funkychateau
Enlightened
I've been using rechargeable batteries in devices for about 25 years, and the series pitfall has always been the same.
If one cell has slightly lower capacity, then it becomes depleted first and the other cells continue to source current. For NiCd and NiMH cells, this results in a reverse-voltage condition that quickly ruins the cell. For Li-ion cells, damage may occur long before reverse polarity.
For setups with only two to four cells, the device being powered (for example, a three-cell walkie-talkie) often can detect low battery voltage and shut down before one cell is over-depleted. But, when a larger number of cells are used in series, the device may continue to work normally for a short time after one cell is substantially depleted, ruining the weaker cell. I've lost count of the battery packs I've salvaged from wireless devices by replacing just one cell (the "new" cell generally becomes the strongest, making the pack last until another of the older cells gives up).
Many flashlights are particularly bad news, as they may continue to work "normally" over a fairly wide voltage range. This drives a nail into the coffin of the one poor cell that depleted first.
Seller "AW" has a thread http://www.cpfmarketplace.com/mp/showthread.php?t=184887 in the dealer section, for "LiMN" cells that are claimed to be "safe to use in multi-cell applications".
Is there something special about this technology that makes these devices immune to the traditional "one-cell-depletes-sooner" damage scenario in series applications?
These cells apparently do not have protective undervoltage cut-off circuits, as they come with the standard warning to not over-discharge.
So, is there something about this chemistry that makes the cells inherently so well-balanced that they all deplete at a uniform rate? I can't think of any other way they would be "safe", in the sense of damage-resistance, in series applications. And even so, wouldn't the problem still exist if cells were mixed from lots with different usage history?
thanks!
If one cell has slightly lower capacity, then it becomes depleted first and the other cells continue to source current. For NiCd and NiMH cells, this results in a reverse-voltage condition that quickly ruins the cell. For Li-ion cells, damage may occur long before reverse polarity.
For setups with only two to four cells, the device being powered (for example, a three-cell walkie-talkie) often can detect low battery voltage and shut down before one cell is over-depleted. But, when a larger number of cells are used in series, the device may continue to work normally for a short time after one cell is substantially depleted, ruining the weaker cell. I've lost count of the battery packs I've salvaged from wireless devices by replacing just one cell (the "new" cell generally becomes the strongest, making the pack last until another of the older cells gives up).
Many flashlights are particularly bad news, as they may continue to work "normally" over a fairly wide voltage range. This drives a nail into the coffin of the one poor cell that depleted first.
Seller "AW" has a thread http://www.cpfmarketplace.com/mp/showthread.php?t=184887 in the dealer section, for "LiMN" cells that are claimed to be "safe to use in multi-cell applications".
Is there something special about this technology that makes these devices immune to the traditional "one-cell-depletes-sooner" damage scenario in series applications?
These cells apparently do not have protective undervoltage cut-off circuits, as they come with the standard warning to not over-discharge.
So, is there something about this chemistry that makes the cells inherently so well-balanced that they all deplete at a uniform rate? I can't think of any other way they would be "safe", in the sense of damage-resistance, in series applications. And even so, wouldn't the problem still exist if cells were mixed from lots with different usage history?
thanks!
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