My module rebuilding process.

The reason I wrote this guide is that there seems to be a lot of conjecture and wrong informaiton when it comes to rebuilding the modules in a prius hybrid battery pack.
The long and the short of it is this: the hybrid battery pack, when installed in a prius, does not fully discharge nor fully charge. As a result, a bit of memory effect occurs from years of the batteries being subjected to this range of charge.

You will hear people say NiMH batteries, which are used in the prius, have no memory effect. This is partially true, NiMH batteries will usually not show signs of a "memory" when fully discharged and charged. If you use NiMH AA or AAA batteries in a digital camera, metal detector, or other electronic device, they will usually end up being fully discharged by the device, and then fully charged by the battery charger.

I am not an electrical engineer or chemist, I am not exactly sure what is the nature of what exactly occurs inside a NiMH module when a partial discharge and charge cycle is repeated 1000's of times like in a Prius, but I do know this process of mine greatly restores lost capacity. Will it save every module? Sometimes, sometimes not. But it surely beats spending $1000+ for a rebuilt pack.

Begin by numbering every module 1-28. I find a regular sized sharpie works best.

Things you will need:

1. A "hobby charger". This device is a multi chemistry charger that can not only charge NiMH batteries, but many other types as well. This charger has two output channels, and I highly reccomend getting a charger that has two output channels, or if roughly the same price, two hobby chargers, as this will cut down your time quite a bit. I am currently using the "Ultrapower 200 Duo".

2. A discharger. I reccomend something like the SkyRC BD250, which can allow you to discharge up to THREE modules at the same time. Having a discharger will allow you to discharge modules while the hobby charger is charging modules, cutting the time down further.

3. Aligator clips, leads, solder, soldering iron, etc. You will want to make good connections to the modules. How you do this is up to you. You can also use butt splices if you aren't good at soldering to splice on alligator clip leads, and I personally recommend using ring terminals to make good connections for the discharger. Use the same gauge of wire as the included harness on the discharger, which I believe is 16 gauge.

The first step is to discharge every module completely to 6 volts. We will not be recording capacity ratings during this step as the pack may of been pulled at different states of charge. So you can hook up to 3 modules at once during this step. I suggest not going over 10A when discharging, although these modules can handle much more.

The next step is to charge every module. One thing to keep in mind is that once charging is finished in the battery, excess charge current is gotten rid of as heat. If you have the time to sit and watch, you can monitor modules when they start to slightly swell, and then change your rate of charge. You can charge at a rate of 5 amps until you notice any changes, then change to a trickle charge of .6A. You can set your capacity cutoff to 7500MAh, which is usually all night. Charging at .6A will not cause any heat or swelling to occur. This longer charging is called "form charging" and aids in capacity restoration.

Another way instead of watching for swelling is to apply a multimeter to the aligator clips of the charger leads, when the voltage hits about 8.6-8.7 volts, switch the rate down to .6A

Once each module is charged, let the last couple of modules rest at least 1 hour for the excess charge to "level off". You can then install the software for the BD250 to start recording capacity.

Keep in mind we are dealing with inexpensive electronic equipment, not multi thousand dollar laboratory grade testing electronics. The capacity you will see is probably not accurate and that's fine. What we are looking for is major differences, and the lowest capacity. The battery management system sets the capacity of the ENTIRE PACK at the capacity of the lowest rated module.

You can record these capacities in an excel spreadsheet.

Once you have discharged every module, we will repeat the process again by charging every module back up. But now we are going to be block discharging to 3 volts. Use your nominal 10A or so to bring 1-3 modules to 6 volts, then set the discharger at a rate of 0.6A and bring these modules down to 3 volts. Remember to combine, if you are discharging 2 modules, you will want to set it to 12 volts for 6 volts per module, and 6 volts for 3 volts per module.

Now, if you have a multimeter you will notice something if you have discharged 2-3 modules at a time. Their voltages will not be even. This is because the discharger cannot see each module, and not all modules discharge at the same time. You can use your hobby charger to discharge each module individually to 3 volts, which should take less than 60 minutes per module.

Now that all modules have been discharged to 3 volts, we will recharge them all, and then discharge each one to 6 volts and record capacity. Remember when you are recording capacity to use the discharger on one module at a time. You should notice an improvement over the last time.

Now we will repeat the charge process, and then discharge again to 6 volts, recording capacity. Then you can block discharge to 3 volts again, followed by individual discharging using the hobby charger. You should see another improvement. You can repeat this process 1-2 more times, but all modules should have a capacity of at least 3,500MAh.

As of the creation of this thread, I've seen quite massive improvements discharging down to 3 volts. Much more than can be accomplished via discharging to only 6 volts multiple times.

 

Here is the data. Now I want to mention a couple things. You will see some negative percentages. There's a couple explanations for this, for starters..I've just now gotten over Covid, and my focus was a bit off. So it's possible I put the wrong value in the wrong cell a couple times. Second, I switched about halfway from the numbers from the hobby charger to the numbers from the discharger, and these instruments will probably not measure the same. I also absentmindedly forgot to enter a few values.

This is what I have done so far, I will update it once done. Red blocks are capacity gains below 30%, green are gains above 30%.
You can definitely see the massive gains from going down to 3 volts on the 3rd discharge cycle, as well as form charging overnight.

I didn't have to drill into my modules and deal with caustic chemicals and risk chemical burns or losing an eye to achieve these results either. Or worry about hobby store epoxy holding in plugs on modules that can reach 100PSI when off-gassing from overcharging.

When I get done completely, I'm going to run a 2nd discharge cycle down to 3V on at least 3 modules and see if any meaningful capacity gains are received. If so, I'll run a 4th discharge cycle. If not, I will start the process of reassembly.I'm not going to spend another week if it only nets me another 100MAh or so per module.

 

I also want to make something clear.
My hobby charger, as well as many others, do not perform a constant current charge cycle on modules. It appears if you set it to say a 5A charge rate, it will sweep from 0A to 5A, then back down to 0 and back up again, these cycles are about 15 seconds long.
I am not sure if this cycling is what causes the modules to be cool to the touch the next morning or not when using .6A or less of charge current.

I am providing this warning as using a charger that provides a steady current may produce different results, but I do not have one that does this, to compare it to.

 

So today I woke up to see that module 28 had swollen slightly over the night.
But..this was one of the first modules I tried charging and swelled up as a result of incorrectly charging it.
So I am not sure if a module has swollen once before, if it makes it easier to do it again.
This module is also on the end of course, which might allow it to swell easier.
I have not noticed this in any of the other 27 modules.

 

Luckily module 28 is fine. Let it level off for a few hours and tested it on my discharger at 1C to 6V.
5020 MAh capacity.

Have read in the past on here and other sites "once a module swells, it's bad". I've found this to be quite false. Unless maybe they are talking swelling to the point it's twice as wide as it originally was.

 

Local hobby store is going out of business. Going to try to snag a cheap dual channel charger to try and speed things up.
Hope to be done by end of next week.

 

Here are some times:
Charging each module at 0.5A to the 7200MAh cutoff with the hobby charger generally takes 15 hours.
Two modules are being charged at a time using the dual outputs.
So one charge cycle generally takes 210 hours, or a little under 9 days.
I would suggest having two chargers with 2 channels, or a quad channel charger to cut this time down to 4.5 days.
At 3 charge/discharge cycles, you are looking at least 26 days for charging. 4 channels of charging would cut this down to around 13-14 days.
Depending on the capacity, discharging each module takes about 45 minutes, so 21 hours per discharge cycle, for a total of 63 hours, or a bit under 3 days.
If you had a 4 channel charger or 2 2-ch chargers or 4 1-ch chargers and the discharger, you could probably have the entire pack rebuilt in about 3 weeks.

 

Here is the data as of this afternoon. 4 modules now have over 5000MAh of capacity, or 77% of original capacity, which is quite good for 14 year old modules with almost 200k miles on them.

 

Picked up a Hitec branded charger for $42 at the hobby store going out of business. Reg like $70.
This one has a built in resistance meter, which will be interesting to see how it works.

 

Still trying to figure out the resistance meter function.
Says 122 MΩ on one module, surely that can't be right? Does the MΩ go down when fully charged?
But I did notice this charges at a constant current. Doesn't sweep up and down like the Ultra Power does. Also feels more solidly built.
Personally don't recommend the Ultra Power brand. Fan started squealing after owning it 3 days, and now it randomly stops working, and I have to use a small screwdriver to force it back into action.
Also I noticed for whatever reason, it's cheaper to buy two Hitec 2 channel charges than one 4 channel.

 

The RDX1 Mini.
https://hitecrcd.com/products/chargers/acdc-chargers/rdx1-mini/product

 

All the cells have been done. The lowest capacity one tested at 4020MAh, so I am redoing a cycle on it to see if it will come up or not. 2nd lowest one was 4494MAh and the next lowest one 4494MAh. Most tested to 4700-4900MAh.

Got all the copper bus bars cleaned using a scotch brite on a die grinder. Here is a before and after.



Also primed some of the sheet metal near the battery area.

 

Happy to report on the 2nd cycle, I took the module that was at 4020MAh down to 2.9 volts. After an all night recharge, it retested at 5093MAh.
So I am going to take the other 2 low performing modules and cycle them once more to see if they improve as well.

 

Here is what the 5th cycle, going down to 2.9V has done so far.
Cell 27: 4020~5093MAh
Cell 13: 4494~5050MAh
Cell 16: 4502~4802MAh

I am doing this process to 3 other modules right now. My benchmark for a good capacity is 4800MAh, but would be nice if I can get them all up to at least 5000MAh, which would be 77% of their original capacity.

It is worth noting..that if you have a few lower rated modules, try cycling them a couple more times, much cheaper than spending money on replacement modules.

 

Here is the data as of now. You can see significant capacity gains were made doing a discharge down to 3 volts, over going down only to 6 volts. Way more green than red on the 3 volt discharge cycles compared to the 6 volt discharge cycle.

 

Would I do this process again? Probably not.
I am broke right now and honestly the only reason I am doing this is because I don't have ~$1500 to get a refurbed battery from green bean and the $50 for the lifetime warranty.
Luckily I have a second vehicle because so far my car has been out of commission about a month.

 

Well this is not good. Module 23 went from 4542 to 3194 MAh.

Not sure if it was on the way out and this killed it, or a fluke with the discharger. Going to try it again before replacing it.

Module 10 went from 4758 to 4917MAh.