Car engine starter inadvertently charges battery?

[john]

I have a car battery charger with 3 settings: 2amp, 10 amp, and 50 amp
engine start. My issue is the 50 amp engine start mode.

When in this mode, the built-in ampmeter goes violently out of scale (full
scale is 12amp) before I even crank the engine.

This means the device has open circuit voltage of > 12V and is charging the
batter with more than 12 amps (not sure exactly how many amps).

I worry this large charging current may damage the car battery or cause an
explosion.

Wouldn't a better design be to provide 11.9V at 50Amps, so that when the
engine is not cranking, the battery will not be subject to a large charging
current, and when the engine is cranked, the battery voltage would drop a
few volts (due to enormous current draw), and the 11.9V at 50Amps will go
towards starting the engine.

Does anyone know if such charger exists?

 

[Martin Riddle]

I have a car battery charger with 3 settings: 2amp, 10 amp, and 50 amp
engine start. My issue is the 50 amp engine start mode.

When in this mode, the built-in ampmeter goes violently out of scale
(full scale is 12amp) before I even crank the engine.

This means the device has open circuit voltage of > 12V and is
charging the batter with more than 12 amps (not sure exactly how many
amps).

I worry this large charging current may damage the car battery or
cause an explosion.

Wouldn't a better design be to provide 11.9V at 50Amps, so that when
the engine is not cranking, the battery will not be subject to a large
charging current, and when the engine is cranked, the battery voltage
would drop a few volts (due to enormous current draw), and the 11.9V
at 50Amps will go towards starting the engine.

Does anyone know if such charger exists?

Car batteries routinely take a 100amp or more charge after you start the
engine, until the voltage comes up to about 13-14 volts, this is normal.
And the engine start mode of battery chargers are meant for a short
duration until the engine is started.
They usually have a amp meter that gets pegged in the start mode, the
voltage will also droop under the starter load.

Its all about cost, since lead acid batteries can handle short high
current bursts. Besides, the battery is probably toast if you have to
use the start mode anyway.

Cheers

 

[Jasen Betts]

I have a car battery charger with 3 settings: 2amp, 10 amp, and 50 amp
engine start. My issue is the 50 amp engine start mode.

When in this mode, the built-in ampmeter goes violently out of scale (full
scale is 12amp) before I even crank the engine.

This means the device has open circuit voltage of > 12V and is charging the
batter with more than 12 amps (not sure exactly how many amps).

any car battery charger that doesn't produce more than 12V is pretty
useless. it takes over 13V to charge a car battery.

I worry this large charging current may damage the car battery

even small altenators produce upto 35A to charge the battery

50A shouldn't be an issue if applied briefly.

or cause an explosion.

to avoid explosions follow the instructions.

Wouldn't a better design be to provide 11.9V at 50Amps, so that when the
engine is not cranking, the battery will not be subject to a large charging
current, and when the engine is cranked, the battery voltage would drop a
few volts (due to enormous current draw), and the 11.9V at 50Amps will go
towards starting the engine.

50A isn't going to be much help if the battery is depleted.

Does anyone know if such charger exists?

It involves a sack-barrow with another battery on it,
and 200A jumper leads, I've seen them see them at used car yards etc

 

[PeterD]

A starter can pull over 500 amps. The battery can handle that amount of
current in either direction.

What is in danger is your charger. However, full scale is only 12
amps. Given the gauge and length of the leads, the circuit is
probably incapable of more than 40-50 amps, well within the limits of
the charger's electronic circuitry. It should be designed to handle a dead short
or have clear instructions that you should disconnect it before
attempting to start the car.

Most likely the meter is way off. Mine does that too, so I tossed on
my Amprobe clampon ampmeter and checked--sure enough it was off by
about twice!

 

[Sylvia Else]

I have a car battery charger with 3 settings: 2amp, 10 amp, and 50 amp
engine start. My issue is the 50 amp engine start mode.

When in this mode, the built-in ampmeter goes violently out of scale
(full scale is 12amp) before I even crank the engine.

This means the device has open circuit voltage of > 12V and is charging
the batter with more than 12 amps (not sure exactly how many amps).

I worry this large charging current may damage the car battery or cause
an explosion.

Wouldn't a better design be to provide 11.9V at 50Amps, so that when the
engine is not cranking, the battery will not be subject to a large
charging current, and when the engine is cranked, the battery voltage
would drop a few volts (due to enormous current draw), and the 11.9V at
50Amps will go towards starting the engine.

Does anyone know if such charger exists?

50 amps isn't enough to start an engine anyway.

My experience of a charger with an "engine start" function suggests that
they'll only be effective if the battery is only just incapable of
starting the engine on its own. If the battery is dead, then you've got
Buckley's chance. I had a battery go from being capable of starting the
engine on its own, to incapable, even with support from the "engine
start" function, in the space of an hour.

A 3 kW device might be able to do it, but would no doubt come with a
price to match.

Sylvia.

 

[Rich Grise]

Most likely the meter is way off. Mine does that too, so I tossed on
my Amprobe clampon ampmeter and checked--sure enough it was off by
about twice!

I've seen DC ammeters that are essentially a compass. It's a permanent
magnet inside the case, on a pivot, connected to the pointer. The meter
case has a clip, and it just clips over the wire. When I used to install
them, we'd calibrate them by moving the #2 wire around until it was
"close enough", then RTV it in place.

Cheers!
Rich