| Al Skierkiewicz |
27-08-2014 08:53 |
Re: Batteries
Allen,
The mechanics of this bad cell failure (in my best guess) is that through damage, either drop or robot hit, some of the plates in a cell become dislodged from the connecting terminals. As such the remaining plates still function normally but the cell has a lower amp hour rating as it will become depleted before the other cells. If we were to test at full current, it might show as a change in available current at 400-500 amps. When testing at the manufacturer's spec, you will see the battery run a normal discharge curve, in some cases more than hour, before the bad cells actually become evident. To visualize, say a cell has 10 positive plates and 10 negative plates. The battery gets dropped and one of those plates either cracks or comes completely off the output terminal plate. Since the plate area is directly proportional to the available current and the amp hour rating, any reduction will reduce both of these measurements. The CBA testing will establish a fixed current load that will calculate the amp hour rating without the need to measure max available current. Since both are affected by damaged plates, just knowing that a reduced cell amp hour rating is enough to declare the battery "practice only". Close examination of the battery case will almost always point to case damage from the drop or hit. If you search "AGM internal structure", you will find a variety of pictures and other links to get an idea of what could go wrong internal to these batteries when dropped.
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