How many charges do you get on a tablet and mobile phone batteries before they need replacing?

So my previous phone iPhone 6 Plus I had 3.5 years, daily charging, and the battery on this unit is still at 94% capacity.

It's hard to answer this question as its a very generic one, it varies based on device, how much power it draws, what is the charging protocol used, how is the battery treated, what environment it's being charged in (temp, humidity)

Generally speaking between 500 and 1,500 charge cycles, for most tablets/phones that equals to about 3 years of general use within 90% capacity and then its a pretty steep downfall from there

For example, I recently swapped a battery in my old iphone5 (that's almost 6 years old) .. it was quite beaten up and didn't hold the charge for very long, it also had swollen up and began to crack. I like to replace batteries in old tech I have as I keep them as mementoes, and perhaps would like to fire them up again in 10 years time :)

I once read/heard somewhere that for best rechargable battery life expectancy, you are best to completely deplete the battery and full recharge it once a week. 

Not sure if this is a old-wives-tale or not, but wondered if it is still true with modern rechargables?

ElusiveMarlin said:

I once read/heard somewhere that for best rechargable battery life expectancy, you are best to completely deplete the battery and full recharge it once a week. 

Not sure if this is a old-wives-tale or not, but wondered if it is still true with modern rechargables?

 Fully discharging a lithium-ion battery is a HUGE NO NO, they can become unstable. this is why all products that are shipped with lithium-ion batteries will have charge in them (around 30-40%).

Li batteries work best with partial discharge cycles. (so not fully discharging and not fully charging)

3 things to keep in mind for a healthy li battery

1. Do not fully discharge, you should start recharging at around 5%
2. Do not fully charge the battery, if you can once it gets to 90%-95% disconnect the charger
3. Do not attempt to charge a fully discharged li battery that has not been used for a long time, it might become unstable

"There is one exception. Battery experts suggest that after 30 charges, you should allow lithium-ion batteries to almost completely discharge. Continuous partial discharges create a condition called digital memory, decreasing the accuracy of the device's power gauge. So let the battery discharge to the cut-off point and then recharge. The power gauge will be recalibrated."

This only applies to li batteries, for acid batteries (like Ni) a full discharge and recharge is more favourable 

Cool fact, an old li battery or one that has been overcharged and mistreated will begin to swell, the swelling is due to CO2 being produced by the nodes. There is a safety device called a CID (Current Interrupt Device) at around 145–200psi, this kicks in and prevents any more current entering the battery, should this safety system fail, the battery will rapture in a specific way and produce a flame, the rapture is the last fail-safe and is designed to prevent the battery from becoming a bomb, however as we all know this can also fail.

Ni-Cads liked being drained before recharging, because they develop a memory and to get full capacity you have to use full capacity. 

@LZoltowski - What about lipo batteries? I suppose they have to be discharged because I get about 10 minutes flying my Quadcopter with them. I take it they have depleted when the Quadcopter drops out of the sky are they not? I usually store them at about 30% charge if left in storage for a month, but would you say that's the right thing to do?

RedRizla said:

@LZoltowski - What about lipo batteries? I suppose they have to be discharged because I get about 10 minutes flying my Quadcopter with them. I take it they have depleted when the Quadcopter drops out of the sky are they not? I usually store them at about 30% charge if left in storage for a month, but would you say that's the right thing to do?

lipo is the same as li, most li batteries have complete depletion protection in them now, so I think at something like 0.5% charge they out shut off, but you should obviously not leave it at that for too long as li batteries discharge even when not in use, slowly, but they do.

RedRizla said:

@LZoltowski - What about lipo batteries? I suppose they have to be discharged because I get about 10 minutes flying my Quadcopter with them. I take it they have depleted when the Quadcopter drops out of the sky are they not? I usually store them at about 30% charge if left in storage for a month, but would you say that's the right thing to do?

Nope, that will usually be the low voltage cut making it "Drop out of the sky" and by flying that low you're damaging the battery.  Common noob mistake.