Automatic load tester for batteries

[mikets]

As an off season project, our team decided we need a battery tester that can tell us if a battery is bad or going to be bad. So we did some research and came up with a simple load tester circuit. Since I haven't designed any serious circuit in over a decade, I would like to have somebody do a design review on the circuit before we build it. The detail of the design can be found here:
http://proj.titanrobotics.net/docs/Robot/BattTester

[Al Skierkiewicz]

Mike,
Robot batteries are best tested for the manufacturer's specifications. Although our batteries are rated at 18 amp hour, that capacity is only valid when discharged at 0.1 C or about 1.8-2 amps. The AH rating falls rapidly when discharged at higher than that rate. Many team are now using the West Mountain Radio CBA-III battery analyzer. One it's features is the ability to record discharge curves and compare them over a period of time by overlay. You can find it here...
http://www.westmountainradio.com/CBA.htm
We have been using it at the 7.5 amp discharge setting and that is sufficient to give us approximate curves that relate to published discharge curves from the manufacturer. One of the greatest failures of our batteries is reduced cell capacity. This is shown on the curves by a two volt drop in terminal voltage during the test. Each of our batteries is serialized when received and we test at least bi-annually to track battery life. When capacity starts to fall of, we relegate the battery to practice only. I do not recommend high current discharge testers of the type normally found in automotive stores. These are designed to test CCA ratings of car batteries and can damage our batteries with repeated and/or unsupervised use.

[mikets]

Al,

Thanks for the info. One question, what is the typical current drawn when the robot is running in competition? I think 20A current draw is not too far away from the typical draw of a four-motor drive train. So drawing 20A for 15 seconds shouldn't cause too much stress to affect the battery life, should it?

[Al Skierkiewicz]

Typical intermittent current draws can go into the 400-500 amp range but are very short duration. The seven amp continuous draw is a good alternative when trying to approximate the manufacturer's spec and maintain battery life. The more you pull from a battery and the number of charge/discharge cycles tends to reduces the battery from about 400 cycles (typical) to something less. I am guessing as low as 200 but with care 300 or more cycles. You can compare Enersys and MK charts and you will see that they differ in curve units but essentially produce the same curves for both battery types. A bad battery will show good for 15 seconds but may die by one or two cells after that.
Check this post http://www.chiefdelphi.com/forums/sh...6&postcount=19 for a plot of several batteries tested with the CBA. The black curve is a good battery plotted against a battery that has two cells die after some time, a battery with an intermittent connection internally and a battery that has two cells die almost immediately. For a short, high current test, all of these might show the same.

[DonRotolo]

A quick review of the test proposal seems good.
I would extend the test period to 1 minute instead of 15 seconds. You'll catch more faults that way.
20A is a good load for these batteries. 7A is good too (that is, 1A is too small, 40A is too large)

What is missing here is your definition of "bad". You are using a State of Charge measurement (terminal voltage) to evaluate battery Condition. While this is a common practice, it is not ideal. A battery with a good state of charge can be in poor condition, and vice-versa. The concern for FRC teams is battery condition prior to attending a competition, and state of charge at a competition.

(some) Modern battery test systems use internal resistance as an indicator of battery condition. I can't get into the details, but Google "Midtronics Battery Tester" to get more info at the Midtronics site.