# charging Li ion battery

I want to ask if this is possible.

I have a Li ion battery that I want to charge. It’s one two cells in parralel. So from what I have read is I must charge it at a constant current till the battery is 4.0volts then do a constant voltage charge till 4.2v and never over charge.

Question. If I only want to do the constant current charge till 4.0v what must I limit my voltage to? I’m slightly confused because a constant current adjusts the voltage to keep a constant current. Will this not cause the charging voltage to go above 4.2volts? Is that allowed? If not what is the maximum voltage allowed when doing a constant current charge. I’ve read these batteries are very sensitive so I just wana be sure. Thanx

It would be easier to simple use a dedicated chip with all the logic (and safety) built into it like an MCP73831 or the like. There are breakout board on the net that are cheap.

If you are limiting the current, the voltage will be less than 4 volts.

lemming:
It would be easier to simple use a dedicated chip with all the logic (and safety) built into it like an MCP73831 or the like. There are breakout board on the net that are cheap.

Thanx but I want to use what I have I don't want to go buy chips. The exact same can be achieved with an arduino and 2 LM317's and some transistors and resistors.

michinyon:
If you are limiting the current, the voltage will be less than 4 volts.

I don't think that is correct. If I limit the current to 1A which is 0.2xthe battery capacity then I'm pretty sure the voltage will go above 4.0 volts

The way it's usually done (on my commercial chargers) is put a constant current into the battery while monitoring the voltage across the battery. When that voltage reaches 4.2 volts, they change from a constant current to a constant voltage of 4.2 volts and then monitor the current until it reaches a very low level (on my charger that is below 10% of the constant current level that was used in the first step)

pegwatcher:
The way it's usually done (on my commercial chargers) is put a constant current into the battery while monitoring the voltage across the battery. When that voltage reaches 4.2 volts, they change from a constant current to a constant voltage of 4.2 volts and then monitor the current until it reaches a very low level (on my charger that is below 10% of the constant current level that was used in the first step)

Thank you that makes perfect sense, do you perhaps know what voltage the battery gets during constant current charge? Can this voltage be above 4.2v?

““Question. If I only want to do the constant current charge till 4.0v what must I limit my voltage to? I’m slightly confused because a constant current adjusts the voltage to keep a constant current. Will this not cause the charging voltage to go above 4.2volts?””

You have already answered your own question. You raise voltage to keep a constant current but once you reach 4.2 you revert to constant voltage - hence voltage never exceeds 4.2v

Read that explanation again. The battery voltage must be monitored and when it gets to 4.2 volts, you switch to a constant voltage of 4.2 volts. The battery can never get above 4.2 volts this way. The charging circuit must be two stage. First stage is constant current, Second stage must be a constant voltage of 4.2 volts. It doesn't matter what the voltage might be at the input of the constant current source, that will vary. What matters is the voltage measured across the battery. It must change from constant current to constant voltage of 4.2 volts when the voltage across the battery reaches 4.2 volts during the constant current source. When you first apply the constant current to a discharged battery, the voltage across the battery will measure well below 4.2 volts. As the battery takes a charge, the voltage across it will increase until it reaches 4.2 volts, at which point you stop using constant current and switch to constant voltage.

You cannot properly charge a li-ion with only a constant current source. It must be a two stage charger: First a constant current source and then a constant voltage source.

pegwatcher:
You cannot properly charge a li-ion with only a constant current source. It must be a two stage charger: First a constant current source and then a constant voltage source.

I charge my lithium's the same way ultrafire chargers do it, using a pnp (wtith an complementary npn) and cutting the voltage / charge every couple of seconds to monitor it's voltage level, it all runs from 5v on an Arduino...

Ok I am pretty confused. Does constant current mean the current is limited but not to say the device will put the max current allowed? Or is constant current when the current to the device is constant at the max that u set it to be?

Example. Say I use a 10ohm resistor with an lm317, this gives me a constant current of 125mA. Now say I put a device on that only requires 20mA. Will that device only draw 20mA or will the constant current source force the whole 125mA on it?

Your circuit can supply up to 125ma ... but will consume only 20ma (up to 125ma) if you shorted the outputs 125ma is the max current you'll get.

cjdelphi:
Your circuit can supply up to 125ma ... but will consume only 20ma (up to 125ma) if you shorted the outputs 125ma is the max current you'll get.

Awesome ok thanx that clears a lot up for me! So even if I use a 100v power supply and limit current to my battery, my battery will not necessarily see 100v?

calvingloster:

cjdelphi:
Your circuit can supply up to 125ma ... but will consume only 20ma (up to 125ma) if you shorted the outputs 125ma is the max current you'll get.

Awesome ok thanx that clears a lot up for me! So even if I use a 100v power supply and limit current to my battery, my battery will not necessarily see 100v?

Correct, in constant current charging the charger manipulates the voltage to whatever is required to maintain that current value. in constant voltage mode is where the charger must not apply higher then 4.2 vdc.