I am no expert, but I have experimented with battery revival half a dozen
times. This is not an expert-opinion but merely sharing my non-expert insights from those
fruitless efforts.
Car jumper battery is probably SLA (seal lead acid) or GEL. 12 volt is actually 6 battery lead acid cells in series each of 2 volt. So 12V car battery is a battery pack consist of 6 cells. My understanding is GEL and SLA are similar in the way they work except for how the acid is "held" inside the cell, as a gel or as a liquid.
When it is below 10V, you are more than 2V from 12V. When you have only 9V, likely, the weakest cell is stone-dead, and at least one more cell is below 2V. The dead cell(s) will work against you when you try to use the remaining "working" ones. (see notes at bottom if you want to know how it work against you.)
In my tests in trying to revive 1/2 dozen of old SLA's (three are from car-jumper packs), the other three from UPS - all are SLAs (not gel) based on visible fluid presence. All but one were dead for > 1year. Some holding voltage at 5v ish, some holding below 2v. The best dead one was last known to be alive and charged about a 1 year ago (before date of attempted revival). That one was holding voltage at 10-10.5v.
- For some, slow (drip) charge often bring it back to 12V. it could hold some charge, but single digit percent of rated. Best case I got from the not-too-dead one (rated at 22AH) was below 1AH.
- for some, it may rise to 12V in mere seconds. It is because the charge it can hold is low so it is "full" quick. Like a gas thank full of rocks, you can fill the tank quick but holds little gasoline. With just a small charge, it drops back to way below 12V really quick -- in sub-second or within seconds when discharge started, it is done discharging (my worst was under 1V, so all cells were dead - upon disconnect of charge cable, self-discharge will bring it back down to 1V within minutes).
- Renewing the battery acid (you can buy that at auto stores) doesn't help... reason below later
- Acid in the battery is sulfuric acid, around 50% pure sulfuric acid if memory serves. Discharged battery start reacting and deposit the sulfur on the lead plate. De-sulfation with those "pulse desulfator" modules does work but just very very little - it gives a flash of life (a few percent more) for a cycle or two than back to darkness. Further de-sulfation will do less and less... same reason, below
- It appears to me that the lead-plate is too well coated with sulfation. Pulse is not going to "break it up". One could consider sanding it off (as some battery re-manufacturers may do), but that is an extensive job and a rather dirty one. Then there is also the consideration: is there enough lead still left on that lead-plate to be useful.
So, all in all, it is not worth the effort. My best was "revive" a 22AH rated battery to give me less than 1AH (At below 10.5V, I terminate and sum up the numbers), but it would be easier and more predictable to use a few AA or 18650 batteries for that.
Notes: How the dead cell work against youIf you want to understand how it works against you: Draw a diagram of 6 cells and a load (so 7 boxes) with a line joining the boxes and the load in a circular chain, write down the + and - on each box that represents a cell. Now follow the current in the circle, you will see that on a dead cell (regardless of which one of the 6 you pick as bad), the current flow for that box will be reverse charging it. It can't do much "push against you" since it has little charge, but it certainly will "resist" your effort.
The old post here I wrote on that desulfation test:https://www.eevblog.com/forum/chat/trying-out-battery-desulfator/Edit: Reviewed my old thread and correct a few numbers I type here. (Number typed here came from memory whereas the old tread was from actual notes at the time, so the old one is accurate). Yes there were 6 total. My old thread said 3 because the other 3 is so "stone-dead" I did not seriously tried to revive them. It did get the desulfation but no serious notes was taken after seeing how little charge it held.