How to charge Lithium Batteries.

Lithium-ion Battery Charging Basics

Lithium Ion Charging

Basics

These remarks apply equally to lithium ion and lithium polymer batteries. The chemistry is basically the same for the two types of batteries, so charging methods for lithium polymer batteries can be used for lithium-ion batteries.

The lithium ion battery is easy to charge. Charging safely is a more difficult. The basic algorithm is to charge at constant current (0.2 C to 0.7 C depending on manufacturer) until the battery reaches 4.2 Vpc (volts per cell), and hold the voltage at 4.2 volts until the charge current has dropped to 10% of the initial charge rate. The termination condition is the drop in charge current to 10%. The top charging voltage and the termination current varies slightly with the manufacturer.

However, a charge timer should be included for safety.

The charge cannot be terminated on a voltage. The capacity reached at 4.2 Volts per cell is only 40 to 70% of full capacity. For this reason you need to continue to charge until the current drops, and to terminate on the low current.

It is important to note that trickle charging is not acceptable for lithium batteries. The Li-ion chemistry cannot accept an overcharge without causing damage to the cell, possibly plating out lithium metal and becoming hazardous.

The question occasionally comes up "What is the effect of charging with less than 4.2 volts?" Unlike other battery chemistries the battery will only be partly charged. The reason for this is that stuffing the ions into the anode or cathode crystals requires more voltage than the simple electrochemical cell voltage. The higher the voltage the more ions can be inserted. The page linked page here some quantitative data on the relative capacity of lithium-ion batteries that are charged below 4.2 volts.

Safety

Every lithium ion battery pack should have (must have?) a safety board which monitors the charge and discharge of the pack, and prevents dangerous things from happening. The specifications of these safety boards are dictated by the cell manufacture, and may include the following:

Reverse polarity protection
Charge temperature--must not be charged when temperature is lower than 0° C or above 45° C.
Charge current must not be too high, typically below 0.7 C.
Discharge current protection to prevent damage due to short circuits.
Charge voltage--a permanent fuse opens if too much voltage is applied to the battery terminals
Overcharge protection--stops charge when voltage per cell rises above 4.30 volts.
Overdischarge protection--stops discharge when battery voltage falls below 2.3 volts per cell (varies with manufacturer).
A fuse opens if the battery is ever exposed to temperatures above 100° C.


	[Home] [PowerStream] [Site Map] [Tech Resources] [Policy and Privacy] [Contact Us]

PowerStream Technology: 140 South Mountainway Drive, Orem Utah 84058 Phone: 801-764-9060, 9062, 9063, or 9064 Fax: 801-764-9061
		PowerStream Technology 140 South Mountainway Dr. Orem, UT 84058 Phone: 801-764-9060, 9062, 9063, or 9064 Fax: 801-764-9061 Map & Directions
© Copyright 2000, 2002, 2003, 2005, 2007, 2008 Lund Instrument Engineering, Inc. All rights reserved This material is copyrighted original work. It is forbidden to use this information, text, or graphics in full or in part on another web site without written permission. This prohibition forbids making derivative text using automatic thesaurus substitution software.