I am trying to get a ruff indication of the condition of a battery used in Automatic Gates and Alarms.

The battery capacity is 7 Ah and charged to 13.8 volts.

I used a Load of 36R 5 Watts across the terminals to apply a load.

The battery voltage dropped to 12.3 Volts. The information I read indicated that if the battery was a minimum of 12.1 Volts it was ok.

What I am trying to find out is from that voltage what is the actual capacity of the battery. If that's possible.

This will help me determin possibly how much longer to expect from the battery.

 

hi Rodney,
If you could monitor and record the battery voltage while the battery is discharging down to 10.8v via the 36R resistor , that would be indication of that batteries capacity, at that discharge rate.
Post what you record
Do NOT let the battery discharge lower than 10.8v

E

 

Ideally i would use a constant current load, drawing 1amp, and measure the time it takes to get down to 10.8V, which in your case should be 7hrs.

 

hi Rodney,
If you could monitor and record the battery voltage while the battery is discharging down to 10.8v via the 36R resistor , that would be indication of that batteries capacity, at that discharge rate.
Post what you record
Do NOT let the battery discharge lower than 10.8v

E

Thank You, I will do this test and post the results. I was hoping for something a little quicker but I will do the test

 

hi Rodney,
As Dave points out, if you could build a 1 amp Constant current load that will give you a more meaningful reading.

If you know that actual current drain in service, you could configure a CC load for that current.
Note the battery should be fully charged before the test and it will only apply to that battery.

As you know battery efficiency and capacity deteriorate with age..

E

 

You could build a two step-load battery tester for gell-cell batteries that have intermittent high current demands.

A 1 amp load to check for the general battery condition base line and then pulse a 10 amp load to measure the voltage drop as a means to roughly calculate the battery internal resistance.

 

I regularly test various types of batteries with one of these http://www.westmountainradio.com/cba.php . They work great, but definitely not for the one-off test.

 

A 12W, 12V incandescent lamp makes a good 1A load, since its resistance drops as the voltage drops, tending to keep the current more constant as the battery discharges, and as a bonus, it gives a visual indication of the discharge state.
For example, for a 10% drop below the bulb's rated voltage, the current only drops about 5%.

 

It is my understanding that a 7AH lead-acid battery normal discharge would be at the 20 hour rate.
For an accurate test one needs to discharge the battery after a full charge. Also if the subject battery is discharged @750mA it should take about 9 hours. A a discharge current of 1.5A should take about 4 hours and at the suggested discharge @ 1.0A it should take about 6 hours. These times and discharge currents are from a chart in document I have, I tried to copy the chart but could not. The values I mentioned are
my interpretation of the chart. A couple of other things to mention are: If you discharge the battery at 7.5A, it will last about 30 mins. When I use any lead acid battery of any size, the first thing I do is put a fast blow fuse between one terminal and a connection, because the battery has the capacity to supply a great amount of current if the leads get shorted. If that happens the fuse is a safety factor. I have seen 12 guage insulated wire get red hot on a battery short.