This all started a few days ago when I evaluated some iClever USB chargers (see Meet my 4-Port beast of a USB charger).
As part of that column, I made the following comment:
I don’t know if this is still true, but I remember being told that the batteries in things like iPads can exhibit a sort of "memory" effect that impacts their charging ability -- also, that it's best to let the charge fall to around 10% and to then recharge to 100% in a single sitting.
I've informed my wife (Gina The Gorgeous) and my son (Joseph The All-Knowing) as to this charging philosophy on numerous occasions. As with most things, however, they haven’t paid the slightest attention to what I've had to say. On the contrary, they tend to charge their devices willy-nilly whenever they happen to come into close proximity with a charger and the mood takes them.
The embarrassing thing is that -- and I'm saying this in a hushed whisper with a brown paper bag over my head -- it turns out that their way of doing things may actually be better than mine. The way in which I discovered this niggling and nagging nugget of knowledge was when a reader, Roger46, posted the following comment to my USB charger column:
As a calculator design engineer in the 70s, I well remember memory effect when we were using NiCad rechargeable batteries. That and a rather high self-discharge rate. Lithium batteries seem to have moved past that era, fortunately (see this discussion about prolonging the life of modern batteries on BatteryUniversity.com).
My personal experience seems to agree with their suggestions. My wife and I have had similar phones in the past. She tended to use hers until it ran down, while I liked to keep mine recharged as often as possible. She replaced the battery pack in one phone twice while I was still on the original with identical phones...
I immediately bounced over to see the discussions in question. Arrgggh! It seems that limiting oneself to partial discharges reduces stress and prolongs battery life. In Table 2 -- Cycle life as a function of discharge -- we discover that a 100% DoD (depth of discharge) results in only 300 to 500 discharge cycles; a 50% DoD offers 1,200 to 1,500 discharge cycles; and a 25% DoD provides 2,000 to 2,500 discharge cycles.
Now, remembering that -- viewing this simplistically -- 1,000 discharge cycles at 50% DoD equates to 500 discharge cycles at 100% DoD with regard to the service life of the battery, it seems to me that the 50% DoD offers the optimum lifetime of the aforementioned options.
As usual, however, there's more to the story. I trotted over to see my chum Ivan in the next bay, because Ivan is a guru on all things related to power. Ivan noted that NiCad rechargeable batteries did indeed exhibit a memory effect and required a deep discharge in order to obtain the best results. Ivan also confirmed that the Lithium Ion batteries used in today's smartphones and tablet computers no longer suffer from this effect (the batteries in your notepad computer and/or batteries of different Lithium chemistries may be another story).
Ivan also proffered a few more sage words of advice. He noted that elevated temperatures also affect battery life and that, running one's iPad (for example) at full whack will cause the battery to warm up a tad. The point is that charging the battery when it's already warm is not a great idea; it's better to wait a while and then charge the device when it's cooled down.
But wait, there's more. Once the battery has reached 100% charge, that's a good time to stop charging it -- continuing to trickle-charge a Lithium Ion battery (like leaving it plugged into the charger overnight) can also degrade its performance over time. Of course, this depends on the sophistication of the charger and/or the thing being charged.
More sophisticated chargers will take the environment into account and properly terminate charging at the optimal point. Some chargers are so smart they can determine the battery's chemistry and make any appropriate adjustments. Similarly, more sophisticated devices will recognize when their batteries are chock-a-block full, at which point they will stop drawing power from the charger.
The bottom line is that I will be changing my charging habits henceforth. I'm not going to become going to try to not become obsessive compulsive about this, but I am going to start charging my iPad when it reaches 50% DoD. Also, if I've been doing something compute-intensive like watching videos or running a simulation, I'm going to give the little scamp time to cool down before commencing the recharge process. (I will do all of this until devices evolve to using different battery chemistries, at which point everything will have to be re-evaluated like déjà vu all over again.) How about you? Will what you've read here change your modus operandi vis-à-vis charging your smartphone and tablet?
Max Maxfield
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