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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The useful voltage range of a Li-Ion is 3.0 to 4.2v per cell, and the charge usually takes 3 hours. 1.5 hours at constant current (about 1C) until the voltage reaches 4.2/cell, then 1.5 hours at constant voltage until the charge current tapers off to C/20 or thereabouts.
Laptops always have a built-in Li-ion charger, a buck converter that runs off the DC input. They usually have power management that reduces the charge rate when the laptop is on, so as not to overload the AC adapter.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
Make sure you have a two layer failsafe, Li cells do not like being trickle charged. 10% current is the cutoff point, after that you are plating out Li metal onto the inside of the cell which can short out then explode if charging continues...
You are better off using a charging IC (IIRC Maxim do samples) and also the cell(s) will last longer.
Especially important is to ensure the cells are balanced by using the correct multi-cell charger, that can read the voltages on each cell.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
The laptop Lithium batteries that I've disected have had a surprisingly complex ammount of electronics onboard, specifically the per-cell (or 2 or 3 in parallel) voltage is monitored as above, charge/discharge current and voltage monitoring, and battery charge status monitoring.
P.S. Have you heard of 'digital memory effect' ? The individual cells do not have any significant 'memory effect' (NiCd cells may) but each charge/discharge cycle causes a slight error, which accumulates over many cycles to the point where the onboard monitor may indicate that the battery has very little capacity left, whereas it's fine. An occasional deep discharge (if the onboard circuitry allows) will reset/fix the problem. So if you've just thrown out your one yer old battery because 'the computer says no' (a tv gag in uk) then Ha Ha ! (Nelson from Simpsons)
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Source of lithium battery datasheets
