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Dave Platt Dave Platt is offline
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Default Hot Batteries in TV Remote

My son noticed that the battery area of our TV remote was quite hot. I
immediately removed the batteries, both were almost too hot to hold
onto. Sorry, I didn't check to see if the batteries were installed
with the polarities correct. They had gotten so hot that the plastic
surrounding the negative terminal that connects to the PCB was
slightly melted and deformed.

A new set of batteries did not cause the remote to function.

I disassembled the remote and completely cleaned it. Scraped gunk out
of the holes in the remote casing with a small jewelers screwdriver
followed with a toothbrush and soapy water. Cleaned the flexible
plastic sheet that functions as the button pad with soapy water and
toothbrush. Completely dried everything with a hair dryer. Carefully
pulled on spring for neg. terminal, stretching it so it will retain a
battery, reassembled and put in fresh set of batteries. All is well.

What caused the batteries to get hot?

The resistance of the poor connections.

The connection resistance would be an issue - it was probably the
local site-of-generation of the heat which started to melt the
plastic. However, I think it's a secondary issue, and not the only
source of heat generation - the internal resistance of the batteries
would also have been a generator.

The real question is, why was so much current being drawn from the
batteries (and released as heat)? A remote control, when working
properly, is a relatively low-current device (a few tens of
milliamperes, I imagine) with a relatively low duty transmission duty
cycle, and it shouldn't be drawing more than an infinitesimal amount
of power from the batteries when there's no button being pushed.

I think there might be several reasons why this overheating might have

- Batteries inserted backwards, in a remote which has a reverse-biased
across-the-battery diode at the input to its electronics. The
diode could act as an effective short-circuit across the batteries
if they were inserted backwards. [Using such a diode, and not
having a fuse or a resettable thermal current limiter in series
with the battery, would seem like a Really Bad Idea.]

- Some bit of metal came loose inside the case (maybe a loose
connector, maybe a stray bit of wire) and short-circuited the
battery connections. Or, possibly, some conductive liquid (salty
broth?) was spilled onto/into the remote, and resulted in a near
short circuit.

- A cat sat on the remote, it started transmitting continuously, and
(due to bad design or some sort of internal circuit fault) it drew so
much current that it overheated.

I tend to lean towards the second possibility, myself.

Dave Platt AE6EO
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