Someone gave me an old car battery charger.
6 volts 10A and
12 volts 2A and 10A, determined by a slide switch.
A friend brings over a dead 12v battery.
On the charger, on 10A, it charges at 9amps.
On 2A, it charges at 7 amps.
Based on the meter on the charger.
What does it mean to be a 2 amp charger if it charges at 7 amps?
After an hour the charging rate dropped to 6 amps on the 10A setting,
and then it was 5 amps on the 2A setting.
On Friday, May 1, 2015 at 12:03:37 PM UTC-6, micky wrote:
Sounds like the battery has had it or if you leave on the 10 amp setting
for 24 hours and it comes down more it might be salvageable...(might).
It could be borderline. Is there water in all the cells? One dry cell
is not good. Top them up with distilled water and see what happens.
I've had successes and lots of failures. The longer they stay dead before
charging then the odds are against you.
Also: For the sake of economy I doubt the mfg would have two scales.
More than likely it's a cheap, non-regulated charger. On an
over-discharged battery, I'd believe the initial output on the 2amp
setting might be high.
If it does not eventually taper down. the battery indeed could have a
Easy enough to check:
After the charger has run a while...
turn it off and let any surface charge dissipate.
If the battery voltage is about 10.5 no need to go further...dead cell
One could investigate further by putting on safety glass, removing the
caps and seeing if perhaps the electrolyte in one of the cells is all black.
I'd take the battery off the charger and use a multimeter to see if the
terminal voltage was over 11.5. I'd let the battery sit overnight and
see if it was still over 11.5. Why fool with a battery if a cell is shorted?
If the voltage was good, I'd use the multimeter to check the charging
current. If it really charges a good battery at 7 amps on the 2 amp
setting, that would explain why someone gave it away. I'd check the
charging voltage because the charger could damage batteries if it's too
On Friday, May 1, 2015 at 2:03:37 PM UTC-4, micky wrote:
It means the battery is very low on charge. IDK exactly how
they come up with those ratings, but I would suspect that it's
rated at putting out 2A at a particular charging voltage. If
the battery is very low, or bad, it will present a bigger load,
the charging voltage will be lower, and the current higher. I
see that on my old Sears charger all the time. If it's on 2 or
10A, on either, if the battery is very low, it will initially
charge at a much higher current. After 5 or 10 mins, it starts
dropping down as the battery charges.
If you're having the same thing, Trader, there's no point in my pursuing
the question anymore. And I'll figure the charger is good enough.
To everyone, yes the battery was very much discharged. I knew that.
The hydrometer shoed 4 cells in the red (semi-cheap hydrometer, no
number assigned to red) and the other 2 very close to the red.
However the battery's voltage was above 12.6 anyhow. (well maybe I
didn't wait until the "surface charge" dissipated, like Philo said to
do. But then the owner came and took the battery away. He has a
charger he thought wasn't working. This is for the truck he rarely
drivesl, but I guess he wanted to drive it the day he got the battery.)
Thanks, and thanks all.
I quit fooling with hydrometers decades ago. In some batteries, it's
hard to get to the electrolyte. Gas bubbles, as in a glass of ginger
ale, will cause false readings.
Before charging, voltage should tell you the state of charge and if all
cells are working. Headlights will show how the battery handles a load
of a few amps. It's also a way to check the state of charge if a
battery has recently been on a charger. Turn the lights on 15 seconds,
In my experience, a deeply discharged battery charges slowly at first
because the electrolyte is weak.
Most DMMs will measure 10A DC. That's a way to check the gauge on the
How do you know the red readings were correct? You said it was a dead
battery. I would have expected a dead battery to draw more current after
an hour, not less.
I remember another thing that can throw a hydrometer off. Sulfuric acid
settles when a battery sits; that's why turning the lights on for a few
seconds can improve starting power. If a battery has been sitting a long
time, the hydrometer may sample the watery stuff at the top.
I know a situation where a hygrometer can be useful. After you charge a
battery and give any tiny bubbles time to clear out, a hydrometer can
tell you the condition of each cell.
The exact voltage depends on the temperature and the kind of battery.
At 60 F, a normal car battery should show 12.63V at 100%, 12.43 at 75%,
12.22 at 50%, 12.04 at 25%, and 11.87 discharged. A lower reading would
show one or more shorted cells. (I use a lithium coin cell to calibrate
my meters. There are better voltage standards, but I don't have one now.)
Another load could be clipped to the battery. In my case, I would have
disconnected the ground from the battery in my car and used jumpers to
connect the test battery to the car.
On Saturday, May 2, 2015 at 8:51:52 PM UTC-4, J Burns wrote:
I've heard that totally dead batteries may draw less current.
But every auto battery I've even tried to charge, the current
is initially higher, then starts coming down in 5 or 10 mins.
The more drained it was, the higher the current it pulled from
the charger. But then I haven't had one that was like connected
to a load to totally drain it down to absolute zero.
If the lights will glow but the engine won't turn over, I'd expect the
charge to start slow. If a motorcycle or riding mower had been left in a
shed for months, I'd expect the charge to start slow. In that case,
stratification of the electrolyte might be part of the problem.
I like the AGM battery I put in my mower. It stays perky over the
winter because the electrolyte doesn't stratify and self-discharge is
Actually, with any battery I've tested. I've never had a problem sucking
up enough electrolyte or having bubbles get in the way. The floating
part of the hydrometer has a semi-spherical bottom, and if there were
bubbles, they would go right past the floating part and up to the
surface of the electrolyte.
How could they not be? There was nothing adhering to the floating
part. What else could cause a bad reading?
How?. What hygrometer changes its reading so quickly it can
distinguish one cell from another? And why would the condition of
the cell aftect the humidity above the cell?
I'm dubious, but it doesn't matter this time. He had hoped to take the
battery with him so I started charging right away.
Do i have another load, other than his car or my car, which I don't want
to bother? A lot of things could have done. My question was "What does
it mean to be a 2 amp charger if it charges at 7 amps? " I don't see
how any of this would have helped to answer that question.
I must have used the wrong word. What's the one that measures humility?
Until 1959, the best batteries were at Humble service stations. A
battery that's all puffed up could explode.
You said the battery was dead. If a 2-amp charger was charging at 7
amps, one possibility was that the battery was dead because of shorted
cells. A DMM would have told you. it would also have told you if it was
really charging at 7 amps.
That's what we need, a humility meter. Really. I can think of lots
But I never had any doubt that 7 amps, or between 6 and 8 amps, were
going into it. A lot more than 2 amps. I could that tell just by the
high number of hydrogen bubbles surfacing in each cell.
When I use the thing again, I'll check the voltages at the 2 and 10 amp
I'm interested, and dadburnit, you've talked me into buying a hydrometer
to hang beside my hygrometer.
Wikipedia says stratification happens in normal use if you don't fully
charge a battery. The bottoms of plates sulfate and the tops corrode. I
want to check that with a hydrometer. Watch me drip acid on my pants!
There's a curb in front of my house. Maybe I should make a habit of
driving over it to keep my battery stirred up. I won't know until I buy
a hydrometer... unless of course I could borrow one...
I have similar 2A/10A charger and on mine the meter does not have two
different scales for 2A and 10A.
On mine, if it's set for 10A it charges more quickly but it also shuts
down the charging earlier. At the 10A rate the green LED indicating
fully charged turns on but if I then switch it to 2A it charges some more.
As to voltage, a fully charged lead acid battery, with no load, will be
2.3V per cell x 6 cells = 13.8V. Once it's under load, it's 2.1V/cell or
12.6V. If you have a shorted cell (or more than one shorted cell) it's
easy to check because under noload the voltage will be lowered by the
number of shorted cells x 2.3V.
An alternator/rectifier/voltage regulator, or battery charger, needs to
put out 14.4-14.8V to properly charge a car battery. While people call
car batteries "12 volts," they are not 12 volts except under load where
they are between 12V and 12.6V.
An article entitled is "Car Batteries Are Not 12 Volts" is available at
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