I need to power some electronics at a trade show for 3 days (total of 26 show 
hours) and won't have an AC outlet.  I need to calculate how many automotive 
batteries it would take to power these electronics.

As a start, the good news is that the electronics are designed to run off 
automotive 12V -- and have been tested down to 8.5V.  Altogether, I estimate 
20W consumption (mostly DC-DC converters, so that power should be fairly 
consistent as battery voltage decreases).

Now, I have a battery here that claims 58AH.  My google research tells me this 
means 14.5A for 4hrs, at which time the battery voltage will be 10.5V.  But 
that is apparently 10.5V *loaded*.

So what happens if I only consume 2A -- does it also mean that I can get ~2A 
(being conservative) for 29 hrs and the ending voltage will still be 10.5V?  
I doubt since the load is different.  Generally a car battery is considered 
0% charged when the open-circuit voltage drops to 11.8V.  And generally 
considered to need charging (75% charged) when the voltage drops below 12.4V.  
But that's for a car -- and I'm sure there's a lot of useable capacity below 
that.

So what happens below 11.8V open-circuit?  Is the discharge (fairly) linear?  
I could not find a battery discharge graph via google.  What is the minimum 
safe voltage to charge a battery?

Bottomline, I'd like to know if one fully-charged battery will be more than 
adequate for all 3 days.  If absolutely necessary, I'll drag it back to the 
motel and charge it, but really want to avoid that -- something about a 
lead-acid battery in an enclosed room (that I'm sleeping in) that does not 
thrill me.

Can any of you lend some intelligence with this?

Thanks,
-Neil.






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