Try & get the datasheet for the battery you are going to use. Typically, a
deep cycle battery will be easier to finfd the data for as the info is
generally required when sel;ecting the battery (rather than finding one
that fits in the space available!)
The charts normally include both constant current and constant power
discharge curves.
The actual capacity will vary with current/power level as well as the
battery construction.

Your 58AHr is probably rated at the 10hour rate. i.e. 5.8A for 10 hours. At
58A you will not get an hours discharge time.

The battery "endpoint voltage" is also discharge rate dependent. At the
higher discharge rates it is permissable to drop to a lower voltage - 1.6
volts per cell not being uncommon in UPS applications. In contrast at lower
discharge rates, the endpoint may be as high as 1.8V/cell. Once the battery
endpoint voltage is reached, there is little energy left - the voltage
falls off quickly and some cells may "reverse" - charge up with negative
polarity. You do not want this.


Recharge voltage / current depends on battery  construction - again the
data sheet will give guidlines. A current limited charger set to ~15V
should be OK for a quick recharge but once the battery is 80-90% charged
you should drop the voltage back to 13.6 or so. The current limit should be
set to no more than than the 1hour rate (58A) but your charger isn't too
likely to handle this anyway! If you can monitor the battery temperature
then this is even better as it is possible for batteries to thermally "run
away" - due to the terminal voltage decreasing with increasing
temperatures. If you overcharge the battery for an extended period it will
loose water and capacity.

A lot depends on battery construction so the data sheet is going to be your
best guide.

Hope this helps.

Richard P





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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