Whereas lead-acid, NiCd and NiMH cells can, and are, designed to be able to convert some over-charge current into heat by a process called recombination; Li-ion cells cannot. Any current forced into a fully-charged Li-ion damages it.
[size=medium]At minimum this reduces cell life. At the extreme maximum some unprotected cells have been known to burst and catch fire. Protection against failed voltage or current regulation should always be present in a Li-ion or lithium polymer charging scheme.
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In float charging, current entry beyond full charge is prevented by choosing a charge voltage less than or equal to the voltage naturally produced by the cell at full charge which depends on the cells exact electrochemistry and temperature.
[size=medium]It is all related to battery voltage, says Robin Cloke, UK MD of GP Batteries. As a general rule of thumb, if you choose around 4.0V it is fairly safe for the cell.
[size=medium]Specifically for his cells, Cloke says: 4.2V is actually what some people do, but if you are going to leave on the voltage, back it off to 4.1V.
[size=medium]Something to be noted here is that there are trade-offs to be made between the voltage chosen, cell chemistry, ambient temperature and cell life.
It is very much on voltage and temperature, says Mike Lain, a battery scientist at the UKs ABSL Power Solutions. We have done some tests charging cells at 4.2V. At 20C there is steady capacity loss, and after two years 80 per cent of capacity is retained. At 60C there is very rapid capacity loss. Dont even think about it.[/size][/size][/size]
