Home > Circuits > Motorbike Battery Charger
Modified:22:06, 07 December 2013
I am a rather proud owner of a Honda VFR800 motorbike and although on the whole I would say it's a pretty good bike, it does suffer from battery issues. It's about 10 years old but has only done 9000, most of them by me, but I am guessing the battery is on its last legs. It does have an alarm on it and after a 3 or 4 days of non use, the battery is close to flat.

So it was an opportuinity to build a quick charger that would charge a 4 A/hr battery for a fixed time and then switch off. That way I can leave it on charge the night before I want to use it.

Nothing to do with the charger but I used to have a GSXF750, and I know some people think they look a bit 'bulby', but they are awesome on Alp mountain roads and long winding valley roads, The handling was fantastic and very confortable.

The battery is a 4 A/hr type and thus will need about 6 A/hr to fully charge it. With a 12VAC transformer, the rectified voltage is about 15.4v but this tends to fluctuate depending on the loading. But when connected to the bike battery, it charged at about 1.2A on average through a 0.47 ohm resistor.

This charger is only meant to be a top-up so the timer is set to about 2.5 - 3 hours. The capacitor C3 was formed from 2 470nF ceramic dipped in parallel. It was close enough.


The circuit is designed to run from a 12 VAC power supply I had laying about. So the power needed to be rectified and smoothed. The smoothing capacitor C1 is quite large and had to be strategically placed on the PCB. The timing is generated by a 14 stage binary counter, the 4060/ This also has the facility to generate its own clock using C3 and VR1. The astable frequency can be adjusted by VR1. On power up, the counter is reset by C2 charging and pulling the 'Reset' high for a few ms. The output Q14 is low and switches on the MOSFET Q1, which is a P-channel IRF9530 type. This charges the battery through R4 to limit the current. When the counter sets Q14 high, Q1 switches off. Diode D3 also provides a voltage to the clock input of the counter, effectuvely freezing it and halting all further operation.

Designed and Written by Phil Townshend
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