Sunday, April 22, 2012

Standby Battery Float Charger for Generator/Car


I run my generator for about a half-hour every 3 months for maintenance purposes. One thing I have noticed is the starting battery is usually dead by the next time I run it. The generator charges the battery when on but this is just too long between runs without any help for the battery; Instead of running it more often which is a boring and smelly chore, I decided to build a standby float charger to keep the battery topped off all the time.

The charger is a regulated onboard float charger that is powered by a wall wart. This could just as easily be powered by a solar panel or any other unregulated DC source. The circuit is based on the LM317T voltage regulator, and can be adjusted to provide any float voltage for the battery. I bolted the regulator to a scrap heat sink I had and then zip-tied it just below the generator front panel. Other parts are super-glued on. This heat sink it much larger than what is required but is what I had on hand.

The circuit is easy to design from the datasheet: LM317

Parts list:

- LM317 regulator
- Heat Sink with hardware
- 1N400X diode
- 100 Ohm resistor
- 5k Ohm trimmer (anything greater than 1K will work, but anything more than 10K is going to be hard to precisely adjust)
- DC input jack

A couple of things to note:

1.) The LM317 in the T0-220 package (is what I used) has a pinout from left to right of of Adjust, Out, In.

2.) Charging is not a critical application, so the capacitors are not needed.

3.) You need to provide an input voltage  to the charger of at least 3V higher than whatever output voltage you choose at full load.

4.) The float voltage can be calculated based on temperature for flooded  lead-acid battery:

Voltage table for standby use charging:
Battery Temperature Charge Voltage per cell Charge Voltage for 12V Battery Gassing voltage
-30 °C * 2.44 14.6
-20 °C * 2.34 to 2.38 14.04 to 14.28 2.97
-10 °C * 2.32 to 2.37 13.92 to 14.22 2.65
0 °C 2.30 to 2.35 13.8 to 14.1 2.54
10 °C 2.28 to 2.33 13.68 to 13.98 2.47
20 °C 2.26 to 2.31 13.56 to 13.86 2.415
25 °C 2.25 to 2.30 13.5 to 13.8 2.39
30 °C 2.24 to 2.29 13.44 to 13.74 2.365
40 °C 2.22 to 2.27 13.32 to 13.62 2.33
50 °C 2.20 to 2.25 13.2 to 13.5 2.30

I set 13.4 Volts as the charger voltage. I calibrated it with the 5k trimmer resistor which allows you to get pretty accurate - you can use a multimeter at the output to measure it.  You want avoid approaching gassing voltage which is temperature dependent - since this will cause bubbling in the flooded battery and water loss over time - which could expose the plates and cause problems.

The charger is wired directly into the battery and has DC plug so it can be plugged into an unregulated wall-wart or a solar panel. This will charge a battery from dead but very slowly - maximum output of the regultor is 1.5Amps but most wall warts are < 1amp capable. The generator should not be operated while the charger is plugged in, but a diode prevents reverse voltages from being applied to the regulator just in case. The charger will not operate anyway if the generator is on, since the charge voltage applied by the generator to battery will be higher than the float voltage.

I store my generator in my garage, so keep it plugged in all the time now. No more dead batteries!

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