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'[EE]: Automotive battery capacity'
2005\02\07@002807 by

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.

PicDude writes:

> 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.

First off, you do *not* want to use an automotive (or starting)
battery.  Automotive batteries are designed to provide a lot of
current for a short period of time, and are not designed to be drained
more than 10 or 20% from full capacity.  Draining an automotive
starting battery to empty is a good way to kill it, or at least reduce
it's capacity dramatically.  Instead what you really want is a deep
cycle battery -- the kind used in golf carts, or as the "house
batteries" in sailboats and RVs.

Off-the-cuff, your tech info seems ballpark, but be sure to check out
the car and deep cycle battery FAQ:

http://www.uuhome.de/william.darden/carfaq.htm

> 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.

Deep cycle batteries come in all different sizes.  If you
underestimate, you can always pick up an additional battery.  You will
find quite a bit of difference in price WRT deep-cycle batteries
depending on construction.  There are several types of sealed
batteries that offer many advantages which you may or may not need.
Properly cared for, though, the cheapest golf cart battery will often
last just as long (overall lifespan and total number of recharges) as
sealed battery costing two or three times as much.

-p.

>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*.

I'm not 100% certain about that last point.

>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?

It will last a bit more than 29 hours. The less aggressive you run the battery,
the better efficiency it has.

On Monday 07 February 2005 12:20 am, Peter Johansson scribbled:
> First off, you do *not* want to use an automotive (or starting)
> battery.  Automotive batteries are designed to provide a lot of
> ...
>
> Off-the-cuff, your tech info seems ballpark, but be sure to check out
> the car and deep cycle battery FAQ:
>
> http://www.uuhome.de/william.darden/carfaq.htm

Ah-ha! I did not know the difference between regular and deep-cycle batteries.
But your mention of them now helped me also locate other useful info, and I
should have access to (borrow) an Optima deep-cycle battery.

What's curious/interesting though is that the FAQ you link to mentions in #1.7
that a regular battery should not be discharged below ~12.0V, but #1.12 says
that a deep cycle battery should also not be discharged below ~12.0V.  Huh?
So where's the advantage of being able to be deep discharged?  It also
mentions that shallow discharges should be avoided for deep-cycle batteries,
but I will be discharging at low rates frequently.

I have a voltmeter with an alarm that I can setup to alert me when the voltage
level gets to the critical point.  But I've still not found a clear enough
answer that tells me how much useful Amp-Hours I can get off a deep-cycle
battery without damaging it, and what that critical level actually is.

Cheers,
-Neil.

Car batteries are designed to provide a LOT of current for a short period
of time, then be recharged by the alternator and kept that way.
Deep cycle batteries are designed to provide a moderate amount of current
for a long time, then be recharged.

The internal construction is quite different.

A deep cycle battery can be discharged to the 50% capacity level and then
recharged several hundred times during it's life. A car battery would
perhaps last a few dozen times under these circumstances.  A deep cycle
battery can be discharged  80 or 90% safely; doing this regularly merely

At 50% capacity, a battery will read about 12.1 volts open circuit, AFTER
RESTING FOR A WHILE (several hours). When it has only 20% left, it will
(http://www.trojan-battery.com/customercare_batterymaint4.html)

At 02:47 AM 2/7/2005 -0600, you wrote:
{Quote hidden}

>

2005\02\07@144154 by

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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