Lead acid batteries. A novice embarrasses himself.

[SeaMonkey]

Mael,

I hope you're still putting your wonderful curiosity and creativity to useful purposes!

Lead-Acid batteries are truly amazing devices that respond very well to rejuvenating, particularly by the "pulse-discharge" charging method described by Bob earlier.

The crowd of "desulfators" is a growing community with much information available at various locations on the web and several forums for exchange of experiences and ideas.

The type of circuit used is largely a matter of personal preference and is responsive to extensive modification and experimentation.

Many start by building their first circuit around the 555 timer chip and that does indeed work well. I prefer the simpler CMOS pulse generator circuit which uses either the CD4001 or CD4011 chip as I find it to be more tolerant of overvoltage, up to about 18 volts at the input, and somewhat simpler with fewer parts.

The MOSFet switching device used can be one of the many with appropriate voltage and current characteristics and is certainly not critical. In a pinch even a bipolar transistor can be used provided the circuit is able to provide sufficient base drive.

I use the IRF540 variety as they are very inexpensive and have a suitable voltage rating and a decent Rds ON.

I've had good success in restoring old lead acid cells that had been sitting in a state of non-use for at least five years. A heavily sulfated cell will generally show a "charge voltage" of up to 12 volts or more when the process first begins; this will very gradually decrease to around 1 volt per cell and then slowly increase to a normal cell voltage of about 2.4V indicating that the process has finished for the "first run."

This means that when desulfating a 12 volt battery the initial voltage measured across the battery in the beginning will be very high - 30 to 50 volts is not unusual. This will gradually decrease as the desulfation progresses and will dip to lower than normal, 10 volts or less, for a time before it begins to increase to normal full charge voltage around 14 volts.

Test discharges followed by additional pulsing charge cycles will improve the condition of the battery and its ability to hold useful charge.

It is a very slow process and the cells can generate a lot of heat in the early stages of desulfation. It is also very rewarding when you begin to see signs of life in the battery or cell.

The pulse frequency is a minimum of 1 kilohertz for good effect, and some operate as high as 10 KHz or thereabouts.

Some circuits employ capacitive discharge (approximately 50 volts) with a drive pulse to the MOSFet of about 1 microsecond.

The circuits which employ inductive discharge will drive an inductor of 100 microhenries to about 500 microhenries with a pulse to the gate of the MOSFet of from 10 to 40 microseconds. If one has a scope available it is possible to "tune" the circuit for best output just short of inductor saturation by adjusting the pulse width while observing the pulse amplitude.

Without a scope a simple peak voltage detector (diode and capacitor) is used with a voltmeter to monitor the output pulse amplitude for adjustment.

Search the web for further info and details - you'll find quite a lot.

Once you get started with desulfation you'll have a good "new hobby" to keep you busy!

[mael]

[SeaMonkey]