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We have a number of Android mobile phones permanently on charge. After approximately 3 months some of the device batteries expand considerable, splitting the phone & failing. The phones are in a regulated 23 °C environment, so I believe temperature is not the main problem. I believe overcharging is the problem, possibly due to the phones being low cost entry models and the internal regulator circuit not being great. Note that the screens are on permanently at full brightness. My guess is that they are using about 300 mW. The charger which comes with the phone is rated to 500 mA.

I've considered powering the phones directly without a battery however Android has gotten smarter and appears to require signals from the battery circuit board. My current thinking is to balance the charge capacity of the charger itself to the power the phone is using, e.g. a 300 mW charger.

My questions:

Would this strategy of trying to balance the charging current of the USB charger with the power use of the phone be effective at preventing over charging?

If so, could I simply limit the charge current by sticking a resistor on the positive. Would a single resistor do the job? and if so what value range? An online calculator gives me:

  • Resistance 16.667 Ω
  • Voltage 5V
  • Current 300 mA
  • Actual Power Rating 1.5 W
  • Recommended Resistor Wattage 2 Watt Resistor
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    \$\begingroup\$ Phone "chargers" aren't strictly speaking chargers at all, they're constant voltage DC power supplies. The actual charging circuitry is integrated into the phone itself (maybe except some of the oldest pre li-ion devices from the 90's). A phone draws only as much current as it needs to operate and charge; you could even connect one to a 100A power supply with no ill effects as long as the voltage is correct. \$\endgroup\$
    – jms
    Commented Nov 6, 2019 at 1:48
  • \$\begingroup\$ I'm assuming the phone's charging regulator is not doing a very good job. Maybe designed for the scenario where charging happens ever day or so. Would limiting what the phone can draw from the charger to what it is consuming reduce the overcharging problem? \$\endgroup\$
    – Bob Ox
    Commented Nov 6, 2019 at 2:12
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    \$\begingroup\$ @BobOx I think it is too complex a situation to discuss here. In theory, when you buy an Android phone from a manufacturer/distributor, you have a right to expect that the phone, battery system, and its charger are properly mated and well-designed. You've basically said, "No, they are not. So what now?" The obvious answer is to find another device and give these away or throw them out, with a lesson learned for next time. Trying to suss out where the problem lays is probably beyond the scope for you and/or the rest of us. And "quick, blind fixes" are a fools' choice. I wish it were otherwise. \$\endgroup\$
    – jonk
    Commented Nov 6, 2019 at 3:49
  • \$\begingroup\$ @BobOx That said, I pretty much agree with your flow of logic. You've addressed details such as the ambient temperature. You've stated that you keep these devices attached to their chargers. And I think you've managed to provide good evidence that (1) the phones and their chargers and battery systems are not designed well, or (2) you've bought a 3rd party charger to replace the original equipment manufacturer's version and applied it to a situation where it fails to work properly. Your only simple option is to buy a quality power source that meets or exceeds the phone specs, I guess, and hope. \$\endgroup\$
    – jonk
    Commented Nov 6, 2019 at 3:52
  • \$\begingroup\$ The strategy to use a charger that delivers just the right amount of power to balance the consumption is fated to failure. Eventually the phone will go into a low power consumption mode and the battery will fully charge. Modern phones use switched-mode chargers that will deliver to the battery the expected amount of current (in constant current phase) or voltage (in constant voltage phase) over a relatively wide range of input voltage. Therefore, using an external resistor will probably not be as effective as you may think. \$\endgroup\$
    – joribama
    Commented Nov 6, 2019 at 4:53

1 Answer 1

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We have a number of Android mobile phones permanently on charge.
Note that the screens are on permanently at full brightness.

Phones are not intended for such high duty cycle operation. They will overheat. This will cause the battery to be damaged.

Would this strategy of trying to balance the charging current of the USB charger with the power use of the phone be effective at preventing over charging?

No. In fact it would make it refuse external power at all. Working from battery alone due to the voltage drop.

Options:

  1. Buy the correct devices. Search for HMI panels with Android.

  2. Remove the battery and connect to the battery terminals directly. If the phone is picky, pull the management board from the battery and use those BMS battery pins to connect approximately a 4V source, simulating the battery. Success not guaranteed. Warranty most definitely void.
    Do this on a discharged battery and don't puncture or short the pouch.

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  • \$\begingroup\$ Some good points, thanks. I don't think heat is the issue though as I've taken the backs off the phones and the labels off the batteries to expose the aluminium casing, applied a thermal paste and stuck it on 3mm aluminium plate, to act as a heat sink. I havent measured the temperature but I'm guessing, by touch, maybe a 27c. I did look at HMI panels and at the low end I could get them for around US$200 however as I needed 25 and I could get a phone for $35 the economics won out. My thinking at the time was that a consumer mobile phone tends to live a hard life so probably ok for the job. \$\endgroup\$
    – Bob Ox
    Commented Nov 6, 2019 at 23:21
  • \$\begingroup\$ I have had a go at removing the management board from the battery. I could not remove the management board without damaging it. I tried with a small dremel disk grinder to cut away the seamless aluminium casing but almost immediately shorted it and got a nasty whiff of something, so left it to boil away. Interestingly after a few hours I was still measuring a 3v on it. Of the 4 pins the 1st and 3rd are power. I found the phone will work without the 4th pin connected but its looking for a signal from the 2nd pin. If I could somehow emulate this I might be on a winner. \$\endgroup\$
    – Bob Ox
    Commented Nov 6, 2019 at 23:30
  • \$\begingroup\$ I'm trialing reducing the charger current. With the phone at 100% the phone is pulling .34 to .54 A from the charger. It cycles up/down for whatever reason, not a stable current. Rather than a current limiting circuit I've put 2m of USB cable in between. I let the phone drain to 90% and plugged it in. The draw is now .22A according to the meter & the voltage drops from 5.1v to 4.6v. The battery % on the phone, if it can be relied upon, is charging at +3% per hr. I'm thinking that this less vigorous trickle charging regime can only benefit the battery life. \$\endgroup\$
    – Bob Ox
    Commented Nov 7, 2019 at 6:30
  • \$\begingroup\$ @BobOx You're lucky the shop didn't burn to ashes. Never puncture lithium pouches. The current limit isn't going to be a stable long term solution. Reconsider the HMI panels, or maybe you have to admit you've sold something that cannot be done for $35. \$\endgroup\$
    – Jeroen3
    Commented Nov 7, 2019 at 6:57
  • \$\begingroup\$ I'm hopeful as the battery, with the current limited to .22A, is now at room temperature. I was surprised that without limiting the current, even at full charge, the phone was still drawing the maximum current of the charger at times (0.5A). I guess with the screen @ max brightness the regulator does not cope very well. \$\endgroup\$
    – Bob Ox
    Commented Nov 7, 2019 at 23:33

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