Li batteries generally require more sophisticated charging regimens than lead-acid
batteries. You might do some background research on charging Li batteries before
proceeding.
Not true for single cells. They actually have a simpler curve than do lead acid batteries. They are just not as tolerant to abuse. Most cheaper lead acid battery chargers actually significantly reduce the life of the batteries we given them to care for. However, because they will take considerable abuse, it goes unnoticed. If they are properly treated, lead acid batteries can last 10-15 years, or more. Lithium cells reward even a little abuse by overheating and bursting, or even causing fire.
The difficulty with Lithium based batteries are at least two fold. First, you need to keep strict watch on the minimum and maximum voltages you allow the cell to hold. Most high power cells these days actually have built in limiter circuits that will cut of the cell if it pushed beyond these limits. Be careful that you keep within these limits as sometimes the reset process is difficult. This is more of a problem if a multi-cell pack as the voltages of each cell need to be monitored separately during charge and discharge. This brings us the the second difficulty. The internal resistance of Lithium cells can vary considerably, even in the same batch. So much so that, in order to avoid breaching the voltage limits, you must monitor each cell independently and cut off charge or discharge of the entire pack once an single cell reaches a limiting voltage.
The 2nd difficulty can be mitigated by doing 3 things. 1 - Artificially increasing the internal resistance of the cells by a fixed amount so that the relative differences between cells are minimized. 2 - carefully selecting amongst those cells for near identical characteristics, and 3 - imposing tighter charge-discharge voltage limits. So, given a cell that has a V
max of 4.2V and a V
min of 2.8V, run it in-pack between 3V and 4V such that for a 4 cell pack, cut off discharge at 12V and charge at 16V. However, because of the increased internal resistance of the cells, this bag of tricks only works in low demand situations, like phones and other small devices. Even then it's not a 100% satisfactory solution and battery life is much reduced from the ideal.
@ winner10920,
Unless you absolutely need to charge these batteries quickly, I'd recommend 1/4C or less. Charging or discharging at C or above can cause heating issues which will change V
max and V
min and make them difficult to monitor correctly.
As an example, for a 4700mah cell with a V
max of 4.2V, charge it in constant current mode at 1A until V=4, then switch to constant voltage mode at 4.2V for about an hour or 2 at the most then end the charge. This method is for when you need to use the cell right away.
If you wish to be able to leave the battery in the charger and keep it at a reasonable state of charge fore go the constant voltage cycle. Just charge at 1/4C until V=4 then cut off. Continue to monitor the cell voltage and when it drops to 3.4 or so, re-charge it back up to 4 again. For a charger like this, I would add a push-button to signal to the MCU when you want it to fully top up the battery and enter a CC - CV cycle that brings it to the full 4.2V.
One more hint. For long term storage lithium cells like to be at their 'nominal' voltage. For most cells this is somewhere around 3.6V. You could program in a 3rd cycle for pre-storage that begins with a CC cycle to 3.6 volts, then a CV cycle at 3.8V for a couple of hours. Check voltage in storage occasionally and it they drop below 3.4 or so, give them another pre-storage charge cycle.
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