Re: I need a low voltage expert

[andrew@xxxxxxxxxxxxxxxxxxxx (Andrew Gabriel)]

In article <5eed3165-f7ce-41d0-9a22-ff9d6def8c9c@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
	nick markowitz <nmarkowitz@xxxxxxxxx> writes:
> On Jun 30, 7:27 pm, Jim <alarmi...@xxxxxxx> wrote:
>> 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

I didn't miss that point. I agree with what nick said about that,
so didn't see the need to comment further on it.

>> 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.
>
> When a charger is being kept on hard full charge because a battery is
> over sized and can not keep up  it is just like when it keeps charging
> into a dead dried up gel cell the charger will get hot and overheat
> when it is constantly being called on there are times when I have gone
> to change out a low battery the panel board and heat sink is so hot
> you can not touch it with a bare hand.and plug in transformer has burn
> marks on it as well from overheating.
> see it all the time.

SLA batteries for float use are normally charged at a max current of C/7,
so you're looking at 7 hours minimum to recharge (probably 10 hours because
charging isn't 100% efficient). The charger will have got up to temperature
within an hour or two of starting to charge, and stay there until the
battery approaches full charge some hours later and the charging current
reduces. If you put in a larger battery, it won't get any hotter, it will
just stay hot for longer (when charging a completely discharged battery),
because with a larger C, the charging current will be less than C/7 and
take longer to charge.

There's no "not keeping up" - the alarm is not suddenly drawing any more
current or the charger providing less current, just because you increased
the battery capacity. It will take longer to charge, but it will more than
keep up.

A shorted cell in a battery is a different issue, and that isn't what a
larger capacity battery will look like to the charger. Of course, the
charger shouldn't end up in a dangerous condition because of a shorted
cell, but if you buy a cheap product, that's the sort of place you might
find [lack-of-]design compromises were made. Poor quality chargers often
result in short battery life too, and may well be the cause of the dead
cell in the first place.

--
Andrew Gabriel
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