Question of the Week

Letting Batteries Discharge

Sun, 19th Oct 2008

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Alvin, Cambridge, Mass asked:

Is it really better to let a battery completely run down before charging it?



We put this to Patrick Palmer, University of Cambridge, Department of Engineering:

This is a very good question that exercises me most mornings after I've cleaned my teeth and I don't know whether to put my toothbrush back in its holder and charge it or whether to just leave it on the side.  BatteriesThere is some truth in the fact that the nickel cadmium battery which is the light-weight one occasionally needs to be helped by being deep-discharged.  Most of the time just discharging it 20% and recharging it is okay but it needs to be reset once every month or so, something like that.  The lithium battery that’s popular in telephones is also light-weight.  These, however, do need considerable care.  That’s why you find lithium batteries in mobile phones and in laptop computers.  Their charging and recharging has to be monitored very carefully.  They have protection circuits in them usually.  So occasionally it probably is a good idea to let your laptop run flat.  Do that occasionally because that allows the computer to recalibrate itself and be up and running for the future.  The other main type of battery is your lead acid battery in the car.  In fact we know very well that lead acid batteries can work very well if they’re just kept basically topped up the whole time.  Care is required and occasional deep-discharge of nickel cadmium, and for that matter probably nickel metal hydride – probably less often – just by using it in the equipment ‘til it’s flat is probably not a bad idea.


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I think it all depends on the battery technology. For the older NiCd and NiMH cells, I think the general principle is that fully discharging them may help ensure they retain their maximum capacity, but at the expense of a smaller number of total charge/discharge cycles before they're end-of-life. For Li-Ion/Li-polymer (as used in modern mobile phones and laptops) I really don't know. techmind, Tue, 14th Oct 2008

It all depends, on the battery, what sort of battery it is, and what charger you're using and what device is discharging the battery.

First it's important to explain that a battery is different to a cell.

A battery is several cells connected together to power a device, whereas a cell is a single power source.

For nearly all types of cells you would ideally discharge each cell completely before fully recharging it.

A problem comes when you connect multiple cells to form a battery. Then as the battery discharges, some of the cells may discharge before the others. When that happens the other cells force current backwards through the cells. This permanently damages the cells.

So batteries are best off not being discharged completely, but should be stopped short of that. Some equipment that uses batteries can detect the batteries being nearly empty and then automatically stop drawing current. That usually gives much longer battery life, but the equipment may be more expensive to buy, so it's a tradeoff of battery life versus equipment cost.

Then there's the charger. A lot of chargers are simple and just supply current through the battery to charge it, many of them cut off after a particular, fixed time. However, unless used with extreme care these usually overcharge or undercharge the battery. The best chargers can actually measure the battery and cut the charge off when the battery is full. Overcharging can severely shorten the life of some batteries, particularly NiCd. Undercharging is less harmful but means the batteries need recharging more often.

In general, you do best to stop the discharge slightly before the batteries are fully discharged and then charge them using a charger that automatically cuts off when it detects that the battery is fully charged. This gives the best time between charges as well as the best battery life.

That's batteries such as NiMh and NiCd.

However, it does depend on the type of battery, some batteries such as (IRC) lithium ion and lead acid should be kept as fully charged as much of the time as possible, partly or mostly discharging them and leaving them like that for long periods tends to shorten their life. wolfekeeper, Wed, 15th Oct 2008 Pumblechook, Wed, 15th Oct 2008

I stumbled across a datasheet for Li-Ion/Li-polymer batteries recently and it recommended that if you're not going to use them for a while, the Li-based batteries should optimally be left at 40% charged! New batteries would also be supplied 40% charged - hence why new mobiles now have some life in them before the first charge.

Li-based batteries have a single-cell voltage of about 3.6 volts, which means that you can run portable electronics off a single cell. This nicely avoids the reverse-charging of the weakest cell in a chain issue that wolfekeeper pointed out - which is a problem for NiMH/NiCd -powered gadgets.

MiMH hate overcharging. techmind, Thu, 16th Oct 2008

I have just been reading about batteries in a mag. 

Ni-Cads are now banned by the EU.

There are newer Ni-mh ..Imedion and MaxE which claim a much lower self discharge rate.    I use Tronic from Lidl (1/4 of the price of ones form Tesco) but they do suffer badly from high self discharge.  Pumblechook, Thu, 16th Oct 2008

I'd wondered about that. The battery with the new phone I bought in March had some charge in when I got it. Also, the instructions recommended letting it fully run out before charging, which I'd never heard suggested before.
It definitely depends on the type of battery. I used to use Rayovac rechargeable alkaline ones (still would if I could get some new ones) and the instructions for these recommended charging them before they run out. rhade, Fri, 17th Oct 2008

The brother of a friend of mine used to race model (electric) cars. These were powered by ni-cads. I recall coming in one evening to find my friend's brother connecting a massive spotlight to some of his battery packs. I asked him what he was doing and he replied that he'd got a big race the next day and that he was fully discharging the ni-cads before overnight charge in order to improve the capacity for the race.

He said that there was a strong "memory" effect with ni-cads; an insufficient discharge reduced the future performance. He couldn't say why, however.

So what is the basis of this memory effect?

Chris chris, Sun, 19th Oct 2008

"Memory effect" in NiCd is widely talked about, and there are many heated debates on the whys and wherefores.
My understanding (I don't claim to be a real expert here) is that the effect is more of a "voltage depression" during the discharge where you go from the regularly-used to the rarely-used part of the capacity. For example if you fully charge NiCd but only normally discharge to 50%, then if you attempt to discharge deeper than normal (below 50%) you'll find a dip in the discharge voltage as you enter the unfamiliar territory. For some applications (motors, torches) that'll just be a dip is power/brightness, while for some electronic goods which have a well-defined minimum operating voltage it might mean they just shut down early.
Common NiCd wisdom is that they should "periodically" be deep-discharged (1 in 10 to 1-in-20 charges) to avoid this.

But NiCds are effectively history now due to cadmium bans and the better performance of NiMH anyway.
techmind, Sun, 19th Oct 2008

Reading between the lines from the batteryuniversity website, for Li-Ion/Li-plymer as used in mobile phones, laptops, etc it would seem that keeping them permanently "topped up" will accelerate their aging. On the other hand, you shouldn't discharge them then leave them unused for weeks/months as they may then over-discharge and never recover.

Unless there's a specific reason to do otherwise, I normally only recharge my mobile "when it's getting low".
For both Li-Ion and NiMH, I think there's very little to be gained in "squeezing out the last drop" on a regular basis, and to do so could be detrimental.

A further rule of thumb is that general-purpose batteries (both rechargable and single-use) are best stored somewhere cool, but used at room temperature.
You shouldn't be storing batteries in the cupboard with the hot water tank (as some people do) - while this may eek out the last energy from nearly-dead cells, it will also greatly increase the rate of self-discharge and overall life of otherwise good batteries. techmind, Sun, 19th Oct 2008

The one thing I am struggling to comprehend with this area is the actual chemical basis for these battery effects. Why are lithium cells so fussy, and why do NiCads have nightmares if you don't deep discharge them?

Chris chris, Mon, 20th Oct 2008

I can't vouch for the veracity of this page -- but it does address NiCad memory:

John Blinke , Wed, 31st Dec 2008

There are conflicting ideas on how the different types of battery should be charged etc.. 

Good site.

I think I had read that Ni-Cads are being banned in some countries..  Toxic??

I still prefer sealed lead-acid coz you can charge at any time and if you have good constant voltage charger with current limit you can't over charge or shove too much current in when they are low. Pumblechook, Wed, 31st Dec 2008

Lead Acid accumulators don't like being discharged more than 50%, I've heard. lyner, Sat, 3rd Jan 2009

Li-Ion cells are often shipped at 40-60% full as fully charged they can contravene freight regulations due to the amount of energy per cell they hold.

Li-Ion cells should never be deep discharged as they contain a cut off ciruit built into them to prevent deep discharge. Deep discharge of most cells (NiMH & Nicad excluded) will greatly kill their life.

If you discharge them down to say 2.3V then let them sit for a few months, they will self discharge to a point the circuit cannot operate from the cell and therefore cannot be charged. They can be recovered, but that requires opening it up and charging straight onto the cell terminals thus bypassing this protection circuit. Doing this, you need to know what you are doing as Li cells are nasty when abused. Waldo Pepper, Fri, 16th Oct 2009

Yes for nickel cadmium cells and batteries
A good ni cd battery usually has high resistance resistors placed across each cell so that's dormant battery will flatten over a long time.
Ni cd sintered electrodes and the chemistry of the KOH electrolyte creat a memory effect reducing the capacity and hence life.
Li ion cells are more effected by temperature variables on use and storage as will be found in electric cars battery life in the near future

I spent 12 year in hi tech batteries  30 years ago the lithium cells were more risky but in 2014 the risk has reduced but not entirely. allan marsh, Sat, 21st Jun 2014

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