got some young 2012 modules

I filled out the spreadsheet with these assumptions:

• 68.0 ohms - more accurate if the resistors per module are known
• 8.10 V starting - more accurate if known
• end volts as posted by module
• 8:40 duration of discharge, 8 hrs, 40 minutes - more accurate if exactly known
• 2.39 to 2.63 Ahr - estimated range of capacities, not great but all are close . . . very important
• 14.63 to 14.60 V - paired voltages, difference 0.03 volts, very good!

These are a very good set of matched modules. The estimated capacities are acceptable and certainly usable.

Bob Wilson

 

I've seen stronger but remember the spreadsheet is using a linear approximation. Take a peek at this:

Notice that at the peak starting voltage, there is a steep, downward curve. The discharge duration puts the straight-line on the steepest discharge rate which projects to a low capacity. However, had the discharge gone on longer, closer to 6.5V, the slope of the discharge line would have flattened out, increasing the Ahr capacity. The flaw is in my spreadsheet. A longer discharge would have given a more accurate and higher Ahr capacity.

When you get a chance to retest with your automated charger/discharge system the weakest and strongest, you'll get a better estimate.

FYI, the plot of modules versus capacity is consistent with what we've seen in the past for used, traction batteries. The center ones tend to be weakest because they are often the hottest. To me, it looks like the low numbered modules were from the extra 10 and one them is very strong compared to the others that are more consistent with the pack.

Bob Wilson

 

Hi Bob,

I wasn't able to download the Excel spreadsheet posted in #105, as I get a corrupted file error msg.

I assume that the chart you attached in post #109 is intended to be generally representative of NiMH behavior but not necessarily of the Prius modules themselves, since the chart depicts a battery composed of eight cells of 1.2V each, vs. the Prius modules which contain six cells of 1.2V each. How precise is that chart intended to be?

I was also trying to recreate how your spreadsheet might work:

Working with an "average" module, it looks like the starting voltage was 8.13V while the ending voltage was 7.31V for an average voltage during the test of 7.72V. The resistance of the load is 70 ohms so the average current flow is 0.11A. The test consumed 8.75 hours so 0.965 Ah were produced by the average module during the test.

The challenge is extrapolating 0.965 Ah to the capacity of the average module. The chart you provided indicates the "halfway point" is reached at 1.2V per cell.

8.13V - 7.31V = 0.82V
8.13V - 7.2V = 0.93V

0.82V / 0.93V = 88.2%

Based upon these ratios, the test of 8.75 hours took the modules to the point where they were at 88.2% of the way towards the halfway point.

Then it would seem that the ultimate capacity would be 2 x 0.965 Ah /88.2%, or 2.19 Ah. I assume your spreadsheet is working in a somewhat more complex fashion since scotman27 says he is getting a result of 2.29 to 2.51 Ah.

I understand your concern that if he continues the test to reach the point where the module voltage declines to 5.4V, he will risk causing permanent damage to the modules.

I guess the bottom line is that (as you suggested) the spreadsheet may not accurately model the actual Ah capacity and that he will have to use the automated charger to get a better idea of the module capacity. This also means that he should not be quick to decide the newly acquired modules were incorrectly described with 9K miles of service.

 

Rename the spreadsheet from ".xlsx" to ".xls" and the problem is cured.

Correct, the starting point should be 1.2V per cell or for six cells, 7.2V. But in this case, the modules were 'over-charged' to 8.1V or about 1.35V. This over-charge region is on the extreme left edge of the discharge curve where it slopes down abruptly.

Had the starting voltage been what was in effect the ending voltage, we would have been in the flat part of the curve. The slope would then have projected a much greater Ahr capacity. In one respect, the discharge was stopped a little too soon. Getting closer to 6.5V would have given a much better estimate.

The normal range for a NiMH cell is:

• 1.2V ~= 7.2V/module
• 1.0V ~= 6V/module

But we know from experience these modules can take up to ~8.0V, an over charge to ~1.35V/cell. In effect, we started out 'off the page' above the 1.2V left edge curve.

A proper metric counts the amps over time until the module is discharging. But we also know the NiMH modules are sensitive to over discharging. It happens so fast, a blink of an eye, you really want an automated charger to take the metric . . . and he bought one, a Hitec X4.

I am sure the Hitec X4 will report substantially higher capacity. But from this initial screening test, he can start with four weakest and four strongest. Whether he does all 38, well 38/4 ~= 10 sets. Then he'll know.

You know GOOD, FAST, CHEAP . . . my understanding is we were going for FAST and CHEAP and the initial test was that. GOOD will be known from the Hitec X4 results. So don't set your hair on fire, yet.

Bob Wilson

 

One amp is perfectly fine. I would also limit the peak voltage to 7.2V to keep it within the normal NiMH range. Going up to 8.0 or 8.1V is still in the overcharge region. But you've got the right idea.

FYI, here is a better set of NiMH curves:

We were running the left side of these curves, no where near the middle discharge region.

Bob Wilson

 

A little rough but 3A is OK. As a general rule, the capacity shows up higher using a fractional charge rate, 1A. A 3A charge, 0.5C (half the 6.5Ahr rate), is OK but going to show a lower capacity than more typical 1A. But I also understand GOOD, FAST, CHEAP.

Bob Wilson

 

Yes.

Bob Wilson

 

Don't forget to keep those modules that are being charged/discharged under physical compression, if you removed them from the traction battery case.

 

Keep in mind that the maximum discharge rate on the X4 is 5 watts, or less that 1 amp. The maximum charge rate is 50 watts, or about 1C for a single module. Everything I've read says that a 1C charge rate will do no harm - maybe you'll get slightly less total charge, but I'm not convinced that there is any advantage to be gained in throttling back the charge rate. Guess it depends how much time you have and what you're trying to accomplish. At maximum rates you're looking at 11 hours per cycle assuming that the modules are in good shape. Limiting the charge to 1 amp increases that to 16 hours.

Nexus 5 ?