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Old 06-20-2018, 09:18 PM   #1 (permalink)
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My Prius HV Battery Rebuild Experience/Guide

This is how I refurbished my 2005 Prius’ HV battery and my tips for anyone wanting to refurbish theirs. The car had 136k miles on it when the dreaded Red Triangle popped up. It lived most of its life in the El Paso, TX heat before I bought it (as far as I can tell) and until I moved to northern Utah a couple years ago. The lights all came on just after starting a drive in Dec ‘17. I returned home and plugged in my mini-VCI cable into the car and my laptop to see what was wrong. This of course happened the day after putting the snow tires on and just as we were leaving on a trip to visit family (swapped our luggage over to the Volt and hoped the roads would be nice enough for its all season tires).

Disclaimer: Don’t attempt this repair if you’re not comfortable working with high voltages. While this is a fairly easy and safe (in my opinion) repair for the average DIYer, you can get a couple hundred volts through you if you touch the wrong things at the wrong time, which can be bad for your health.

Here’s the video I followed to remove the battery:
I used two Turnigy Reaktor 250w chargers for the charging and discharging. https://hobbyking.com/en_us/turnigy-...e-charger.html I also bought some banana plugs and alligator clips to make my own charging leads. I powered each charger with an Xbox 360 power supply. I could have powered both chargers with one, but I liked being able to turn them off/on individually by plugging into the wall. Any 12v power supply will work, just make sure it can put out enough amps.

The chargers I used have many different modes and can be used with just about any chemistry. I mainly used the charge/discharge mode and forming charge mode. Charge/discharge will do just that to the parameters you input and you can also have it cycle however many times you want. You can scroll through the mAh used for the charge/discharge for each cycle. Forming charge is a slow overcharge used to get the weaker cells a full charge, which is what’s usually done to recondition NiMH batteries. I used 2 or 3A to charge (I can’t remember for sure which, shouldn’t make much of a difference besides time) and discharged at 2A down to 6v. For forming, I told the charger it was dealing with a 6.5Ah battery and 6s/7.2v.

After a little experimentation to make sure everything was configured correctly, I started doing 3 charge/discharge cycles for each module and recorded the results by taking pictures with my phone and plugging everything into a spreadsheet. I confirmed that my bum module, number 4, was indeed bad, while the other modules all gained some capacity with each cycle (note, I’m using the discharge mAh as the capacity, not the charge mAh). I then did a forming charge on all the modules, followed by the 4th c/d cycle. I saw much higher gains after a forming charge versus just the plain c/d cycles over and over. I continued doing a forming charge followed by a c/d for the weakest modules until they were similar to the better performing modules. I ordered 3 modules for ~$120 on ebay and did a c/d to check their initial health, followed by a forming charge and another c/d. I sorted the modules from weakest to strongest and rebuilt the pack pairing the strongest and weakest modules, then the next, and so on until the two middle performing modules were paired together. I then put the busbars on one side back on and did a final forming charge on each block to get everyone acquainted with their new block buddy.

A few months and ~1000 miles later (my Volt gets most of the miles now) I performed a HV battery check using the Hybrid Assistant app. It estimated the battery capacity at 6.21Ah, not bad for a 13 year old car that was rated at 6.5Ah new.

------- GUIDE -------
Now on to what I would do if I had to do it again. This is basically just the charging procedure that will maximize capacity gains in a given amount of time. Pretty much just what I did, but without the extra c/d cycle I initially performed. I'm not getting too much into the details, they're either self explanatory or in other guides. If you need your hand to be held, this might be over your head (I say this in the interest of safety, not to be rude).
Charger settings: 2A or 3A for charging (probably stick to 2A if your work area is warm to keep the heat down)
2A discharge to 6v
6.5Ah forming charge for 6s if doing a module or 12s if doing a block

All charging/discharging needs to be done with the modules compressed either in the HV pack or another clamping device to keep them from swelling. Try not to charge or discharge adjacent modules at the same time or immediately after each other in order to reduce heat build up.

1. A single charge/discharge for each module to see where they’re at and how many modules may need replacing. Put the little copper busbars in some vinegar for a day or two to remove the corrosion. Don’t leave them in too long or they get nasty, not that I would know…
2. Order replacement modules, preferably one or two more than you anticipate. This will save you time if one of the replacements is junk and give you spares in case modules don’t respond well to the reconditioning. My 12 and 14 modules weren’t the worst originally, but they didn’t improve as much as the rest.
3. Perform a forming charge and a c/d for each module and see how they improve.
4. Continue forming charges and c/d for each module until its capacity is to your liking or it plateaus. I went with 4500mAh for the minimum in my pack, but I would shoot for 5000mAh if you have the time.
5. When you receive your replacement modules, do a c/d to see where they’re at followed by a forming charge and another c/d. Mine were performing similarly to the best of my original modules at that point.
6. Rebuild the pack and integrate the replacement modules. I put the strongest with the weakest because I already had to take most the pack apart to remove the weak modules. If your bad module(s) are on the end, you’re probably fine with just swapping in your new ones instead of tearing the whole thing down.
7. Put the busbars back on one side and do a forming charge on all of the blocks (I can’t remember which side will get the modules paired up into their blocks).

This may not be the best way to rebuild a Prius battery, but I think it’s a good compromise of cost, time, and effectiveness. I only spent about $250 for the replacement modules, chargers, and other charging supplies and I could do it in a week or two with maybe 8-10 hours actively working, mostly checking and changing the chargers.

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Last edited by vskid3; 06-20-2018 at 09:23 PM..
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Old 06-21-2018, 08:59 AM   #2 (permalink)
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Great write up. Thanks vskid.

My 2004 Prius seems to be loosing some capacity last time I drove it. The SOC meter goes up and down a lot more than it used to. Still not bad for a 14 year old car.
Current project: A better alternator delete
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Old 06-21-2018, 12:40 PM   #3 (permalink)
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Originally Posted by Daox View Post
My 2004 Prius seems to be loosing some capacity last time I drove it. The SOC meter goes up and down a lot more than it used to. Still not bad for a 14 year old car.
In hindsight, I noticed that the day before the lights came on. On the way home from getting the snow tires installed there's a pass that gains about 1700ft in 8 miles. The battery usually gets down to 2 bars about halfway up, but that time it only took a mile or two and I think it hit 1, maybe even 0, bars. It also seemed to deplete quickly after startup; it would go from 5 or 6 bars down to 2 or 3 in less than a mile of driving at 25-30MPH. That's about what you'd expect in EV mode, not with the ICE running.
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Old 06-21-2018, 03:13 PM   #4 (permalink)
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Thanks for the info!

I am sure it will come in handy one day.

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