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  #11  
Old 10-01-2012
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Re: AGM Battery Upgrade and Charger

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Originally Posted by Minnewaska View Post
Rated at 105ah, with 2 you can roughly assume you have 100ah usable before you need to top them up fully. The ratings are done assuming you draw at a specific rate and, therefore, their capacity will differ if you draw more or less at a time. Your specific systems will dictate how sufficient. The point is that you don't really want to get below that or you will shorten the life of those expensive batts.

A battery monitor is also a great investment, of you don't already have one.

This makes me wonder - Not many boats actually draw 20 amps a hour while sitting around -
So, do you get more actual AH when drawing less per hour, or more ?

and why can't a battery maker give us a AH capacity based on somewhat closer to real use?
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Old 10-01-2012
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Re: AGM Battery Upgrade and Charger

Very helpful!

My electrical panel has two needle gauges - DC Amperes (showing the current load) and DC Volts showing the state of the two battery banks (using a little bank selector dial). So ah is Amp-Hours right? Do fancier batter monitors calculate load x amp hours and give you a projected rate of decline? With LED lamps, stereo, and nav equipment I've never seen the load go over 4 amps. But I'll start paying more attention.
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  #13  
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Re: AGM Battery Upgrade and Charger

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Originally Posted by chucklesR View Post
This makes me wonder - Not many boats actually draw 20 amps a hour while sitting around -
So, do you get more actual AH when drawing less per hour, or more ?

and why can't a battery maker give us a AH capacity based on somewhat closer to real use?
Some do publish capacity at various draws. I don't recall it being dramatically different, but is different..

For those with a chartplotter, radar, fridge, autopilot, radio, maybe a light and or a fan on, draw adds up quickly.
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  #14  
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Re: AGM Battery Upgrade and Charger

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Originally Posted by dvuyxx View Post
Very helpful!

My electrical panel has two needle gauges - DC Amperes (showing the current load) and DC Volts showing the state of the two battery banks (using a little bank selector dial). So ah is Amp-Hours right? Do fancier batter monitors calculate load x amp hours and give you a projected rate of decline? With LED lamps, stereo, and nav equipment I've never seen the load go over 4 amps. But I'll start paying more attention.
ah = Amp hours yes.

The better monitors show usage, monitoring both in and out to provide a percentage of charge. The really good ones allow you to download history to a PC so you can analyze the heck out of it.
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Re: AGM Battery Upgrade and Charger

At the very least this sounds like an obsessive spreadsheet that I can build adding up the draw of all my equipment.
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Re: AGM Battery Upgrade and Charger

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Originally Posted by dvuyxx View Post
At the very least this sounds like an obsessive spreadsheet that I can build adding up the draw of all my equipment.
If you don't have a battery monitor, you really should do that. Then, build in a real margin for error underway, so you don't deep discharge the batts.
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Re: AGM Battery Upgrade and Charger

Quote:
Originally Posted by chucklesR View Post
This makes me wonder - Not many boats actually draw 20 amps a hour while sitting around -
So, do you get more actual AH when drawing less per hour, or more ?

and why can't a battery maker give us a AH capacity based on somewhat closer to real use?

The Ah capacity is NOT based on 20A per hour. To figure it you divide the 20 hour rating by 20. Or 100Ah battery / 20 = 5A

A 100Ah battery can support a 5A load for 20 hours before hitting 10.5 volts.

A 50Ah battery can support a 2.5A load for 20 hour before hitting 10.5 volts.

A bank of four 100Ah batteries in parallel can support 20A for 20 hours before hitting 10.5V....

Anything drawn at less than the 20 hour rate will result in slightly more Ah's in capacity. Anything drawn at more than the 20 hour rate will result in less available capacity.This is called the Peukert Effect..

This is why battery monitors, properly calibrated and used can be so useful...

I explain it more clearly here: Effect of Current on Amp Hour Capacity
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Last edited by Maine Sail; 10-01-2012 at 09:28 PM.
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  #18  
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Re: AGM Battery Upgrade and Charger

Ah so... As usual I had it backwards.
Thanks.

Now off to find a good monitor for my 500 ah or so of batteries.
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Re: AGM Battery Upgrade and Charger

David,
Are you on the Magothy side of SP, or the Severn side?

I'm on the Magothy, right at the #2 / #16 and #MR triangle (where the river splits to Cockley, Old Man and Magothy river)
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Re: AGM Battery Upgrade and Charger

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Originally Posted by btrayfors View Post
Paulo,

I think you're thinking of gelled batteries, not AGMs. In fact, AGMs can and should take a higher absorption voltage...14.6-14.8 VDC and, the faster they can be charged the better (i.e., the largest charging source you can fit on your boat).

As was noted above, AGMs need to regularly be fully charged or they will sulfate and suffer a shortened service lifetime.

This is hard to do on a mooring, unless you have adequate solar power and a proper charge controller.

I'd not choose AGMs for a boat on a mooring UNLESS it had a sizeable solar charging system.

Also, it's true that if the batteries are significantly discharged they may place a very heavy load on your undersized alternator and, with no protection in the way of temp sensors, burn it out. This is because AGMs can take an enormous amount of amperage, will demand it from the alternator, and alternators typically fitted on boats are not capable of putting out high power for a long time.

Battery chargers will not likely be burned out by attaching them to AGMs, but they'll take a long time to charge. This is no problem if at dockside for a long time. If your charger doesn't have an AGM setting, use the flooded setting. Flooded batteries and AGMs have very similar charging profiles.

Bill
Well, it certainly seems you know a lot more about this than me

I was taking the Mastervolt recomendation as a rule and it seems that it is a particular case, as you explain.

I am quite satisfied with my set up (360ah for house and 55ah for the engine) of mastervolt batteries, using also mastervolt regulator, mastervolt battery distributor and mastervolt port charger. For the engine I run an Iskra 120a alternator (the only one that fits on the engine compartment) and even if I certainly could get better results with a Mastervolt or other dedicated marine alternator, the measured charging results, even with the alternator hot are surprisingly good.


Give me just an opinion about this: With the boat on the hard, without anything connected to the battery it will be enough to fully charge the battery once a month or it is better every week?

Regards

Paulo
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