Battery mythbusting

[Jon86]

I'm sure for a lot of you guys this is pretty simple stuff, but I just thought I'd ask about a few little things to do with batteries that I've heard from various people.

You should totally discharge a battery before you charge it
Some batteries have circuitry that slowly kills it on purpose
You should fully cycle your batteries (laptops for example) every month
You should take out a battery if you're not using it

Is any of this true? If so is it worth doing it?
Feel free to add any more on, personally I don't 'look after' my batteries in anything, and I've never had any problems 
Sorry if I'm being stupid asking about these, I'd just like to know once and for all what the truth is about batteries.

[mariush]

You should totally discharge a battery before you charge it

Was valid (and still is) with batteries that use Ni-Cd and  Ni-MH chemistry. They have a "memory effect" : http://en.wikipedia.org/wiki/Memory_effect  Lithium based are not affected by this (or there is a negligible effect)

Some batteries have circuitry that slowly kills it on purpose 

No... there usually isn't such a thing. Maybe some manufacturer made something to the effect of including in the battery protection circuit some battery health status and will refuse to charge batteries after x cycles. 
I know the battery of my IBM T40 laptop reports to the laptop how many full charge cycles there were, how much mAh it can provide and other stuff. But I doubt someone would make the battery get damaged on purpose.

You should fully cycle your batteries (laptops for example) every month

It wouldn't hurt  but i don't think it would do any good either.

You should take out a battery if you're not using it

These days I don't think it should make a difference.
I had friends who noticed that on older laptops (mostly with Ni-Mh batteries), the batteries would last longer if they were removed when laptop was plugged in the wall socket. Could be those laptops had a crappy charging circuit that kept pumping energy into batteries all the time or something else, like the heat from the laptop keeping the batteries warm all the time... I don't know.

[grumpydoc]

You should take out a battery if you're not using it

These days I don't think it should make a difference.

I'd take them out - alkaline batteries are especially good at leaking and destroying whatever they're in but I think any battery type can potentially have problems.

[Jon86]

Thanks for that, that's the sort of thing I was hoping to hear 
It sounded more believable a few years ago, but now with better charging circuitry and li-ion batteries, I didn't think it would be the case. Thanks for clearing that up

and grumpydoc, I was mainly thinking of li-ions and lipos; laptops, cameras and such.

[Neilm]

Many battery myths come from older types of battery technology that is applied to all batteries by people who don't understand what was actually happening.

For instance your first myth totally discharging a battery prior to charging probably comes from older battery monitoring circuitry that would coulomb count the charge in the battery. With time and repeated partial charging and discharging the value in the count would become more and more inaccurate leading to misleading reported charge levels. Discharging the battery prior to a full charge cycle reset this circuit as it could detect when the battery was fully empty.

Myth 2 - circuitry that slowly kills the battery. Well I've never seen one. There are circuits in some battery packs but that tends to be for protection.

Myth 3 - fully cycling batteries. With older technologies it was recommended, mostly to get around the issues I described above. More modern Li-Ion batteries should be kept around 80% charged. Indeed, if you cycle a Li-Ion battery hard you will get less charge cycles out of it. I have seen data that says if you discharge a Li-Ion battery to 60% of full charge you will get about 4k charge cycles out of it. If you take the same cell and completely flatten it, you will get about 500 cycles.

Myth 4 taking the battery out will prevent over discharge and reduce the chance of the cell leaking and damaging the equipment so I would do it if you are not going to use the equipment for a while. Also, you don't know what the "off" current draw is. It might be so large it will flatten the battery in a month.

[Jon86]

What I meant by the 4th one was taking the battery out of a device when it's powered up from the mains, I guess to avoid it float charging. I know a lot of people that have a habit of doing it with laptops.

[Neilm]

What I meant by the 4th one was taking the battery out of a device when it's powered up from the mains, I guess to avoid it float charging. I know a lot of people that have a habit of doing it with laptops.

I wouldn't bother. With modern Li-Ion batteries the chargers do not float charge - they charge until the current drops below a set level then turn off to prevent overcharging the batteries.

[grumpydoc]

and grumpydoc, I was mainly thinking of li-ions and lipos; laptops, cameras and such.

I figured you were only interested in rechargeables but you didn't actually say in your original post.

I'd still take the batteries out of anything I knew I'd not be using for a few months, assuming a) I planned it that way and b) I remembered, that is. Coming back to a dead battery and a gadget which needs a new battery is rather less annoying than coming back to a dead battery and a dead gadget that has battery juice corroding it's insides whatever the battery chemistry.

NiCd memory effects might possibly be mitigated by discharge before charge and I've resurrected a few packs by discharging at a fairly high current followed by charging at about 1xC for a few cycles.

Fully cycling a battery every month is a complex one - I've heard that doing so helps keep the battery protection circuits in some laptops "calibrated" so they don't loose track of the true state of charge (and therefore shut themselves down when the battery is still actually OK) but I'm not really sure I believe it.

Edit:

What I meant by the 4th one was taking the battery out of a device when it's powered up from the mains

OK, dunno. I do have a laptop that was basically always used on mains and the battery's useless on it, runs for 15mins - whether that's because it was "float charged", not cycled or just got old, I don't know

[xygor]

All kinds of info here:
http://batteryuniversity.com/learn/

[Neilm]

Fully cycling a battery every month is a complex one - I've heard that doing so helps keep the battery protection circuits in some laptops "calibrated" so they don't loose track of the true state of charge (and therefore shut themselves down when the battery is still actually OK) but I'm not really sure I believe it.

Batteries lose capacity all the time. The the state of charge circuit is a complex one (some of the Li-Ion monitors do all sorts of things including impedance monitoring) then fully discharging the battery will allow it to get an idea of the remaining battery capacity. The trouble is, if you do it too often with Li-Ion batteries you will reduce the capacity of the battery quicker.

[dannyf]

Some batteries have circuitry that slowly kills it on purpose

I don't think you can conclusively disapprove it without taking apart every battery out there.

To me, that's a possible but improbable scenario.

[Jon86]

I see, thanks very much 

[Legit-Design]

You should totally discharge a battery before you charge it

Last cordless device I got mentions in manual (yes I actually read the manual) that it should be completely discharged every 6 months or so. And after that charged for at least 16 hours continuously. It probably has Ni-Cd or Ni-MH cell/s inside.

[con-f-use]

I don't know if that has been said but:

- All batteries (self-)discharge over time. This can lead to a deep discharge which is especially bad for Li-based batteries and NiMHs. So you should avoid having those batteries lying around for a long time.

- As a consequence: Before storing your Li and NiMH batteries, charge them to about 50% of their capacity (self discharge is faster for fully charged batteries) and store them in a fridge. Temperature needs to be as low as possible without reaching the batteries specific freezing point.

- NEVER use a battery that was frozen at one point in it's life.

- Alkalines can leak and destroy the device they are in, so either take them out or look at them from time to time. They even sewell up and get stuck or break their casing.

- Alkalines don't like to be charged. Putting an old one between a few fresh ones equals "charing" them and can blow them up. So make sure no voltage greater than their present voltage is across it's contacts.

[amyk]

Nickel-based batteries are best stored completely discharged (0V on all cells) and almost has no degradation in storage, while lithium-based is best stored at half capacity and will degrade no matter what. Lead-acid needs to be stored at full charge.

[Psi]

Nickel-based batteries are best stored completely discharged (0V on all cells) and almost has no degradation in storage, while lithium-based is best stored at half capacity and will degrade no matter what. Lead-acid needs to be stored at full charge.

At 0V you run a high risk of “growths” forming inside and creating a short circuit which prevents you from charging them up later.
A high current pulse can blast the growth away sometimes but its not ideal

[Maxlor]

You should totally discharge a battery before you charge it

Was valid (and still is) with batteries that use Ni-Cd and  Ni-MH chemistry. They have a "memory effect" : http://en.wikipedia.org/wiki/Memory_effect  Lithium based are not affected by this (or there is a negligible effect)

With lithium batteries, it's actually the other way round. They age most quickly when in their sub-10% and above-90% charge range. So if it's practical to keep it, say, between 20% and 80%, you'll get significantly more charge cycles out of it. According to Battery University, you get 3 times more charge cycles out of your lithium battery if you only charge to 80%.

[Kjelt]

modern Li-Ion batteries should be kept around 80% charged. Indeed, if you cycle a Li-Ion battery hard you will get less charge cycles out of it. I have seen data that says if you discharge a Li-Ion battery to 60% of full charge you will get about 4k charge cycles out of it. If you take the same cell and completely flatten it, you will get about 500 cycles.

What I know you want to charge the Li-ion batteries when they have still  20-25% charge in them, indeed don't run them flat as it decreases their lifespan.
Keeping them at 60% or 80% is new to me and that won't give much use for the battery.
Apple in their manual for the Ipad devices state that the lifetime is depending on the number of chargecycles, that it doesn't matter if it is a complete charge from for example 30% to 100% or from 60% to only 70%, doesn;t matter both count for the same decrease in lifespan.

[Balaur]

Keeping them at 60% or 80% is new to me and that won't give much use for the battery.

Let's see:

Li-ion does not need to be fully charged, as is the case with lead acid, nor is it desirable to do so. In fact, it is better not to fully charge, because high voltages stresses the battery. Choosing a lower voltage threshold, or eliminating the saturation charge altogether, prolongs battery life but this reduces the runtime. Since the consumer market promotes maximum runtime, these chargers go for maximum capacity rather than extended service life.
...
Avoiding full charge has benefits, and some manufacturers set the charge threshold lower on purpose to prolong battery life.

My Vaio laptops have an optional "Battery Care" function that stops the charge at a preset level (80% by default).

[minime72706]

You should take the battery out if you're not using it mostly because it will almost definitely be dead when you go to try to use the related device. Many devices continue to drain microamps of current continuously which would definitely exceed the (lithium) battery's own discharge rate.

As for fully cycling a laptop battery - the main reason for doing so is to recalibrate the battery monitoring circuits. A lot of empirical guess-work goes on inside the battery "fuel gauge" circuits and the errors compound over time. You shouldn't do it that often since it just wastes a charge/discharge cycle, but you should do it once in a while.

[Lightages]

Do not fully discharge NiMH batteries. This shortens their life considerably. Recharge them directly after each use and if they are going to be stored for a long time then don't store them fully charged.

[Rigby]

everyone has something to say on this.  I think only one person has mentioned what their goal was; maximum battery life or maximum power storage.  your choice there will determine how you treat the battery.  most folks assume that maximum battery life is the goal, and it's certainly my goal, but I've never once stopped my laptop charging at 80% intentionally.

my point is that it's a pain in the ass to take care of batteries properly.  someone needs to rectify this.  i'd pay more to know that my charger took better care of my batteries.

[Neilm]

my point is that it's a pain in the ass to take care of batteries properly.  someone needs to rectify this.  i'd pay more to know that my charger took better care of my batteries.

Most (but by no means all) of the Li-ion battery chargers I have seen have the ability to be set to a specific level of charge for the cell - we don't let the user know that we are only charging to 90% of the battery capacity and discharging to say 5%. That will mean they can charge it up when they think it empty and will not have totally discharged the cells and it will ensure that the cell will not overcharge.

A handy side effect is when management complain that the battery life is not long enough, I can tweak some resistor values / firmware constants to use more of the cells life. I just have to explain to everyone that the expected lifespan of the batteries will be considerably reduced.

[captbilly]

In hybrid cars, whether using NiMH or Li ion or LeFePO4, the charging/discharging algorithm is quite complex, but as a general rule, to get the longest life out of the batteries they are virtually never fully charged or discharged.  If you go to some manufacturers sites you will see data that show dramatically longer life for the batteries if they are kept in the 20%-80% charge range.  I can't swear that it isn't a good idea to fully charge or discharge a battery sometimes (although I am fairly certain that for lead acid/SLA/gel cell/AGM batteries, it is never a good idea to run them all the way down), but for long life you definitely do not want to regularly fully discharge (down to the shutoff voltage for lithium types) your batteries.

As to removing batteries from devices that are plugged in.  Like so many tings in engineering, the answer is complicated.  Depending on exactly what type of battery and what type of charging circuit and algorithm that was used, it is possible that the battery could be somewhat overcharged if left in the device.  A properly implemented lithium ion battery charger should not ever overcharge the battery but some very cheap charging systems don't absolutely cut off, and some do not balance the cells well, so the battery could be hurt if left charging continuously.  Several lead acid battery powered devices that I have (including my Irobot Roomba, and some big flashlights) definitely overcharge the battery if left plugged in all the time.  My Roomba will actually get the battery quite hot if it is left in the dock for several days without running a cleaning cycle.  The Roomba will actually wear out the battery faster if it is not used for cleaning, than if it is run every day.

[AndrejaKo]

One thing I feel should be mentioned about removing batteries from AC powered devices is that today some manufacturers are actually trying to save on AC power supplies in some extreme circumstances and use batteries as a sort of energy buffer.

For example, I've seen a recent cheaper Dell laptop (can't remember the model number at the moment) which, when its GPU and CPU are fully loaded, will start draining battery even though it's connected to its power supply.

[macboy]

...
Apple in their manual for the Ipad devices state that the lifetime is depending on the number of chargecycles, that it doesn't matter if it is a complete charge from for example 30% to 100% or from 60% to only 70%, doesn;t matter both count for the same decrease in lifespan.

You have misinterpretted their information. According to Apple, one "charge cycle" means using 100% of the charge. So if you use 20% then re-charge, and repeat this, you can do so five times before it counts as one cycle. In other words, using 20% of the battery capacity, then charging it, counts as 20% of a 'charge cycle'.

From http://www.apple.com/batteries/ :
"A charge cycle means using all of the battery’s power, but that doesn’t necessarily mean a single charge. For instance, you could listen to your iPod for a few hours one day, using half its power, and then recharge it fully. If you did the same thing the next day, it would count as one charge cycle, not two, ..."

I have seen other research that indicates that more shallow cycles degrade the battery less than the fewer deep cycles (with equivalent watt-hours used in each case).

[Kjelt]

Apple in their manual for the Ipad devices state that the lifetime is depending on the number of chargecycles, that it doesn't matter if it is a complete charge from for example 30% to 100% or from 60% to only 70%, doesn;t matter both count for the same decrease in lifespan.

Then they must have changed the information, I can almost with full certainty remember when I bought my first iPad in 2010 I was astounded to read online that each time you connect it to the charger it would count as a cycle.
Ok good for them that they now have clear and precise info with examples what counts as one cycle. 

[abbotsmike]

everyone has something to say on this.  I think only one person has mentioned what their goal was; maximum battery life or maximum power storage.  your choice there will determine how you treat the battery.  most folks assume that maximum battery life is the goal, and it's certainly my goal, but I've never once stopped my laptop charging at 80% intentionally.

my point is that it's a pain in the ass to take care of batteries properly.  someone needs to rectify this.  i'd pay more to know that my charger took better care of my batteries.

My Samsung laptop has an option to only charge the battery to 80%, so I use it. No real hardship as I can squeeze out 6 hours on one 80% charge if I keep it in power saving mode! Every so often I will fully charge it if I know I need the extra runtime without a charge available.

[ejeffrey]

The memory effect of NiCd and NiMH batteries is mostly a myth.  While NiCd does have a memory effect, it is rarely observed in practice.  The reason for the 'discharge before charging' rule is simple:

*all* rechargeable batteries hate being overcharged.  Early NiCd and NiMH chargers were simple timers with a fast charge cycle followed by a trickle charge.  If you discharge a battery half way and then put it in a dumb charger, it will be overcharged and damaged.  Modern NiMH chargers that charge faster can't do this.  Instead they detect voltage or temperature changes to detect when charge is complete.  With these chargers, there is no need to do a full discharge.