what is a forever battery? a forever battery is a standard lead acid battery that has been cleaned out and alum used as the electrolyte. I have seen other people rejuvenate their dead batteries and so I thought I would give it a whirl to see exactly what is going on.
As a control I purchased a new battery which was dry and had never had any sulfuric acid in it. this allows me to compare power between the two. The old battery had its sulfuric acid removed and was cleaned out with baking soda to neutralize as much sulfuric acid as possible. after a few flushing, New alum electrolyte was added to cells on both new and old batteries. in a mixture of 8oz alum per gallon of distilled water.
Batteries were charged with 20A for 2 hours and trickle charged to full capacity. Since the batteries are now based on alkaline instead of acid, the voltage per cell goes from 2.24 down to 1.6V normal cell voltage, so unless the battery is a bit overcharged it will equalize out to about 8.5V instead of 12.4 Volts for a normal lead acid battery.
A curious observation is that the new battery when placed under a 1Amp load over time would discharge quicker than the old lead acid battery, so there is clearly some latent chemistry going on in the old battery that the new clean battery does not have, I can only assume that some of the lead sulphate on the plates turned back into sulfuric acid during the charge process and thereby increasing the overall amp hour capacity of the used battery.
Both batteries require a tending charge or they will within a week or so self discharge to just a few volts. So if you were to use them for solar lighting out in your yard, they should suffice very well for such purposes and last virtually forever due to the fact that the low acid content would prevent the lead plates sulphating and thereby shorting out. they may work as service batteries if you had enough of them to offset the reduced AH capacity per cell with the use of more cells. Its use would be of course up to you and what you expect form the batteries. It seems to be a great way to get a lot of life out of otherwise useless batteries.
some people have had great luck in using alum batteries in their car for starting and everyday use. although it would mean you should run your car at least once a day to keep the battery charged. It reminds me a lot like the use of supercaps as a battery car replacement.