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What is the algorithm used in charging lipo batteries?

From watching my charger, I see that the amperage is close to the maximum specified (i.e. what I keyed in) at the beginning and gets smaller as each cell nears its maximum charge, but I haven't noticed a direct correlation between the cell voltage and input amperage.

Is there some "standard" algorithm, or does each charger manufacturer implement their own?

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Constant current of x amps (x is usually a fraction or multiple of capacity, e.g. 0.5C, 1C, 2C. Usually never any higher than 5C or cell heating results) until 4.2V ±0.5% is reached. This is called the "CC" stage.

Then a constant voltage of 4.2V ±0.5% is applied until charge current drops below a specific current, usually 0.2C to 0.05C. This is called "CV".

If cell voltage is below 2.8V, a precharge is performed at about 1/10 to 1/20 normal charge current until cell voltage reaches 2.8V; from then on CC takes over. Discharging a li-ion below 2.8V can usually cause a loss of capacity or permanent damage, so avoid doing it. Not all chargers implement pre-charging; some will refuse to charge over-discharged batteries.

This is for newer li-ion cells with 3.7V nominal voltages; for older ones with 3.6V nominal voltage, use 4.1V ±0.5% instead of 4.2V as the CV point.

This is per cell. For multi-cell packs at low charge currents, cells are charged in series. With higher charge currents, they are still charged in series but if they become out of balance, a balancer draws a small current from the appropriate cell to re-balance the pack. Some lower-current charges charge each cell separately, but this increases the cost.

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1: trickle charge until approximately 3 V per cell (for really flat cells), 2: constant current charge until 4.2 V per cell, 3: constant voltage charge until full.

actual values may vary dependent on cells - refer to manufacturers datasheet.

This is fine for upto two cell batteries. Greater than two cells and some charge balancing may be reuqired.

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    \$\begingroup\$ Where "really flat cells" means very low voltage, or deeply discharged, depending on your lingo. (Kept using it well into out of charge) \$\endgroup\$ – Kortuk Jun 22 '11 at 3:26
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It's not an "algorithm" in the sense that software is deciding how much current to permit into the battery at each moment in time. The battery is connected to a power supply that is both voltage and current limited and allowed to take as much current as it likes. At first (if empty enough) it will take all the current it can get, the current limit of the power supply, and the voltage will slowly rise. When the battery voltage rises to the voltage limit of the power supply, the power supply will switch from current limit mode to voltage limit mode and the current going into the battery will start decreasing. [Thus far no software is required at all.] When the current falls below some level and/or enough time goes by, charging needs to end, which could be handled by software, hardware, or a human turning off the power supply. For safety there may be other features of the charger such as refusing to charge above or below some temperature range.

It's possible to build a battery charger entirely in hardware, which is how older battery charger chips worked. Now that microcontrollers are so low power and inexpensive, it's becoming more popular to use them, as a host of new features are enabled, such as keeping track of battery state of charge (how full it is), checking to see if the battery is "counterfeit" (aftermarket) before charging it, and so on.

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Easy way with a bench power supply:

1) Read the rated capacity on the cell, let's say it's 1500 mAh.

2) Set the current limit to 1500 mA, this should give you about an hours charge time, ideally.

3) Set the voltage limit to 4.2 V, measure it with a Volt meter, this is critical.

4) Hook up the cell and let it charge until the current has dropped to less than 200 mA.

5) ... but stop if 2 hours pass, in that case the cell might be worn out.

If the cell is really flat deeply discharged, then you need to start off charging at 1/10 C, but cells should never be discharged that far in the first place.

I've had to do this many times when I've misplaced chargers for some shitty equipment I have that doesn't want to charge from USB.

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  • \$\begingroup\$ Where "really flat cells" means very low voltage, or deeply discharged, depending on your lingo. (Kept using it well into out of charge) \$\endgroup\$ – Kortuk Jun 22 '11 at 3:26

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