Battery Preservation

[Al Skierkiewicz]

Ether,
This recommendation is a poor man's fix to get around leaving the battery connected to a smart charger. The smart chargers generally turn off once full charge is reached and won't restart until power is removed or the battery is disconnected and reconnected. The general practice of topping off the charge every couple of months is then simulated every 24 hours. Light timers that allow turning devices on once a week is a better substitute as long as they are non-dimming types. Not stated yet is to store batteries where they won't be subject to temperature extremes particularly preventing freezing or temperatures above 100 degrees. In other words don't store them in a garage or shed.

[Jon Stratis]

Al/Ether -

Based on our experience, the batteries seem to slowly discharge when hooked up to an unplugged KoP charger... could one of those normal 24-hour timers (or 1-week timers) that basically "unplug" the charger then be of any great use?

It's still a great idea, and one I might use in the future with our battery cart. When we built it, we noticed batteries discharging over night when left plugged in. To get around that, we added some appropriately sized relays between the charger and the battery, and set it up so they would automatically disconnect the two when power was turned off to the cart. Simple and great solution!

[Al Skierkiewicz]

Jon,
I suspect that the charger has some leakage in one of the output components and that is discharging the battery. Using a timer to disconnect from the power line likely won't help that. It will restore the charge though when the charger turns back on. If this takes place over several months, it might shorten the life of the battery.

[Ether]

Originally Posted by Jon Stratis

Al/Ether -

Based on our experience, the batteries seem to slowly discharge when hooked up to an unplugged KoP charger

Jon, would you be willing/able to run a simple test? Put a small resistor (1 ohm or less¹) in the circuit (as shown in the attachment) and measure the voltage drop across it² with the charger unplugged.


¹ A 1-foot length of #10 gauge solid copper wire is about 0.001 ohm.

² Make sure you measure the drop across the resistor as shown. Do not include the drop across the resistor's connection to the wires on either end.

[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!