Questions about CC/CV li-ion charging

[moimies]

I'm thinking about making my own microcontroller based li-ion charger, but there's some point that aren't fully clear. I've seen this kind of CC/CV graph many times, but that shows only the cell voltage. So the questions are:

• What is the charging voltage during constant current stage? Is it 4.2V, or something else?
• Does the cell "automatically" limit the charging current in CV stage, or does the charger do it?
• In a simplified scheme, if I have a lab power supply with voltage set to 4.20V and current limited to 1C, would it work if I just connect the cell to the power supply and wait till the current drops to 0.1C?

 

[Mugrunty]

hooking your battery to a bench powersupply works great, but you have to monitor it. I've done it many times before at home and at school! It's better to charge a li-ion battery at about 2/3C, though. 1C is pushing it, and it really doesn't help shorten charge times by much because then you would reach 4.2V sooner....therefore start the CV part of the process earlier.
Here's a quote from www.powerstream.com

"The lithium ion battery is easy to charge. Charging safely is 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."

You would have to design a circuit that limits the current at 0.6C (or similar) and have a max charging voltage of 4.2V. You could use the microcontroller to monitor the current and voltage. When the current falls to 10%, simply stop the charge. You could use a transistor based current limiter, and maybe a voltage regulator set at 4.2V output. The transistor circuit would make sure the current doesn't rise too high.

A circuit similar to the one above would work for the current limit part. You vary the 25 ohm POT to adjust the current. Just an idea, might not work for your application. It's not entirly microprocessor controlled...more microprocessor monitored.

Oh well, I hope this helped a little.


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[linterno]

I have been working in my own built chargers too and I am going to tell you my experiences:

What is the charging voltage during constant current stage? Is it 4.2V, or something else?

If you work with 4.2 volts from the beginning then the battery won't reach 4.2 during the CC stage. The current will be dropping as the battery voltage approaches 4.2 volts. If you work with something higher as 4.25 or 4.3 then the 4.2 volts are reached relatively fast and before the battery is full. As soon as the battery voltage reached 4.2 volts you must change to CV.

Does the cell "automatically" limit the charging current in CV stage, or does the charger do it?

In my test the current starts dropping as the battery is becoming full. I guess the battery limits it. But, the charger should have a mechanism to limit the current.

In a simplified scheme, if I have a lab power supply with voltage set to 4.20V and current limited to 1C, would it work if I just connect the cell to the power supply and wait till the current drops to 0.1C?

Yes, it should work. Probably you won't implement the CC phase the best, as my experience in my response to your first question, but finally your battery will be charged almost 100%. You should use a multimeter to measure the current to the battery.

 

[rushnrockt]

I'm thinking about making my own microcontroller based li-ion charger, but there's some point that aren't fully clear. I've seen this kind of CC/CV graph many times, but that shows only the cell voltage.

It might help to have not only a graph of voltages/currents, but an operation flow chart like one of the two examples below.

Just wondering, what exactly are you trying to achieve with a micro controller based charger? What kind of MCU are you using and will it be a linear charger or a switcher? Have you checked out the existing dedicated battery chargers chips to build around? Most of them have quite a few feature that might take a little while to implement in a microcontroller based setup.

Besides charging alone, consider that there are other modes such as pre-conditioning.

Charging current does not have to be limited to ~0.7C if the battery is rated higher, so if that's a spec that you can check, do so. Charging at specified rate should not degrade battery's performance.

 

[moimies]

Thanks for the answers, those clear things up.

hooking your battery to a bench powersupply works great, but you have to monitor it. I've done it many times before at home and at school! It's better to charge a li-ion battery at about 2/3C, though. 1C is pushing it, and it really doesn't help shorten charge times by much because then you would reach 4.2V sooner....therefore start the CV part of the process earlier.

Yeah, that was just a example. I was going to charge 2200mAh 18650 at 1A. But thanks for the info about bench power supply.

I have been working in my own built chargers too and I am going to tell you my experiences
--

This helped a lot, thanks!

Just wondering, what exactly are you trying to achieve with a micro controller based charger? What kind of MCU are you using and will it be a linear charger or a switcher? Have you checked out the existing dedicated battery chargers chips to build around? Most of them have quite a few feature that might take a little while to implement in a microcontroller based setup.

Besides charging alone, consider that there are other modes such as pre-conditioning.

I'm trying to make a multi-chemistry charger using L200 linear regulators and AVR-based Arduino. I like to tinker with electronics and I guess (or hope ) that it's cheaper than a commercial solution. I've looked intensively through available charger chips, and TI's BQ2057C is the best one for me. Most charger ICs are in ridiculously small/multi-legged SMD packages, which I don't want try to solder.

I'm aware of other aspects of li-ion charging like pre-conditioning and temperature monitoring. I think if MCU just regulates and monitors charging voltage and current, it would be pretty easy implement these different modes in software. With MCU and LCD I can monitor charging process and have some user interface to change chemistry and charging current. That TI's chip I mentioned charges only li-ions and has just a single led as indicator

 

[rushnrockt]

I'm trying to make a multi-chemistry charger using L200 linear regulators and AVR-based Arduino. I like to tinker with electronics and I guess (or hope ) that it's cheaper than a commercial solution. I've looked intensively through available charger chips, and TI's BQ2057C is the best one for me. Most charger ICs are in ridiculously small/multi-legged SMD packages, which I don't want try to solder.

I'm aware of other aspects of li-ion charging like pre-conditioning and temperature monitoring. I think if MCU just regulates and monitors charging voltage and current, it would be pretty easy implement these different modes in software. With MCU and LCD I can monitor charging process and have some user interface to change chemistry and charging current. That TI's chip I mentioned charges only li-ions and has just a single led as indicator

I am going to be biased here due to my "affiliation," but you can check out a PIC based switching solution here. There are some solutions based on MCP1630 and MCP1631HV that you can either use as-is or may be get some ideas from.

Do you have a basic schematic of how you are going to do it yet? The L200 might have a bit too high of a dropout for your purposes. Keep us updated, seems interesting.

 

[moimies]

I am going to be biased here due to my "affiliation," but you can check out a PIC based switching solution here. There are some solutions based on MCP1630 and MCP1631HV that you can either use as-is or may be get some ideas from.

Do you have a basic schematic of how you are going to do it yet? The L200 might have a bit too high of a dropout for your purposes. Keep us updated, seems interesting.

Your link isn't working . I don't know how to control switching power supply with microcontroller. Switching would be more efficient thought. I have no schematics at all, just some examples from data sheets. I'm not even sure if I use L200 or something else. And because I don't have acute need for a li-ion charger, I don't think I'm building this soon. But in the summer (or so) I'll start a new thread and keep you updated.

 

[rushnrockt]

Your link isn't working . I don't know how to control switching power supply with microcontroller. Switching would be more efficient thought. I have no schematics at all, just some examples from data sheets. I'm not even sure if I use L200 or something else. And because I don't have acute need for a li-ion charger, I don't think I'm building this soon. But in the summer (or so) I'll start a new thread and keep you updated.

Not sure what happened to my linking ability. Here it is again: http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en020449

The actual switching is provided by another chip (MCP1630 or MCP1631HV in this case) and the microcontroller provides the brains that you were talking about.