This is probably the most significant cause
of premature battery failure, and applies to battery packs, or groups of
cells. A group of cells is where there is more than 1 rechargeable battery (AAA,
AA, C or D cell) in the device, for example torches and TV remote controls.
Battery packs are cells that are grouped together within the one package, like
mobile phones, cordless phones, power tools, digital cameras, etc.
In any battery pack or group of cells, one cell will always be weaker than the others.
As the cells discharge, the weakest cell will discharge first. As discharge continues, the current through the flat cell becomes
a reversed charging current, which damages the cell, and shortens it’s capacity and life. When the pack is next charged, that
cell may be overcharged (further shortening it’s life), and the problem just gets worse each cycle from then on. The whole
battery pack then has a reduced voltage output, and much shorter life. If possible, always charge separate cells individually,
and never let a battery pack or group of cells flatten too far.
Some devices with battery packs (eg. mobile phones) will shut
off when the battery is too flat, but don’t allow this to happen often, and certainly don’t try to use the device beyond this
point. Either charge it immediately, or remove the battery pack so it won’t continue to deplete in the device.
It is best not to keep batteries on charge all the time, like portable vacuums, cordless
phones, electric toothbrushes. It is best to drain batteries a bit before putting them back on charge. Even a constant slow
charge (trickle charge) creates a chemical structure with bigger crystals, and converts the energy storage chemicals to a
slightly different type. Both these problems result in lower voltage and useful stored capacity, which is one of the two main
causes of “memory effect”.
To reinvigorate cells, it is best to drain them nearly flat (around 10% capacity left) then
give a full charge. This allows the chemical structure in the cells to be reformed, and maximises their capacity and useful life.
However, be careful not to drain battery packs or groups of cells completely, as this will do more damage than this deep cycling
Partial cycling is where the battery is only partly drained before being
recharged. This is not good to do all the time, as the chemical material deep inside the cells never gets
used. Over many recharges, the crystalline structure of this unused material
gradually changes, with the crystals growing larger. Large crystals are not
good, as this results in lower output from the cell, and is one of the two main causes of “memory
effect”. Draining the battery to nearly flat, and then fully charging the cell results in a more complete
reformation of this material, which will now have a microcrystalline
structure, leading to better release of energy when it’s needed.
NiCad’s self-discharge fairly quickly. Expect about 50% of the capacity left after 2 months,
and being completely flat after 6 months. A normal recharge every 3 months ensures maximum life of the cell.
Li-Ion batteries have a very low self-discharge, and on their own, can happily last a year
without recharging. However, many Li-Ion batteries contain some circuitry that remembers the charge state of the battery
to assist with smarter charging. This also provides charge state data to the device. This circuitry requires energy to run,
and depending on how complex or efficient it is, it may drain the battery if sitting on the shelf for even 6 weeks.
So, it's best to keep an eye on your Li-Ion batteries when stored on the shelf, to determine how long they can safely last.
Make sure you recharge them before they are drained too far, otherwise permament damage will occur, see tip 1.
If a cell is charged or stored at high temperature (eg. in a car in summer), the chemical structure inside the cell
changes with the formation of larger crystals. These larger crystals find it harder to release the stored energy, and result in
lower voltage and useful stored capacity. Normal room temperature is best for charging and storing.
When a cell is recharged, chemical material
inside the cell is redeposited onto plates inside the battery unevenly, which
is unavoidable. Over many cycles, this material soon forms hills and valleys
on the plates. When there are hills directly opposite each other on the positive and negative
plates within the cell, this raises the
current density at those points, which leads to more material being deposited at those
hills during charging. Those hills grow sharply to form needles, called
dendrites, which eventually bridge between the positive and negative plates, shorting the cell out. A cell that appears to
self-discharge in a couple of days has dendrite problems, and will soon completely short out, and not accept any further charging.
These cells can be “zapped” to vapourise the dendrite causing the problem, but this is only temporary. Another dendrite will not
be far behind. Also, the vapourised dendrite leaves behind permanent damage
that allows energy to leak between the positive and negative plates, which leads to problems with charging, and a faster
So, when a battery hits this phase of it’s life, there’s nothing really you can do to save it, it’s time to replace it.
Most types of rechargeable batteries (eg. NiCad, NiMH, Li-Ion) are capable of delivering
hundred’s of Amps for a short time, and can turn even screwdrivers into molten metal! So, always ensure these batteries are not
stored with metal objects that could bridge across the terminals.
Sealed Lead Acid Batteries
These types of batteries should last at least 3 to 5 years in normal use.
Completely discharging a lead acid battery will reduce the effective life of the battery,
and should be avoided. Some devices using these batteries (such as Uninterruptible
Power Supplies for computers) will shut off
when the batteries are nearly empty, which is great. Other devices such as torches will just get dimmer and then completely fade
out, which if allowed to happen, will permanently damage the battery. A 12V battery is considered flat at 11.64V under