Testing cells from an '05? Prius , Am I overcharging?

The battery is coming from what I believe is an '05 prius. I purchased this off a client (where I sold him the un-altered battery pack from an 08' prius) and provided installation. I am using a 1000W Reaktor Charger/Discharger, it has a NiMH charging feature. I read on here and other sites that it was safe to charge these cells well past 8 volts when charging. I read on BatteryUniversity each cell can take 1.5V, thus making 9V even acceptable. I've also read that 6.5Ah is the rated capacity OEM from Toyota on these cells. From another post, I've read users can expect 0-5% degradation on their NiMH batteries per year (I personally believe it depends on RH%).

On my charger, I used the NiMH function to first charge ~1.5Ah. Then was able to discharge 2.5Ah but then the charger was able to pump in 6,500mAh peaking at 8.91 V. Not sure if this is too high, it was still going at a current of 4.0A toward the end. I know for longevity, NiMH batteries should be charged around 0.1C (so 1/10 of 6,500mAh I presume would be 650mAh only. Do I harm charging or discharging at 4A trying to determine the batteries max capacity [or attempting to deter memory effect, if these are prone]. When should the charger enter trickle state, I would assume it would already be before 8.9V, if max V is 1.5V per cell, the trickle was set to 100mA. Or is this where the DeltaV cutoff takes role? Will have to check the manual on this.

I also left the battery at this max capacity state, and had an ending read Voltage of 8.53V for the pack. I was then planning on checking it close to 18-24h after to determine the loss potential in that time span (to maybe see if it correlates to battery health/condition). Is it harmful to leave the pack at this max state? As I know some NiMH batteries can self-drain up to 25% in one day. Given I think the internal resistance was only 81m Ohm.

I am looking to resell each of these individual cells (or in packs of 4 on eBay). But I am hoping to determine the capacity or see how degrated each cell is before listing (and adjust price accordingly). So what would be the best way to determine the health of these, anything besides what I am testing?

Also does anyone have tips for shipping? Not sure if PriusChat has a community forum for selling items, or people interested are allowed to PM me?

Thanks in advance!

 

I did not realize the IR was rated so low between two in series. Will recheck the internal resistance, and make sure I wasn't measuring my Pb-acid battery instead. I know most say to charge these modules at 0.1C, which would mean around 650mAh instead. Is this recommended? Or only for the trickle charge aspect. Thanks

 

Yea at least this is a spare I get to work on, rather than worrying about having it rebuilt asap. Thanks again TMR on the info, I forgot to account for the resistance of the [extension] wire I was using, which accounted for about 0.060 of the 0.080 ohms. The resistance is around 0.019 ohms now, on a few that I tested. The suspect cells are in the upper 20's if not higher. It seems suspect cells may depend on the resistance of their neighbors as well (as the modules get put in series). Will have to do more testing and will try to report back with the results.

How important is having cells/modules near each other with the same resistance? I know the ECU will warn if 3 sets of 2x modules have too large of a voltage potential difference. But internal resistance would merely drain each pair of modules. Hypothetically, if someone was repairing their HV battery, but their "good" modules were all 21m Ohm. Would it be bad to replace the defective units with ones rated at 18m Ohm instead? Or should the two in series have the same IR .. that was the voltage difference doesn't climb out of proportion? Thanks

 

There is a special function that is built into the Turnigy 1000w Reaktor (the 300w model may have it, not sure) to check for internal resistance of a battery. Since the device can self-discharge cells upwards to 60 or 80 watts, I'm assuming it then does the calculations internally via a known resistance.

I can recheck the manual to see how accurate the test is, but the tests have been precise down to the 0.001 Ohm so far. The wires from the unit are still about 6in long -- not sure on gauge of wire or specific material. I could calculate its specific resistance, unless the machine already factors those leads in for me, will check the manual. I just had a shotty (self-built) extension cord to charge/discharge the batteries, which that wire seemed to add about 60m Ohm to initial reads.

Currently I am working on discharging the first module after I've charged ~7 of them over the past few days. The voltage seems to taper around 8.10V after a module sits for 24 hours (from about 8.48V after a full charge) and 8.00 V after 48-72 hours. This seems close to being in-line with what I read on NiMH self-discharge rates. But I don't have my notes with me, they are in the shop, so don't quote me exactly

Also curious to see if the internal resistance 'seems' to change with a unit at rest (initial read), vs a fully charged, vs a dead cell. I have a feeling this can help to determine how accurate the machine is. But my goal is to drain the modules after 72+ hours to see how much capacity they still retain, when discharging at various rates (from 0.1C to 1C).

 

After a bit more testing, here are my results. Please feel free to correct me if I make a mistake.

1) I started with discharging Module 1. Then I charged the module with 6,500mAh. The ending voltage read 8.53V.
2) 72 hours of leaving module 1 fully charged, the resting voltage read 8.00V after returning. With an internal resistance of 17mOhm.
3) I was able to drain 4,450mAh at 3Ah (~0.46C with an assumed 6.5Ah for C). This left me with a resting voltage of 7.27 V and the resistance read 23 mOhm.
4) I was further able to get 180mAh at 2Ah (.41C) and another 82mAh at 0.6A (.092C). After which the voltage read 6.28V (and climbing) with an internal resistance of 25mOhm.
5) I ran a few cells like this, and recharged module 1 with 6,500mAh. After taking a break for 3 weeks from the battery, module 1 still read 7.72V. I was able to drain 4274mAh at 5Ah, and stopping near 6.6V.

From the results, it seems that internal resistance does climb with lower voltage potential sources, or is this caused from heat dissipation?

It also seems that the battery was able to retain ~4,712mAh or loose about 25.8% capacity, after 72h from a 100% SOC. From another post on here, I read typical NiMH naturally lose about 7% to 28% capacity over 3 days anyhow. Mind you this is from a 12-13 year old battery pack.

Although I did find a table titled "Inaccuracies of Estimating Remaining Cell Capacity with Voltage Measurement Alone". It appears that a cell when discharged at a rate of 0.1C and finishes around 1.21V per cell (at 26°C), should have a capacity of around 450mAh left. This similarly reflects result #4, and would lower my loss of capacity to only 20.5% (after 3 days that is). And a typical NiMH battery loses approximately 11% naturally over 3 days.

Result 5 would lead me to believe there is about 110mAh left in this module (based on the above link).

Does this module seem fine, especially since the resting voltage was >7.7V after 3 weeks? I simply plan to resell the modules and want to make sure I have the math/method right to test these beforehand. Feel free to PM me if you're interested in any modules. Thanks!