Exploring Smart Battery Pack Data

Enthusiasts with a variety of interests, ranging from flashlights to electric vehicles, repurpose rechargeable lithium ion cells from laptop battery packs to power their projects. In order to assess the age and health of the cells, people use a succession of tests, ranging from quick tests of initial cell voltage, voltage ~12-24 hours after charging, and discharge tests on analyzing chargers. I looked at the initial cell voltage measurement in the context of information about pack age and history that hasn’t been available to most hobbyists.

To obtain this information, I used PackProbe. The PackProbe project allows quick access to the information contained in each pack’s “smart battery system,” using inexpensive, widely available hardware. PackProbe is part of a larger effort to improve the tools and information available to people re-using lithium ion cells.

 Materials and Methods

PackProbe was used to extract data from a lot of 34 used Lenovo ThinkPad 9-cell 84Wh battery packs (IBM-42T4619) obtained from a seller on the Budget Light Forums. I don’t know more about the provenance of these packs, other than that they apparently came from a single source in a few different shipments starting sometime this summer and totaling hundreds of packs.

Observations and Commentary

General

  • Most of the packs were manufactured on the 22nd and 23rd of October, 2009. Two on the 27th October, and three on the 8th of September.
  • The packs all report :
    • Manufactuer: SANYO06
    • Chemistry: LION
    • Design Capacity: 84.24 Wh
    • Design Voltage: 10.800 V
    • Charging Voltage: 12.600
    • Charging Current: 5000 mA

Cycle Count

The median charge/discharge cycle count is ~88, distributed as shown:

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Initial Cell Voltage

We used the self-reported pack voltage from each pack to estimate initial voltage of each cell. Some of the packs had their PCBs damaged in shipment, which made it difficult to get data out of them, but by applying ~6v to the pack, I was able to read out data. The reported voltages for the damaged packs were quite low, and from my investigation, it appears that they only report the series voltage for two of the banks of cells. If I take this into consideration, and look at the average cell voltage for all the packs, I see the following distribution:

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I suspect that all these packs were last charged at about the same time, likely just before their previous users gave up the machines in a round of upgrades (or layoffs). The variation in the voltages could be due to a number of factors, including variations in cell quality, pack wear, charger accuracy, storage conditions, and date of last charge.

Cell Voltage as a Proxy for Cell Wear

Exploring the idea the variations may be the result of pack wear, I’ve tried plotting cell voltage against the number of charge/discharge cycles for each pack:

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I don’t see an obvious relationship here. It seems like differences in self-discharge rate due to cell wear don’t explain the variations in voltages within the packs sampled, which all have cycle counts within the normal service life. It may be more useful for packs with large numbers of cycles (>300).

Smart Battery Capacity Measurement

SmartBattery packs maintain an estimate of pack capacity based on run-time data in order to account for aging of the pack. I wondered if initial cell voltage had a relationship to relationship to the pack management circuitry’s estimate of the packs remaining full-charge capacity:

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Again, I see no obvious relationship, perhaps not surprising, giving the broad distribution in the reported Full Charge Capacity (in Wh*10):

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Further, the reported full charge capacity doesn’t seem to have a strong relationship to cycle count:

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It is hard to know what to make of this, other than that there isn’t a clear relationship, which isn’t surprising, given that these packs haven’t been used for some time, and likely require a full charge/discharge/charge cycle before the full charge capacity estimates are accurate.

Discharge Test

I’ve only torn down one of these packs so far and tested the cells. Based on the tests, the full pack would have a capacity of 65 Wh, or 77% of the original capacity (at a relatively high 1C discharge rate). That’s not bad for less than $2.50, but its a pretty severe drop-off over the 103 cycles reported by the pack. It could be worse though, the reported full charge capacity was much lower, just 50Wh, or 60% of the original capacity.

Conclusions

This feels a little too much like writing up a lab report for a not-too-successful organic chemistry lab experiment back in college, lots of  trying to explain inconclusive observations. Even so, I think this work has important implications.

Foremost, the results are a strong indication that initial cell voltage from used packs is not a good indicator of cell wear for used packs, at least with packs of the age and cycle count distribution of our sample.

Second, it should be clear that PackProbe provides useful information for assessing the value of a pack without necessitating tearing the pack apart.

Future Work

I intend to do more work to improve PackProbe and use it to better characterize used and aged lithium ion batteries. I hope others will join in the effort. Opportunities for community contribution include:

Join the Power Cartel forum to contribute and stay abreast of PackProbe development.

An Interview with Tesla Battery Hacker [wk057]

Hack a day has an Interview with the guy who got a battery pack from a wrecked Tesla Model S.

One interesting take-away he paid about $20K for it. I think those packs have 6-7000 cells, so that works out to about $3 per 18650 cell.

That’s not bad on a per-cell basis. New old-stock laptop packs with similar capacity cells seem to go for $3-5/cell, though I’ve scored a few for ~$2.25/cell.

Since he was able to use the packs more or less intact, though, $3/cell is a great deal, considering all the labor he saved disassembling packs and then building a large pack for his project.

Dell and Thinkpad Lithium Ion Pack Irritation

I got a bunch of Thinkpad Lithium Ion battery packs yesterday and dumped pack data out of all of them. The new packs had the same issue I saw with a pack for a different module, they don’t report individual cell voltages in response to commands that work with many other packs.

I did some research and found that the linux tm-smapi module provides access to individual cell voltages but from a little reading, it looks like this information may come by way of an embedded system controller. I figured there was still a good chance that this information was originally gathered from the battery via SMBus, so I wrote a simple arduino sketch to scan through a wide range of SMBus commands and look at the data returned. Unfortunately, I don’t see any values that look like cell voltages.

With any luck, the data is still there, and just packed in a way that isn’t obvious. I think I’m going to need to collect data from multiple packs to see which values differ, particularly if I charge or discharge the packs. The worst possible option is that reading the data requires putting the pack in an undocumented mode and using undocumented commands.

The Dell packs I have don’t yield individual cell voltages either, so while I was at it, I also looked to see if any of the Dell packs might report the data in response to non-standard commands. Again, nothing obvious. I couldn’t find any confimation on line that Dell makes this info available via any utilities, so I may be chasing something that isn’t there.

The humble Smart Battery reveals its secrets

It took me a little longer than I’d hoped, but I’m able to get most of the information I want out of most of the laptop batteries I’ve tested.

ASUS AL32-1005

Manufacturer Name: AS085NJ35E
Device Name: 1005-28
Chemistry LGC0
Design Capacity (mAh): 5400
Design Voltage: 11250
Manufacture Date (Y-M-D): 2009-6-21
Serial Number: 937
Specification Info: 49
Cycle Count: 254
Voltage: 10.28
Full Charge Capacity (mAh): 1680
Remaining Capacity (mAh): 0
Relative Charge PCT: 0
Absolute Charge: 0
Minutes remaining for full charge: -1
Cell 1 Voltage: 2642
Cell 2 Voltage: 3820
Cell 3 Voltage: 3817
Cell 4 Voltage: 0
State of Health: 0
Charging Current: 0
Charging Voltage: 0
Temp: 20.25
Current (mA): 0

You’ll see that this pack is 5 years old, and has had 254 cycles, which probably puts it near the end of its useful life. Looking at the individual cell voltages (actually banks of parallel cells), you’ll see that that one of them is quite a bit lower than the others, suggesting those cells are closer to failing.
I’m still having trouble with the Dell packs I’ve tested. I can get most of the information I want from them, but they don’t report the capacity of the individual cells properly. The individual cell data isn’t part of the official smart battery standard, but it seems pretty standardized. It may be the Dell packs don’t report that information at all, or it may be that they use a different set of commands to reveal it.

Dell 9T48V

Dell 9T48V
Manufacturer Name: SMP-SDI2.8
Device Name: DELL YXVK234J
Chemistry LION
Design Capacity (mAh): 8400
Design Voltage: 11100
Manufacture Date (Y-M-D): 2013-4-19
Serial Number: 181
Specification Info: 49
Cycle Count: 44
Voltage: 10.03
Full Charge Capacity (mAh): 8428
Remaining Capacity (mAh): 0
Relative Charge PCT: 0
Absolute Charge: 0
Minutes remaining for full charge: -1
Cell 1 Voltage: -1
Cell 2 Voltage: -1
Cell 3 Voltage: -1
Cell 4 Voltage: -1
State of Health: -1
Charging Current: 4214
Charging Voltage: 12900
Temp: 23.25
Current (mA): 0

HP Packs have been a mixed bag. I’ve been able to get a full compliment of data out of some of them, and none at all out of others. I’ll work on fixing it after the initial release.

The code runs on an arduino Yun now, and should be easily adapted to any arduino compatible. I’m going to write a little documentation and release it while I continue to work on it.  If you are interested in getting early access, leave a comment here.

Inateck 120cm USB to Micro USB / Lightning Cable

I recently purchased an Inateck (EC2001L) 120cm USB to Micro B cable that comes with a Lightning adapter for recent Apple iOS devices from Amazon. I picked the cable for its dual-purpose nature, but it is also set apart by its flat profile, which reduces tangles. IMG_6634 My plan was to keep it in my bag with a USB powerbank so that I could use the cable to charge the powerbank and iOS devices.

Unfortunately, it isn’t up to the task. First thing I realized is that I missed the fact that it wasn’t Apple MeFi certified. In the past I’ve had trouble with non-MeFi lightning cables leading to slowed charging rates.

The Inateck cable seems to have the same problem. At first I thought it wasn’t allowing the iOS device to properly determine the current delivery capability of the power source, but when I moved the lightning adapter to another Micro B cable, it worked fine, so, the problem must be the resistance of the cable itself.

I did some tests using a multimeter and some USB testing equipment and I found that the cable does, indeed, have high resistance 370 miliOhms or more, leading to a voltage drop of 0.37v at the 1A charging currents used my iPhones, and even more at the 1.5-2A demanded by an iPad. Other cables had less than half the resistance, and correspondingly, less than half the voltage drop.

This cable is fine for data transfer and occasional charging, but it isn’t what you want to use for charging if time or power are precious, because it will take longer than a lower resistance cable.

Battery Deals: $1.66/cell $0.19/Wh New Genuine Dell Precision M70 80WH 7200mAh Battery

Note: There is a report that cells from this pack came out with a voltage of 1.7-1.9v. New cells at this level pre-charged at 50-100mAh for a few minutes until they reach 3v before charging can have a good useful life, but you should keep an eye on them during charging. They should get to 3v within 10 minutes or so, and shouldn’t heat up significantly during charging. Once charged, check the voltage after a few days and make sure they aren’t lower than 4.1v or so.

Over on Ebay, Red Planet Trading is selling Dell 9 cell, 80Wh, 7200mAh battery packs for $14.99 with free shipping. They currently show more than 10 available.

The pack is labeled that the cells are made in Korea, which means Samsung, of LG. Capacity is probably 2,400-2,500 mAh/cell.

This isn’t the pack to tear down if you are looking for high capacity cells, but at $1.66/cell and $0.19/Wh, its a good value.

Looks like the same pack is listed a few different ways:

Battery Deal: Genuine Dell Battery Inspiron 13R N7010 N5010 N4010 9T48V M511R GK2X6 | eBay

I ordered one of these “Genuine Dell Battery Inspiron 13R N7010 N5010 N4010 9T48V M511R GK2X6” from eBay seller for $17.99 and free shipping. Its a 9 cell, 90Wh pack, which, I estimate, probably has 2,800 mAh cells. It arrived quickly and was packed well, but I haven’t opened it yet.

Works out to be $2/cell and $0.19/Wh, which is a pretty good deal.

There are a couple of downsides though. The pack I got had ~40 cycles on it. Not bad, but probably 10% of the lifespan. It wasn’t a surprise either, since the pack wasn’t described as new, it was listed as manufacturer refurbished.

Which brings me to the second problem. Before purchasing, I asked the seller for more information about what “manufacturer refurbished” meant and they didn’t reply. After purchasing and receiving the pack, and checking the cycle count, I decided that it would be worthwhile to get another couple of packs if I could get them at a lower price, so I took advantage of the “Make Offer” featured on the listing and offered a lower, but still reasonable price for two. They ignored the offer and let it expire.

Battery Deal: Acer AS09D70 5600mAh 11.1v battery pack

I found a seller with a decent price on a few Acer AS09D70 5,600mAh battery packs on ebay.

These appear to be new, old-stock, packs. The listing has conflicting information. The title states they are 5,600 mAh and 56wH, which doesn’t quite make sense, since the nominal capacity is 11.1v. The listing explicitly states 4,000 mAh capacity, but the pack markings in the listing look like 5,600 mAh.

Someone cracked one of these packs one of these packs open, and found six Samsung 18650-28As, which are 2,800 mAh cells, which make these  62Wh.

The seller has these for $16 with free shipping, which works out to $2.66/cell. That also works out to  $0.26/Wh which isn’t bad, though I’ve seen $12 packs with 6 2,600 mAh that are a bit better deal.

One caveat is that these cells are supposed to be charged to 4.3v. Typical Lithium Ion chargers charge to 4.2v. If you charge to 4.2v, you’ll give up a bit of capacity.