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Statement of the problem
1. When many cells are assembled
into a single pack the pack often will experience an early failure. This problem starts when
you have just 10 cells in series, and gets worse for 20, 30, and 40 cell packs. Some
applications need over 100 cells.
The reason for the early failure is that when you
have cells assembled into a pack there will always be stronger and weaker cells. One of these
cells will be fully discharged before the rest of them. In a 3 cell pack this is easy to
determine, since the voltage drop will be 1/3 of the total. With 10 cells the voltage drop will
be 10% of a voltage that varies between 14 volts and 10 volts. When the pack reaches 10 volts
it can mean that all 10 cells have dropped to 1 volt each, or that 9 cells are 11.1 volts, and
one is zero volts. For a 30 cell pack, a pack that has dropped to 30 volts may mean that 29
cells are at 1.03 volts, and one cell is zero.
Nickel Cadmium and Nickel Metal Hydride
cells can be individually discharged to zero volts without damage, but they cannot be reverse
charged without generating gas. Eventually pressure will build up to the point that the cells
start to vent, which results in permanent capacity loss.
This is a particularly
difficult problem for applications like electric bicycles, electric wheel chairs, and dive
lights where you might have 20 or 30 expensive NiCad or NiMH cells in a pack. It is the reason
that most projects to replace lead acid batteries with NiCads or NiMH have failed.
Several Solutions to the
Problem
1. Matched Cells The most common way to solve the
problem of early pack failure due to imbalance is to make sure the cells are matched before you
assemble them into a pack. This is useful for packs with few cells. As the cell count grows the
matching needs to be more and more accurate.
At low discharge rates, there is about 3%
of the capacity of a typical cell between 1.1 volts and zero volts, about 1% between . 1 volt
an zero volts. So if you match within 3% you should be safe if you discharge to 1.1 volts per
cell, and 1% if you discharge to 1.0 volts per cell. (Note, not ±3%, but 3%
absolute).
However, the cells don't stay matched and after a moderate number of cycles
the difference can grow to cause trouble. And finding 10 matched cells is a lot easier than
finding 30 or 40 matched cells. For larger packs you would need a large pool of cells to draw
from.
2. Cut off Voltage. The second way to protect the pack from overdischarge
damage is alluded to in the last section. If the cells were perfectly balanced you could
discharge to zero volts without any problem. But as you can't perfectly balance the cells, and
you can't expect them to stay in balance over a large number of cycles.
So a common
approach is to keep the pack voltage from dropping below the level which is dangerous. So what
level is dangerous? Usually for small packs, up to 10 cells the rule of thumb is 1 volt per
cell. Panasonic recommends a cut off voltage of 1 volt per cell for packs less than 7 cells,
and 1.2 volts for packs from 7 to 20 cells. At low rates of discharge a cut off of 1.2 volts
only loses about 2%, but at a discharge rate of 1C you are leaving about 8% in the pack, and at
3C you are leaving 80% of the useful charge in the cells when cutting off at 1.2 volts, and
only using 20%.
3. Active monitoring. The best way to solve this problem is to
monitor each cell in the pack. When the first one drops to a predetermined level (depending on
discharge rate) the entire pack is disconnected from the load. This has the advantage of making
the battery last as long as the weakest cell, which is 1000+ cycles for a NiCad and 500+ cycles
for NiMH. It also ensures that you can get every last bit of power out of the pack without
damaging the pack.
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This is a testbed of 20 NiMH D Cells with the active monitoring board. |
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