Limiting current draw from Lead Acid Battery

 

Need a little help.

Say I have a device that draws 90 amps to operate.  It will function
with 25 amps as well.  I want to limit the current draw from a lead
acid battery.  I've read about rheostats but with this many amps I
understand that quite a bit of heat is generated.  I also read abit
about poteniomters but am not quite sure of the difference between the
two.  Can anyone point me to a source where I can purchase what I need
to do this?

Thanks,
Bob

 


Mr Bob wrote:

Need to know nature of device...

 


They are both variable resistors.

Not wishing to sound rude but if you don't know basic electrical info you
are best off not touching this with a barge pole.

12V batteries are not likely to kill you but when they push 90A then that is
just over a kilowatt of power going somewhere. Enough to heat wires hot
enough to melt insulation and flesh.


Maybe get an engineer to rip a motor speed controller from one of those
invalid scooters? Though make sure you ask the invalid for it first :-)

 


Ho can someone do that when no one, including yourself, knows what you
need? The only way to reduce the current draw from the battery is by
developing a voltage to counter the battery voltage. Your device
develops some voltage VD with 25 Amps through it, and VD may even be
zero, so you will need to develop a counter voltage VC that makes
BATT=VC + VD. This can be done with a resistance R so that 25Amps x R
equals the required VC counter voltage. R has to sized to withstand a
power dissipation of 25Amps x VC Volts which may be quite hefty. Going
back to what you have said, the device and the battery resistance
develop all the counter voltage at 90Amps, so that it must be
12/90=0.133 ohms. Then at 25Amps they will develop VD=25x0.133=3.33V. So
the counter voltage from an added series resistance must be
BATT-3.33=12-3.33=8.66V. The required resistance is R=8.66V/25Amps=0.346
ohms, and this must dissipate 25Amps x 8.66V=220Watts. Where you go from
here depends on other information like is this battery being charged by
an alternator so that the output voltage rises, how important is it to
maintain 25Amps exactly or is this a ballpark figure, how is the device
connected into the circuit, operating environment temperature range, etc...

 

On Sun, 23 Oct 2005 07:18:55 -0700, Mr Bob wrote:


What is the device? If it is a commercial product with a detailed
Datasheet, maybe someone here can help you. If it is a simple device, then
just tell us what it is.

--Mac