Touch Battery Indicator is not accurate

[Ken Maltby]

First off, deep discharging Li-Ion batteries would kill them. But the manufacturers are aware of that and set limits on how low their batteries can be drained to. So what you see when the battery appears fully drained, is often about 40% of the battery's capacity. (Even that level is not safe if left too long, as self discharge will start killing off individual cells as they drop below a certain voltage, and they then can't be recharged.)

That said calibration is a real issue for devices, even if not for the batteries themselves. The calibration of monitoring circuits for Li-Ion are very straight forward and work within the limits built into the charging circuits of the device. It should not need repeated charging at the top end. In fact Li-Ion batteries can also be damaged by over charging, the harmful plating effects can start at relatively low levels of overcharging.

It would a good idea to let your device discharge to zero once in a while, but be sure to fully charge it to 100% soon after. The majority of the time though keeping the charge level up over 50% will be better for your battery.

Luck;
Ken

[TechniSol]

As Ken said, the lower limit is set either by a hardware circuit preventing discharge below a certain lower limit or through software looking at an A/D converter. The upper limit is key to the issue of capacity as the voltage level at which charging should be stopped varies with aging -it's not simply voltage dependent but sensed by looking for a change in rate of rise(dv/dt) or actual drop, as I recall, depending on the method. It's been a while(years) since I researched data sheets on lithium ion charging/power management chips so cut me some slack if I mangled the exact method.

The things is that the curve chosen to represent the approximate relationship between voltage and remaining current capacity has to be modeled properly and you have to modify it to match the change in upper charge cutoff(terminal charging voltage) as the battery ages. As was said by someone else the proper way to do it would involve monitoring current supplied during a full discharge from 100% to the hardcoded minimum voltage level and then base charge level on the most recent capacity determined through that full discharge cycle, or through it averaged with a previous cycle or two to moderate the change in capacity in case it was skewed through low temperature effects, or the like, temporarily affecting capacity(current).

As Ken said, once every couple of months I run lithium batteries down, especially if they have been charged at 50-75% or higher a lot, just to give the charge metering a chance to recalibrate through experiencing a full cycle.(I mean 50 -75% of capacity as depicted by the battery gauge on the device, not actual capacity as there is always a bottom end that doesn't get discharged to prevent damage to the battery.)

I don't mean to be absolute on this, but I fail to see the benefit of charging to termination, unplugging and repeating, unless the device has a faulty final termination detection algorithm or a thermal cutoff is getting in the way and interrupting the process -been known to happen, but I don't really want that to be the method chosen to prevent my device, possibly my home, going up in smoke...

[filmo]

I was charging Nicad battery pack using bench power supply. Had to leave room and upon return about all that was left of battery pack was the wires. Must have set charge a little high or had a bad cell. These were sealed nicad batteries. I knew a guy that recharged Alkaline batteries that were labeled do not charge but he was lucky that they didn't explode.

[Aydan]

NiCd and NiMH charging is pretty simple actually:
Use a bench supply with current limit and set the current to say 1C and set the voltage limit to less than 1.5V per cell. Now you leave it until the voltage has been reached and the current has gone down to about 0.1C (C=Capacity means 1C for a 2000mAh battery is 2A)
I've done this for years and never had a batterypack go bad on me.
Lithium Accumulators, be it LiIon or LiPo, are much mor critical in regards to charge profiles and voltages and they really do react explosively if mistreated.

Regards
Aydan

[TechniSol]

Originally Posted by filmo

I was charging Nicad battery pack using bench power supply. Had to leave room and upon return about all that was left of battery pack was the wires. Must have set charge a little high or had a bad cell. These were sealed nicad batteries. I knew a guy that recharged Alkaline batteries that were labeled do not charge but he was lucky that they didn't explode.

I'm guessing that pack had no thermal cutoff, or you'd probably still have been fine. Maybe not gotten too far with a reversed cell, but no going up in smoke.

[TechniSol]

Aydan,

Sounds workable. My NiMHs generally hit a terminal voltage of around 1.414 if memory serves. You can get away with 100maH of trickle with NiMHs pretty easily, but Lion wants to go into meltdown if you try that. Great technology, but a little finicky. You still hear stories now and then about some dingdong that put an iPhone or other cellphone in a back pocket and sat on it hard enough to short out a Lion pack and had it set their pants on fire. Pilot error, but still scary.

[Ken Maltby]

Originally Posted by TechniSol

Aydan,

Sounds workable. My NiMHs generally hit a terminal voltage of around 1.414 if memory serves. You can get away with 100maH of trickle with NiMHs pretty easily, but Lion wants to go into meltdown if you try that. Great technology, but a little finicky. You still hear stories now and then about some dingdong that put an iPhone or other cellphone in a back pocket and sat on it hard enough to short out a Lion pack and had it set their pants on fire. Pilot error, but still scary.

With resulting brain damage I presume.

Luck;
Ken

[PeterT]

Rather than brain damage, hopefully some anti reproductive damage...

[TechniSol]

I think Ken has a good grasp of the anatomy of the bargain basement Dingdong... brain damage, indeed! In my way of thinking, if you can't keep from sitting on your own phone you have bigger issues to resolve than how to get it fixed...