Note: Any important changes made to this document since the original publication have been noted in a separate change document.
When maintaining 12v batteries as an emergency backup supply in any application, it is very important to know the battery's real amp-hour capacity. This circuit which uses the Low Voltage Disconnect circuit described elsewhere on this site, makes the amp-hour test simple and cheap. The amp-hour reading obtained should be accurate to within 10% if the load resistor (lamp) is properly calibrated.
The operation of this circuit should be pretty obvious. The Low Voltage Disconnect circuit connects the battery to a load resistor and at the same time applies power to the clock. When the battery voltage drops below 10.5 volts, the load is disconnected and the clock stops. The average load current multiplied by the elapsed time gives the amp-hour capacity at that discharge rate. (The amp-hour capacity will vary depending on rate of discharge. Most batteries are rated based on a 10 or 20 hour discharge interval.) This automatic mode of operation avoids the problem of having to continuously observe a voltmeter and timer as is done when manually testing the battery capacity.
A novel form of load resistors are the 12v incandescant lamps that are available for use in RVs and camp lighting. They provide a slightly non-linear voltage current curve that makes the current drawn more constant as the voltage drops. (The current variation is about half of what it would be for a regular fixed resistance.) They provide a convenient way of changing the load by simply screwing a different bulb into a socket or by paralleling two or more bulbs. As there may be quite significant variation in current draw between bulbs of the same type, individual calibration will be necessary. As an added benefit, these bulbs provide a visual indication that the battery is being discharged.
Calibration is quite simple. Place an DC ammeter in series with the battery lead marked "Cal" and wait until the battery voltage is between 11.5 and 11.8v during discharge. Note the ammeter reading and mark it on the bulb using a suitable felt marker. If you have additional bulbs to calibrate, replace the bulb in the unit and repeat the measurement.
As an option, if you have an appropriate DC ammeter kicking around (or if you find a cheap one at a fleamarket), wire it permanently into the circuit in the line indicated "Cal".
It is very important that an old fashioned mechanical 115 volt clock or functional equivalent be used here. Electronic digital clocks often do not stop timing when the power is removed. We need to maintain the reading and be able to read the time without any power applied to the clock. An equivalent timer would be any of the electronic or mechanical elapsed time meters available. A 12vDC (or lower) elapsed timer would replace both the clock and the associated relay. Many of these timers have a reset function as well which is very convenient. These elapsed timers are often available surplus at little cost.
If you are already using a low voltage disconnect on your equipment, the batteries can be tested using only the load and timer part of the circuit. Disconnect the equipment lead and any charger lead that may be in use. Attach the load/timer set and allow the battery to discharge. Calculate the AmpHour capacity and then reconnect the normal equipment leads. This circuit is so simple and inexpensive that a separate load/timer set could be made for each site that uses the LV Disconnect.
Good luck and good testing.
I have made one unit according to this design and it seems to work just fine. It used the options mentioned and in addition I added a couple of switched resistors to the load bank for lower currents. A 25 ohm 30 watt and the 12.5 ohm 60 watt (two 25 ohm 30W in parallel) resistors (they were the wattages I had in the junk box) allowed 1/2 amp and 1 amp load settings. I used screw-in fuse holders from an old kitchen stove as the lamp sockets (don't use these at 120v - there is too much exposed metal base on the lamps but they are quite safe at 12 volts). The only problems I had was a 12v DC timer that stopped running after 3 hours. Fortunately I had a spare.
If you build up the circuit and find any problems, please let me know.