Can I replace 1st generation battery cells with 2nd generation battery cells?

People have built out Gen 1 batteries using 38 modules from Gen 2 batteries (meaning you need more than one Gen 2 battery to pull from, because there are only 28 modules in a Gen 2 battery). Working at the level of modules turns out to be tidier than individual cells.

I don't think the modules are exactly the same size; they are a smidge longer, which might lead to some creativity in getting the cover back on.

 

This is what I know, about six months ago I had my mobile mechanic who specialists in hybrid batteries come out because I was having problems with my 2002 Prius. He told me that if enough of the cells (they were Gen 1 cells) were bad he would put in all Gen 2 cells and then charge me extra of course. Only one cell was bad so I didn't need the Gen 2 swap. So based on that experience I would say most likely you can use Gen 2 cells (hes been doing hybrid repair for many years, knows his stuff and my car is fixed again). I'm leaving a caveat that don't yell at me if its not the case, lol, just sharing my experience. Good luck.

 

David,

You and Chap are correct. The Gen 1 modules can be replaced by the Gen2/3/4 modules, but to do the job right, all 38 modules need to be replaced. Dimensionally, the modules are all the same thickness and height. The difference is the width(or length, depending on how you look at it). The bottom of the modules is the same, and they match bolt patterns for mounting on the steel base plate. Gen 1 uses 2 mounting bolts per module and the other Generations use only one. The upper portion, where the (+) and (-) terminal lugs are located is slightly wider on the Gen 2/3/4 modules. That's the main reason you cant mix Gen 1 with 2/3/4. The busbar harness will not sit flat and will not have the right spacing due to zig-zagging in and out. So, the upper portion of the Gen 2/3/4 modules are a bit wider, but they will fit under the steel top cover. There's just a bit less clearance between the black plastic terminal covers and the inside surface of the steel cover. The other issue that arises is that the interlocking "nubs" on the modules are different on the gen 1. This causes a problem only for the outer 2 modules, as they must mate to the steel clamp plates on each end of the Gen 1 case. The steel plates are made to match the Gen 1 modules, not the Gen2/3/4, so the "nubs" don't match and deform the Gen 2/3/4 modules a little.

 

Right. A cell is the smallest unit of a chemical battery, always producing a certain nominal voltage that's characteristic of the chemicals used. For NiMH chemistry, one cell is nominally 1.2 volts.

The modules in a Prius produce 7.2 volts nominal, so by simple division you know each module has six cells in it. You can see the six cells if you cut a module open, as seen here.

But no one cuts up the modules and replaces cells. Messy. And the stuff it's messy with is potassium hydroxide, which will eat you.

 

Be careful with Gen 4 modules. You can only use the NiMH modules from a Trim 1 or 2.
2 Eco and above use Li-Ion modules.

 

A grid charger is simplest, charging & balancing the whole pack. @jeff652 designed a good one. I do not see a discharger for the Gen 1 Prius though to totally recondition the pack.

Prolong® Battery Charger - Hybrid Automotive

 

see the battery rebuilding threads. they use hobby chargers and light bulbs to charge and discharge the individual modules.

 

I was babying my batteries at first and could not get results. The Dr. Prius app has a life estimate test. It hard charges the battery pack first: Park, foot on brake, Drive mode, full gas until fully charged. Takes about 5 minutes. Then they have a discharge process to follow for about 15 minutes. The charge process forces current into the pack hard stressing the batteries, that rejuvenates them. Not something you want to do a lot but maybe once or twice a year. MOST IMPORTANT is examine the pack. Humidity will collect and corrode the copper plates between the batteries and will drip into the battery ECU (orange connector). The water will corrode the sense pins shorting them together.
That at first will mess up the ECU ability to manage the pack and as the corrosion progresses, start a fire at the ECU connector....
Lots of youtube vids on removing the pack and examining the elements. good luck, be safe....

 

I just thought I'd mention some successful repairs to the Gen 1 Prius traction battery. I bought my Gen 1 Prius in 2001 (I'm UK based) and sold it to my brother in 2009 who has had it since then, he doesn't do much mileage. In October 2024 it came up with the battery warning light. Fortunately I knew a guy Chris Read who lives locally to me and has great experience with Prius traction batteries. Initially with his diagnostic equipment he was able to determine that one of the modules (no.5) was faulty. Each module consists of two cell stacks, each cell stack with a nominal 7.2V but when fully charged can be above 8V. Chris ordered two cell stacks from ebay and on stripping down the HV battery pack found that in fact only one of the cell stacks in module 5 was faulty, that was replaced with one of the good replacement cell stacks (which subsequently by looking at the manufacturing date code I found had actually been made in December 2010, even though production of the NHW11 stopped in 2004, so the cell stacks were presumably from a later Toyota manufactured factory replacement spare HV battery). The car then ran well, putting on a further 1000 miles, before the warning light re-appeared in January 2025. Once again due to a faulty cell stack, this time in module 15. Chris replaced the cell stack and the car was fine again until the warning light re-appeared in June 2025 (this time the faulty cell stack was in module 16). Another cell stack replacement was needed in August (module 7). All good until the end of September 2025 when the warning light re-appeared. At this point it became clear that replacing individual cell stacks was no longer an option as (a) the original (manufactured December 2000 according to the date code) ones were successively failing one by one, and (b) there are no currently available gen 1 cell stacks available on ebay anyway.

So in October last year Chris and I made the decision to see if he could replace all the gen 1 cell stacks in the HV battery housing with gen 2/gen 3 cell stacks, which there are plenty of available on ebay, a Prius "heart transplant" so to speak! I'd done some research on this Prius chat forum and it seems that a few people have successfully done this. The gen2/gen 3 cell stacks are superior to the gen 1 ones in several regards. They have the same nominal voltage and 6.5Ahr capacity as gen 1, but the sealing of the cell stack terminals is improved (gen 1 sometimes suffered from electrolyte leakage from the terminals, which is why Toyota issued a service recall for the NHW11 for their less than successful application of epoxy sealant), and also gen2/3 have better thermal properties as the sides of the cell stacks are metal for better heat transfer rather than plastic as in gen 1. HOWEVER there is one big complication in that although the thickness and height of gen2/gen 3 cell stacks is the same as gen 1, the length of them is 10mm greater. The Prius chat forum said that this meant that refitting the top HV battery cover was a very tight fit, though doable.

So we decided to go ahead. A local recyclers near to me was selling an HV battery from a 2008 gen 2 Prius which I bought for £400. That would provide us with 28 gen 2 cell stacks once Chris had disassembled it. The gen 2 Prius used 28 cell stacks, the HV battery in the gen 2 had a nominal capacity of 7.2Vx6.5 Ah x 28= 1.31kWh. The gen 1 Prius however used 38 cell stacks for a nominal capacity of 7.2Vx6.5Ah x 38= 1.78kWh. So I bought 10 additional individual cell stacks from ebay for an additional £173.60, making a total parts cost of £573.60.

Before attempting the "transplant" Chris and I had measured the voltage of each cell stack, the 28 ones in the recycled gen 2 HV battery pack were all about 7.65V so with a NiMH charger I have I made sure that the additional 10 were all at a similar level (of course while charging the individual cell stacks I had them under compression). When Chris and I got together at my brother's place to start the work we were pleased to find that they were all within 0.1V of each other, so the cell stacks were well balanced.

Chris spent all day on the various tasks, disassembling the Prius NHW11 HV battery, removing the old cell stacks then replacing them with the gen 2 ones and refitting the HV battery cover, as anticipated this was problematic as the resultant "bowing" of the top cover meant that the fastener holes didn't align well and needed somewhat forceful re-alignment to engage. Chris then re-installed the "transplanted" HV battery back in the car, connected up everything and then the moment of truth, try starting the car! To everyones relief the car started first time, no warning light appeared and the battery took a very healthy charge as the engine ran. We did this a couple of times and by the time Chris left the battery was in a state whereby after the engine stopped it didn't restart again for anothe 19 minutes, indicating a very healthy battery.

This was done back in October last year, and 6 months on everything is running fine. I'm hoping the car will coontinue running well for many more years. I really like the Gen 1 Prius, there were only ever a few hundred sold in the UK between 2001-2004 and it must now be an increasingly rare "Classic". Incidentally the original stainless steel car exhaust system still passes the annual MoT check required in the UK, not bad after 25 years!

 

Right ideas, wrong terminology. "Module" is the term for a single 7.2V unit of six cells (what you're calling a "cell stack"). The term "cell stack" isn't used. The term for a pair of modules with a single monitoring connection to the ECU is a "block".

There are 14 blocks (hence 14 voltages monitored by the ECU), meaning 28 modules, meaning 168 cells.

Your guy found that one of the modules making up block 5 was faulty, and so on.

Glad to hear your gen 1 is still going strong. I miss mine, and probably would still be trying to nurse it along, if it hadn't tangled with a Dodge Dakota.

 

I'm also proud of my temporary HV battery repair. I bought a Prius with a damaged battery. One cell had literally melted on the side (number 37, to be precise). I decided to quickly repair the batteries by replacing one cell with a newer type. In the photos below, as you can see, the new cell (at number 38) isn't perfectly centered, but slightly offset. I tried to deform the bus bar as little as possible. There was already a wire on the other side. I also ground down the locating pins on the adjacent cell. Of course, all the cells were balanced. I had the repair done in January and since then the battery has been working without any problems, although it is clearly lacking capacity (current Delta SOC 30%). I plan to experiment with cyclic charging and discharging of the cells (I already have two CQ3 chargers). I suspect it's pointless with such old cells (this is a European unit from June 2000, where production started in May), but I'll do it for fun and I'll be happy to share the results and how much life such cells have left.