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Battery Choices - what is too small AH capacity?

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klehman(at)albedo.net
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Posted: Wed Aug 13, 2008 6:34 am Post subject: Battery Choices - what is too small AH capacity?

Obviously current draw is a critical number that is best measured by 

you. However in general the number for most auto engines is in the 8 to 

15 amp range from what I've seen for 3 or 4 cyl. engines. Higher with 

some EFI engines. Perhaps a bit less with a carb. and gravity fuel feed. 

The cruise number may be significantly higher than at idle. I would 

expect there are some Suzi users that have measured this.



If you have independent low voltage warning and change out the battery 

every two years, I still would not count on more than 50% of rated 

energy out of a battery. ie 4AH out of an 8AH battery which means be on 

the ground within 15 min. with an 8 AH battery and a 15 amp load. 

Regular battery capacity testing might extend that a bit but I don't 

know anyone who does that. Testing at the required load or running the 

engine at cruise rpm until it quits will tell the story with some 

further degradation as the battery ages. In extreme temperatures, don't 

count on even 50% capacity. The battery AH rating is usually for a lower 

current than you will need, so you can't count on gettng 8AH of useful 

energy out of an 8AH rated battery even at room temperature.



Ken

Beemer wrote:

 	  Quote: 	
		   

 

 Mike,

 I have the same question for my Kitfox with a G10 Suzi, I am planning to install Z-19RB, and have issues with weight. Right now, I'm planning on installing a 17ah in the tail for the Primary Eng bus, and a 12ah behind the seat for the Secondary bus.

 

 My real question is how long will they last once the alt goes dead? no way to know, unless we figure out how much amperage the ECM, coil, and fuel pump are using. Right now, I have no idea.

 

 I suppose it's a situation where an 8ah and a 12ah, or two 8ah would work, but would it leave enough time, say 15-30 minutes to descend and reach an airport? No idea.

 

 I don't want to carry more weight than I have to, but I don't want to design in an off-field landing, either. I guess if we were to figure out how much juice is being used to keep the engine alive, we could calculate the time given by the battery size(s).

 

 Sound right?

 Bradley

 

 --------

 Beemer

 KF2 (and now an M3!)

 Middle Georgia

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mikefapex

Joined: 05 Jul 2007
Posts: 70
Location: Colorado

Posted: Wed Aug 13, 2008 11:06 am Post subject: Re: Battery Choices - what is too small AH capacity?

Ken,



Your conservative approach is probably a good one to begin estimating. I plan to experiment with actual battery only run times to back up the theory. 



I am also thinking about purchasing one of those West Mountain battery testers. At $109 it is probably money well spent. I know we go on forever with the 'little extras' in OBAM but I don't want to be looking for a field and thinking 'damn, just $109.....' .



Mike

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klehman(at)albedo.net
Guest

Posted: Wed Aug 13, 2008 3:21 pm Post subject: Battery Choices - what is too small AH capacity?

They are 3/16" bolts I believe. Might be a metric size and a bit larger 

but the bolts come with the battery. I have #4 welding cables on them 

which they are quite suitable for. Should be some pics on the Dekka 

site. They are made by East Penn battery in PA I think. I picked up the 

cable end terminals from a marina. I doubt there is an advantage to #2 

cables on these little batteries unless you have a very long cable run. 

They are sturdy terminals but obviously smaller cables stress them less.

Ken



mikef wrote:

 	  Quote: 	
		   

 

 Ken,

 

 You wrote: 8 AH Dekka AGM batteries

 

 Can you describe the battery terminals on these Dekka batteries? Even a digital photo would be helpful. I have #2 welding cable with #2 lug terminals from the main contactor to the main battery terminals. I'm a bit concerned my existing battery terminals are too small for long term use & vibration resistance.

 

 Thanks,

 

 Mike

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