Consequence, effects and solutions - mobile-phone always connected to AC power

Question

Would like to ensure that a mobile phone is always charged, without requiring manual intervention (s.a. disconnect/reconnect charger, switch on/off charger etc.). One thought is to put the phone permanently, with it's wall-wart charger connected to AC mains, always. 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 okay to make changes to the circuit of mobile charger.

Edited: 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 fixed-line replacement (since area isn't served by fixed line phones). A care-taker would use it periodically to report well-being, and share some video / pictures of the place by sending MMS. The place has bad power-supply, and thus the objective of keeping phone always charged. I do not want to hand-over the phone to the care-taker, since he doesn't live on this property.

Answer 1

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:

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.

Answer 2

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 )

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.