From an old post of mine on another forum:
on Wed May 19, 2010 adrenalinejim wrote:
I'm currently (get it?) using a CSB brand 1217W AGM battery. It's rated at 4A-h, where the original was 5.5, which just means you are limited to idling with the headlight on for an hour or so instead of an hour and 20 minutes.
I chose this battery because it is readily available and is only around $25 +shpg. It has recessed male push-on terminals, so you need to modify the harness by removing the original battery terminals and putting on the right size female push-on ones. It's a little smaller, so I was able to remove the rubber bumpers in the battery box at the sides and bottom (not ends) and add about 1/2" of foam rubber in their place.
AGM batteries are damaged by overcharging, since they don't have any electrolyte to spare, and you can't replace replace it once it's lost, so check your charging voltage and compare it to the charging information that's usually printed on the battery.
A Werker brand version of the same battery is $19.25 plus about $15 shipping here, and it's rated at 5.1 A-h:
http://www.replacementupsbattery.com/Product_pages/HC1217W.html
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FAQ
Normally, the liquid lost from use is water, leaving more concentrated acid behind, so unless you think you spilled the acid out, it's water that needs to be replaced. When I've let a battery get the acid level below the tops of the plates, the battery is usually about useless, even after refilling. As they say, "Your mileage may vary".
And Jim is right, engineers are very smart! 
