got some young 2012 modules

Certainly do your research. The real answer is when you charge them all to the same voltage and then do the parallel load test.

If I remember correctly you know how to parallel wire and charge them up to 8.0 to 8.1 volts. As a sanity check, take them off the charger and see what sort of 'self-discharge' you see.

Self discharge should not be much but it is an unknown load. The discharge resistors will be the true test.

Bob Wilson

 

Hi Jeff,

In a perfect world, we would test each module, mark its capacity, and then order the pairs. But I got the impression FAST was important to the OP:

Bob Wilson

 

So let's go over the electrical stuff:

• 8V / 68 ohms -> 0.118A
• 8V * 0.118A -> 0.94W
• 7.5V / 68 ohms -> 0.110A
• 7.5V * 0.110A -> 0.83W

Working close to the designed power rating of these resistors, make sure there is 'free air' around them. They will get HOT and you want to have some gloves or oven mittens handy. When they first heat up, you'll probably smell a 'hot electrical' smell but as long as they don't discolor or char up, don't worry about it. This is what it is all about.

Now the discharge curve of NiMH batteries is fairly flat. For a first approximation:

• ~0.114A/hour
• ~8.8 hours, 8 hrs 46 minutes is 1 Ahr

Just spot check them to make sure they are all staying above 7.5 V. If one reaches 7.5V, it is OK to let it go down to 6.5V but never let it fall below 6.0V. If these were brand new modules with a full 6.5Ahr capacity, discharging 1Ahr should bring them down to:

• ~7.75V ## Great!!

I'll work up the spreadsheet tonight and post it to the thread. But understand these are 'back of the envelope' numbers. I would recommend spot checking against your RC charger, three weakest and three strongest. This just gives confidence that the '3d world' engineering approach has given usable metrics.

Later,
Bob Wilson

 

Yes. You have the modules and know how to parallel charge them. Git'er done while waiting for the resistors.

The parallel charge and using matched resistors for controlled discharge with a clock, these are basic, applied physics. But you also have a smart RC charger and it makes sense to compare what basic physics reports versus the RC charger.

Bob Wilson

 

Comments inserted
* * * * * commented posting * * * * *
So when I get the resistors ill parallel charge to 8.1v (probably 8.16v and let it bleed off to hopefully 8.1v?) let the pack rest for 8 hrs, (an hour or two would fine but use the rested voltage as the start) record voltages and do the discharge with the resistors and check it with a voltmeter once every 45mins or so up until about the 8 hr 45 minute mark (the longer the run, the more accurate the numbers but yes, closer to 9 hrs). then record voltages and remove the resistors(or should i remove resistors then record voltages? also any rest time before recording voltages after removing resistors?) (remove resistors and take the measurements there will a small rebound but go run some cold water over your blistered fingers and then measure the voltages . . . the resistors will be HOT).
but once i get done the discharge test and record voltages and post the values here. ill put the 3 strongest that you say through a discharge/ charge, discharge charge cycle, record voltages and mah.
then do the same with the 3 weakest correct (this is only to get a second opinion, a scaling, between this manual process and the r/c charger, I would trust the discharge metrics but this is to give you and everyone confidence that the two match).
would that be considered the spot check with the r/c charger? (correct, an optional step.)
* * * * * commented posting * * * * *

I've attached the promised spreadsheet inside a zip file. All of the columns in italics are inputs and will calculate the expected Ahr capacity of the modules. Now the longer the discharge, the more accurate the estimated Ahr capacity. Just monitor the voltages over time to see if any of the modules show discharge voltages falling below 7.5V. You really want 6.0V to be the absolute lowest ever seen and even then, it is not good.

Bob Wilson

 

Gloves are good! These resistors are going to be just under their 1W power rating so they are going to be HOT!

As for the rest period, I don't know a hard value to use. You might find something on the internet but certainly give the a pause. My thinking is don't be in a rush. Put the resistors away, drink a cuppa coffee, look at the spreadsheet and 'go to town.'

Bob Wilson

 

My guess is the 28 modules came from at least three donor battery assemblies. That doesn't mean that they are not OK.

 

Yes, that is a good sign but until the modules are put under load, who knows whether they are matched.

 

Can I assume that you've compared the battery modules from the Prius v to the modules from the Prius c, and have found them to be identical with regards to physical form factor:

1. height/width/depth of the modules?
2. Do the terminals come out of the battery case at exactly the same height?
3. Have you tried to fit the new modules into the Classic Prius battery case to see if any modification of the sheet metal will be required for the modules to fit?