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Forgive my ignorance, I'm not an electrical engineer. Can someone explain why there apparently are no decent chargers for 9V blocks? There are two issues I would like to clarify:

  1. Normally, you can not charge LiOn batteries on a NiMh charger and vice versa. Also, I have never seen a consumer grade charger that can handle both types. However, with 9V, some claim that they can charge both NiMh and LiOn. I suspect faulty product description, and find it dangerous, but what does an expert say? Is it even possible for the charger to detect the type of cell? Can a 9V LiOn block blow up in such a charger?

  2. Why is it so difficult to charge 9V blocks with the same comfort as 1.2V cells? I have yet not seen a charger that displays the current charge status & progress bar and has a reliable charge stop. With 1.2V cells, almost any 30EUR charger can do all of these things. Are there technical issues at work, or is it just a matter of poor/cheap product design to increase profit? Because even the "high end" devices do not do a better job at this.

I'm talking about consumer grade devices in the price range below 50 EUR.

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  • \$\begingroup\$ What is your “comfort” parameter? \$\endgroup\$
    – Solar Mike
    Feb 17 '20 at 11:21
  • \$\begingroup\$ @SolarMike Reliable charge stop, being able to see the progress of the charging process, being able to rely on the safety of the device (will know how how fast to charge, when to stop, not charge anything unsuited for the charger...). With AA/AAA, that is all very standard nowadays. With these 9V cells, it's just a mess as far as I can see. \$\endgroup\$ Feb 18 '20 at 9:51
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I can mostly answer your Q2: the reason you have

yet not seen a charger that displays the current charge status & progress bar and has a reliable charge stop.

is because 9V-rechargeables are a very niche product, which also has a lot of fragmentation:

  • 6 NiMH cells (require charging to about 9V, nominal average voltage 7.2V)
  • 7 NiMH cells (the most common I've seen, require charging to 10.5V, nominal voltage 8.4V)
  • 8 NiMH cells (require charging to about 12V, nominal voltage 9.6V)
  • 2 Li-Ion cells (require charging to 8.4V, nominal voltage 7.4V)

So typically the manufacturer of the battery would issue some cheap charger with it, and warn in big letters that you should not try to charge other brands (I've had a Tronic (LIDL) one, which even had percentage indication IIRC, but all other ones I have are noname cost-cut trickle-charging ones, which ruin incompatible batteries very quickly; you get what you pay for, i.e. almost nothing).

The other important reason it is such a niche product is because of their very small capacity:

  • A typical primary 9V is ~600 mAh (with alkaline cells), while a typical NiMH 8.4V rechargeable is just shy more than 250 mAh.
  • Compare with AA and AAA where the rechargeable versions nearly reach the capacity of their alkaline counterparts.

On your question 1, I don't know for sure, but I suspect the fragmentation alone makes the detection almost impossible. Compare a 1-cell NiMH vs 1-cell Li-Ion - the detection is a piece of cake, just a voltage comparison. For the 9V the voltage ranges overlap, so you may try to do some sort of internal-resistance detection trickery, yet I suspect that would be quite unreliable. In the end it would probably be best just to avoid mixing charger/battery manufacturers for the sake of safety¹, and we arrive at the status quo.


¹ yes, unprotected Li-Ion 9V block could definitely explode if charged on a 8.4 NiMH charger. OTOH I've only seen protected Li-Ion 9Vs but I have not dared to test that part.

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  • \$\begingroup\$ Thanks a lot for the info. At least some clarifcation. However, there are tons of 9V rechargable cells now available, mostly without a dedicated charger, and chargers that mostly say "NiMh" only, but some have LiOn mentioned "somewhere", but not in a way that seems 100% intentional, some claim to handle both (which I doubt for the reasons you clarified), and a few LiOn only (they come with the batteries). So it seems there is a "standard", but it is not really made explicit in any of the product descriptions. Best to follow your advice then and stick to charger+cell sets of one company. \$\endgroup\$ Feb 18 '20 at 9:46
  • \$\begingroup\$ I've only seen NiMH chargers, for Li-Ion I had to knock up a DIY one (but that was 8 years ago). Can you post some examples of the ones that claim compatibility with both chemistries? I'm definitely curious what are they saying about the detection. \$\endgroup\$
    – anrieff
    Feb 18 '20 at 10:08
  • \$\begingroup\$ There's this one: amazon.de/gp/product/B07F58PXYR that claims both (Li-ion is mentioned way down, only once, very suspicious). There are 9V Li-cells from this company that apparently can be charged with this (amazon.de/dp/B07CTKSCPQ). This one claims Li and NiMh also: amazon.de/dp/B07HKXCT2Q. But they say nothing about the method of detection. That one says "LiOn 9V" but "NiMh AA/AAA", not sure about NiMh 9V: amazon.de/dp/B07W1137NW. \$\endgroup\$ Feb 18 '20 at 12:15
  • \$\begingroup\$ ... then there's this:amazon.de/dp/B07XDTGHSC which looks like a clone of the first one (from Bonai). \$\endgroup\$ Feb 18 '20 at 12:20
  • \$\begingroup\$ Yeah, for the first one it does seem they got it wrong in the Q&A section, it is really a Ni-MH/Ni-Cd charger, and won't work with Li-Ion. Also 20hrs charging time for 600mAh is 30mA, which hints at simple trickle charging (they probably use -ΔV for the AA/AAA, but not here). Li-ions can be charged much faster, the charging algorithm allows it. \$\endgroup\$
    – anrieff
    Feb 18 '20 at 13:23

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