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I want to ensure that a mobile phone is always charged, without requiring manual intervention: e.g. disconnecting/reconnecting the charger, switching the charger on/off, et cetera). One idea would be to leave the phone always connected to its charger, which in turn would always be connected to the AC mains. How feasible / practical is this? And are there better approaches?

Note that I am unable to make changes to the phone, because it is an off-the-shelf, commercial phone. But I can make changes to the circuit of the charger.

Edit: This is not a specific phone model I am talking about, but potentially a phone I'd buy, for a very specific purpose. This might be lower-end Android smartphone. The phone would be left in an unattended remote property, and serve as a landline replacement (since the area isn't served by fixed-line phones). A caretaker would use it periodically to report well-being, and share some video / pictures of the place by sending MMS multimedia messages. The place has bad power supply, and thus the I want to keep the phone always charged. I do not want to hand over the phone to the caretaker, since he doesn't live on the property.

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  • \$\begingroup\$ Found a somewhat related (but not exactly same) question here \$\endgroup\$ – icarus74 Jul 21 '12 at 18:43
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Most modern phones have a charging control circuit which takes care of what you are proposing. When the battery is charged, the charge control IC will terminate charging of the battery, then monitor to see if it needs charging again. So all the charger has to do is make the power available.

Below is a flowchart from the datasheet of MCP73831 Li-Ion charge controller IC from Microchip:

MCP78131

You can see the various stages of the charge cycle. This is a pretty basic IC (I just picked it since I've used it a few times) but does the job and is nice and cheap - many phones will have a pretty complex power management IC, possibly a custom one.

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    \$\begingroup\$ Thanks for taking time to answer my question. This is precisely what I was hoping to hear. Now, is there something that one can observe and determine, without ripping a phone model apart, if they do have such power management IC's. For all the mobile phones I've come across, I've been disappointed to find that the "battery-charging" animated icon starts showing activity, on being plugged-in, indicating that they lack such intelligence to prolong battery life! \$\endgroup\$ – icarus74 Jul 22 '12 at 7:27
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    \$\begingroup\$ You could rig up a little test jig to put your multimeter in series with charger and phone to measure/plot charge current. If it's something like the above you should be able to see the various stages, eventually dropping to 0A (or a very low value of leakage current, e.g. ~1uA or so) Either that or see if you can find a teardown of your phone, check what PMIC it uses and refer to the datasheet. \$\endgroup\$ – Oli Glaser Jul 22 '12 at 8:08
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While all that has been said is on the right lines, one also needs to understand the high internal temperature of the battery and adverse affects when fully charged and continuing to be in that stage. To that extent charging fully and retaining in that stage well not cause electrical damage to the battery but will definitely affect the capacity of battery and hence long life for to the temperature induced stress.

Coupled with this is a related aspect of not charging fully and not letting the battery discharge fully before charging

There are a lot of articles on this at battery university apart from this here.

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My experience tells me that rechargeable batteries have a finite life that for benign conditions is the cumulative sum of charge rate * time and discharge rate * time. i.e. There is almost a constant finite cum. Amp-hr life in rechargeable batteries unless you exceed the rate or depth of discharge recommended by the mfg of that technology. I am not talking about the Amp-hr of a single use, but the Amp-hr life of all uses is finite. If you want to estimate its cost. here's a test. compute the MTBF of rated battery / cost of replacement and come up with a cost/hr of using the battery. It may surprise you .. or not.

i.e. 1000x -50% discharge is close to 2000x discharges at -25% depth but this relationship gets nonlinear below 50% discharge depths and depends also on Discharge rates. Here assuming C/5~ C/10.

So running on the charger 100% of the time should give you infinite lifetime? not quite.

THere is a supplementary rule of thumb, that says "if the battery temperature increases for every 10'C" {from ambient heating of nearby electronics} "it's lifetime will degrade by 50%" even if not in use. Quite often you will see storage of batteries in fridges because the self-leakage tends to be lower and this extends lifetime. Self leakage is a temporary thermal characteristic but also becomes permanent over time.

The 50% reduction every 10'C or "Arrhenius Equation" rule for all chemistry applies to all electronics as it does on Li-Po batteries, but there are also threshold effects of damage from over-temperature that also accelerate failure rates as well if discharge rates increase defect rates and average lifetime will accelerate, meaning that no or light use of batteries is the smartest way to extend your battery life.

Notice how resistance increases with age of battery. THis also affects self heating and is another key indicator of a battery getting old. ( Gee if never used to get this hot )

enter image description here

Even if it is a little inconvenient to keep plugging it in. Of course you must be gentle on the charge plug and remember it can fail too near each end from over-stretching and moving a sharp bend radius. So be gentle on your cables.

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    \$\begingroup\$ Thanks for taking time to answer. Phones are probably not designed to keep charger plugged-in all the time, but there are indeed devices with in-built battery back-up to ensure uninterrupted operation for in-frequent power outages. I believe, those are designed differently then ! I have such a large LED wall-clock gifted by ex-colleagues, with inbuilt battery, that's been running fine for past 4 years. \$\endgroup\$ – icarus74 Jul 22 '12 at 7:14
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Good going!

Batteries are after all power units. When a source is available (like you are near a wall socket) always use the source. The charger usually has an extra capacity (unless your mobile is fully drained and you are charging it) and hence if the mobile is used it does not discharge the battery.

Better to remember that the battery life is determined by the number of cycles (especially the deep ones where your dip below 30% and recharge) and it should in principle prolong your battery life. It is best to use the phone with battery between 95% (100% too is okay but not needed) and 45% for a long battery life.

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  • \$\begingroup\$ AFAIK, Li-Ion batteries do not really suffer from the low discharge problems you describe. \$\endgroup\$ – Rev1.0 Oct 28 '15 at 13:38

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