# How can you monitor the charge of a sealed, lead acid (UPS) battery

I'm building a robot that I plan on running off of 12 volt sealed, lead acid batteries ( UPS Batteries )
One of my 'things to do' is to figure out how to have the
robot recognize that the batteries are running low.
Is there a simple circuit ( that won't consume a lot of power it self ) that will report the battery charge level via a couple of I/O pins or RS-232 signals?
Thanks - jack
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The voltage of most batteries goes down as the battery is discharged. A simple battery monitoring circuit merely samples that voltage.
One older method is to use a bargraph IC. These ive you output pins that can indicate voltage within a fairly narrow range. The IC is intended to connect to LEDs, which you can use for visual feedback. Pick one or two pins to connect to your microcontroller as "getting low" and "really low." Look on Google for some circuit ideas.
If you are connecting to an MCU with an ADC, you can connect a resistor across the battery and load (a fairly high value to limit drain). Use Ohm's Law to calculate the voltage.
Maxim and others make battery monitor ICs you might try out. Many of these companies will send free samples. Some of these have 1- and 2-wire serial interfaces. They're made for polymeer and rechargeable lithium cells, and are probably more sophisticated than you need for lead acid, though.
A somewhat crude but workable method is to use a zener diode, which "folds back" if the voltage drops below a certain level (essentially, outputting a HIGH or LOW depending on battery level). You select the zener based on the minimum voltage desired from the battery, and the characteristics of the zener. Google should have circuit examples.
RS-232 wouldn't be as easy, but it could be done with a PIC or AVR using just the analog comparator inputs. Some PICs and AVRs have serial UARTs, but you may need a MAX232 chip or equivalent if you need real RS-232 signals.
-- Gordon Author: Constructing Robot Bases, Robot Builder's Sourcebook, Robot Builder's Bonanza
Jack Snodgrass wrote:

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On Sat, 27 Dec 2003 17:52:55 GMT, Jack Snodgrass

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A 12 lead-acid battery is usually considered discharged when its
terminal voltage falls to 10V, so this will give you a low-going output
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On Sat, 27 Dec 2003 14:30:25 -0600, John Fields wrote:

Thanks for the info.
jack
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On Sat, 27 Dec 2003 22:43:59 GMT, Jack Snodgrass

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You're welcome. Unfortunately, there's an error, since 8333 ohms is
bracketed by 8250 ohms on the low end and 8450 ohms on the high end.