Charge completion LED for Li-ion charger


Old 02-01-14, 11:16 AM
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Charge completion LED for Li-ion charger

Hello everyone - I have a Li-ion charge controller PCB that I'm hoping to modify to include a charge completion LED. I believe I need to use a simple FET based circuit to achieve this but don't know where to start choosing parts and working out what resistors I need etc... I have the desire but not the knowledge sadly!

The charger is based on the S8254A charge controller IC. When charging is complete pin 1 is pulled high to stop the charge via this P channel charging MOSFET.

I hoped to use an N channel MOSFET on that same pin to switch on a charge completion LED when the pin is pulled high. However reading the datasheet for the charge controller (links above) tells me this would mean the LED remained illuminated when the battery and charge controller were disconnected from the power supply. It would stay illuminated as long as the battery is in a fully charged state and over time this would obviously drain the battery while in storage until the voltage fell enough to release the IC from the overcharged state.

The battery is a 4 x 18650 Li-ion pack with a fully charged voltage of 16.8v across the input/output pins of the pcb. The wall plug power supply I use supplies the charge controller board with 18v 500mA to those same input/output pins.

I hope to use some simple logic to illuminate the charge completion LED only when both the following conditions are met:
1) charge controller IC pin 1 is high (18v) indicating the cell voltages have all reached 4.2v
2) pcb is connected to power supply (not sure how to detect this. there is only a slight voltage difference when charger is connected 17.9v vs 16.8v!)

The whole setup of cells and charge controller has to fit inside a small canister of the bike light that it drives. There is some room for additional circuitry to control the charging led. Wall charger connects via a standard centre positive round DC jack - I could maybe modify the socket to detect when the charger is attached?

Any suggestions to help me out?
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Old 02-01-14, 11:27 AM
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Welcome to the forums.

Does that board use 4 charge controller IC's ..... one for each cell ?
If it does then you'll have to monitor pin 1 on all four of the IC's.

Coaxial power jacks usually have a switch built into them which is why they have three connecting pins.
Old 02-01-14, 11:42 AM
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Hi, thanks for your reply.

No just one charge controller IC monitoring all 4 cells. The 4 chips you can see on the board are the MOSFETs controlling charge/discharge (2 of each in parallel for higher current I presume).

The jack is moulded into the case and only has 2 pins sadly I have considered adding a "test" button that can be momentarily depressed to show the status LED. That's probably easiest.

In that setup how would I go about wiring up the LED and MOSFET? Something like this presumably: only with a switch in series with the load. Just need to choose a suitable N channel MOSFET I guess.
Old 02-04-14, 12:14 PM
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Proposed circuit

Ok so I have come up with this circuit that I think will do the job...

The circuit diagram shows the basic setup of the unit at present plus my proposed modifications (boxed).

The battery is managed by the charge controller PCB which has two pins (P+/P-) that serve as both charging pins and also the output pins from the battery to the LED driver board that it powers. An 18V DC wall adapter can be connected to charge the battery as shown.

I have added diode D1 (low drop, 0.35V) so that my charge status LED (LED1) should only light when the 18V DC adapter is present - do you think this will work?

The P-FET gate is attached to the control pin of the charge controller (pin high (20V) = fully charged) to control the status LED.

Can someone check that I have the source and drain of the FET connected the right way around? I plan to have the load on the drain and the source connected to +18V.

Do I need a zener diode for protection of the MOSFET? How would I fit one if I do?

Thanks, Chris
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