Battery care

Golden Rules to Help Your Batteries Last

The following tips will help you achieve the best useful life for your rechargeables. Click a tip to get an explanation of why it is important.

NiCad, MiMH, Li-Ion Rechargeables

These types of batteries should last about 1000 cycles, if treated properly.

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.

If the battery is hot after a charge (not just warm, but more than 10°C above ambient temperature) then it has been overcharged, and permanently damaged.

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 does good.

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.

NiMH batteries generally have a very high self-discharge, and need to be recharged every month, even if unused.

However, some types like Sanyo’s Eneloop batteries have a very low self-discharge, and are still almost fully charged after a year! This means they’re great for applications like remote controls.

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

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 self-discharge.

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.

To ensure a good battery connection, occasionally clean the battery contacts, and the battery compartment contacts. You can rub the contacts with a pencil eraser, or rough cloth.

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.

These batteries can be trickle charged constantly for at least 3 years, without any appreciable deterioration in performance. Trickly charging is the best way to keep these batteries at their peak.

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.

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 no load.

A completely flat battery left in this state for a long period will show little response to normal charging, and deliver a fraction of the original capacity.

Lead acid batteries will self-discharge after a year or so. Using the battery as normal and then fully charging it again will restore the battery to its full capacity.

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.