There are many ways to determine the state of charge. Some controllers measure the actual energy leaving the battery and calculate what should be remaining. However, a volt meter can be used to get a pretty good idea. You are on the right track with your idea to make an empty-to-full meter, but you have over-simplified it. A resistor in series will not do what you want, but there are other circuits that can use components like zener diodes. For example, if you want the meter to read empty at 10 volts and full at 13 volts, you could use a 10 volt zener to "subtract" 10 volts from the actual voltage. The result would go to a meter that reads 3 volts full scale thereby giving you your gas gauge. Components like op-amps can be used as well. Both require some knowledge of electronics. I recommend looking for battery gas gauges on places like ebay and amazon. They have them for 12, 24, 36, and 48 volt systems. They don't cost much and someone else has already calculated all the resistors, zeners, etc to get the job done.
Pat
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of king_of_neworleans
Sent: Saturday, September 06, 2014 8:51 AM
To: electricboats@yahoogroups.com
Subject: [Electric Boats] A couple of battery monitoring questions
The proper way to read state of charge of a bank of flooded lead acid batteries, other than checking the s.g., is to read the open circuit voltage. That I know. But what about when there is a load on the batteries? Do I simply have to shut down the controller to take a reading, or is there another way? Also, if I want an analog panel type voltmeter to read open circuit state of charge, could I add a series resistor to make the scale zero out at the zero charge voltage, and a parallel resistor to make the 100% charge voltage read full scale on the meter?
Posted by: "Forums" <forums@greensdomain.com>
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