by mael » Wed Dec 07, 2011 5:18 pm
Although of course I don't know everything about lead/acid batteries, I would like to state how I feel about the process of bringing old batteries back into service.
There is no doubt in my mind that the death of a battery is the deterioration of the positive plates. The negative plates are likely to remain servicable for many, many years because they are basically solikd lead. But the positive plates are built using lead oxide on a lattice made from a hardened form of lead - but it doesn't last long.
On a practical note, I am sure it is easily possible to keep an SLI battery cranking for a decade - and maybe more. Though towards the end it'd better be on a car with an engine that starts easily because slowly those positive plates rot, and bit-by-bit those positive plates have less effective surface area.
The positive plates rot from the bottom up. I suppose this indicates it's probably due to the relatively high concentration of sulphuric acid in the bottoms of the cells due to the higher specific gravity than the less concentrated electrolyte nearer the surface. Stirring by applying a controlled over-charge occasionally might help this situation, as might manually moving the battery so as to mix the electrolyte. I gather some of the most expensive and longest-lasting lead/acid batteries are fitted with a pump which continuously moves and mixes the electrolyte, and that this pump is always 'on.' And those long^lasting batteries will have positive plates which are far-sturdier than the crap batteries mostly in the shops nowadays.
I am sure that desulphating the battery is never a bad thing. Desulphating using (my method) of pulsing will mean the battery will remain useable until those positive plates corrode away - and that could well mean a decade's use or more. So all-in-all desulphating is a great idea. an arbitrary 'battery' will have the life of two, three, four or maybe even more than the normal if it is periodically desulphated and the cells equalised through a controlled over-charge.
Batteries where I am are particularly suited to desulphating because so many people here only use the car for short hops or infrequently use the car, so they tend to sulphate. Batteries in the US, for example, where the car is used day-in and day-out on long drives - are unlikely to be a good candidates for desulphating because the problem with such batteries is likely not sulphation so much as the batteries being physically being worn-out!
- And to an extent, the mixing of (alleged) maintenance-free batteries and wet-batteries is causing some problems:-
M.Free batteries are charged to only 14.1 (or so) volts. This is not a full charge, and inevitably sulphation will arise as a result. More cars now seem to have the battery-regulating circuits set to suit MF batteries. - Though if you fully charge a MF battery it'll soon gas itself dry.
Me? I make holes in MF batteries and replenish the electrolyte with distilled water and they last probably a decade. But most everyone will just throw them away because the process of replenishing a MF battery is hazardous, heavy and dirty. (I can't blame anyone deciding they don't want to fool with a potentially explosive lump of toxic liquid and metal).
The batteries in the US where desulphating would be well-suited (I think) would be special batteries which are not available easily or cheaply that are sitting on under-used earth-moving equipment or classic cars - and boats are likely an area where desulphating would work well.
In my experience commercial desulphators are as good as useless. You need a really good whack with a sharp-edged pulse to do the desulphating in a reasonable length of time (my opinion of course).
* In a pinch you can try a spoonful of EDTA in each cell and afterwards manually adjust the electrolyte to compensate for the loss of effective (non-bonded) sulphur the EDTA rips out and sinks to the bottom. Also you can try a bit of mag-sulphate to get some more life out of a knackered battery. Both the above will poison your battery, but it'll do its job for a bit longer - and that'a a good thing.
I'm not at all disappointed to have found a limit on getting old batteries back in service - far from it! Pulsing to remove the sulphates will mean at least double the life from your batteries at least, and it doesn't cost much in the way of electricity to do it.
What has been won by the brave at great cost can be lost cheaply by fools and once gone can rarely be regained and only then at great cost.