>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>Sent: 11 August 2005 13:09
>To: Microcontroller discussion list - Public.
>Subject: Re: [EE] Signalling low battery voltage
>
>
>I am in need of a simple (read low cost) way to signal that a 
>lead/acid 12 volt gelcell is in need of recharging. I have in mind a 
>device or whatever that will turn on an LED when the recharge point is 
>reached. Is there such a critter?
>
>Any enlightenment will be appreciated.
>
>
>Circuit here
>
>        
>http://homepages.which.net/~paul.hills/Circuits/BatteryMonitor/
BatteryMonitor.html

Something even simpler that works moderately well could be designed. Lead Acid voltage is fairly flat across most of capacity and drops 
more sharply towards end of capacity. How close to end you get for a 
given voltage depends on discharge rate and temperature (and other 
factors).

Does this need to be connected permanently to the battery, or will it be a "press to test" type arrangement?

If it has to be connected all the time, consider (as I have!) a nanowatt PIC arrange to wake up from sleep every few seconds or so.  To minimise current consumption, power the voltage reference from a PIC pin so you can easily turn it off when you go back to sleep.  Temperature compensation or terminal voltage is then just a cheap thermistor away.

Regards

Mike

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