Geza Polony typed:
> I've been using an Optima Yellow Top for the past year or so as aux battery
> and have always been surprised at how little reserve current it has. This is
> based more on intuition than anything else. (It seems like it ought to run
> an 800-watt 120 v espresso maker for 5 minutes, but no way...)

Intuition is Tinkertoys compared to analysis. You know that 800 watts @
120V requires something like 85 amps from the battery? Use the Battery
Demand Calculator
(http://www.altavistaaudio.com/Westy/Vanagon/battcalc.html)

85 amps is a lot of current. Unless you have some fat wire feeding the
inverter I bet you are losing a lot in the wire. What's the voltage
across the inverter's input terminals while your espresso maker is
running?  Does the battery voltage bounce back after you turn off your
espresso maker?

> But I've just been testing it with a meter and find that the refrigerator
> draws it down from a starting voltage of 12.30 to 11.99 in 20 minutes, to
> 11.84 in 60 minutes. Is that typical?

12.3 isn't a fully-charged battery. If you are loading it with a
Dometic, which draws 7 - 8 amps, I would expect to see the voltage sag
due to internal resistance in the battery. Whether the batter is really
run down can be seen by turning off the refrigerator and seeing if the
voltage bounces back up to 12.3V

>
> Also, when the alternator is charging, it shows 13.9 to 14.4 at the
> cigarette lighter (vehicle battery) but only a max of 13.7 on the aux
> battery. Why wouldn't they be the same?

Charging current into the battery working against the resistance of the
wire feeding the battery -- some of the current gets turned into heat
resulting in lower voltage at the battery terminals. Let it charge
longer so its current draw drops or try fatter wire if you don't have
the time to let it come up. Of course, if the charging wire is also
feeding the Dometic, then that current draw is also adding to the wire
losses.

>
> The standard relay seems to be isolating them as it should.
>
> No, the Optima isn't sealed! There's white powder all over the ground post
> and you can see where liquid's been running out, probably eating holes in
> the floor.

Well then -- maybe you DO have a hosed battery!