Indicator Light For Worn Out Battery Needing Replacement
This circuit is a possible way to turn on a LED whenever the battery voltage falls below a certain level and keep it lit. It has been suggested if battery supply falls below 13v after half an hour use, this might indicate a bad battery (see thread "Neato with a Voltage monitor",http://www.robotreviews.com/chat/viewtopic.php?f=20&t=16111
) . In other words, excessive drop in voltage from discharge is the indicator; discharge always drops the voltage, but the bad batteries presumably drop too much, too fast. If the degree of drop alone, and not just its speed, observed sometime during a typical vacuum run can indicate battery condition, this circuit might be useful.
It is inconvenient to observe the shortfall because readings must be taken after running some time, and because once returning to the charger the indication is lost. Running Neato while out of the house certainly produces that situation. So having a voltmeter may be useful to discover what shortfall indicates a bad battery, but does not make it easy to monitor the condition.
This circuit uses a voltage comparator chip with output through a pair of open collector inverter drivers to latch an LED on whenever the voltage falls below a certain level, and keep the LED lit until the reset switch is opened (normally closed momentary button), even if the input voltage rises again. The comparator relies on Neato supplying regulated 5V for logic circuits, such as the wall and cliff sensors, which remains unaffected by small drops in 15V supply. I think all the parts are at Radio Shack. Some tinkering required to set the voltage divider resistances to fix the proper trigger level; a potentiometer might be needed.
The inverters were chosen for the latch merely because already in use for a wall following circuit; all sorts of latching chips exist, but this one also happens to provide drive power for an LED. This is just one chip I happen to know, and am no expert in such circuits generally.