Charging unknown li-ion cells salvaged from old laptop  - Page 1
Forum section: electronics / beginners

author: yin
author_rank: Newbie
timestamp: March 15, 2015, 08:39:36 pm
content: Hi all,I salvaged 6 nice li ion cells from an old laptop battery pack. I want to include one cell in my current project (which includes an charging), I have no idea about capacity and C rating of individual cells (pack capacity was 58Wh @ 10.8V => 5.2Ah). How can I safely charge them?I tried to google the washed off codes on the cells, but found no info on these.

author: IanB
author_rank: Super Contributor
timestamp: March 15, 2015, 09:06:13 pm
content: You need a multimeter and a laboratory power supply with voltage and current regulation.Measure each cell and check the voltage. It should be between 3.6 V and 4.2 V. Put to one side any cells with a voltage below about 3.0 V. These are probably weak or failed cells and should be recycled.If you have six cells that were wired to give 10.8 V and 5.2 Ah, then the pack would have been a 3s2p arrangement and the capacity of an individual cell should be about 2.6 Ah.Take one of the cells with a voltage above 3.6 V and try charging it with the power supply. Set the voltage limit to 4.2 V and initially set the current limit to 500 mA. Put the cell on charge and check that it accepts charge. The power supply should be in CC mode, the cell voltage should slowly rise and it should not get warm. If this is OK, you can increase the current limit to 1 A or even 1.5 A and continue charging.Eventually the voltage should reach 4.2 V and the power supply should switch from CC mode to CV mode and the charging current should start tailing off. When the charging current decreases to 200 mA you can stop charging.If you want to check the capacity of the cell you will need to conduct a controlled discharge and measure the current over time. A data logging meter is helpful for this. Otherwise you can just use a load like a 6 V 6 W lamp and record the current every 5 minutes or so by hand. Discharge the cell down to about 3.3 V under load and then figure out the capacity as average current times discharge time.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 15, 2015, 09:20:18 pm
content: If you are using the cells as singles, even the ones that read below 3.0 volts may be usable. Try charging them at 50 mA for a while. You must monitor them while doing this. If the voltage climbs fairly rapidly above 3.0 volts and then begins climbing much more slowly you can then proceed to charge and evaluate the cells as recommended by IanB.The low voltage cells presumably did have a lower capacity than the other cells in the pack, and the charging system in the pack did not succeed in keeping the charges well balanced. These cells were therefore at the low end of the voltage range (or below) when the battery pack was last actively used. And further draw from the pack while the equipment sat waiting for you (supporting battery monitoring for example) drew those cells down more quickly. Nothing you can do will restore the original 2600 mAH capacity of the cells, but they may well have a residual capability of operating as a 1000mAH cell, which may or may not meet your needs.

author: Ian.M
author_rank: Super Contributor
timestamp: March 15, 2015, 09:40:14 pm
content: You also need a fireproof enclosure to charge them in till you are sure they are good. Four bricks in a square, with some duct tape strapping round the outside and two small concrete paving slabs, one as a base and one as a lid will do well enough. Tape a thermocouple probe to the cell you are charging and monitor the temperature. Shut off the power if you see an excessive or sudden temperature rise, and don't lift the slab until the cell's cooled to near ambient. If one does fail, you'll get a lot of caustic (alkaline) toxic smoke and may get some small flames from the cracks between the bricks, so charge them outside!

author: yin
author_rank: Newbie
timestamp: March 15, 2015, 10:23:40 pm
content: Thanks guys,You are almost correct IanB, they were in sort of 2p3s configuration, tough divided into groups of 4 and 2 (both groups shared one common wire to the pcb). Anyway, I broken them up into 2p blocks and measured each pair separately, each measuring exactly 4.03V if I can trust my crusty multimeter. I would trust these cells.So, IanB, you say I can simply try putting more and more charging current in the CC phase into the cell, until the temperature starts elevating? But I'd also like to get most out of the capacity and battery life time.

author: IanB
author_rank: Super Contributor
timestamp: March 15, 2015, 10:31:33 pm
content: Quote from: yin on March 15, 2015, 10:23:40 pmNo! The cells should never get warm when charging.There is a maximum charging current which depends on the cell type and manufacturer, but keeping it to 0.5 times the capacity is fairly safe (0.5/h x 2600 mAh = 1300 mA).

author: Rick Law
author_rank: Super Contributor
timestamp: March 15, 2015, 11:53:18 pm
content: Quote from: IanB on March 15, 2015, 10:31:33 pmIt has to get a little warm unless you have 100% efficient. Charge absorption is typically is around 60-80%. At 80% efficient, that leaves 20% heating the battery up.

author: Rick Law
author_rank: Super Contributor
timestamp: March 16, 2015, 12:18:27 am
content: Quote from: yin on March 15, 2015, 10:23:40 pmOn the first go, you should not measure battery voltage in 2P configuration. In parallel, one battery may be obscuring the other. It is better to measure each individually. Once you make sure "they are safe" individually, you can put them back in pair.>> I am reluctant to use the term "they are safe" - they are only safe for that moment. Battery can always fail at a later time. I savaged a couple of old StarTek phone lithium cells. They were safe for a month of two, then, after using it a couple of months, one during charge got too hot to touch. It was "safe" for a while.Also, it is a good idea to install a PCM (protection circuit module). Those module ensures the battery will not over-discharge and some PCM protects against both over discharge (<2.6V) and over charge voltage (>4.3V).

author: IanB
author_rank: Super Contributor
timestamp: March 16, 2015, 12:25:04 am
content: Quote from: Rick Law on March 15, 2015, 11:53:18 pmLithium ion batteries don't obey the same rules as other batteries. For sure, charging can't be 100% efficient, but there is a very small loss of energy in charging. If you charge your smartphone you will notice that the charging circuit gets much warmer than the actual battery.

author: Rick Law
author_rank: Super Contributor
timestamp: March 16, 2015, 04:07:43 am
content: Quote from: IanB on March 16, 2015, 12:25:04 amInteresting that they differ. I didn't know that.When you say categorically that it doesn't get warm, it gets me worried here. Mine always gets a little warm, about +5F to +10F over ambient temperature at 1C charge rate. So unless I am totally wrong, getting a little warm is ok, it doesn't (shouldn't) get hot.

author: IanB
author_rank: Super Contributor
timestamp: March 16, 2015, 05:37:14 am
content: Quote from: Rick Law on March 16, 2015, 04:07:43 amSure, "a little warm" is fine. But it should be a steady temperature that does not keep increasing, and for sure not hot.

author: Rick Law
author_rank: Super Contributor
timestamp: May 20, 2015, 01:47:18 am
content: Quote from: IanB on March 16, 2015, 05:37:14 amHello,IanB, I finally encountered exactly what you described.As a relatively new newbie, I do not have much experience. I was exposed to just:(a)works fine,(b)slight warm, stay slightly warm but otherwise works fine, and(c)an old StarTac phone lithium cell that heat up instantly to scary temperature - too hot to touch by hand. within a minute or two max.I have a new crop of lithium 18650. One got warmer than usual. So, I stop the bugger on the TP4056 and dug up my old setup. My old setup is an Arduino controlled LM317 with 16 bit ADC to read voltage and 12 bit DAC to set voltage. I added an LM35 as temperature probe to see what is going on relative to temperature.The bugger(s) would charge the CC phase normal. During the CV phase, its operated normally with voltage held constant and current decreases. Normal batteries would reach end-of-charge which is a preselected "saturation current XXXmA". With these buggers, when it decreased to around a certain mA, current starts to increase as much as CC limit allows. Temperature starts to rise at the exactly moment current switch from decreasing to increase. My "normal" batteries gets slightly warm, but it does not couple with a current increase.I took some charge data (room temperature was about 75F):1st time I cut off when after it risen to 101F. Discharged it using a plan resistor.2nd time I cut off charge as soon as it reach 92F. Discharged it using the same resistor.3rd time I cut off charge at 85F. Discharged it using the same resistor.All were discharged to battery-protection cut off at 2.6V. The difference in cut off temperature translate to about 30 minutes more between 85F to 92F and about 25 minutes for it to rise to 101F. So in theory, the 101F was charged about an hour longer.Since it is the same resistor, time_to_discharge alone can compare relative charge. With all 3 tries, there was no appreciably difference in run time - that suggests to me that the power going in after the inflection point is just heating rather than charging. Had I allow it, current would have risen increasing the heat, and the temperature might have kept rising.I also tried two separate charging (charge to inflection point, wait, charge again). That seem to "get past" the inflection point with no fanfare, but does not result in any increase in run time (ie:charge stored).It is interesting. While I did not doubt your words, but seeing it happen in real life is interesting. Now with this crop of 6x18650, looks like not too many are useful. This "bad yielding crop" part is disappointing.Rick

author: amyk
author_rank: Super Contributor
timestamp: May 20, 2015, 04:09:52 am
content: Quote from: Ian.M on March 15, 2015, 09:40:14 pmElaborate fireproofing is more highly recommended for lipos, which can vent and flame much more violently than hard-cased 18650s due to their more fragile and higher-power-density construction; for the latter, keeping them on a heat-resistant/nonflammable surface and pointing their vents (+ terminals) away from anything flammable should suffice - there's a reason almost all of lithium cell fire videos on YouTube feature lipos, not 18650s...

author: eas
author_rank: Frequent Contributor
timestamp: May 20, 2015, 05:39:38 am
content: Quote from: IanB on March 15, 2015, 09:06:13 pmLaptop packs are usually pretty well managed, so if cells are below 3v, then its likely that its due to slow-self discharge, rather than an extreme event. In those circumstances, putting aside cells that come out of the pack between 2v and 3v is a waste of what are likely to be perfectly good and safe cells.I have seen some terrible packs though, mostly older Dell packs, with all sorts of indications of shitty engineering and manufacturing, like reversed polarity cells, parallel banks of cells where the component cells have different voltages, and broken welds on the strips tying the pack together.Quote from: CatalinaWOW on March 15, 2015, 09:20:18 pmIn the packs I've torn into, its rare to have cells with significantly lower voltage than others in the pack without them being pretty much trash, and often the other cells aren't in great shape either. The best cells in a problem pack may have half their nominal capacity or less. Even so, they can definitely be useful. I decided to cycle some Samsung 28A cells that were probably at 60% of their nameplate capacity. I thought they'd decline pretty quickly. Instead, after 100 cycles or so they'd only faded by a couple percent, at most. I wasn't exactly treating them gently. I was charging and discharging over the full range (2.75 to 4.3v, as I recall) at charge/discharge rates over 1A.Quote from: IanB on March 16, 2015, 12:25:04 amLithium ion batteries are close to 100% efficient with respect to electrons-in and electrons-out. The parts per million of inefficiency is basically due to the reactions that lead to capacity loss. From a power perspective though, its a different story. I looked at some data on cells I was testing at about 1.6A charge rates (0.5C for the cells in question), and the losses are about 10-12%, which works out to less than a Watt of heat, which in my casual checks has been hard to detect.Quote from: amyk on May 20, 2015, 04:09:52 amMoreover, a lot of RC pouch cells are treated as consumables, bought as cheaply as possible, and abused for MOAR POWER. Even so, I have to wonder if their safety is any worse than the methanol/nitromethane fuel they replaced.To the original poster, a cheap way to test your cells is to get a USB power meter that keeps track of mAh of current ($10), and a TP4056 charger module ($1-2). Power the module off a USB power brick through the meter. The IC on the module handles all the details of properly charging a LiIon cell. They usually ship with a fixed charging current of 1A, but you can change the resistance on the ICs programming pin to charge at a lower rate, something you'll want to do when testing.You can get a few more of the boards to use to charge the batteries in your projects. Some add protection circuitry to keep your battery from discharging to far, or overcharging if the charger IC malfunctions.

author: Rick Law
author_rank: Super Contributor
timestamp: May 20, 2015, 07:34:44 pm
content: Quote from: eas on May 20, 2015, 05:39:38 amI think on the initial go and until one is comfortable that the cell is AOK, tracking just mAH may not be enough. There needs to be a temperature cut-off.The thing that raise the alarm with this 6-pack (from a Fujitsu) was temperature. The log shows CC ran fine, CV was normal for a while. At some point, mA stops dropping and did a U turns and start increasing. Without a temperature cut-off and at high enough charge current, it may turn into a runaway thermo event.If the charger has a 1A limit for instance, such U turn may let it rise back to 1A. Charging at 4.2V would mean 4.2W of heat has to be dissipated for the cell not to continue the temperature increase.The cheap USB TP4056 charger works nice if the cell is good. While the TP4056 chip supports temperature cut off, most of the cheap modules doesn't implement the temperature cut off. So if it is left charging and unattended, it could be a danger.