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Taipan
01-04-2006, 10:25 AM
Hi,

I need to monitor the capacity of my batteries as well as the drain that my motors are putting on them. I want to calculate how long till the batteries are dead taking into account the current drain on them. The motors are speed controlled so the drain on the batteries will vary and hence the remaining running time will also vary as the load changes. I don't think it would be a matter of monitoring the voltage of the battery. Any ideas would be appreciated.

Bruce Bates
01-04-2006, 11:31 AM
Taipan -

Use Google or your favorite search engine for "battery gas gauge" and you'll come up with exactly what you're looking for. Much depends on the number of cells, and the battery chemistry.

A few years back, Texas Instruments bought out a company named BenchMarq. They made dozens and dozens of these "battery gas gauges" in all different flavors.

Regards,

Bruce Bates

Taipan
01-04-2006, 01:18 PM
Thanks Bruce, most of what I found were for small batteries for cell phones etc, my project will be using larger batteries, something like an electric golf buggy battery. I'll have to do more searching.

Bruce Bates
01-04-2006, 01:43 PM
Taipan -

There are certainly those, but I would think (without looking) that any of the lead acid qualified "gas gauges" would work for utility batteries such as you're planning to use. If not, shoot them an email, and ask for their suggestions. They have always been very prompt in answering any questions I've had about these units.

I'm SURE there is something available, and probably from the BenchMarq line. I'll have to try and dig out my old BenchMarq Catalog and Data Book to see what I can find there.

Regards,

Bruce Bates

Tracy Allen
01-04-2006, 11:51 PM
Monitoring the current is usually done with a shunt resistor in series with the battery. Here is an example of a 50 amp/50 millivolt shunt:
www.mpja.com/directview.asp?product=15322+ME (http://www.mpja.com/directview.asp?product=15322+ME)
The resistance is 0.001 ohm (0.05 / 50), and notice that it has two terminals for the high current connection and two more terminals where you connect your voltmeter (BASIC Stamp ADC).

You need an amplifier for the 50 millivolts. Usually this is done with a circuit like the following, which uses the MAX472 current monitor chip:
www.emesys.com/BS2rct.htm#B_current (http://www.emesys.com/BS2rct.htm#B_current)

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Tracy Allen
www.emesystems.com (http://www.emesystems.com)