The magazine of the Melbourne PC User Group Rechargeable Batteries Alan Stevenson

0ver the Christmas break I bought my first digital camera, a Nikon Coolpix S4 which has all the characteristics I feel I require in a digital camera — i.e. it acts as a portable scanner as well as a camera. It has 10x optical zoom, 4x digital zoom and will focus down to 4 cm or 1.6 inches. Its main attraction was that it operates from two AA batteries and comes with a fast recharger.

Therein lies the problem. The batteries are 2500 mAh, or 2.5 ampere hours. At 1.2 volts, this gives a total of 3 watts for one hour. This is about four times the power of the normal non-rechargeable AA battery. I am not a battery expert but have studied electricity (4 years in the Navy as electronics engineer and 5 years in Honeywell as a computer engineer). I went to the Internet and here is the result of my research.

The manufacturers of my charger claim to charge two AA batteries in 2.5 hours. Now I am aware that there are many fast chargers on the market. It's no big deal to increase the charge rate — simply increase the voltage. However, other factors come into play. One is temperature and the other, the gasses given off at both the anode and the cathode at high charge (and discharge) rates. The temperature should never get above 40° C because breakdown begins to occur and the battery can be permanently damaged.

Therefore, most (but not all) high speed rechargers have a temperature sensor designed to reduce or cut off the charge if it gets too hot. Some even have a fan. The better chargers use a pulsed charge which keeps the temperature down since most of the time there is no current flowing — the pulses are of very short duration but of high current. However, even the best of these can only charge the battery to about 9096. The remaining 10% is achieved through a trickle charge. Total duration 4-5 hours.

All high speed chargers reduce the life of the battery. Some also reduce its capacity as well.

We are all aware of the extravagant claims put out by hard disk manufacturers regarding the life of their product. The figures are reached by using some fairly creative maths. Some manufacturers of batteries use the same logic for calculating the capacities of their batteries. For instance the true capacity of a 2500 mAh battery can be between 1700 and 2300. Add to this the fact that the battery will loose about 30% of its charge in the first month and 10% of the remainder each month thereafter. This means that the capacity is only valid for a very short time after charging.

New technology has significantly decreased the internal discharge rate. Eneloop batteries retain 90% of their charge after six months, 85% after a year and 70% after two years. They are estimated to last 1,000 cycles and have four times the energy capacity of alkaline batteries. Other manufacturers now offer low self-discharge rate batteries usually as 'pre-charged' rechargeable batteries. For example, the Ansmann MaxE range, Panasonic R2 and Varta Ready2use. These appear to be available in AA and AAA sizes only.

A comparison of popular chargers showed Maha C-204W as the best. The big advantage of the C-204W is that it uses a modified charging profile (Maha "tops off" cells after a rapid charge cycle). Another excellent charger seems to be the Lightning Pack 4000N from RipVan 100, which is more in the fast/gentle camp (although its overnight trickle charge does do a fairly good job of topping- off batteries).

However, overall, it appears that batteries operate best if you only charge them at the normal 8-12 hours required for the average (and very much cheaper) chargers.

What's best? This can be very subjective, but my research tells me you can't go far wrong with Energiser or Sanyo.

A new, non rechargeable battery on the market is the Li-ion type which has a very long shelf life and delivers nearly five times the power of the normal non rechargeable. It is therefore ideal for backup.

Warning. Most of us probably put batteries in to a device and run them until they are more or less dead. Doing this with rechargeables is not a good idea. The reason is that not all batteries are created equal. The electricity will leave the top of one battery, go through the device and back into the bottom of the other battery, thereby closing the circuit. When one battery fades, the other takes the load resulting in the 'dead' battery being charged in reverse. This can cause permanent damage to the battery. It they are in a torch, replace them as soon as the globe begins to get dimmer. If in a camera, replace as soon as the 'battery low' symbol comes on. In this way you will get a lot more use from your batteries.

Reprinted from the April 2007 issue of PC Update, the magazine of Melbourne PC User Group, Australia

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