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Are NiCd batteries superior to NiMH batteries for emergency lighting unit use?..........
the following seems to suggest that it is...do you agree?
As discussed, i was looking into NiMH vs NiCd cells for our Emergency Lighting Unit......
Unfortunatley, NiMH is not looking suitable for Emergency use, because Emergency use requires the battery being charged up and then staying charged up by being continuously trickle charged.....NiMH cells cannot generally tolerate being trickle charged at a low rate without their capacity lifetime being too badly affected.
NiMH is smaller than NiCd, however, for a similar capacity (4000mAh) we would need a SAFT "VHT 7/5 Cs" NiMH cell, which is the same length as our current NiCd D cell (60mm). However, the NiMH "VHT 7/5 Cs" cell is slightly thinner at 22mm diamater, compared to our current NiCd's 33mm diameter.
NiMH , "VHT 7/5 Cs" Datasheet (SAFT):-
**broken link removed**
Its worth noting from the datasheet, that a "trickle" or "permanent" charge rate is not offered for this "VHT 7/5 Cs" NiMH cell, and this is likely to be because NiMH's cannot handle being trickle charged well.
However, SAFT do say that they are able to make custom built NiMH batteries for Emergency Use.
The only off-the-shelf SAFT NiMH cells whose datasheet offers a "trickle" or "permanent" charge rate , are those NiMH's whose cell size is 'D' and above. -(Though our current NiCd cell is a 'D' size).
The NiMH "VHT D" cell from SAFT offers a trickle charge rate, and four of these D cells would be needed for the Emergency light unit, instead of the current five NiCd 'D' cells being used. (-so the NiMH solution would mean saving the space of just one D cell (60mm high by 33mm diameter).
NiMH , "VHT D" Datasheet (SAFT):-
**broken link removed**
Other reasons for using NiCd over NiMH for emergency use are:-
#Lower self discharge rate.
#Better recovery from deep discharge such as may occur in prolonged mains outage
#Cheaper
#NiCd can generally be used with cheaper chargers..The NiMH cells cannot be simply charged at double the trickle charge rate, and then trickle charged therafter....unless they have cell
temperature monitoring during the double-trickle charge rate. NiCd's do not need cell temperature monitoring during the C/10 (double the trickle rate) charge.
#NiCd has better high temperature performance than NiMH
#Individual cell monitoring, is less likely to be necessary for a NiCd battery pack, than a NiMH battery pack, because NiCd Is more robust than NiMH.
#NiCd cells can be C/10 charged at below freezing temperatures....NiMH cells cannot be, unless they are custom built to do this.
#Though NiMH cells do exist for emergency use, they are not that much smaller than similar capacity NiCd's, because the NiMH cells that are "emergency-capable" have to have thicker seals and
bodies so that they can handle the trickle charge....all in all, NiCd looks best, and is not that much bigger than "emergency capable" NiMH's.
Conclusion that NiCd is the way forward for Emergency Lighting Battery.
...is this all correct?
the following seems to suggest that it is...do you agree?
As discussed, i was looking into NiMH vs NiCd cells for our Emergency Lighting Unit......
Unfortunatley, NiMH is not looking suitable for Emergency use, because Emergency use requires the battery being charged up and then staying charged up by being continuously trickle charged.....NiMH cells cannot generally tolerate being trickle charged at a low rate without their capacity lifetime being too badly affected.
NiMH is smaller than NiCd, however, for a similar capacity (4000mAh) we would need a SAFT "VHT 7/5 Cs" NiMH cell, which is the same length as our current NiCd D cell (60mm). However, the NiMH "VHT 7/5 Cs" cell is slightly thinner at 22mm diamater, compared to our current NiCd's 33mm diameter.
NiMH , "VHT 7/5 Cs" Datasheet (SAFT):-
**broken link removed**
Its worth noting from the datasheet, that a "trickle" or "permanent" charge rate is not offered for this "VHT 7/5 Cs" NiMH cell, and this is likely to be because NiMH's cannot handle being trickle charged well.
However, SAFT do say that they are able to make custom built NiMH batteries for Emergency Use.
The only off-the-shelf SAFT NiMH cells whose datasheet offers a "trickle" or "permanent" charge rate , are those NiMH's whose cell size is 'D' and above. -(Though our current NiCd cell is a 'D' size).
The NiMH "VHT D" cell from SAFT offers a trickle charge rate, and four of these D cells would be needed for the Emergency light unit, instead of the current five NiCd 'D' cells being used. (-so the NiMH solution would mean saving the space of just one D cell (60mm high by 33mm diameter).
NiMH , "VHT D" Datasheet (SAFT):-
**broken link removed**
Other reasons for using NiCd over NiMH for emergency use are:-
#Lower self discharge rate.
#Better recovery from deep discharge such as may occur in prolonged mains outage
#Cheaper
#NiCd can generally be used with cheaper chargers..The NiMH cells cannot be simply charged at double the trickle charge rate, and then trickle charged therafter....unless they have cell
temperature monitoring during the double-trickle charge rate. NiCd's do not need cell temperature monitoring during the C/10 (double the trickle rate) charge.
#NiCd has better high temperature performance than NiMH
#Individual cell monitoring, is less likely to be necessary for a NiCd battery pack, than a NiMH battery pack, because NiCd Is more robust than NiMH.
#NiCd cells can be C/10 charged at below freezing temperatures....NiMH cells cannot be, unless they are custom built to do this.
#Though NiMH cells do exist for emergency use, they are not that much smaller than similar capacity NiCd's, because the NiMH cells that are "emergency-capable" have to have thicker seals and
bodies so that they can handle the trickle charge....all in all, NiCd looks best, and is not that much bigger than "emergency capable" NiMH's.
Conclusion that NiCd is the way forward for Emergency Lighting Battery.
...is this all correct?
