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I have started looking at LiFePO₄ rechargeable batteries as a replacement for NiMH or alkalines since they appear to have some advantages over other types of common Li-ion cells for this purpose, and wanted to start a discussion on that, so here are some of my initial thoughts--
(In this discussion I refer to the other commonly used Li-ion rechargeable battery chemistries such as Lithium Nickel Manganese Cobalt, Lithium Cobalt Oxide, and Lithium Nickel Cobalt Aluminum Oxide as "other".)
The LiFePO₄ cell chemistry is used as a replacement for lead-acid batteries, since its voltage (for 6, 12, and 24V batteries), charging characteristics, and safety profile are similar to lead-acid, thus the charging rules, safety considerations, and limitations regarding the use of other Li-ion batteries don't necessarily apply.
Of course it only takes two LiFePO₄ cells to match the voltage of three lead-acid cells.
A LiFePO₄'s nominal cell voltage is 3.2V with a quite flat discharge profile and only slight change with temperature, so it can likely replace two NiMH or alkaline series cells, whereas the 3.6-3.7v of other Li-ion cells may be too high for that direct replacement.
The LiFePO₄'s voltage may also be preferred over the lower 2.4V nominal of two NiMH cells when replacing two 1.5V alkalines.
LiFePO₄'s can be charged similar to a lead-acid battery with a simple constant-current(limit), constant-voltage profile, and don't necessarily require cutoff of charging at the end, as long as you don't need to charge them above 80-90% of their rated capacity.
No specialized charging profile or IC is thus necessarily required unless you must charge them to 100% capacity.
LiFePO₄ is generally the safest of the Li-ion battery types and, even if abused, don't present the danger of a catastrophic fire that other Li-ion cells do.
LiFePO₄ cells can safely stay on a 3.4V float-charge, so can be used to directly power a device while also being charged, such as UPS applications, without specialized charge-control circuitry or requiring switching between main's power and battery.
A 3.4V/cell maximum charge/float voltage should charge and maintain the cell at 80-90% of its rated capacity.
This is similar to using a lead-acid battery for such applications, except an LiFePO₄ doesn't need temperature compensation of the float voltage as a lead-acid battery does (although that compensation may not always be done with typical lead-acid float charger designs).
Other Li-ion or NiMH cells generally can't operate reliably or safely in such a charge/float configuration.
The main disadvantage of LiFePO₄ 's it that they are larger/heavier than other Li-ions for a given energy rating, so may preclude their use if that is a significant consideration in the application.
Relative cost is also a consideration of course, but that, based upon similar Wh capacities, seem to be generally in the ballpark of NiMHs.
Further thoughts?
(In this discussion I refer to the other commonly used Li-ion rechargeable battery chemistries such as Lithium Nickel Manganese Cobalt, Lithium Cobalt Oxide, and Lithium Nickel Cobalt Aluminum Oxide as "other".)
The LiFePO₄ cell chemistry is used as a replacement for lead-acid batteries, since its voltage (for 6, 12, and 24V batteries), charging characteristics, and safety profile are similar to lead-acid, thus the charging rules, safety considerations, and limitations regarding the use of other Li-ion batteries don't necessarily apply.
Of course it only takes two LiFePO₄ cells to match the voltage of three lead-acid cells.
A LiFePO₄'s nominal cell voltage is 3.2V with a quite flat discharge profile and only slight change with temperature, so it can likely replace two NiMH or alkaline series cells, whereas the 3.6-3.7v of other Li-ion cells may be too high for that direct replacement.
The LiFePO₄'s voltage may also be preferred over the lower 2.4V nominal of two NiMH cells when replacing two 1.5V alkalines.
LiFePO₄'s can be charged similar to a lead-acid battery with a simple constant-current(limit), constant-voltage profile, and don't necessarily require cutoff of charging at the end, as long as you don't need to charge them above 80-90% of their rated capacity.
No specialized charging profile or IC is thus necessarily required unless you must charge them to 100% capacity.
LiFePO₄ is generally the safest of the Li-ion battery types and, even if abused, don't present the danger of a catastrophic fire that other Li-ion cells do.
LiFePO₄ cells can safely stay on a 3.4V float-charge, so can be used to directly power a device while also being charged, such as UPS applications, without specialized charge-control circuitry or requiring switching between main's power and battery.
A 3.4V/cell maximum charge/float voltage should charge and maintain the cell at 80-90% of its rated capacity.
This is similar to using a lead-acid battery for such applications, except an LiFePO₄ doesn't need temperature compensation of the float voltage as a lead-acid battery does (although that compensation may not always be done with typical lead-acid float charger designs).
Other Li-ion or NiMH cells generally can't operate reliably or safely in such a charge/float configuration.
The main disadvantage of LiFePO₄ 's it that they are larger/heavier than other Li-ions for a given energy rating, so may preclude their use if that is a significant consideration in the application.
Relative cost is also a consideration of course, but that, based upon similar Wh capacities, seem to be generally in the ballpark of NiMHs.
Further thoughts?
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