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#31
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Re: Batteries
[tangent alert] You could say that about much of the robot though.
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#32
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Re: Batteries
I'll admit I could have worded that better. What I meant is that it's the beginning of the power path so it affects the performance of all other systems.
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#33
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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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#34
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Re: Batteries
Quote:
Part of the reason we don't test that low is to save time and part of it is because, though observation only, batteries that are drawn that low (always on the robot or powering an inverter) don't seem as healthy afterwords but that could just be observation bias. |
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#35
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Re: Batteries
Yes,
The cell may be just shy of running out of energy at 10.5 volts. For that matter even the lower cutoff voltage may not show a partially damaged cell. The lower voltage will catch more defective cells than at 10.5 volts in my opinion. In some cases, internal temperature may exacerbate the intermittent connection. I had one battery that confused me for a while because when the cell got close to dropping off the graph, it suddenly produced a lot of "noise" over a period of 20-30 minutes before dying. These voltage peaks and dips were several volts. I came to the conclusion that the cell had partial damage and the noise was caused by the plate intermittently making and then breaking contact with the terminal plate. |
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