Prius battery NiMh cells puff up a bit during charging - how to stop it...

Puffing means the battery is shot

 

Are you charging the modules outside the OEM clamping mechanism? If so, do you have the side of the modules braced so they cant swell while being charged?

If you're charging modules with no side support, you're seeing exactly what would be expected and will likely need to replace any that have swollen.

If you're charging he modules while they're still in the OEM clamp and you're seeing swelling after only charging 1000mAh, then that module is likely bad. It's also a good idea to always discharge a module first, so you know how much energy it contained prior to charging. It's a good reference for comparing to the other 27 modules.

 

Are you clamping them? You gotta.

 

If you have time to search back through my posts, I've put plenty out there with charger settings. I've used the HiTec 4 channel model #44167(?) and the CQ3 4 channel models. They all have very similar firmware and similar setup menus.

 

The clamp arrangement shown in the photo was a neat idea from Bob Wilson. Along one of those boards he put a couple layers of metal foil with a plastic sheet between, making a capacitor. As pressure within the module increases, squeezing those sheets closer together, the capacitance goes up. Monitoring the capacitance, he could monitor the pressure in the module.

 

I was charging a spare module once with a hobby charger and one of the modules started puffing slightly. The voltage was barely 8.4volts.
I stopped the charge and let the pack cool down. Then put it in a fridge for 15-20 minutes to help it with cooling. The swelling was gone entirely after about an hour.
Once it swells doesn't mean that the pack is shot. The ones in your car would swell like a balloon if there weren't restrained from both ends.
Those packs are getting a charge rate way more than what you used.

 

If you want to charge a module until it swells (and there are degrees of swelling, obviously) and then build a battery with it, feel free. In my opinion, having built one or two, it would be an ignorant thing to do. It may appear normal on the exterior once the swelling dissipates, but is it ok on the inside? Not a risk worth taking for a $25 module.

 

I have a habit of building Gen 2 battery packs using late model Gen 3 and Gen 4 modules from low mileage wrecks. I have one on the bench right now from a 5k mile 2018 Prius. Although the exterior case of the battery is different, the internal modules are interchangeable with the Gen 2. Install it in a customers car at a very fair price with a 3 year guarantee and I also keep the old battery pack. Sometimes I've been known to also buy a pallet of batteries from a salvage yard. Depends on what comes my way, lol.

I have about 12-1300 modules right now and can easily provide you with replacement modules matched to yours (approximate date of manufacture and tested capacity). It's especially easy to match if you're about to actually measure the capacity of the ones you have. I can use those numbers to get an average value for your pack and pick replacements that will fit right in. I've shipped quite a few to other members.

 

Sure, but they are, so they don't. So they aren't subject to the mechanical effects on the electrodes and insulators of having their physical shape changed.

 

I'll be the barer of the bad news.

That is a weak pair. probably don't want to use those two as well as any others that respond like those two did.
When you put modules like those back in the car, the pack doesn't last very long at all, even if by cycling them their specs look better.
Make a list and write down what you get and how long it took for each one of set of two modules you're working on. Number each module and the same number on the case where the module came from.

If all the modules from the cars pack show readings like those you've listed in your posts above ( not typical ), you'll probably need more replacement modules than just the 4 you've already got. You could also put the replacement modules through a discharge charge cycle to get an idea how good they are.

what you're seeing when discharging is what's called a surface charge at 7.6 volts and the 6.1 volts or the 4.5 volts posted earlier is where the voltage starts to hold at it's real voltage level.
It's similar at the top end. Going up to 9.7 volts in a minute is a pretty high voltage. Did you noticed how my xx.x mAh it took the charger to bring the module to 9.7 volts? A couple hundred at most and most likely a lot less than a hundred.

Sometimes when the voltage goes up that high and hits the modules real voltage level it will back off a 1/10 of a volt or two tenths while continuing to charge. It still doesn't really fix a module that shows that kind of behavior no matter how many cycles you throw at it. Or how low you back the charger setting down.

You can lower your charge and discharge settings way down low. It takes longer but you'll get a better feel for which modules are stronger and can handle higher setting and which one most likely will not respond to cycling.

after you run though a cycle or two of each module and write down the voltages pf each module - charge and discharge settings used - time each discharge - and time each charge session you ran per module, well get into measuring each module for self discharge. But worry about self discharge readings later.

 

How long did you wait, after discontinuing any charging/discharging, before loosening the clamp?

Going back to the Gen 1 repair manual (that's a long way back, but it had instructions on disassembling the battery, and the NiMH modules and basic design are still pretty similar), its disassembly instructions include:

 

And keep a fan blowing over the modules while they're being cycled ( discharge / charge cycling ) ...
@PriusCamper I think I read in one of your reconditioning posts that mentioned "heat is the enemy" ....

Cooling while cycling is IMPORTANT ! It can save your butt if any of the cells are really weak and get too hot without external cooling. The same thing that happened to the module while it was being charged by the 12 volt charger, can also happen at much lower charge levels. There are pics of full packs destroyed scattered around the web.

 

One thing I can see that is going to really mess with your voltages is the tiny wire you're using for charging and discharging and all the alligator clips. That's going to cause all kinds of problems and whacked readings. Keep in mind the voltage being displayed by the charger, and used by the charger to control the sequence, is the voltage being seen at the charger, not at the module. A very small wire and potentially high resistance connections means the charger is going to need to "push" a higher voltage to get the setpoint current flow, and will need to drop voltage down low to "pull" the setpoint current out of the battery.

I use 16 gauge speaker wire, crimped "fork" style terminal lugs and fasten them to the threaded terminals using nuts.

The repair manual says to wait 4 hours before unclamping. I can tell you that doesn't work for modules that have just been significantly charged, especially fully charged. I always leave a pack in my rig for at least 24 hours after the last charge cycle, with non-stop cooling flow. Even at that point, if the clamps are removed, the 28 modules will expand enough to make it hard to get the pack back together. I usually let the pack sit for a week or more, sometimes even a couple months. This also allows me to check voltages to see if there are any modules with self-discharge problems.