Re-hydrating the battery modules.

I have been experimenting with re-hydrating Toyota hybrid battery modules after reading earlier posts by Bob Wilson. The results on 15 year old gen1 modules kindly donated by UK member "Hybridbatt" are remarkable. These modules have recovered capacity from under 2000 mah to 7175 mah with an internal resistance falling from 0.022 down to 0.013 ohms.

First a word or two on safety the chemicals used to re-hydrate the cells are very corrosive so suitable protection should be worn, and at minimum a large bucket of clean water is at hand to dilute any possible spillage or contact with skin or eyes. It is advised that mixing the chemicals produces lots of heat possibly with fumes dangerous to breath so this should be done outside. The chemicals should be added to water not the other way round. Plastic or steel containers should be used for mixing. Other metals will react with the chemicals, and the chemicals etch glass so this also should be avoided. I used disposable plastic milk containers
There is plenty of information on the web for handling the chemicals KOH or NaOH so I will not fill this thread further with information.

The modules were checked for capacity, and internal resistance. They had already been cycled previously at leased 3 times.
Module 1:- 1950 mah resistance 0.022 ohms.
Module 2;- 1825 mah resistance 0.022 ohms.

I tested the amount of pressure the modules are likely to operate at. It was found with the modules clamped in a vice the pressure relief valves stick somewhat after a period of non use. The pressure required to open them was 120 psi + or - 5 psi measured with both a mechanical, and an electronic digital gauge. After the initial opening this fell to a constant retest figure of 80 psi. This gives a module sidewall load of 1.7 tons imperial at the lower pressure.

The chemicals should be mixed at this point (see the safety paragraph) to allow time to completely dissolve, and also to cool after mixing as this produces plenty of heat.
I used 2 different chemicals to see what if any difference it made, and partly because in my research more than one article implied a mixture was used in the modules.
The first chemical is KOH (Potassium hydroxide) mixed by weight 20% KOH to 80% distilled water adding the chemical to the water!!!!
If KOH cannot be obtained use NaOH (Sodium Hydroxide also know as Caustic Soda or drain cleaner)
This is mixed in the same proportions, and in the same way as KOH. Again produces lots of heat.
In my final results no measurable difference was found using either chemical.

The amount to mix guide:-
Each module takes about 60 ml depending on how dried out it is. Imperial pints of water (20 fluid ozs) takes
4 ozs of chemical, and a total of 3 pints will be needed with 12 ozs of chemical to refill 28 modules.
2 ltrs (3.5 imperial pints) would need 14 ozs of chemical.

Method.
Each module will need to be drilled on the top surface near each end with as small a hole as you can inject into. I used a 1 mm drill. The hole will be tapped for a 1.4 mm screw. Any small size can be used that can be tapped using a fine thread such as BSF. Small holes seal more easily. The drill should be fitted in the chuck so that no more than 3/8 of an inch of the drill protrudes from the chuck (see picture).This is so that when you drill through the top of the module the drill does not come into contact with the cell plates shorting them out.

The positions of the holes are marked on the plan below by the crosses in the circles. You will notice the hole in the left hand end is about 2/3 the way across the module while the one on the right is on the center line. This plan is for gen2 modules. Gen1 are slightly different but the principle is the same. The process exploits the fact that all the cells are joined by a manifold to the pressure relief valve



Lay the module to be filled on it's side preferably on a plastic tray with the offset hole uppermost. Raise the end with the offset hole with a small block about 2 inches off the tray (not critical). With a hypodermic syringe (I used a 20ml version) draw in your mixed chemical and inject steadily into the raised offset hole making sure you do not push the needle to far into the module and touch the cell plates. I found this took about 60 ml before the liquid begins to run out of the hole at the other end. Stop at this point and draw up any spillage with the syringe. Stand the module upright and leave for a few minutes.
At this point I was going to put the module in a home made vacuum chamber, but found it totally unnecessary.
You can now fill the next module. After filling that go back to the first one and lay it on it's side again without raising it on a block. Add more chemical through the same hole as before until it appears from the other hole. repeat the mop up with the syringe then stand the module back up.

At this point I discharged the module down to 6 volts. I then charged the module at 3 amps regulated for
3 hours at which point it was warm (about 100F). This was then discharged down to 5.4 volts (0.9 volts per cell) at a 5 amp rate. The module was then recharged as before and discharged to 7.2 volts at 5 amps for an internal resistance check then continuing down to 5.4 volts at 5 amps then further discharged down to 3 volts (0.5 volts per cell) at 300 ma. These settings are not critical, but when discharging down to 3 volts keep the current low, and it does take some time. Also charge the module till it gets evenly warm about 100F.
The module was charged again and checked for capacity being discharged at a 10.5 amp regulated rate and timed down to 6 volts. This gave a capacity of 7175 mah.
All this can be done without clamping the modules as pressure cannot build with the holes open. A very small amount of fluid may appear at the holes ( about the size of a rain drop) this can be removed with the syringe.

(Edit) With cell capacity information supplied by member "Royfrontenec in a later post" another 1ml of electrolyte per cell has been added to the modules.This is to bring the fluid between 1 and 2mm above the cell plates. This should be added after the refresh charging to avoid the possibility of fluid being ejected while on charge with the modules open. The method is as follows:-

Lay the module on it's side on the plastic tray. Lift the edge with the holes drilled in about 1/2 inch. Inject 6ml of electrolyte into the fill hole (the offset hole). lower the raised edge a small amount at a time until fluid appears at the drain hole. Immediately stand the module up. Job done. Seal the holes as below.

I have sealed the hole with 1.4 mm steel screws as steel is not affected by KOH or NaOH. I have pressure tested the steel screws at 160 psi, and have no leaks. This is double the release pressure of the safety valve on the module, and as high as I was prepared to go with the modules clamped at the sides but no protection around the edges.



I may replace the steel screws with resin screws bonded in with plumbers plastic pipe weld, but really see no reason to do this.
Once the work is completed clean the module thoroughly to remove traces of electrolyte.

I used KOH on one module and NaOH on the other. No measurable difference could be found between them. In some articles researched before starting the work it was said the modules contain a mixture of both chemicals. In theory KOH should have a slightly lower internal resistance because it is more conductive, but it is possible that only the water content of the electrolyte is lost anyway in the ageing process.

There are bound to be things I have inadvertently left out of this post so if you have questions please ask.

John (Britprius)

 

You could refill all the cells with the battery in place but you would have to drill above every cell one hole, and put say 10ml in each.

John (Britprius)

 

I only had two modules to work with donated by PC member "hybridbatt" so I did not have enough modules to try just using water.
To pressure test the module after drilling the holes I plugged one hole with a steel screw and used a mini tyre compressor fitted in a jump start pack complete with pressure gauge. The pack came with a needle type adapter for blowing up foot balls (see picture below). This was a tight fit in the hole drilled in the module. It was then just a matter of turning on the compressor and watching the pressure rise. This was only a few seconds perhaps 15 because of the small area above the cell plates.



John (Britprius)

 

The safety valve had obviously stuck closed on that module. This is why in my experiment I only took the pressure up to 160 psi just in case the same happened. You will note in my test the valves did not open initially till 120 psi. Repeatedly after that the pressure released at 80 psi.

You cannot top up via the safety valve as this sits above only one cell, and only lets pressure out and will not let air or liquid in. The method in my post uses a waterfall principle. When the first cell reaches a level of about half full the liquid flows to the next cell as you keep topping up the first cell. This is why I drilled the fill hole at a slightly higher level, and why I finish with the module horizontal all the cells get to the level of the lower drain hole.

John (Britprius)

 

To get the pressure up to 160 psi I just plugged the safety release hole.

John (Britprius)

 

Yes it would be possible to do rehydrating on a complete battery by standing it on end. The problem is that some electrolyte will spill out when topping up the modules, and it would be difficult to clean the modules to stop leakage currents. If washing with a mild acid would overcome that problem there is no reason why this could not be done.

John (Britprius)