On Wed, 12 Oct 2005 19:35:30 +0100 "Dave Plowman (News)"
wrote:

Think every car alternator ever made will limit the current to a safe
value for it.

I think you're right, but I'm not sure what one would do if run into a
dead short. There's probably some minimum safe impedance that they
expect.

Indeed, even early ones had voltage limiting which meant
they simply wouldn't recharge a sulphated battery with a high internal
resistance.

A sulfated cell does appear to have a high internal resistance, but
that's not the whole story. The reason it looks this way is that the
lead plate, in the process of discharging, has been turned to lead
sulfate, taking the sulfate ions out of the sulfuric acid solution.

The lead sulfate (a solid) is deposited in some particular state, but
slowly changes crystalline form over time. Unfortunatley, the latter
crystal form is not nearly as soluble as the one originally deposited,
so when the current is reversed, you can only remove the sulfate ions,
taking them back into solution, as fast as the new crystal state will
let go of them.

Unfortunately, that's slow.

You can push the voltage up, but that doesn't increase the solubility.

In the end, increasing the voltage just quickly puts you above the
point where hydrolysis occurs, and this just turns out to be a waste
of energy, because it does nothing toward recharging the battery. High
voltage, and hydrolysis, also tend to damage the porous sintered
battery plates.

If you want to reverse sulfation, you just have to set the voltage at
about 2.35 V per cell and wait for the ions to come back into
solution. If you're patient, it usually works.

I rather regularly recover sulfated batterys and it generally takes
about a week. I had one which took 2.

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Jim Adney
Madison, WI 53711 USA
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