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I was measuring the voltage across a 12v VRLM battery when I noticed something : the battery had 11.7 volts without the load and when the load was added, the voltage dropped to 11.4. I don't want to have to disconnect my battery just to get a low battery indicator to work properly. How can I know if my battery is low when I have it "in use"?

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  • \$\begingroup\$ How close to the battery were you measuring this voltage? When current is drawn by the load, there will be resistive loss between the load and the battery, and this will lower the voltage measurement, in proportion to the current drawn. There is also a loss within the battery, in the connection between cells. \$\endgroup\$ Commented Oct 26, 2013 at 5:09

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You decide what the "low" threshold voltage level when the battery has its load on it. This way you will know when the battery gets to critical level of capacity corresponding to what load it is expected to power. Trying to do this when the battery is unloaded will not give you a realistic in system usage scenario.

Think about your car. Its battery stays connected to the alternator / voltage regulator all the time -- unless you remove the cable to do service work.

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  • \$\begingroup\$ So If I had a varying current draw at different times, I would have to have an amp meter to help calculate? \$\endgroup\$
    – skyler
    Commented Oct 27, 2013 at 2:22
  • \$\begingroup\$ @skyler - The in system usage of the battery may have to be characterized for a certain load. Some systems have an average load that does not change too much. Others have loads that will vary a lot depending upon what is happening at the time (i.e. sometimes a big motor may be on and other times it may be off). One strategy you could use would be to measure the voltage for low battery tests when the system is in the known highest load mode. It would not necessarily be necessary to have an load current measurement if your system can be characterized at that load. (continued) \$\endgroup\$ Commented Oct 27, 2013 at 5:22
  • \$\begingroup\$ (continued from above). You could also choose to use another known system state at a lower battery draw to make your measurements. It would also be possible to measure at multiple states if each was characterized. \$\endgroup\$ Commented Oct 27, 2013 at 5:24
  • \$\begingroup\$ On the other hand you could just determine the worst case voltage that you can let your battery get down to and then measure the voltage frequently while the system goes through various levels of current draw on the battery. \$\endgroup\$ Commented Oct 27, 2013 at 5:26

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