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Does mAh rating of a battery affect the device at the same rated voltage?

Discussion in 'Power Electronics' started by patkim, Oct 23, 2018.

  1. patkim


    Dec 7, 2017
    I have a Panasonic Cordless phone. Its batteries went dead after about 2 years and I bought a new set of batteries from an online shopping site. The originals were 1.2v 600mAh pair whereas the newer ones were 1.2v 800mAh pair.

    A few days down the line the display failed and I took it to the service center. The engineer said this has happened because of new batteries having higher mAh rating that the original ones.

    I am under the impression that for the same rated voltage mAh rating just defines the duration how long the battery can supply current. So 1.2V 800mAh shall last longer than 1.2V 600mAh for the same load.

    Is my understanding correct? Are there any further complexities or caveats that exist in this scenario that a layman like me is not aware?

    Thanks and regards.
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The "engineer" is making excuses.
    Ian, hevans1944, patkim and 2 others like this.
  3. Tha fios agaibh

    Tha fios agaibh

    Aug 11, 2014
    Yes, your understanding is correct. The larger mAh just means it will last longer.
    The current that's draw is determined by the phone load not the battery. Wither it's a 600mAh or a 800mAh battery it will still draw the same amr of current.

    The only potential problem I could see, is if the battery capacity was way too large,and therefore made the charging circuit work beyond its intended duty. (Not shutting off/overheating)
    patkim, 73's de Edd and Bluejets like this.
  4. dave9


    Mar 5, 2017
    These phones trickle charge the batteries, and overcharge them most of their life. You will get more lifespan out of them if you leave the phone off the charger base once it is fully charged.

    Low mAh cells were chosen because they tend to survive more cycles and overcharge (and are cheaper towards that end).

    I suppose it is possible that newer model phones have changed that, now have a proper charger circuit but I doubt it as long as they stay with NiCd and NiMH.

    This trickle charging is also relevant to your prior post about what the right charge voltage would be, because it would float well above 3.0V because the PSU DC input voltage is limited by a resistor and would float higher than 3.0V depending on the load on it on the charging circuit and how drained the battery is if connected and charging.

    Your display died for some other reason, depending on how it failed. Most likely the phone was dropped damaging the connection to the display.

    I don't understand this "take it to the service center"? Panasonic warranties for only 1 year which it is long past, and it can't be worth the time to take it somewhere and pay parts and labor now. I also doubt that you talked to a (real) engineer.

    You state that the display failed but does the phone work otherwise? If there is some kind of internal short circuit that could kill your battery life.

    It might be time for a new phone. ;)
    patkim likes this.
  5. patkim


    Dec 7, 2017
    Display died naturally! I did not do anything specific. The only co-relation I have is it happened a while after replacing the batteries. While the phone had only 1 year warranty, I took it to the service center to get an idea of extent of the issue because except for display the phone was working just fine. It was capable of making and receiving calls.

    After charging me for minimal inspection charges they said that it requires full PCB replacement as display is integrated with PCB and that would cost me almost 80% of the price of new phone. So you are right, it's time for a new phone.

    When I handed over the phone to them, their engineer opened the battery cabinet and inspected the batteries first before me and made that statement after looking at 800mAh rating.
  6. BobK


    Jan 5, 2010
    Nitpick. I am not saying this caused your problem.

    You cannot always replace a battery with a higher capacity battery and have it work correctly.

    I replaced NiCd batteries in a drill driver with NiMH of higher capacity, but it did not work because the new batteries could not supply the needed current.

  7. ChosunOne


    Jun 20, 2010
    Bob is absolutely correct, and IMHO, it's not a nitpick. One thing no one asked yet is, what kind of batteries (NiCd? NiMH?) did your phone have originally, and did you replace them with the same kind of batteries? If you replaced NiCd with NiMH, then they won't charge properly, regardless of mAh rating.
    40 years ago, if it physically fit, then it was probably compatible. Those days are long gone.

    If you would name the model number of your Panasonic cordless phone and/or the model number(s) on your original and replacement batteries, we could give you better advice. Details matter!

    Also, assuming both sets of batteries were NiCd, decades ago I used to work with NiCd-based power supplies for small units comparable to cordless phones, and I seem to recall that we couldn't just raise the mAh rating of the standby batteries without upgrading the power supply/changing circuit to match, or they wouldn't charge properly. My industry, security alarm systems, eventually got away from NiCd batteries altogether (partly because they were so finicky about how the batteries had to match the charger).
    But my experience may be completely out of date. I'll leave that to the experts here.
    Last edited: Oct 26, 2018
  8. dave9


    Mar 5, 2017
    ^ It may be somewhat of a nitpick in this particular case. These phones most likely use a crude trickle charger, a wall wart (wall plug AC/DC adapter) somewhere between 6VDC and 12VDC, then a resistor in series to charge 3 x AAA series cells.
  9. ChosunOne


    Jun 20, 2010
    Seriously!? Okay, my bad, I guess. I have no experience with cordless phones of any brand or model. I'm just surprised that any electronic device in this day and age has that primitive a charging circuit. Apparently it works well enough, though.

    I have an old cordless phone stored away somewhere (haven't had a land line in over 10 yrs). I might be curious enough to dig it up and take it apart to check out its charging circuit---if I can find it.
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