Home-made Li-ion charger

Posted on 2015-01-27
Last Modified: 2015-01-27
I'm building an Arduino 10,8 Volt li-ion battery pack out of standard 18650 cells and have spent a lot of time researching charge protection circuits.
Suitable battery protection circuit boards (PCMs) are easy to find, but it's somewhat unclear how to configure the complete setup with charger.
The PCM boards provide over/undercharge protection, but it seems they don't regulate the charging process itself. Somewhat confusingly, they also specify a single discrete input voltage; not a voltage range.
These are my questions:

Are there voltage regulators on the PCMs ensuring they operate at a constant voltage or do I have to charge at a constant voltage in order not to exceed the PCM voltage spec?
A constant voltage charger is essentially a power supply. Is this a reasonable/safe alternative ?
I guess some chargers have their own internal "smart" circuitry that regulate the charging process. Could this functionality interfer with the PCM causing unexpected results?
The PCMs I've found don't seem to have connections for thermistors or regular temperature sensors like most power tool battery packs have. Could I still rely on their ability to limit overcharging?  

Grateful for any help.

Question by:EISTO
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LVL 95

Expert Comment

by:John Hurst
ID: 40573091
A constant voltage charger is essentially a power supply. Is this a reasonable/safe alternative ?

I would say yes. Why? Because the constant voltage will only allow the battery pack to draw the current it needs. So long as the voltage is not allowed to rise, the battery pack will draw what it needs to charge. You do need to provide for adequate current in the power supply.

I don't think computer battery packs are a good analogy to high current power tool fast charging supplies. These have a different design.

Author Comment

ID: 40573190
@John Hurst:
Thanks for the speedy reply. I have a follow-up question on your answer. I'm not an expert on electronics, but charging a 10.8 volt battery pack with at a constant voltage of 10.8 volt would mean never fully charging the batteries. Am I right?

Additionaly, a 10.8 volt power supply is not easy to get hold of, so presumably the common 12 volt power supply would be an alternative, and then we're back to the problem with exceeding the voltage spec of the PCM.

Also: Do I have a catch-22 situation with the current rating of the power supply?

Using a 4A PCM and a 6A power supply would probably trip the over-current protection of the PCM in the beginning of the charging process
Using a 6A PCM and a 4A power supply would probably overload the power supply in the beginning of the charging process
Choosing a PCM and power supply that greatly exceeds the maxium possible charging current drawn by the batteries would solve this issue, but it would also mean no real over-current protection and the associated risk of overheating the batteries?

 Just thinking out loud here.

LVL 95

Accepted Solution

John Hurst earned 500 total points
ID: 40573206
If you use exactly 10.8 volts, the battery will charge fine but not necessarily 100%,

You can use 12 volts and I think that should be fine. You should be able to determine the maximum current capacity of the battery and put a current limiting resister in the supply line to the battery. This will be a relatively low resistance resistor and should not impede rated current or voltage.

Author Comment

ID: 40573460
That's a good suggestion. I'll try to measure the internal resistance of the batteries when discharged and (if necessary) chose an appropriate resistor to make sure the current stays below 4A. As soon as the internal resistance increases, the supply voltage should gradually increase to the nominal 12 volts and make sure the batteries are fully charged.
Thanks for your help!

LVL 95

Expert Comment

by:John Hurst
ID: 40573474
@EISTO  - Thanks for the update. If 12 volts is insufficient, you could even try a 14 or 15 volt supply with a higher value limiting resistor.  Good luck with your project.

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