We make these things for telephone exchanges. There are a few things to
watch out for.

1. Maximum disconnect current - whatever you use must be capable. With
constant power loads the current increases as the battery voltage drops.
2. Maximum fault current. The disconnection device must be able to
withstand a fault, particularly if replaceable fuse or MCB is in circuit.
It's no goo resetting the MCB just to find the battery disconnect has
failed anyway. It's generally not a good idea to have it try & disconnect a
fault.
3. Timing. You probably don't want it to disconnect on a transient low
voltage spike. A "recognition time" delay is normally inserted (1 - 10
seconds).  Also you don't want it to work like a buzzer if a battery cell
fails as this will probably stuff your disconnect device and possibly the
load.
4. The current goes both ways - battery charge & discharge. FETs can be
used by bipolars are probably not feasible. Relays work best if you can
handle the standing current. A latching relay that only requires a
connect/disconnect pulse is possibly best - if you can find one in the
price & ratings category you need.

Richard P




                    Jinx
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                    Sent by: pic            Subject:     Re: [EE]: Low battery voltage switch
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                    28/01/03 22:05
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> Look at Maxim's micropower comparators w/reference chips like
> MAX8211/8212. The Maxim parts use low microamps, so they are
> insignificant in comparison to the self-discharge of the SLA battery.

Maybe I'm being a bit too precious about the quiescent A of a
battery monitor. The preference was to have all circuitry disconnect
from the battery, including the monitor. After all, once it's done
the job it's no longer needed until the battery is recharging

I'm told that the power had been turned off on the poor thing 2 1/2
years ago (the building is part of an occassional council restoration
scheme). Now, even if the Maxim or similar chip had been there it
would have only drawn ~0.1Ah in that 30 months, so maybe total
battery isolation isn't necessary. It just needs the addition of an
electronic (ie automatic) reset for when the mains is restored

RS have quite a selection of BMs from TO92 upwards, could be
time to look up a few data sheets

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