Charging Batteries at different modes

[polashd]

while charging Li-ion battery at constant current mode do I need to limit the voltage also, like 4.2v max.

When the battery reaches to 4.2v the constant voltage mode is to be started.

My question is when battery voltage and charger output voltage are equal (ie. 4.2v) why battery draws current.
Theoretically when voltage difference is zero no current should flow.

 

[MrAl]

Hi,

There is still a very small voltage difference, that creeps up slowly until it gets even closer to the target voltage which is usually 4.15 to 4.20 volts. The small voltage difference still produces some current because of the very low cell impedance, which is around 0.1 Ohms or even less.
Note that using I=E/R, 0.1v/0.1 Ohms equals 1 amp. That's 1 amp with only 0.1 volt difference. That means that with 0.01v difference we could still see 100ma.
The "constant voltage" here is not the battery voltage, it is the target charger voltage.
If you watch the actual battery voltage with a decent digital meter you'll see it creep up.
When they say "constant voltage mode" what that means is that the current has cut back from max to some slightly lower current. So if the charge current is 1 amp, when it cuts back to say 0.99 amps that is an indication that it is in constant voltage mode. So you can tell by looking at the current not the voltage.

 

[audioguru]

The charging voltage for a lithium battery cell MUST be limited to 4.20V because if the voltage is higher then the Lithium might explode or catch on fire. You do not want a Lithium fire because it is very hot and water on it makes it burn hotter. Think of the similar metal called magnesium used in flares.

 

[MrAl]

Hi,

I have read that the normal Li-ion cells can be put out with water, just not the special type which i'd have to look up to remember (later: lithium metal cells). Good to know what kind you have. Most of the common kind are not lithium metal, and the common kind have less li metal.

Just to note, when the voltage set point is set to 4.200 volts, the current might start to drop off at 4.100v, or maybe 4.150v, or even 4.160v, depending on how much internal resistance the cell has. This is because the charger acts like a voltage regulator which (can) regulates the output to 4.200v, but with 0.05 Ohm internal resistance we cant put out the full 1 amp anymore:
(4.200v-4.150v)/0.05=1 amp, but
(4.200v-4.175v)/0.05=0.5 amps which is cut back to 1/2 full current.
So you see how this works, as the voltage creeps up near 4.200v the curent starts to cut back, and as the voltage creeps up more and more the current cuts back more and more, until the "cutoff current" which is maybe 50ma. At that point the cell is considered charged.

There is one more view point though, and that is that if you dont mind charging to only (say) 4.15v instead of 4.20v, once the cell reaches 4.15v with the charger set to 4.20v it may be considered charged to some because they want to get more cycle life out of their cell. But here the charger is still set to 4.20v, and the charge is terminated at 4.15v using the same meter to measure both voltages (for accuracy)

 

[eTech]

Hi

This simulation shows a typical 4.2 Li-Ion Battery Charge profile.
During precharge mode, a small amount of current is fed to the battery if battery voltage is below the charge threshold of about 2.9v.
Once the battery voltage rises above the threshold, constant current mode begins and continues until battery voltage reaches about 4.1.
Then constant voltage mode begins and is terminated when battery voltage reaches 4.2v. Different manufacturer chargers vary the duration of each mode somewhat. Some also detect the presence of a connected battery before beginning a charge cycle.

 

[MrAl]

Hi,

Very good illustration of the charging profile.

The exact point where the switch from constant current to constant voltage occurs is:
Ich=(Vset-Vbatt)/Rbatt

and solving for Vbatt we get:
Vbatt=Vset-Ich*Rbatt

and this is where
Vbatt is the internal voltage of the battery,
Vset is the set point voltage of the charger,
Ich is the set point max current of the charge (normal charge current),
Rbatt is the internal series resistance of the battery.

Vbatt is not directly measurable, but Ich is, and Ich drops just after this point.

 

[bountyhunter]

while charging Li-ion battery at constant current mode do I need to limit the voltage also, like 4.2v max.

When the battery reaches to 4.2v the constant voltage mode is to be started.

My question is when battery voltage and charger output voltage are equal (ie. 4.2v) why battery draws current.
Theoretically when voltage difference is zero no current should flow.

You should study the difference between constant current (CC) and constant voltage (CV) charging. The Li battery charges CC until the battery reaches 4.2V, but understand this carefully:

The battery has an impedance so when it first reaches 4.2V, the battery voltage is not 4.2V (there is a V drop across the impedance). If you turn off the current the voltage will drop immediately below it. The CV charge mode tapers off the current holding at 4.2V, but the cell is not charged until it's 4.2V at a VERY LOW charge current.