12V Battery Maintenance

[eugenebrooks]

Originally Posted by cbolin

Hi,
How do you guys maintain the 12V batteries?
Do you put a load on them and measure voltage over time to determine if they can 'hold' a charge?
Have you built a load tester?
If so, what did it consist of and conceptually how did you use it?

Regards,
ChuckB

If you want to get moderately serious about sorting out your batteries you can build a reasonable "capacity" tester using the robot controller, a spike, and a high power load resistor from Digikey. Last years RC can be used for this, or you can do it with the EDU using a suitable relay wired to a solenoid output.

You can use a one ohm, 225 watt resistor, part number FVT200-1.0-ND costing $12.36. The mounting bracket for this resistor appears to be B1001-ND, you would need two of these per resistor.

At 13 volts, and the battery won't be there very long, the resistor will draw 13 amps and dissapate 169 watts, within spec. It is also within the spec of the current limit for the spike. Pointing a small muffin fan at the resistor will be a good idea!

If you want to load at higher currents, you use more than one spike controlled resistor in parallel. The concept can actually be taken to a rather insane level using high power bipolar transistors to load the battery with a constant current independent of battery voltage, and programming the load current in binary using small relays to drive the transistors. A serious senior project, indeed. Some of us are quite crazy...

You sense the battery voltage through a voltage divider hooked to an analog input. You measure the time that a fully charged battery takes to drop to a specified voltage, drop the load, and display the time in suitable units. A reasonable choice for voltage limit, examining the discharge curves for the battery in question, would be 11 volts.

The task of programming the RC and calibrating the voltage sense for the analog input is not difficult. It is a nice pre-season project that will "get used" to sort out your batteries. A setup using one load resistor and a voltage divider is cheaper than a battery, assuming that you have last year's RC and a spike laying about.

You can calibrate the measured capacity, the time to reach 11 volts, by checking the new batteries that come in this years kit (after charging them over night). A good rule of thumb is that any batteries that are lower than %75 of the capacity measured for the new ones should be used only for practice, but you can come up with your own rule of thumb based on your own experience with your batteries.

These batteries don't take much maintainance as their self discharge rate is low, but you should fully charge them before storing them, store them in a cool dry place, and give them a charge when taking them out of storage, before using them.