Re: LEAD ACID BATTERY

On 29 Mar 2006 10:50:17 -0800, "john" <conphiloso@xxxxxxxxxxx>
wrote:

Hi,

I have to design a voltage source ( +/- 18volts, 13A ) using lead acid
batteries. My circuit draw is 700mA and requires plus minus 18 volts to
operate efficiently. I need batteries that can atleast run for 10 to 12
hours before the voltage drops to +/- 17 volts. I am thinking of adding
three 6 volts, 13AH ( rated for 20AH ) batteries in series to produce
+18 volts and adding three 6 volts to generate -18 volts. I choose the
battery ( BP13-6V ), http://www.zbattery.com/zbattery/ub13-6.html.

Can anybody advice me that am I doing the right thing that will these
six batteries last for 10 to 12 hours maintaining +/- 18 volts @
600mA.

---
The capacity of most lead acid batteries is defined by how long they
can deliver current into a load, at a particular rate, until the
voltage across the battery terminals falls to a particular value,
usually about 5.25V for a 6V battery.

the battery you've chosen has a capacity of 13 ampere hours at a 20
hour rate, which means that if you start off with a freshly charged
battery, the voltage across the battery's terminals will fall to
about 5.25V in 20 hours if you discharge the battery at 13 AH/20H =
0.65A.

Since you have six batteries in series, that corresponds to 31.5V,
and since you're using the center tap of the string as common, you
can expect that the voltages on the ends will decay to 15.75V after
20 hours.

Since I don't have the discharge curves for the batteries in front
of me, I can't say what the battery voltage will be after 12 hours
of discharge, but I'd guess that you're cutting it kind of thin.


--
John Fields
Professional Circuit Designer
.

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