-----Original Message----- From: Mike Keitz To: PICLIST@MITVMA.MIT.EDU Date: Thursday, 16 October 1997 1:37 Subject: Re: Sensing Low Battery Condition (Thanks!!) >On Tue, 14 Oct 1997 17:58:00 -0300 Ricardo Seixas > writes: >> >> I'd like to thanks all that replyed to my question. >> In this circuit I'll use the method described by Mike Smith >>to store "battery used time" since I'm using nicad. > >I believe in the original post you said the project was a portable >measuring device. What does your device do as the battery runs down? >Does it (a) just quit all of a sudden, or (b) give inaccurate readings >when the battery voltage is low? The latter condition is very bad. >Others have proposed schemes to give an early warning that the battery is >failing. This is somewhat difficult on NiCds and may not be of much use >anyway. Users will correctly deduce that a mode (a) condition is a dead >battery. But you need the circuit designed to guarantee that >mis-operation in mode (b) will not happen. Regardless of the battery >type, this is done with a simple absolute voltage detector or design of >the circuit so it always goes to mode (a) first. > >> Using one or two EEPROM locations I can create a counter that >>increment every minute and, when a determined count (time) is reached >>I can display a message with a safety margin to recycle the battery. >> When the battery is recharged I can press a button before >>turn-on >>the device then, at power-up the start routine check the button and if >>pressed reset the counter. > >This assumes that the battery will be fully recharged every time. The >time needs to be decreased as the battery loses capacity due to age, or >you could set it rather short from the outset so significant capacity >will be sure to remain. This will lead users to ignore the warning >though. > >Users often only charge enough to get a little more use. This is rough >on the battery, but what do they know? Also they may have to if they >can't afford the time to take the device out of service for a full >recharge. They may ignore the warning and continue using until the >battery is dead. That's why it needs to be designed to stop gracefully >when the battery expires. My suggestion was to use a dedicated battery minder chip - these monitor degradation of battery with time, and measure discharge via a series resistor temp transducer. (they also keep battery charged nicely:) MikeS