ORIGINAL: olnico
ORIGINAL: PaulD
I have been using pairs of A123 batteries with 2 heavy duty switches. Never had a problem but only concern is I don't have any isolation if one battery were to go south. Been thinking about some form of diode arrangement to provide isolation but then I would have to ask - which has higher chances of failing - an A123 pack or a diode?
PaulD
Paul, this is THE question: how simple does it have to be to stay reliable ?
Personally, and given the testing protocol I use for my batteries I would rather say that the diode would be the weak point.
A battery does not die suddenly: you see a sudden consequence of a slow degradation of the internal components. A thorough monitoring of the battery internal resistance will show an impending problem.
In case of one cell short-cutting on a pack, the voltage will drop from 6,6V ( on A123 ) to 3,3V. The internal resistance will drop significantly ( from 30 mOmh to around 20 mOhm ) thus the remaining pack will see the shorted pack as a load and will start to slowly discharge into it. I have made an experience with a 1 cell pack and a 2 cell pack plugged in parallel on a Rx. The current transfer on A123 is around 0,5A. This is an equivalent of two additional servos being plugged to the system. So I believe that if the power source is sized with sufficient buffer, a cell shortcut will be handled correctly by the remaining pack. Note that this experience does not truly simulate the reduction of internal resistance of a shorted 2S pack but seems close enough to me.
Also note that I have not had a single shorted cell event for the last 15 years.
I tend to agree. Since switching to the A123's, this has been my set-up and I have had no problems. When I charge my A123's I allways check the cell voltages to see how they are balancing. I have not seen a problem but do expect that it would be evident during charging. I also have a cell voltage meter that will show individual cell voltages when I check the pack's charge condition.
The Diode would likely be the weakest link.
PaulD