Smart Phones Should Measure Charge Speed

My first mobile phone lasted for days between charges. I never really found out how long it’s battery would last because there was no way that I could use it to deplete the charge in any time that I could spend awake. Even if I had managed to run the battery out the phone was designed to accept 4*AA batteries (it’s rechargeable battery pack was exactly that size) so I could buy spare batteries at any store.

Modern phones are quite different in physical phone design (phones that weigh less than 4*AA batteries aren’t uncommon), functionality (fast CPUs and big screens suck power), and use (games really drain your phone battery). This requires much more effective chargers, when some phones are intensively used (EG playing an action game with Wifi enabled) they can’t be charged as they use more power than the plug-pack supplies. I’ve previously blogged some calculations about resistance and thickness of wires for phone chargers [1], it’s obvious that there are some technical limitations to phone charging based on the decision to use a long cable at ~5V.

My calculations about phone charge rate were based on the theoretical resistance of wires based on their estimated cross-sectional area. One problem with such analysis is that it’s difficult to determine how thick the insulation is without destroying the wire. Another problem is that after repeated use of a charging cable some conductors break due to excessive bending. This can significantly increase the resistance and therefore increase the charging time. Recently a charging cable that used to be really good suddenly became almost useless. My Galaxy Note 2 would claim that it was being charged even though the reported level of charge in the battery was not increasing, it seems that the cable only supplied enough power to keep the phone running not enough to actually charge the battery.

I recently bought a USB current measurement device which is really useful. I have used it to diagnose power supplies and USB cables that didn’t work correctly. But one significant way in which it fails is in the case of problems with the USB connector. Sometimes a cable performs differently when connected via the USB current measurement device.

The CurrentWidget program [2] on my Galaxy Note 2 told me that all of the dedicated USB chargers (the 12V one in my car and all the mains powered ones) supply 1698mA (including the ones rated at 1A) while a PC USB port supplies ~400mA. I don’t think that the Note 2 measurement is particularly reliable. On my Galaxy Note 3 it always says 0mA, I guess that feature isn’t implemented. An old Galaxy S3 reports 999mA of charging even when the USB current measurement device says ~500mA. It seems to me that method the CurrentWidget uses to get the current isn’t accurate if it even works at all.

Android 5 on the Nexus 4/5 phones will tell the amount of time until the phone is charged in some situations (on the Nexus 4 and Nexus 5 that I used for testing it didn’t always display it and I don’t know why). This is an useful but it’s still not good enough.

I think that what we need is to have the phone measure the current that’s being supplied and report it to the user. Then when a phone charges slowly because apps are using some power that won’t be mistaken for a phone charging slowly due to a defective cable or connector.

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