Battery Charging

Motorcycle Battery Charging

I'm going to make a battery charger to trickle charge my motorcycle battery. I understand (from from this excellent article by David Searle) that a voltmeter connected across the terminals of a conventional lead acid battery should show between 12.3 and 12.6 volts on a 12-volt battery. Therefore, I may use a PIC chip's ADC to monitor the voltage level and use this as my guide.

It doesn't need to maintain the battery at 100% charge, just make sure it sits somewhere between, say, 80% and 90%, which allows me a margin of error in my design. Similarly, and again to protect the battery against my potential incompetence, I'll use a very low current like 150mA.

If I do indeed choose to use a PIC chip, then this will give me further options of controlling the voltage or current in response to the current state of the battery. This could allow, for example, a configurable charging profile based on the capacity and type of battery.

Another good article is at

4V Sealed Lead Acid Batteries

  • Open-circuit (quiescent) at full charge: 4.2 V to 4.27 V (2.10-2.135V per cell)
  • Open-circuit at full discharge: 3.93 V to 4.0 V (1.96 V to 2 V per cell).
  • Loaded at full discharge: 3.5 V (1.75 V per cell).
  • Continuous-preservation (float) charging: 4.47 V for gelled electrolyte; 4.5 V for AGM (absorbed glass mat) and 4.6 V for flooded cells
  • Precise (±0.017 V) float voltage is critical to longevity; too low (sulfation) is almost as bad as too high (corrosion and electrolyte loss)
  • Typical (daily) charging: 4.73 V to 4.83 V (depending on manufacturer's recommendation)
  • Equalization charging (for flooded lead acids): 5 V for no more than 2 hours. Battery temperature must be monitored.
  • Gassing threshold: 4.8 V
  • After full charge the terminal voltage will drop quickly to 4.4 V and then slowly to 4.2 V.


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