Phone Batteries – Some Things You Might Not Know

By March 8, 2018Technical


Batteries are surprisingly fussy things. The different types (Alkaline, Li-Ion, Ni-MH etc) each have their own strengths and weaknesses. Today I’m talking about some simple battery facts. And by battery, I’m specifically talking about the Li-Ion / Li-Polymer type of battery…so the battery in your smartphone, power banks and a lot of consumer electronics today. The aim here is to provide some researched advice and bust a few myths. Nothing more.


This is probably the simplest because it’s a fairly well-known term. If you don’t fully flatten some battery types before recharging them, the life will be permanently impacted. For example, if you only let your battery drop to 50% and then charge, the battery will begin to see 50% as 0%. Over time, this will leave you with a battery that thinks it’s empty but isn’t. So here’s the advice. Smartphone batteries do not suffer from memory effect in any way shape or form.


I stressed the word batteries there because some ‘smart batteries’ (batteries which include additional circuitry to monitor and report capacity to a unit) can fall out of calibration over time and it is possible for a phone to misread the battery life. The result of this isn’t a reduction in battery life though. It’s usually just an error in reported battery life. So a device may report it has just 20% left and activate all necessary power saving measures for that level. This can be a problem, if only because seeing 20% left is a bit unnerving on a phone. I recently updated my iPhone to iOS 11. After doing this I noticed a massive drop in battery life. Out of curiosity, I let the battery run down much lower than I would usually. The phone lasted a staggering 20 hours on just 3% of battery with normal use. Now, this may not have been exactly related to a calibration problem but it shows how reported battery life can be very different from actual battery problems. Anyway, the phone eventually died and following a full recharge had much more accurate reported life. You shouldn’t really ever need to do this but I thought I’d include this little anecdote in case anyone has experienced this.



This is when your phone says you have 40% remaining but it then drops suddenly and turns off. This can be a calibration issue but it is more often than not a symptom of high internal resistance. I’m not getting into internal resistance in this video but it’s usually caused by one of three things.

  1. A faulty battery
  2. An old battery at the end of its life
  3. A cheap and poorly manufactured ‘bargain’ battery

Higher internal resistance is actually an inherent part of certain low-cost battery chemistry but much less so with Lithium cells.


How long a battery will remain useful is as difficult to answer as how long will a charge last. The lifespan of smartphone batteries is usually quoted in cycles. How many times you can recharge a battery (a cycle) and what percentage of its original capacity it will achieve after this number of cycles. This sounds simple enough but it’s a terribly misleading number. It’s easy to think that if you have 500 cycles on a battery, surely letting your battery discharge more each time would extend each cycle, hence lengthening the life of the battery. Not so. Do not worry whatsoever about performing little bits of charges here and there. Li-ion batteries do not care at all. It’s all about something called Depth of Discharge. The same battery sold at 500 cycles will probably be running just fine after 5000 cycles if your depth of discharge is low. The advice here is discharging a bit and then recharging is fine.


Linking back to the idea of battery cycles, there is no need or benefit to fully charging a phone battery. In fact, depending on the charger or phone model, it’s better not to. When you take a battery to full charge, it stresses the battery more than e.g. an 80% charge. The drop in stress massively decreases the degradation of the battery over time. Of course, you get less power to use each time but the increase in life expectancy more than offsets this.


Quick start instructions provided with products still ask you to fully charge a battery before first use. Simply put, this is a myth. From a battery viewpoint, there is no need whatsoever to do this. You are told to do this to reduce potential technical support calls on devices where the battery is flat or it doesn’t last as long as a consumer expects. Do it if you want but don’t think you have to.

Batteries Icon


With a correctly operating charger, leaving a device on charge for long periods of time is perfectly fine. The best example is a work laptop which gets docked and is fully charged in 20 minutes and then left for the rest of the day. There’s no reason this should degrade the performance of the battery. A small caveat here though as this does assume the right charging environment. If you leave a device on charge and it’s getting excessively hot, you could have problems.


Battery voltages are usually marked on the label. Most people know that an AA Alkaline battery is 1.5V. This value is not really as fixed as you might think. It’s an average and the voltage actually varies quite considerably depending on how charged a battery is. Battery charging has very distinct stages and it is quite normal for a charger, phone and even cable to get quite warm when you begin charging a fairly flat phone. The current drawn from a Li-Ion battery can be pretty high. By the time the phone reaches a decent charge, this current will have dropped off and the charger should feel much cooler. If you’re at 90% and everything is still very hot, you might have a problem.

iPhone on Wireless Charger


Sometimes batteries fail. Sometimes chargers fail. In reality, many of the horror stories you see are down to human error. People forget to respect the fact that a battery is a massive ongoing chemical reaction sealed away inside your device. Put simply, Li-Ion batteries are fussy. They hate excess heat, stress, over voltage, under voltage and short circuit. Chargers have essential circuitry built into them to protect against this as much as they can. This is one of the reasons why it’s worth avoiding cheap chargers. It becomes a gamble how much of this protection is in place. The second reason is that there’s no guarantee that the whole USB output won’t become live at mains voltage due to poor design…but that’s another topic that I’ll leave to BigClive to cover.

I guess the advice here is to consider and be aware of these flaws when you charge your devices. For example, if you let your phone charge from flat wrapped up in your bag inside a scarf and put it next to a radiator, it’s going to get hot. It might be fine but when batteries get hot, they can rupture and explode causing quite serious damage. In the least, the heat will stress the battery and reduce its useful life. Equally, if you don’t pay attention to where and how you use your chargers you can short circuit a battery. Something silly like dropping a staple into the USB socket of a power bank and not noticing can cause some pretty catastrophic results.

This isn’t meant to scare you. Most phones and chargers can deal with these type of things and charging is becoming more and more aggressive but also more intelligent. Anything can fail but cheap replacement batteries? Well, you’re taking a risk. Cheap chargers from eBay? Again, maybe not such a good idea.

These are just a few basic safety aspects relating to batteries that people easily forget. Your phone just works for years with the same battery and it’s too easy to take for granted how much is going on to keep it that way.

So I hope that you found these battery tips and advice useful. They’re not essential because battery charging is something we do every day and probably get on just fine with. Hopefully, these will give you a bit more knowledge to know what helps and what doesn’t. If I’ve made any errors, please do correct me in the comments below or on the video page.

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