Checking a deep cycle battery
Jeff,
I have a deep cycle battery which we use with an inverter at craft shows to power the credit card machine and sometimes lights or fans. After using it all day and before going to the next show, I would like to be able to check the battery level.
How can I check the battery level?
I plan to get a second battery as a backup since we do multiple day shows and can’t afford to lose function of the credit card machine, but it would be good to know how much drain has occurred after a day’s use.
Thanks for your help.
Sam Allen
Sam,
Knowing how much amp-hour charge is remaining in any deep cycle battery has been a problem still waiting for a solution, as there is no perfect meter that can tell you this. The only way to know the exact charge level of a lead-acid battery is to insert a calibrated hydrometer with temperature correction into each cell. Obviously that’s not going to happen, so you could take a voltage reading with a digital volt meter.
The problem is the volt meter will not give a true voltage reading while the battery is under load, as it will read too low, but it will also read too high when there is no load and the battery is at rest or was just charged. Many companies make a battery amp-hour meter that keeps track of how may amp-hours you put into the battery when charging, then will subtract from this total during discharge, but this also has to keep correcting itself due to temperature changes and how the “rate” of discharge can affect this reading and these meters cost several hundred dollars.
You did not say what inverter you are using, but many will include a battery volt meter which will give you a rough idea of battery charge.
Here is my suggestion. Buy a volt-meter from Radio Shack for about $15 that provides a digital display. Set for the lowest DC voltage range it has that can easily read in the 8 to15 volt range, but not more than 50 volts.
Add a “small” load to the battery. Nothing large and do not connect the inverter. Maybe a small 12 DC light or small DC fan you indicated you had, just enough to pull off the surface charge but not enough to start drawing down the voltage.
Note the voltage reading and compare with the following:
20 to 30% charged – - – - 11.60 volts
30 to 50% charged – - – - 11.90 volts
50 to 60% charged – - – - 12.20 volts
60 to 70% charged – - – 12.45 volts
80 to 90% charged – - – - 12.50 volts
Over 90% charged – - – 12.66 volts
The above chart is just a starting point for you. Once you take a few voltage readings and then see how long your equipment will run, you will be able to customize this chart for your specific battery. Please also note this chart is based on your battery being at 77 degrees F. If the actual battery temperature is higher or lower, this voltages will shift some, but not major until it is over 90 degrees or below 50 degrees.
Good Luck,
Jeff Yago
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