got some young 2012 modules

The first part will be quick because the cells at 8.1V are actually a little over charged. Once they reach 7.8V, the rate of discharge will rapidly flatten out. But you are seeing exactly what the load resistors should do. We want the current rate through the load resistors to be >>10x the self-discharge rate. This is the only way we can get a reasonable metric.

Wait an hour, and survey the modules, and do it again an hour or so later. Those two points will be on the 'straight line' part of the discharge curve. We can use that rate and the lower limit to estimate the Ahr capacity.

The rule of thumb is 6.5V should be your 'floor' for a normal discharge. Anything under 6.5V and you're getting close to the DO NOT DROP BELOW 6.0V range. Once you get below 5.8V, you are in 'uncharted territory.'

So are the resistors pretty HOT?

Bob Wilson

 

You are good to go!

Do you go to work at 3:00 PM, about 2.5 hrs from now?

Bob Wilson

 

Excellent!

You are in great shape. Check them every 1-2 hours and life is good. Heck, put the numbers and time in the spreadsheet and we can look at the discharge graph.

Bob Wilson

 

That is absolutely working as expected. Think about it, you've only dropped .03 or .04 volts or 3/100ths. Not significant at all. A bad module would likely drop to 7.5 or less in as little as ten minutes (depending on capacity).

 

After you are done with the battery module testing, measure the resistance of the resistors and see how close to 70 ohms they are, after cooking for so many hours.

 

Ok,

Let's 'back of the envelope':

• 7.89V @1:12 PM (posting time), highest voltage
• 7.58V @3:38 PM (posting time), lowest voltage
• 0.31V / 1:26 hr ~= 0.22V/hr.
• 5 hr * 0.22V/hr ~= 1.1V drop, expected
• 7.58V - 1.1V ~= 6.48V around 3:38 + 5Hr ~= 8:38 PM, earliest expected stop

Now these are 'back of the envelope', which is engineer for an estimate and using the worst case metrics for the posting time stamps. You may have more accurate times. The real proof is to monitor the voltages and if any of them get down to 6.5V, take the resistors off and call the experiment done. But I suspect you won't be in that range until closer to 8:38 PM this evening.

At that point, take a short break, 5-10 minutes, and measure the module voltages. You will then have a list of strongest to weakest. Then use the spreadsheet to calculate the expected Ahr capacity of each module.

My experience with the MRC 989 is:

• 4.5Ah or higher - great!
• 3.5Ah or higher - servicable
• 2.5Ahr or higher - hummm
• lower - uh oh, marginal, avoid using

Now this is not a 'glass half empty or half full' but you have already verified there are no shorted cells in the modules. Having the weighted values, you'll have enough to identify the pairs for the new stack.

The "new" specifications for these modules is:

• 7.2V @1.2V per cell for 6 cells
• 6.5Ahr discharge, typically at 1C

So using 70 ohms and 7.89V (worst case), 0.113 A, which is 0.018C, a gentle discharge. We are getting quality data to measure the Ahr capacity of these modules. This is one of the reasons why I suggested taking a sample of low capacity and high capacity modules and using your RC charger to see what number it reports. <grins>

This is good, in fact, a very good, well controlled test. Patience, monitor the voltages and as I mentioned earlier, don't let the weakest drop below 6.5V and you're in high cotton.

Bob Wilson

 

Stop at 8:38, OR stop if any module voltages get down to 6.5V, whichever comes first.

 

Correct, disconnect the resistors, let the modules rest for 5-10 minutes, then measure their voltages.

After the resistors cool down, pls measure their resistances and see if they have drifted from the original 70 ohm values that you measured. Thanks.

 

Close enough since the original target was 9:15 . . . we're looking pretty good already and 33 minutes is not going to be a 'show stopper.' The 6.5V should be considered the floor only to provide 'head room' or pad away from 5.8-6.0V that should be treated as a hard limit. More technical details later.

Questions are always OK and they have been good ones. Often technical folks forget not everyone 'plays' with our toys. <grins>

To put things into perspective, these modules are considered 100% charged at 7.2V and your latest metrics are above that number. You haven't even reached that level. <grins>

Now looking at the worst case voltage difference, 7.41 - 7.34 ~= 0.07V. This is early but excellent numbers for matched modules. Remember the Prius threshold for a throwing a code is 0.30V between module pairs. Stacked badly, two strong and two weak, the worst case would be ~0.14V, less than half the 0.30V that would throw a code. But you'll pair the weakest and strongest pair, which make the difference well under the 0.30V. This preliminary data is very good!

Ok, we went with basic, simple load resistors and an alert technologist with a clock and interest. But there are two other load approaches that would work for less committed or skilled technologists:

1. 6.0-6.5V Zener and current limiting resistor - the Zener would stop current flow once the voltage reached the limit. However, most of the power would be absorbed by the Zener. A 6.0V Zener sinking 100 ma would be about 0.6W . . . owch! The current limiting resistor would barely heat up.
2. 6.0-6.5V Zener wired a pair of bias resistors to the base of an NPN transistor and 50-70 ohm load resistors - the Zener controls turning ON the NPN transistor until the voltage drops enough. Then the transistor turns off and the module is safely discharged. Best of all, the transistor drops ~1.2V and the rest is dropped across the resistor . . . a good thing!

One variation is to put a LED and current limiting resistor across the load resistor. When the Zener stops conducting, the LED goes out. The Zener provides an automatic, shutoff that protects the module from over discharge. Setup a video recorder taking one shot every 1-5 seconds and the play back will easily identify the modules in order. BUT these are the idle musings of an engineer thinking 'if we ever have to do this again . . . in French.' <grins>

Bob Wilson