got some young 2012 modules

It sounds reasonable but GOOD, FAST, CHEAP. Always go for GOOD so you won't have to go through this again. <grins>

Bob Wilson

 

I think Patrick's suggestion had to do with improving on the 5% tolerance on the resistors you're buying. If they are labeled 100Ω, when you take them out of the package and measure them one by one with your multimeter, you'll find them coming out anywhere between 95Ω and 105Ω. That would affect your idea of the pack balance, because if you happened to get two resistors at the extremes, the two modules you used those two resistors on could end up looking 10% different from each other even if they were perfectly matched.

If you only bought 38 resistors, you'd be stuck with whatever mix of actual resistances you got, but if you bought more than 38, you would be able to measure them all, line them up in order by resistance, and pick the 38 that were all closest to each other in value. You might get them all within 1% or even better.

On the other hand, if you don't buy that many, you could consider just using math to get the same effect. You could buy only 38 resistors but you have to measure all their resistances as precisely as you can, note them all down, and be sure to keep the resistors in that order. Then after going through Bob's procedure to get capacity figures for the 38 modules, multiply each module's capacity by 100/(actual resistance of that resistor) and there you go.

-Chap

 

We'll work up and post a spreadsheet but you'll have to plug-in the numbers. Don't wait for the spreadsheet as much as get the parts and start finding out what you've really got.

BTW, I mentioned Radio Shack only because they are nation wide, like McDonalds. If there is a real electronics store anywhere close, go there! You'll get more and pay less. <wink> Tell them you're rebuilding a Prius traction battery pack and they will assume you are 'a special person'. It does no good in a Radio Shack as you'll never see that clerk again unless they get promoted to Burger King. Heck, they'd probably ask,"You can rebuild a 12V battery?"

Bob Wilson

 

I would not buy those resistors on eBay because some of them are going to be counterfeit, poorly made product from China.

If you are going to place an online order, I strongly suggest you use one of the suppliers I earlier provided links to, because you will receive quality product. Resistors are not very costly.

 

Sounds OK then, and the $ amount involved is pretty minor in any event. A 1% error is 0.68 ohm, so you'll need to use your digital multimeter to confirm that the resistors all fit within that range.

 

Each resistor should measure 68 ohms, +/- 0.68 ohm. That is the meaning of a 1% tolerance.

 

If you've got a multimeter of your own, you might just want to save him the trouble and do the measurements yourself, so there's no question about them. You should also be aware that fractional ohms like that are very small resistance measurements, not something you can do casually. You might want to knock something together with screw terminals you can tighten down on your multimeter test leads and on the resistor you're testing. Then you need to start with a short sturdy wire between the terminals and jot down what resistance is showing on your meter. It's the resistance of the test leads and terminals, and you'll need to subtract it from your actual resistor readings. Depending on your meter, there may be a "relative" or "delta" button on it that remembers the current reading and automatically subtracts it from the readings you make later.

Also check your meter manual for the mode that gives the most resolution in the reading. On my Fluke 87, for example, if you hold the yellow button while turning it on, it goes into a special mode where the display only updates once a second, but gives an additional decimal place in the reading.

If you rely on the seller to measure the resistances, you're also sort of counting on the seller to be that detail-oriented.

-Chap

 

As opposed to the high-quality domestic sourced resistors available at Radio Shack

 

I agree that the Radio Shack resistors also will come from China, but hopefully they are sourced by approved vendors, and are not parts rejected by the quality control process.

Further, the benefit from buying Radio Shack is that the OP doesn't have to wait for the parts to be delivered and can easily return to the store to buy more, if he finds the resistor tolerance to exceed his acceptable range.

It seems that the problem with Radio Shack was that the OP is not able to obtain 68 ohm resistors - the 100 ohm resistors that are available would substantially add to the expected discharge time.

 

Not to hijack the thread - this talk of resistors is informative and sort of interesting - but there are other options based on the original description of needs. Setting up parallel discharge as discussed here satisfies the FAST condition. But I don't sense any urgency in the original post other than registering a claim if the modules are defective.

Given that the r/c charger will do charge/discharge cycles on individual modules and provide useful metrics on module capacity/performance to allow matching, if I were in Scott's shoes I'd be tempted to just randomly order the modules, and hook them up on-by-one to the charger, setting it for a couple of cycles and recording the results. At 24 hours per module assuming you're around to swap them daily the time to measure performance on all modeules is more than 30 days, but if the results looked good on the first 50% tested the likelihood of having to file a claim is pretty small.

Am I missing something? Maybe it takes six weeks before the battery pack gets assembled, but this method seems simpler and more robust to me.

Jeff