I will admit to being a little skeptical about the capacitive charger for 2 basic reasons.
1) batteries can readily explode when too much potential and current are applied to them.
2) it just seemed to utterly simple.
On the other hand I knew there was a reasonable chance it should work because batteries have a capacitance as well as a resistive factor to them. based on that I proceeded with the simple circuit below:
I knew the resistive chargers worked because I have used them on and off over the past 35 years or so. They have an established track record. Primarily the Reverse Current Charger, the Alkaline charger was simply a new approach with an old design. So I was willing to give it a shot. In part because the capacitive charger also tends to desulphate a battery as it charges.
My preliminary battery chosen was a motorcycle battery that had been sitting outside over winter, and was likely 4 to 7 years old and therefore an excellent candidate for sulphated plates. The charger itself is nothing more than an industrial bridge rectifier I could only infer was a 9 Amp 900 volt PIV as I could not locate a datasheet. The capacitor was a 250 VAC, 3 microfarad capacitor- non-polarized. That was it. Not counting cord or wires. The capacitor was wired to the line side of the mains power, in the event of a failure, it was the shortest potentially exposed path for AC current in the event of failure.
Ideally in a failure situation it would ground to the metal case I used from an old printer power supply and blow a fuse. At worst, it would hang free inside of the housing.
After the initial 90 minutes of the first test, the battery read just over 10 volts.
After 6 hours of charging the battery read 11.1 volts.
After about 14 hours it was up to 11.9 volts.
Further charging did not change this, so it is safe to asume one cell likely shorted from dendrites. If it will start lawn tractor, vibration from use might break up the offending dendrite. Otherwise it may see service powering radios if otherwise healthy.
I suspected the shorted cell, due to the low voltage after 6 hours. This was borne out by the final voltage after 22 hours on the charger.
The next test will be some golf cart betteries that are so sulphated they will not take even a minimal charge off a conventional charger. As I write this update, the first golf cart battery is under charge. There is a significant risk of explosion with these due to some unintentional rough handling. There are a number of other batteries in the lineup for testing as well, including potentially some fork lift cells that also sat out over winter. All of these will be updated in posts as the results are known.
Also to be tested with smaller capacitor values will be Alkaline cells NiMh and NiCads.
I may construct a second charger with aproximately 8 microfarads of capacitance just to speed up the charge rate for the larger batteries. The capacitor is what limits current through the battery. Using larger values of capacitance raises charging current, and reducing capacitance reduces current.
One thing I have noted is while a charging battery will have its’ plates expand; for the amount of charging this motorcycle battery has experienced so far, the plates have expanded at the same or a slightly faster rate as the electrolyte has been reduced due to the electrolysis of the water in the electrolyte. In short, the electrolyte level has increased within the cell.
In short- the capacitive charger seems to work rather well in the initial test and holds a lot of promise. I do not know how well it would work on European 250 VAC standard which is also used elsewhere in the world like Japan. I would think a suitably sized resistor of sufficient wattage could be emplyed to create an adequate voltage drop across this circuit, and I do think it could work but I would be inclined to use a transformer to reduce the circuit voltage to around 120/125 VAC rather than the resistor. I do not believe the frequency difference of the two standards is an issue.
In areas where 20 and 25 Hertz AC is in use, I do not know how effective this circuit would be, but I do believe it could still work, and I would be inclined to favor larger capacitor values by a factor of about 2. The dry polyester/mylar types are preferred over the oil filled types.
If 400 hertz AC is the only AC available, a smaller capacitor value should be used, and expect a moderate failure rate of capacitors unless the capacitor is suitable for long term pulse or RF application. Power factor correction types “might” work, I would tend towards using the dry types as opposed to the oil filled types to reduce fire risks. I would be inclined to reduce capacitance by at least half or even one fourth.
This is yet another charger style that fools and idiots need to avoid- this one can readily kill you if you do not excercise caution. With all of these, and ideally with any charger, situational awareness will avert most issues that might occur. with it or cause a fire. This is a charger that will never earn any CE/UL safety stamps.
If you build this, you accept all responsibility if something goes wrong. It is a reasonably safe charger in competent hands. If you are not a careful individual, do not build this.
It is a fast solution that has the potential to be cost effective. The drawback is the inherent danger of being connected to a high enough potential that can easily kill someone who is careless.
If you have a wind generator that has the ability to be tapped at a point for single cycle/phase AC, that tap can be used as the AC source for this charger. Lower potentials reduce the energy envelope, but does not eliminate the desulphating ability. It just means a longer charging period.
More battery chargers and desulphaters to come, as well as some other miscellaneous projects as well.