Li ion end charge detection

[digik]

I want to build a Lithium ion battery charger and i don't know how to implement a led that turn on when the current absorption drops near to zero. I need to charge 4 18650's battery in series with protection circuit and a power supply set at 16.80V (4.20V/cell). When the battery reaches the end of charge the current drops, but i want a led to turn on instead of having a multimeter hooked up and measuring the current. What's the simplest way to do that?

[Buriedcode]

Charging lithium batteries involves a constant current phase, then constant voltage phase (depends on cell type, but usually as you stated, 4.20V per cell of lithium polymer) with charger termination when the cells draw less than 10% of the rated charge current.

I sounds as if you're just providing a constant voltage - if the supply has current limiting then this *can* work, because the constant current doesn't have to be accurate, but if it has quite a high current capability you could be charging your cells at over 2C (twice their rated capacity).

Because such charging circuits use a current sense resistor for the constant current phase, this is also used to indicate charge termination.  Given the sense resistor is chosen to have a low voltage, to minimize losses during constant current, detecting the cut-off current means sensing for a smaller voltage.  Example, charging at 2A using a 0.1ohm resistor - you have 0.2V across it during constant current charge.  But once in constant voltage mode, it will need to detect C/10 = 2/10 A = 200mA.  that means a voltage across the sense resistor of 20mV.  At that level opamp offset voltages and input bias currents can have a significant effect - but it does not need to be that accurate.  The most accurate part should be the maximum voltage output.

Also, if you're charging in series, you might want some form of cell balancing to prevent some cells being overcharged.  The web has many examples of this in varying levels of complexity.

[Kilrah]

I use a 1A power supply and the battery are rated at 3400mAh, therefore no current limiting is necessary.

What kind of power supply? If it's just a common one and the 1A is just what the supply is rated at this is wrong. It would just mean the supply can safely provide 1A constantly, not that it will never put out more. I.e. if you connect your battery without a limiting circuitry you'll likely damage the PS as it will overload it.

[Buriedcode]

Ok, well, do you have much experience designing electronics?  Just because a PSU is 'rated' for 1A, does not mean it can act as a constant current supply of 1 Amp.  If it is voltage regulated then it could shut off from short-circuit protection, reduce its voltage or anything else when too much current is drawn - the point is, you can't just rely on a PSU's inherent limit to make a constant current supply.

Secondly, if you are stepping this up to 16.8V from 12V then clearly the input current would have to be higher for an output of 1 A.  P = I*V.

Thirdly you seem to want to just 'turn the damn LED on'... for indicating fully charged.  If you don't have much experience with DIY, do yourself a favour and buy a battery charger.  Or, at the very least, by some 'modules' like a CC/CV supply board (they go for a few $$ on ebay).  These often not only have current and voltage limit adjustments, but also indicator LED's to determine whether its in constant current or constant voltage mode.  They also tend to have a low-side current sense resistor which you can use, along with a comparator to indicate when the current draw is below C/10.

Whilst many seem to go over the top with regard to the 'danger' of poor charging practices, these cells really can 'pop'.  But for lower charge currents, generally it will just destroy your batteries, which harms your wallet more than yourself.

[KL27x]

When the battery reaches the end of charge the current drops, but i want a led to turn on instead of having a multimeter hooked up and measuring the current. What's the simplest way to do that?

If your charger gets its power from a linear regulator, you can directly measure current from the power supply. E.g. put a shunt resistor in series between the PSU and the charger circuit and setup a comparator to trip when current drops below XmA.

Even if the charger uses a DC converter, you can still do this. You will just have to fudge for the power conversion and loss of efficiency.

...but

The 12V 1A MeanWell branded power supply is connected to a step-up converter which has the voltage adjust feature. The PSU is regulated and with the step-up can output slightly less than 1A current which is perfect to charge the battery at less than 1C to prevent stress.

... if your steup-up output is maxed out, it may saturate and overheat the inductor, and you will possibly not sustain 1A, I think. It may be best to limit the current, pro-actively, for the fastest charge.

[Kilrah]

The 12V 1A MeanWell branded power supply is connected to a step-up converter which has the voltage adjust feature. The PSU is regulated and with the step-up can output slightly less than 1A current which is perfect to charge the battery at less than 1C to prevent stress.

Still sounds wrong. You might want to describe everything more precisely, what you used exactly and how it's connected and set up.