Problem With Battery Charger

Thread Starter

Frank Spigner

Joined Mar 17, 2018
3
I'm trying to make a relatively simple battery charger for a rechargeable 9V lithium ion battery.

I wanted to make a charger that will not cause overvoltage or damage to the battery and will also provide LED indicators that will show when the battery is being charged and when the battery has finished charging.

I've decided on using a Zener diode that will shunt current above the maximum battery voltage to ground and an LM317 configured as a constant-current source.

The idea is that once the battery reaches the charged voltage (around 8.7VDC), the "charged" LED will turn on.

When I breadboarded the circuit, being powered with a 12VDC power supply (as I intend for the final circuit to use) the LED is just always on. (The 100F capacitor on the output is only for simulation purposes.)

The following attached are the schematics and a chart from a voltage sweep simulation.

https://www.circuitlab.com/circuit/n533pn7gekzv/zener-charge-circuit-3/

Any help would be greatly appreciated!

zener-charge-circuit-3.png
chart.png


Edit:
This is the type of battery I was testing with - I couldn't find an actual datasheet though: https://www.eblmall.com/product/ebl...t-lithium-ion-600mah-li-ion-batteries-4-packs. I took apart one of them, and I see that there are two Li-ion cells and what looks like a very simple battery management PCB which includes an 8205A Dual N-channel MOSFET (https://www.maritex.com.pl/product/attachment/91261/8205A.pdf) and an "20L8 K60S", which I can't find any info on. I'm not sure if/how this circuit actually has any bearing on how my charger circuit is designed though.

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Last edited:

dl324

Joined Mar 30, 2015
13,505
That circuit isn't suitable for li-ion. Li-ion batteries like to burst intp flames when abused, and sometimes even when they're not.
 

Thread Starter

Frank Spigner

Joined Mar 17, 2018
3
That circuit isn't suitable for li-ion. Li-ion batteries like to burst intp flames when abused, and sometimes even when they're not.
Any idea what would be a more suitable circuit, or a better type of rechargeable battery in a small physical size that would supply 9-12VDC that could use this circuit?
 

bassbindevil

Joined Jan 23, 2014
201
If it's a 2-cell 8.4V battery, there's zillions of 2S chargers out there already; check thrift stores or yard sales for obsolete digicam chargers. But, I think it would be OK if you set the LM317 as constant voltage 8.4V (or a bit less if you believe in charging to 80% for maximum life), and added a series resistor to limit the current to a safe level, at the expense of slower charging. The battery should already have a protection module that will prevent dangerous overcharging, unless it's a cheap thing from China with an imaginary mAh rating.
Alternatively, there are dedicated lithium-ion charger chips. SY6912 is one that can do 1, 2 or 3 cells at 500 mA to 2A (configurable by jumpers and removing shunt resistors); I was favorably impressed, until one day I hooked the board up to the battery and smoke came out of the SY6912. Those boards are mostly what show up when you search ebay for 1s 2s 3s lithium charger; the designs look different than mine, so there may be variant on that chip, or a knockoff, or an improvement. I bought another to replace the dead board but haven't put it into use yet.
A more promising chip could be the MAX745 from a little company called Maxim. Those boards cost more (maybe that means they have genuine chips?), but have a dipswitch for setting number of cells (1S to 4S), and trimpots for setting current and trimming voltage.
How small does your battery need to be? The 18650 form factor cells are common and can often be salvaged from recycled laptop and tool packs. They should be used with a module that provides balancing, over-charge, and over-discharge protection (those will be the boards that have rows of power MOSFETs and SMD resistors and chips with 4 to 6 legs. 3S (12.6V fully charged) is close enough to 12V for most purposes.
Alternatively, use a single lithium cell and a boost converter to power your circuit. That's common in many portable devices. Single cells are easy to charge from USB, and don't need balancing. You may be able to find a spare battery for some old cellphone or salvage one from a dollar store item; those will nearly always include a protection board.
 

dl324

Joined Mar 30, 2015
13,505
Any idea what would be a more suitable circuit, or a better type of rechargeable battery in a small physical size that would supply 9-12VDC that could use this circuit?
I don't think it's worth the bother. I bought chargers for all of my batteries; li-ion, NiMH, NiCd, lead acid. I built one for lead acid and it wasn't even worth transferring the prototype to a board.

BU-409: Charging Lithium-ion - Battery University
 

Audioguru again

Joined Oct 21, 2019
3,828
Read about Lithium-ion and Li-PO batteries at Batteryuniversity.com.
1) A 2 cells battery is 8.4V max, not 8.7V.
2) When the battery voltage reaches 8.4V it is not fully charged, it is still charging until the charging current drops to about 10% then the charger must detect that and turn off the charging.
 
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