Replace your project power supplies with recycled Li-Ion cells and a switching regulator


[Dr. Iguana’s] experience moving from projects powered by disposable Alkaline cells and linear regulators to recycled Lithium Ion cells using the buck regulators seen above might serve as an inspiration to make the transition in your own projects.

The recycled cells he’s talking about are pulled out of larger battery packs. As we’ve seen in the past, dead battery packs for rechargeable tools, laptops, etc., are often plagued by a few bad apples. A small number of dead cells can bork the entire battery even though many perfectly usable cells remain. Once he decided to make the switch it was time to consider power regulation. He first looked at whether to use the cells in parallel or series. Parallel are easier to charge, but boosting the voltage to the desired level ends up costing more. He decided to go with cells in series, which can be regulated with the a less expensive buck converter. In this case he made a board for the RT8289 chip. The drawback of this method requires that you monitor each cell individually during charging to ensure you don’t have the same problem that killed the battery from which you pulled these good cells.

15 thoughts on “Replace your project power supplies with recycled Li-Ion cells and a switching regulator

  1. It’s not an Out-of-balance situation that kills a Laptop battery. It’s because they are getting too hot and mistreated. Either a Laptop battery sits empty or full for long periods, and that kills it. Lithium Ion/Polymer cells have it best at around 3,85V and nothing else. The electronics inside a laptop battery pack DO balance out the cells, otherwise it would be a recipe for disaster….

    1. You hare halfway right. The batteries in laptops do go out of balance since one set of cells will die first. I have recycled something like 50 laptop packs and almost all have one set of cells under acceptable voltage.

      One interesting fact is that the cells retain most of their capacity even when very old and worn out, what they lose is peak current capacity. I have several cells from laptops that won’t output more than a few hundred mA but still retain most of the original capacity. They can’t produce 2.2A for one hour, but they can produce 100mA for 22 hours.

      As for the tip in the article, i usually prefer to use a boost circuit for low current 5V applications (less than 500mA), this eliminates the need for balance charging. For high current or higher voltage applications a buck circuit would obviously be preferred.

      I would like to put in a tip about this boost circuit with built in battery protection, it has a couple of flaws like any chinese product, the main one being that the 5V booster is always running, but this can be remedied by cutting a trace and putting a switch between the battery protection and booster. A minor flaw is a too small output capacitor.

        1. How do you charge it? From a laboratory power supply set to exactly 3.7V output?
          Sadly on the ebay page, the schematic contains no chip numbers so can’t really decide which one is the protection part of the circuit, and where to cut the trace.

          Also by switch you mean a plain old switch, which you operate *manually*?

          1. I either use a lab supply set to 4.2V (which is the peak voltage of a 3.7V battery) and current limited to the capacity value of the battery. Or i use a dedicated lithium charger.

            The cut needs to be made on the underside where the current comes into the inductor. The manual switch would be to turn the battery power off so that the battery isn’t constantly draining.

            If you don’t want to mess with that module you could get protection circuits loose, for example works fine. The great thing about that combined circuit is that the price and size is very good.

    2. Yes! Amazingly, I have found the majority of paralleled cells in a pack are still charge balanced after 15 years of storage. I mark all parallel sets before I cut them apart. BTW, all of the two cell LiPo chargers I have bought run the 2 cells paralleled, so charge balanced sets are preferred on these $8ppd chargers.

      1. Oh yeah, for $12ppd you can buy a small device which holds 2 x 18650 and outputs a regulated 5V @ 1A from a USB connector. It also (correctly) charges the LiPo cells if the unit is plugged into a USB port. To store long term, briefly disconnect the cells and the processor goes into sleep, with very low current drain. Hit the reset button to make the unit automatically come on when it senses that something is plugged into its output USB connector. It looks like it could keep a cell phone going a week or more.

  2. I’ve been meaning to do this with some wireless devices I have (wireless keyboard and mouse etc)
    The main thing stopping me is the protection circuitry. You can get some nice 18650 cells that protection built in. But the raw ones from laptops don’t. And I managed to find that out the hard way when attempting to make a li-ion pack for my cordless drill lol.

  3. About the ballancing issues.
    Current li-po battery packs you buy online and pretty much any charger worth more than 20$ will balance the cells. I find the problem to be more of keeping the batteries at too low of a charge for too long is the main problem.

  4. Although just 3.2V (& with lower energy),the FAR safer LiFePO4 version may better suit? These are very rugged,cheap & tolerant, & claim 1000s of charge/discharge cycles (rather than the 100s of Li-ion)

  5. Personally, I’m not messing with Li-Ion batteries outside of using them with provided charging circuits given the fire hazard of overcharging them leading to a fire. To each their own, of course.

    1. I have made lots of things with rechargeable Li* and i have never even been close to creating a fire, you need a faulty charger and a faulty protection circuit to get a serious fire, it is nearly impossible to get a battery with a proper protection circuit to catch fire.

      The chinese 18650 protection circuits can be had at about $5 for 10 and they work fine, i have been through something like 50 of them and not had one problem.

  6. Small math mistake, but it doesn’t matter (practically speaking). The resulting voltage is 5.98V instead of 6.01V. This is because 10k/2k66 is 3.76 not 3.92. My bet is a quick typo on the calculator, because doing 10k/2k55 gets the same results as is on the blog post.

    Overall, nice job.

  7. I have 18650 batteries dated 1995 that work very well! These cells are the most frequently found in laptop battery packs. I get a 60-70% usable cell count out of retired laptops. Many cheap Chinese electronics now use the 18650. NOTHING needs to be built. I absolutely LOVE my 200 lumen head lights, 18650 powered, about $24 ppd. The 300 lumen UltraFire WF-501B flashlights are wonderful, simple (desirable) on/off. Under $10ppd. If that isn’t enough, the body OD is exactly one inch and fits perfectly into 1″ gun scope mounts. Chargers run about $8, but overcharge a bit, 4.24V instead of the correct 4.20V, This voltage is critical for LiPos. I saw a ref saying that a 4.15V termination nearly doubles cell life charge cycles while dropping capacity about 10%.

    Usually, the cells are connected with spot welded nickle strips. You cut it flush with the negative end and smush what remains flat. The longer strip on the + end is then folded several times to make the titty. These cells are NOT protected! Do not short! Do go to WalMart where in camping dept. you will find a tube of waterproof matches for under a buck. The screw top plastic tube (with rubber gasket) if an absolutely PERFECT fit for the 18650 and allow you to carry a mess of them in complete safety.

    You can find peddlers on Ebay, or just Google 18650. If you have problems with these foreign sellers, just post on and next day they will be tripping over themselves to make you happy! The 18650 battery is an inexpensive store of big energy.

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