Re: [A/S] Boondocking Battery specs, which is most important?

["Maurice Riggins" <maurice.riggins@xxxxxxxxxxxxxxxx>]

There are two specs you need to look at:

Amp-Hour Rating:  This is typically given at the 20 hour discharge 
rate (i.e. a 100AH battery/20hours = 5A rating from 100% charge to 
essentially dead 10.5V).  Keep in mind you'll be lucky to charge 
batteries to more than 85-90% of their capacity in just a few hours 
of generator or solar, and you don't want to discharge them below 
50%.  So this tells you that at a 5A discharge, that 100AH battery 
will give you 35-40% of its rating between charges, or 35-40AH.  You 
can discharge them down to as low as 20% charge for 65-70AH, but 
you'll be buying batteries twice as often.

As discharge rate goes up, battery capacity goes down.

Reserve Capacity: This is the number of MINUTES of total battery 
capacity at a 25A discharge rate, what you would have powering a 250W 
inverter at its rated output.  If that 100AH battery has an RC of 
180, divide that by 60 to come up with 3 hours.  And you'll see a 
100AH battery (at 5A discharge) is now a 3 hour X 25A = 75AH battery. 
Again, if you want to keep the charge above 50%, and won't have more 
than 90% to start with, you're looking at 75AH X .4 = 30AH usable at 
this rate, or a hair over an hour's use at 25A.

That's why using a pair of 12V batteries gives more than twice the 
capacity... it's cutting the amount of current through each battery 
in half, and as a result, the capacity goes up.

They're only 1/4" longer than Group 27 batteries, but Group 31 12V 
batteries can give you 130AH and 225 minutes of RC EACH.  A pair of 
them is as good as 6V T-105 golf cart batteries at 25A discharge, and 
better at the 20 hour rate.  However, given the same depth of 
discharge, the T-105 will still give about 40% more cycles (life).

Optima batteries are great if you have a dune buggy or 4-wheeler that 
might get tipped upside down off-road.  But at what, 3 times the 
price, and 1/2-2/3 the capacity of flooded cell, you need a lot of 
them so you can baby them with shallow depth of discharge for longer 
life, considering their cost.
--
Maurice
WBCCI 5446