|We specialize in using "D" & "F" cell NiMH batteries on all of our 500W Series applications. Earlier we had designed systems using sealed name brand lead acid gel cells, but in our present opinion these are just too heavy for bicycle type applications. especially after one beefs up the vehicles' structure to accommodate the heavy batteries. Its counterproductive and increasing less desirable as the price of NiMH comes down. We offer individual "D" & "F" cells at minimum mark-up for custom kits to promote their wider use and acceptance, which will lead to further price reductions. Like anything else, NiMH batteries have their own peculiarities that you must accommodate in order to use them successfully. IMPORT: First and foremost, remember that they are not toys, but are infact little concentrated chemical factories that can store and deliver enough electrical energy to cause shock and or serious bodily injury and property damage if not handled properly. For example, never ever: 1) attempt to measure a cells' length with a metal caliper or micrometer!!; 2) puncture, penetrate or crush a cell; 3) allow the cell or cells to heat up externally or internally higher than about 135º F; 4) even momentarily butt two cells together positive to positive or negative to negative as this can instantly cause irreversible internal chemical changes in the weaker cell and most likely destroy that cell; 5) allow a once charged cells' voltage to drop below about .65 volt, as this permits irreversible internal chemical changes to occur and the cell will be impossible to re-charge - e.g. it is dead; 6) store in a damp place or on a metal (e.g. conductive) surface, and 7) attempt to charge the cell with a charger not specifically designed for NiMH batteries or one without an automatic shutoff.
If you follow the above suggestions, you will have a reasonable chance of success, but of course there is more to learn. First, the cells' published voltage (1.2 V) is a nominal voltage, actual voltage can range from 0.6 to about 1.5 volts. If the voltage of a cell is allowed to drop below about 1.0 volts, serious irreversible chemical reactions can start taking place, reducing its life and/or capacity; if this cell is in a string of stronger cells, it can experience cell polarity reversal, causing the death of that cell and its string. We initially individually charge and match cells before putting them into a string and we highly recommend this practice.
Second, the cells heat up during discharging, recharging and the environment. Many come with just the plastic covering as shown on the "F" cell above on the right; at low levels of service (4-5 amps), this is quite suitable, but for the high performance service (20 amps plus) we experience, this plastic covering will start to melt and shrink away allowing the battery to electrically short out against its case. We have found that this can be fixed by wrapping the battery O.D. with electrical tape; however, a better fix for this problem is to get the batteries wrapped in a tight fitting, high temperature specialty paper tube by the factory, which we recommend and do. This paper tube cannot be too loose, or it will not efficiently transfer the heat out of the battery, to its outer case and to the atmosphere.
Third, the cells have a considerably diminished performance below the freezing temperature (32º F). If they are on a vehicle left outside in cold weather, we recommend Q-charging them for about 15 minutes before use, to warm them up, thereafter, the heat generated during discharge generally will be enough to allow them to perform normally.
Fourth, the slower one discharges these batteries, the more total energy (and hence range) you can extract from them. For this reason, our mating electronic controller, senses when our battery packs are about two thirds depleted and reduces the maximum power level that you can withdraw, warning you and allowing you to pedal home with some power for a longer distance than otherwise.
Fifth, the battery manufactures state that these batteries can be "totally" discharged and recharged about 500 times depending upon the application, and more times if partially discharged. Even though these NiMH batteries have no memory effect like some others, we recommend and believe it good practice to periodically (say at least once every 5 to 10 charges) "chemically exercise" the entire battery by totally discharging it (down to about 1.0 Volt or when the controller quits functioning) instead of just "exercising" the atoms nearer the reactive surfaces via shallow charging and discharging.
Sixth, the cells will self-discharge, once charged, just by sitting on the shelf or if left dormant in a battery pack, sort of a "use it or lose it" phenomenon. We highly recommend that once charged, you regularly fully recharge them at least once every three months. One of the problems with any string of batteries, no matter how closely matched, is that they are all slightly chemically and electrically different and will discharge at different rates; if you let the voltage of one of them drop too low, the more powerful cells in the string can cause the weaker cell to go into a polarity reversal, killing that cell and string. Then there is no fix to this, except to replace it; on our battery packs (see above) we offer a service to do just this, prolonging the average life and lowering your battery costs. If you faithfully observe the above practices, these little batteries will prove to be the dependable rechargeable workhorse electrochemical factories that they are.
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