got some young 2012 modules

GOOD! This means you aren't under a time-gun to get things ready. In GOOD, FAST, CHEAP, pick two, you can get GOOD and CHEAP.

Sounds good. But understand your buying the modules, not the capacity. The actual module capacity is going to be a function of the prior usage and there are no guarantees. But the good news is:

• later modules have lower internal resistance - lower heating during operation
• much improved terminal seals - they are better able to handle heat and pressure, less likely to leak like the original NiMH modules

Ok, if you choose fast, the process:

• parallel charge to get them as close to the same SOC (State of Charge)
• consider a timed, discharge, "n" identical load resistors (see Radio Shack or eBay)
  • 1/4W, 256 ohm or higher, one resistor per module, 0.031A initial rate (32 hr ~= 1Ahr)
  • 1/2W, 128 ohm or higher, " " ", 0.063A initial rate (15 hr ~= 1 Ahr)
  • 1W, 64 ohm or higher, " " ", 0.125A initial rate (8 hr ~= 1 Ahr) (*)

The protocol is to use parallel charging to bring all of the modules to same voltage. Prepare the resistors, one per module, with hookup wire to connect to the modules and have them ready to go. Make sure the resistors are as close to the same values as possible. If any are low or high, put them aside and get a closely matched set. The following has to be done WITHOUT INTERRUPTION:

1. Isolate the modules after parallel charge
2. Put a load resistor on each module.
3. Start monitoring the voltage as they discharge
4. After an hour or no more than the 1 Ahr limit or when one module reaches ~7.5V, remove the load resistors from all modules

Let them 'rest' and measure the partially discharged voltages. This gives a relative strength of each module so you can pair the strongest and weakest.

The Super Brain can give you metrics, numbers, to use for a module but doing all 38 is going to take time. The procedure I outlined before allows a 'parallel' metric to support pairing of the strong and weak modules. Only you can determine if there is enough time to test all modules:

• discharge to 6V
• charge (record this Ahr)

For the faster procedure, it makes sense to take one or more weakest modules and use the Super Brain to get quantitive numbers for worst-case, Ahr capacity. Two or three would give a good average. Doing the same for two or three of the strongest modules would also give a range. BUT this is not necessary for your purposes.

Use the earlier procedure to balance charge all the modules and then assemble a pack with weak-strong modules paired together. I would probably pay for a fresh set of buss bars since the earlier ones probably have some trace KOH from the old modules. But I'm not hard on this if the old ones are throughly cleaned and the delicate sense wires checked. Also, I understand the cover may need some 'metal adjustments' to handle the slightly larger modules.

The 30 day schedule pretty well negates the opportunity to individually measure each module with the Super Brain. It would take time to do all 38, probably measured in weeks. It would be the best way and give good Ahr capacity readings. But the procedure I've outlined is fairly safe, fast, and the matched load resistors a reasonable approach.

Bob Wilson

* - Resistors are also identified also by precision. Cheaper ones have more variability and more expensive ones, higher precision. If buying cheaper ones, get some extras so you can manually match them. More expensive ones, fewer extras are needed to match. As for the hook-up wire, I am partial to a stripped, tinned lead on both ends that are 'hard to bend' and sized to really make a tight connection. You don't want the terminal-to-wire junction to be an unplanned resistor. A quick touch test after all are on can identify the 'loose' connections.

 

I don't know the Prius c, and hope that the traction battery modules are the same as Prius v. Good luck.

Please take photos and post if you should find any differences.

 

Regarding the purchase of resistors, you might try one of these suppliers:


Wirewound Resistors | Mouser

Through Hole Resistors Product List | Resistors - Fixed Value | Newark element14 US

 

The problem with the use of light bulbs is that the resistance of the light is highly variable depending upon how hot the filament gets. So you will not have very precise control over the discharge rate.

The 100 ohm 1W resistors are OK, but it will take longer to discharge a module compared to the 64 ohm 1W resistors suggested by Bob.

 

1K ohm resistors will take much longer.

Use the 100 ohm, 1W resistors, then. You should probably buy 70 of them or more and use a digital multimeter's ohmmeter function to select the 38 that are closest to each other.

The 100 ohm resistor will result in 0.08A discharge rate, therefore 12.5 hours required to discharge ~1 Ah.

 

Patrick is correct. Use the 100 ohm resistors but measure them first so we can adjust for any differences. Your initial current will be about 80 ma. So 1 Ahr will be a little over 12.5 hours.

As the modules discharge the ~8V will decay to upper 7.x values. The current will decrease proportional to the voltage drop BUT this is only important to anal engineers. <grins>

We're really interested in ranking the modules so the strongest and weakest can be paired.

Bob Wilson