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I'm having a problem with a circuit that I did not design. The unit uses
Lithium Ion batteries (laptop battery pack) when AC is not available. The
problem is, when the unit is turned on with the batteries, there is a large
initial spike (approximately 8 amps) which the charge controller sees as a
short circuit and immediately shuts down the battery. My first thought was
to use a capacitor to send the spike to ground. Then I realized that it was
the capacitors already in the circuit that are charging up to make it look
like a short. A resistor was added to attempt to reduce the impact of the
spike but that failed.
I then thought of putting a coil in series with the battery. The engineer
shot the idea full of holes and mumbled something about back emf when the
circuit was shut down and also complained that the spike could jump over the
coil. Not having an engineering degree, I shut up but continued to think
about this problem that should be solved electronically instead of settling
for cracking open the $100 battery pack and taking battery power directly
from the batteries to avoid the spike getting into the charging circuit -
which works fine but is kind of hokey and voids the warranty of the
batteries.
In the circuit below I show my thoughts (albeit in a poorly done schematic).
I figured I could use a coil with a ferrite core and a diode to ground on
each side. I figured the diodes would satisfy his emf concerns and the
choice of core would keep the spike from going across the coil. Note there
is an on/off switch (not shown) between the battery and the rest of the
drawing.
Is this an idiot idea? Should I use an iron or any other type of core for
the coil? Maybe only one diode? Get a job not even remotely connected to
electronics? Any better recommendations?
___________nnnnnnnn___________
| | |
+ _ _
BATT ^ ^ Out to rest of circuit
- | |
| | |
______________________________
Lithium Ion batteries (laptop battery pack) when AC is not available. The
problem is, when the unit is turned on with the batteries, there is a large
initial spike (approximately 8 amps) which the charge controller sees as a
short circuit and immediately shuts down the battery. My first thought was
to use a capacitor to send the spike to ground. Then I realized that it was
the capacitors already in the circuit that are charging up to make it look
like a short. A resistor was added to attempt to reduce the impact of the
spike but that failed.
I then thought of putting a coil in series with the battery. The engineer
shot the idea full of holes and mumbled something about back emf when the
circuit was shut down and also complained that the spike could jump over the
coil. Not having an engineering degree, I shut up but continued to think
about this problem that should be solved electronically instead of settling
for cracking open the $100 battery pack and taking battery power directly
from the batteries to avoid the spike getting into the charging circuit -
which works fine but is kind of hokey and voids the warranty of the
batteries.
In the circuit below I show my thoughts (albeit in a poorly done schematic).
I figured I could use a coil with a ferrite core and a diode to ground on
each side. I figured the diodes would satisfy his emf concerns and the
choice of core would keep the spike from going across the coil. Note there
is an on/off switch (not shown) between the battery and the rest of the
drawing.
Is this an idiot idea? Should I use an iron or any other type of core for
the coil? Maybe only one diode? Get a job not even remotely connected to
electronics? Any better recommendations?
___________nnnnnnnn___________
| | |
+ _ _
BATT ^ ^ Out to rest of circuit
- | |
| | |
______________________________