Caution: Lithium-ion (Li-ion) batteries and fire-breathing Daleks!
By Arc Ivor Davies
FOR those contemplating using a power-supply to charge their lithium ion (lion) batteries, I hope this short feature serves to inform and to prevent a journey of discovery – that is, discovering that your house is on fire!
Charging 3.7 volt Lithium cells – my story
Digital cameras are gluttonous current consumers, and as I did not have the cash to purchase the best batteries that money can buy, I decided instead to connect 3 x 3.7 volt (500ma) Lithium-ion batteries in parallel and to charge them off my Roband power-supply.
My plan was then to solder a couple of diodes in series (wattless dropper method) to drop the voltage to just below 3 volts for the cameras.
Charging off my power-supply
The thing to remember when charging these 3.7 volt Li-ion batteries is that the charging voltage must never exceed about 0.8 volts above the rated voltage of the battery. Let me explain:
For a 3.7 volt Lithium battery, the power-supply voltage is adjusted slowly so that the needle rises above 3.7 volts, heading slightly North of 4.5 volts.
Watching out for a current surge before cutting back to 2 amps
Suddenly, at a tad over 4.5 volts, there will be a sudden rush of current of around 3 amps (or more). The voltage must now be reduced so that the meter reads 2 amps.
The voltage across the battery should now be 4.5 volts or slightly less.
Hands off those knobs! the current now begins to drop off on its own accord, eventually reaching zero
At this point, no further adjustments should be made (DO NOT DISTURB), as the battery is now charging. The current should eventually fall to zero, corresponding to a full charge.
Using this simple method, the camera batteries would take about an hour to reach perfection, with the batteries cool to the touch, corresponding to room temperature.
So what went wrong?
Having fully charged the lithium battery, I was suddenly distracted.
On the phone was a ham radio friend, who was bemoaning the loss of his brand new transmitter receiver, having accidentally dropped it down a flight of stairs, “smashing it to bits!”
Having offered him my condolences, I hung up and returned to the workshop. I noticed that the power-supply current was zero (battery fully charged).
I was soon presented with a Dalek-like figure, reminiscent of the BBC TV series: Dr Who!
This is when things started going pear-shaped. Instead of taking the battery off charge, I inadvertently increased the current again to 2 amps, thus beginning another charging cycle!
This action took the charging voltage up to around 6 volts (remember with the power-supply still running, a fully charged 3.7 volt Li-ion battery should be idling at 4.5 volts across, with zero amps going through it).
It took me a few minutes before I realised my error, in that short time the batteries had started to boil.
“Don’t panic!” I told myself; immediately disconnecting the batteries and praying they would cool down – they did not!
Meltdown, then fire!
Within 2 minutes they burst into flames. Thick smoke then fire engulfed the workshop and all the smoke alarms went off!
I waited a few seconds for the flames to subside.
Covering the batteries with a heavy cotton cloth before ramming everything down inside a coal skittle, I chucked the skittle of the window and left it to cool down naturally – outside.
The Li-ions continued to burn, with one battery reduced to a cinder, the other two fused together, morphing into a hideous Dr Who, Dalek-like figure! Just right for trick or treat!
The workshop liveth
It wasn’t the end of the world, but it could easily have been. So, Arc’s message to all you experimenters:
Be very careful, as Li-ion batteries are fiery, unstable creatures!
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[whohit]Caution: Lithium-ion(Li-ion) batteries are fire-breathing Daleks![/whohit]