On 4/5/2019 12:58 AM, Mike S wrote:
I was given an electric drill that uses 12V lithium ion batteries
because the charger didn't work and the batteries died and the owner
lost interest. I hack sawed the transformer case open and found that the
primary winding was open somewhere that I couldn't see, so I didn't try
to repair it.

Also the charge adapter that the battery slides into has a burnt out
resistor on the circuit board. It's a very simple circuit board, there
are 4 diodes on the input side which I assume is a full wave bridge
rectifier. After the rectifier there is a small series resistor (R1)
powering an LED across the dc power lines coming out of the rectifier,
and a larger resistor (R2) which got hot enough to char the circuit
board near it as it was blowing up. I'm guessing it was over-voltaged,
or maybe more likely the outputs were shorted together. The V+ output
voltage passes through R2.

I don't have a schematic, and would like to repair the ct. board if I
can choose a replacement resistor that doesn't unsafely charge the
batteries. If I get a transformer with the same V and I ratings (14.5V
200mA) I'm wondering what value Resistor to replace R2 with.

I am including a picture.
http://tinypic.com/m/k2ck8j/3

I read this, "Most consumer orientated lithium ion batteries charge to a
voltage of 4.2 volts per cell and this has a tolerance of around ± 50 mV
per cell. Charging beyond this causes stress to the cell and results in
oxidation that reduces service life and capacity. It can also cause
safety issues as well."
https://www.electronics-notes.com/ar...n-charging.php


Since the batteries are labeled "12V", if it uses the same LiIon
technology the article is referring to, I assume it has 3 cells totaling
12.6V. The transformer was clearly labeled 14.5V 200mA.

Also from the web page, "Charging lithium ion batteries can be split
into two main stages: Constant current charge:Â*Â* In the first stage of
charging a li-ion battery or cell, the charge current is controlled..."

It seems like it might be a simple matter of trial and error, power the
charge adapter attached to the battery, and find a value of R2 that
leaves 12.6V on the output side, when the battery is fully charged. The
adapter has no current regulation, other than the series R2. Is that a
safe approach?

Would a 12V car trickle charger work safely, with a maximum charging
current of 2A (based on the web page)? The transformer had a much lower
current rating.

I'm surprised the charge circuit is so simple after reading this web page!

TIA, Mike
Short answer...don't do anything until you figger out what you're doing.
Are you sure that they're lithium?
Is the picture showing EVERYTHING that is in the charger path?
That ain't nearly enough stuff to charge a lithium battery.

The picture suggests that you have the typical symptom of what
happens when a NiCd battery shorts and the crap unregulated charger
melts trying to charge it.