Re: I need a low voltage expert

[Jim <alarminex@xxxxxxx>]

On Jun 30, 7:05=A0pm, and...@xxxxxxxxxxxxxxxxxxxx (Andrew Gabriel)
wrote:
> In article <7bdce4ae-930d-44f5-a05a-8aff9b1b2...@xxxxxxxxxxxxxxxxxxxxxxxx=
.com>,
> =A0 =A0 =A0 =A0 nick markowitz <nmarkow...@xxxxxxxxx> writes:
>
>
>
>
>
> > On Jun 30, 5:00 pm, obmcclintock <f...@xxxxxxxxxxxxxxx> wrote:
> >> I have a technical question about low voltage in an alarm system. I
> >> have a 10 year old radio shack 8 zone burglar/fire alarm panel with a
> >> small rechargable back up battery. I would like to replace it with a
> >> larger wet cell battery just in case our power is out for quite a
> >> length of time. I will place it in a vented box close to the panel. My
> >> question is; Can I use a battery much larger in size and amp hours
> >> than the little one as long as the voltage is the same- 12 volts? Will
> >> I run the risk of burning something out as I can not afford to replace
> >> it now that my work is becoming scarce. Please can someone help me
> >> with this question, thank you!!!
> > No can do must use a lead/gel acid battery or risk of fire.
> > the panels charger is not designed for wet cells.
> > could put a bigger lead/gel =A0acid in most panels can handle 7-10 amp
> > battery no problem.
> > if you do put bigger than that =A0the charger in the alarm panel will
> > never be able to keep up proper charge and burn out.
>
> The charger circuit limits the charging current, and the current
> won't increase with a larger capacity battery - it will simply take
> longer to fully charge. This is most unlikely to damage the charge
> circuit.
>
> Note that lead/gel acid batteries used in alarms don't like being
> run completely flat (whipes out some of the capacity each time
> you do it), and the alarm may behave unpredictably as the battery
> voltage drops too low. Some alarms have a battery protection
> function to switch off and cease current draw before running
> the battery too low, but probably not a 10 year old radio shack
> system. If you are using the system in a situation where you
> expect the battery to run out, you might want to investigate a
> battery protection circuit, although it might not integrate well
> with the alarm if it wasn't designed with this in mind.
>
> --
> Andrew Gabriel

Andrew, read Nick's reply. I think you missed the fact that the OP
wanted to use a wet cell battery, presumably a car battery. Aside from
the things that Nick pointed out, the fact that liquid lead acid
batteries emit hydrogen is important to note also. Lack of proper
ventilation and/or a little open flame can cause quite a "pop" and the
ignition of anything flamable. Not to forget about the corrosion
factor with wet cells.

Gel cells that are normally used in alarm panels control the venting
of gas and the UPS also has gel cells. I too have used UPS's to extend
the standby time of alarm panels. Works great and pretty inexpensive
too.