Battery

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Battery Life

If you will be using GPS receiver a lot, you will probably want to use rechargeable batteries - or if you're in a car or boat, a 12v adaptor.

I have found the life of fully charged NiMh rechargeable batteries can be somewhat variable. You should expect a 30% variance between the best and worst cells in a batch. and the odd failing cell with capacity as low as 5 or 10% of the expected.

I had previously shown test results of 10 cells using my home-made battery capacity tester but these results were erroneous as my battery charger had failed badly. I had a tronic charger from Lidl which cooked one cell whilst hardly charging the others! The manufacturers had clearly economised on the copper thickness of the PCB, resulting on the SOT23 SMD transistors failing through overheating.

Without having some means to test the delivered capacity, I would not have discovered why my GPS was giving poor battery life.

Cell Matching

I believe it is well worthwhile using some sort of tester to weed out duff rechargeable batteries, and match pairs of cells of similar capacity. A set of cells will only last as long as the weakest cell. Testing cell capacity into matched sets will often result in an immediate 30% improvement in the life you can get from your existing cells. Weeding out duff cells will give even better results.

Some manufacturers exaggerate the capacity of their cells. The best cell in two batches of 750mAh AAA Vanson cells delivered 631mAh, whilst the average was around 560mAh. In practice, without matching sets of cells, I would typically get 530mAh. On the other hand, my AA Vanson cells performed roughly according to their rated capacity.

Some manufacturers have better quality control. For example, 4 AAA vapextech 1000mAh cells delivered between 942 and 1020mAh. These vapex cells are cheap for their performance and in the UK can be bought from vapextech.co.uk with free postage.

You can perform comparative tests on your cells with a simple and cheap battery tester made of junk. http://www.nickhill.co.uk/nimh-capacity-tester.html . I have also made an accurate mAh tester but is far more complicated, and overkill for getting a good comparative idea of cells to match into sets.

You should also consider a sophisticated charger such as the Lacrosse RS-900 http://www.batterylogic.co.uk/technoline/technoline-i-charger.asp . I have bought one of these, and find it gives enough information about cells to apply proper management of my cells. It uses an intelligent end of charge monitoring system, programmable charge rate between 200mA and 1800mA. It can also apply refresh and discharge/charge cycles, which accurately determine battery capacity and wake up old cells. Each cell has it's own LCD readout for charge/discharge rate, voltage, capacity, charge/discharge period, function. Different programs can be run on individual cells.

Low self-discharge cell types

Normal NiMh cells lose around 10% of their charge in the first day, followed by between 0.5% and 3% per day thereafter. Some cells lose all their charge in 2 months.

Sanyo and Uniross have introduced a new generation of cell under the Eneloop and Hybrio brand names. These lose charge relatively slowly - at around 15% per year. They are marketed as "ready to use". ie they are supplied charged. They cost slightly more than the highest capacity rechargeable cells, and have an initial capacity 19% less than the highest capacity 'normal' rechargeable.

I found the Uniross 800mah Hybrio perform about as well as the 900mah normal rechargeable in the Geko 201 GPS receiver - even freshly recharged. After a week or two, Hybrio should substantially out-perform standard rechargeables. Useful if you want rechargeables ready to use for your digital camera, GPS etc. Also suitable for clocks, remote controls etc. NickH 03:14, 25 December 2007 (UTC)


See also: