Re: Easy Question: Charging Car Battery Removed From Vehicle?

On Thu, 13 Dec 2007 15:29:41 -0500, tnom@xxxxxxxxx wrote:



Measurement complete. Interesting results, actually.

The first test I did was with a 10-amp fully automated (fast then
maintain) electronic battery charger that's about 10 years old. Its got
dual-color LEDs on front to indicate "charging" or "charged." The LED
turns amber ("charging") when the charger is unplugged but connected to
the battery, so I knew it was going to have a measurable reverse current
because that is what lights the LED. It did- 17 milliamps. Personally, I
consider that completely safe to connect while unplugged- the
probability of a spark that could ignite hydrogen from the battery would
be vanishingly small. But there IS a current. I turned out all the
lights and tried to see a spark- no dice.

At 20ma I can see a spark in the dark. At 17ma I would suggest you
look again.

The second charger I measured was a newer fully automatic fast/maintain
charger. It was even better than I claimed- when connected to a fully
charged battery but unplugged from the wall, it showed a whopping 0.2
MICROamps of reverse current. Yes, 200 NANOamps. The meter I was using
only goes down to 10ths of a micro-amp, so really the measurement's at
the limit of the meter's- it could be even less than that. Absolutely
ZERO chance of that charger causing a spark if you connected it (while
its unplugged) to a fully charged 12v battery.

Finally, I dug up the oldest-style charger we have. Its a semi-automatic
Dayton, meaning that it charges until it senses 13.8 volts, then shuts
off (a mechanical relay clicks open) and won't kick back on until the
voltage falls WAY down to something like 11. It also has a 55-amp
"start" setting. When unplugged, it draws about 30 mA reverse current
from a fully charged 12v battery, and that might, conceivably, remotely,
create a spark when hooked up unplugged. Yet its instructions don't say
*anything* but pretty much hook it up, positive first and then negative"
and then plug it into the outlet. Nothing about a long insulated lead or
anything. Since Dayton is still in business without having killed or
maimed any users, and this was the "worst" reverse current of the
chargers I had on hand, I think I will rest my case here.

I have made connections to batteries also without blowing anything up.
It just makes more sense if you completely remove the possibility by
'use of the extra cable.


Modern auto and marine battery chemistry is such that not a lot of
"gassing" occurs unless seriously overcharging.. not the case here and
second, the vents are ususally equipped with some sort of flame trap
type arestor in the openings.

I used to work at a shop where oddball sized, heavy duty, lead acid
batteries were "rebuilt", and before leading the terminal connections
or heating the tar that was used in those days to seal the cells into
the case, the tech used to wave the torch over the open cells to get
any little hydrogen "pop" out of the way so that it wouldn't startle
him later on!.

In my opinion (only), the danger of a hydrogen explosion is MANY times
greater after the battery has been on charge, has reached full charge,
is gassing freely and the charger is REMOVED without turning off the
power... creating a spark when there would likely be gas in the
vicinity of the battery terminals.

This "extra lead on the negative terminal" sounds like someones idea
of a legal CYA more than an absolute requirement if common sense is
used around a lead acid battery that has been gassing.

Bob
.

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