We hate to spend more money then we have to. In the case of batteries we often do - only because we "follow the book". The problem is; The "Book" is often written by an expert in the marketing area and not by somebody who is familiar with the technical side of the problem. In the "Batteries Department" there is a whole score of myths worth investigating.

Form them or not?
    One of the myths seems to stem from some strange analogy between NiCd or NiMH cell and a hard disk of your computer. The later, as we know, is useless without formatting. So, a new battery has to be formatted as well. The lifetime and performance of it will depend how well we perform this initiating ritual. As there is no smoke without fire, we are not looking at a total nonsense but a mere misinterpretation of facts. By definition we are looking at a "confusion of slightly inaccurate truth with a blunder - nevertheless a very accurate one".

    Cell, either new or not used for a long time, isn't capable of delivering its full capacity. The deficiency is somewhere around 10%. Only after 2nd or 3rd cycle the cell is capable of delivering the full 100% of its energy. Only those who can't stand the thought of having a battery with mere 90% of its capacity during the first couple of cycles should go through the trouble of formatting it. As a 10% difference is hardly noticeable, it seems to be wise let the battery form in our instruments during their normal use.

Discharge before charging?
    Another myth often misused by marketing experts is the "memory effect". "Buy the more expensive charger which discharges your batteries before every charging." Sounds smart - and the price doubles at once. You can throw your money through the window - and be better of, because you'll save on batteries!

Memory effect
    NiCd or NiMH cell's capacity does depend on the degree of previous discharge. Again, it is not a total nonsense. If we discharge cells repeatedly (tens or hundreds of cycles) to let's say 50%, at the next discharging process they will not deliver 100% of their capacity. As if they remembered the level of the previous discharges - hence "memory effect". We are looking at only few % of the total capacity and the problem will disappear after one or two deep discharges. It looks like the answer lays in a total discharging every single time. Watch out, it is not that simple.

Number of charging cycles - life span
    There is another important relation here. It is the relation between the maximum number of cycles achievable and the depth of each cycle. During the normal use the useable capacity is slowly rising (by a few %) with the number of charging/discharging cycles. This is true up to the number of 2 or 3 hundred. Then it will become declining irreversibly. The maximum number of useable cycles is usually defined as a point, where the capacity drops below 60%.

    Up to now, we were talking about deep cycles - 100%in, 100% out. We would tend to believe if we use the battery twice on only half of its capacity, we have 2 half-cycles, which should equal 1 full cycle.

    In the real world the situation is much better. For example, if we keep discharging our batteries by only 25%, the maximum number of useable cycles won't increase only 4x, but approximately 10x. This phenomena is being used in satellites, where the power system is designed to discharge the batteries only by some 10% at each cycle. Changing batteries in orbit is not a mean feat!

    So, now we have a drop of capacity (by maximum of some 20% and reversible too) against increase in useable charging cycles and thus lifespan of the cell by hundreds of percent. The conclusion is clear. We should charge our batteries after every significant use and avoid discharging close to empty whenever possible.




Deep discharging
    There is another important reason why discharging batteries to the very bottom is not desirable. Batteries in most instruments consist of two or more cells connected in series. Their voltage and power ads together (not capacity). This applies to sealed packs as well.

    Even if most instruments are designed to switch off when the supply voltage drops below a certain level, there is not a benchmark for this voltage and some instruments will simply keep discharging the battery to zero. No two battery cells are exactly the same. After a period of time inevitably the capacity of individual cells in our pack will show significant differences. And that's where things will start go wrong.

    While the battery is gradually discharging, the cell with the smallest capacity gets flat. The rest of the battery is still delivering current to our instrument. The weakest cell is done for. As the current flows through it, the cell is forced to reverse its polarity. The chemical process during the inverse charging of the already weakened cell is causing another irreversible damage. During the next discharging this cell will get flat even earlier. And so on. We have a chain reaction and that cell is as good as dead. The higher the number of cells in the pack the higher the chance of this problem developing. At the same time, if we avoid deep discharging, it may not happen at all.

    If we use our batteries only little they will last a long time. The longest will last batteries with no use at all. Test by company Sanyo show, their unused batteries lose only 10% of their capacity over a 10 years period of time. Here goes the myth the lifespan of NiCd batteries is only 3-5 years anyway. The determining factor is the number and depth of charging cycles. That is, if we do not overheat the cells. A continuous use at 40dg C shortens the lifespan by 50%. At 60dg C by shocking 90%. Know your enemies!