On 26/10/14 17.31, Jeff Liebermann wrote: > On Sun, 26 Oct 2014 11:08:32 +0100, Robert Roland > wrote: > >> On Sat, 25 Oct 2014 20:45:38 -0700, Jeff Liebermann >> wrote: >> >>> One accidental overcharge and your >>> battery is dead. >> >> Not true. > > I beg to differ. > >> Early NiMH batteries were quite intolerant of overcharging, but modern >> batteries are much more robust. Not quite as robust as NiCd, though. >> >> In fact, some amount of overcharge is necessary in order to keep the >> cells in a battery balanced. > > Let's see if I understand what you're saying. In a series string of > NiMH cells, one must overcharge the good cells in order to bring the > not-so-good cells up to the good cell voltage? Is that what you're > saying? If so, this sounds like a variation on "battery conditioning" > which works well at killing batteries. ...
Hi Robert, Jeff and others
The worst thing for any rechargable (except NiFe) is reverse-charging of e.g. one cell. That happens because cells have different: (1) self-discharge characteristics (2) capacity (3) maybe different internal impedance as a function of charge-level.
Reverse-charging even one cell will shorten its life hugely. And when one cells is dead, the whole battery fails a lot faster than maybe 200 full cycles? - and not the advertised 1000-2000 full (100%) cycles.
The best you can do for your serially connected cells are to monitoring your cells when discharging.
But this will typically cost a microcontroller and some wires to each cell end, and a power switch that is controlled by microcontroller in question.
The system is called a Battery management system (BMS):
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If a cell is overcharged, it will shorten the cell life some, especially if the cell vent - which means that it looses water (=higher internal impedance).
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Li-ion (excluding LiFePO4!) cell must not be overcharged or else it also get a lot shortened life - or will even destabilize.
If a Li-ion (excluding LiFePO4!) cell is over-discharging (0V and not even reverse-charged), it will destabilize. And when later charged it will be prone to burning and explosion.
It a Li-ion (excluding LiFePO4!) get to high a charging current or discharging current, it will destabilize.
That is why (nearly) all Li-ion (e.g. computer) Li-ion batteries have a BMS.
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LiFePO4 can handle overcharging, but this will shorten the cell life some, especially if the cell vent - like any other chemistry - which means that it looses water (=higher internal impedance).
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It is possible to buy battery BMS-controllers e.g.:
- TI: bq20z60-R1, bq29330, bq294xy...
- Atmel: ATA6870N *n +(ATmega32HVE2 or ATmega64HVE2)
Atmel ATA6870N Li-Ion, NiMH Battery Measuring, Charge Balancing and Power-supply Circuit datasheet:
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Look at the sources here:
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There is even an interface standard (Smart Battery System, SBS):
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A battery with a BMS is called a smart battery:
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A charger that can "talk" with a battery with a BMS, is called a smart battery charger:
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Glenn