>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*.

Peter Johansen is right that it is not IDEAL to use a car battery for 
deep cycle applications. However, if you only do it for a limited 
number of times and/or don't mind about battery life (ie the battery 
cost is modest compared to the value to you of the power) then a car 
battery will work fine. Long long long ago my main experimental power 
source was a 12v truck battery. (Also worked well providing high 
current to heat wire fuses to initiate 'explosive' devices - but 
that's another story). When it got too flat I would put it on my 
push-bike and take it to school to charge. Carrying a truck battery on 
a push bike is an extremely 'interesting" experience. Adds a whole new 
meaning to "frame flex". I don't know what finally happened to the 
battery but it served me well for many years as a power source.

The 10.5v end point is probably figured at an automotive load where 
battery impedance will be significantly important due to IR voltage 
drop across it. At 2A you can expect the voltage drop to be lower. 
Look at some battery data, but you will find that a lead acid battery 
is not too far off 12v for most of its discharge time.

You can get ROUGH running time from AH_capacity/Amps_load  = 58/2 = 29 
hours in this case.
If you want to run 3 days that's 29/3 = 9+ hours per day.
Murphy being Murphy, if I was in your position, I'd consider 2 
batteries as an *almost* bulletproof solution.
If you don't mind charging overnight then you are very safe. Charging 
in a room where you are sleeping is safe enough ****IF**** the battery 
does not outgas. I long ago charged a battery in such circumstances 
and it vented significant gas/acid. I had a very very very raw & sore 
throat for weeks. Not at all recommended. Charging at a modest rate 
AND encasing the whole battery in a tape sealed plastic bag would be 
reasonably safe. I would consider doing that myself, notwithstanding 
my increasing aversion to doing life threatening things as I mature. 
There is a small risk (as always) of battery explosion from 
accumulated gas when charging and bagging the battery would tend to 
concentrate any vented gas (which is what the bag is for). I consider 
the danger reasonably OK but mention it for completeness. Turn off 
charger before adding or removing charging leads etc.

You can buy small lead acid chargers intended for gel cell use and 
these may have enough capacity for you. If you run on the stand for 8 
hours and can charge for 12 then you need somewhat under 2A charge to 
fully replenish. Adjust rates according to actual hours.

If they have lighting but no power to the stand you COULD consider a 
solar panel but it would need to be large at the illumination levels 
you are liable to meet - unlikely to be a practical solution.

One battery will probably be OK. Two batteries will definitely be OK 
IF your load figures are correct.



        RM 

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