Battery Preservation

[Jon Stratis]

Ether - That sounds like an excellent test. It looks like radio shack has some 1 ohm resistors in stock in the area, so I can pick some up before our next meeting and challenge my students to figure out how to design a test to determine the rate of discharge if we hadn't added the relays (all of the students who built the cart and helped solve the problem initially have graduated, so this will be a great excuse to transfer that knowledge forward to new students!). Of course, the solution is pretty straightforward (use the resistor to bridge the terminals on the relay, measure the voltage drop across the resistor)... I just hope none of them are following this thread, or they'll come with the answer too easily!

Expect answers in a week... our next meeting is Monday night.

[philso]

It is not likely that the leakage through the output of the charger in it's off-state is more than a few mA, perhaps a few 10's of mA at the most. The 0.001 Ohm shunt resistance would produce a drop of only a few uV which would be pretty difficult to measure without specialized equipment.

You may want to replace the shunt resistance with a DVM set to measure 10's of milliamps. If you don't see anything, set the DVM to a more sensitive range.

Phil

[Ether]

Originally Posted by philso

You may want to replace the shunt resistance with a DVM set to measure 10's of milliamps.

You may damage your meter if there is a larger current.

With the "voltage drop across a resistor shunt" method, all you risk damaging is the resistor. Set your voltmeter to 12v (or higher) range, then make it more sensitive if needed.

[Jon Stratis]

Originally Posted by Jon Stratis

Ether - That sounds like an excellent test. It looks like radio shack has some 1 ohm resistors in stock in the area, so I can pick some up before our next meeting and challenge my students to figure out how to design a test to determine the rate of discharge if we hadn't added the relays (all of the students who built the cart and helped solve the problem initially have graduated, so this will be a great excuse to transfer that knowledge forward to new students!). Of course, the solution is pretty straightforward (use the resistor to bridge the terminals on the relay, measure the voltage drop across the resistor)... I just hope none of them are following this thread, or they'll come with the answer too easily!

Expect answers in a week... our next meeting is Monday night.

Results: We used a 1 ohm resistor (verified by two multimeters). The battery, while connected to the charger (which was unplugged), was measured at 12.811 V. The resistance across the shunt resistor measured 0.002 V with one multimeter, and 1.2 mV with the second.

I have my students do the math at the meeting tonight... I'll hold off in case any enterprising students watching this want to do the math too!