in it.

Number of deep cycles a battery gets depends how deeply discharged. Lets say its rated at 250 deep cycles at 50% and 750 at 25%

Two one battery banks you get 500 cycles ( 250 cycles out of each battery)
One two battery bank you get 750 cycles


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post #10 of 16 Old 04-19-2013 Thread Starter
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Re: Basic marine deep cycle battery questions

This is very helpful. I love this site. I am planning some overnighters in the Puget Sound and just want to make sure I'll have enough juice to use the anchor light all night and use the VHF in an emergency. And also wanted to know how long I would need to run the outboard to get a decent charge back into the battery.
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