08 September 2021

A question about recharging batteries

I use rechargeable batteries for my keyboard, mouse, alarm clocks etc and have noticed a puzzling phenomenon.  My recharger is probably a standard model, with an indicator light that turns off when the process is "complete."

But... if I then test the recharged batteries in a battery tester, sometimes the "recharged" ones are only perhaps 70% max.   It is not the battery's fault, apparently, because when I pair up a couple 70-80% ones and reinsert them, they recharge to the 100% level.

I'm guessing that when two weak batteries are inserted, the recharging process proceeds until one of the batteries is at max, then shuts off, without continuing for the second battery.  If I insert a 75% and a 25%, the 75% reaches 100% but the 25% might be only 60-80%.  They would seem to be charging in series, not in parallel.

It's easy to workaround the problem by using a battery tester, but it's a nuisance.  I don't know whether the design is necessary to avoid overcharging the better one of the pair, or whether my (old) recharger was suboptimally designed.

Someone out there will know the answer.  Thanks in advance.

(Just to clarify, my tester does not give exact percentages; I am extrapolating from a red/yellow/green non-numerical readout.)

Addendum.  Wow.  Lots of useful information in the Comments section, including this link to a comprehensive relevant website.


  1. First of all, I hope you know that rechargeable batteries usually output 1.2V or so (and it declines slowly), while non-rechargeable ones operate at 1.5V (declining faster). If your tester measures the voltage, this might be an issue.
    Otherwise, this model seems to charge the batteries in pairs for one circuit, so it cannot really cope with batteries at different charge levels. Try to pair batteries that were used together, it will yield better results. If you want to buy another charger (and good batteries), I recommend Panasonic's Eneloop system. All of the chargers have one circuit per battery. The white series of batteries last for a very long time and they keep their charge much better than usual. https://en.wikipedia.org/wiki/Eneloop

    1. I am STILL using a set of Eneloops purchased around 2004. They still work wonderfully for my uses. Remote with sound built in (Roku) Mouse (Computer) etc. Low impact stuff.

  2. you want a battery charger that charges the batteries separately rather than in pairs.

  3. Many cheap chargers charge by the pair to save on internal electronics. If you have unequal batteries, this will hurt your batteries.

    You can also buy chargers that will charger individually. I have one from EBL that works nicely, but I'm sure there are others.

    There are also very fancy chargers that have some programming incorporated that really tops of every battery to its individual needs. These are pricey.

    Now here's my question. Why did none of this matter when I charged my walkman batteries hundreds of times with my dumbest of dumb charger in the 80s and 90s?

  4. The batteries must be in parallel. Note that you can charge up to four batteries at once - only a parallel circuit allows this. If in series, they would receive the same electrons or there would be no circuit. Oh, and the charger would have to provide double the voltage desired. In parallel, each battery would receive the same input voltage, but a battery could be rejecting the charge. One of the signs of an aging battery is that it will not take a full charge on the first try. Eventually, it will not take full charge at all.

  5. Get a better charger, the kind that has auto shutoff, a display that shows you what it's doing, chemistry detection, and selectable charging strategies to optimize battery life. Product placement is right out, but a quick Amazon implies that the price floor for a machine worth having is around $30. The El Cheapos will wear your batteries out before their time, sometimes very quickly indeed.

    1. Will do, Frank. Thank you. Your last sentence may explain why I have had to toss so many rechargeables over the years.

  6. Normal batteries typically have 1.5V and rechargable batteries typically have 1.2 to 1.3 V. A normal battery tester is calibrated for normal batteries and therefore never shows full charge for rechargables. You can use a Voltmeter to compare rechargeables somewhat reliably.
    And yes, your charger charges batteries in series, which is a bad thing. It may reduce the lifetime of the fuller battery. This usually is the case with the chargers supplied with the batteries in a pack. You should look into a charger with "individual control" or similar feature. This also allows to charge a single battery.

  7. i check my batteries by measuring their DC current with an analog voltmeter.


  8. I second the Eneloop recommendation.

  9. Because "measuring" the capacity using the voltage is highly unreliable.
    As seen here: http://madscientisthut.com/wordpress/wp-content/uploads/2013/12/Rayovac50ma1.jpg
    The only proper way is to measure the mAh pulled from the battery.
    I fully agree with Frank Wilhoit, you need a better charger.
    I will also not do any recommendations, but instead refer to https://lygte-info.dk/
    He's doing tests of batteries, battery chargers and torches. Extremely dry, serious and full of any facts you can wish for.

    Definitely worthy of an entry on your blog, perhaps if it wasn't for his site being chemically void of design. :-D

    1. TYWKIWDBI is in no position to pass judgment on other websites based on the quality of their design, so I have gladly incorporated your second link into the body of my post. Thank you, Jörgen.


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