Here's the project: Installing fancy complicated BMW seats into my boss's sprinter van.
Here's the problem: I need to feed power to the seats for adjustments/heating.

What's the best way to do this without frying something? I've tested the motors and heating system at 10A@12Vdc , and that seems to work very well. So how do I regulate the current coming from the battery to that level? Is there a purpose-built automotive current regulator available?

Second question: What's the best way to control seat heater temperature? Potentiometer? That's all I've been able to come up with, but if there's a better way please feel free to advise.

Cheers, Dan

 

If the 12V battery full charge is 13.8V then you can using three rectifier diodes to is series from battery to the motors and heating system, if you can find the current of diodes at least have 25A or some more, it should be ok.

Using Ne555 and n mosfet to do the pwm control, it's easy to find the circuit.

 

don't drop voltage, use fuses to limit current in circuits...

potentiometer is just adjustable voltage divider (current is very small, specially wiper current). common way to control loads is using PWM.

 

... So how do I regulate the current coming from the battery to that level? Is there a purpose-built automotive current regulator available?

You dont. All you have to do is feed the 12V to the power seats. The current takes care of itself. Use #12 awg wire. Install a 20A fuse at the feed end. They are designed for automotive application; they can deal with anything from 11.5V to 14.8V (normal automotive range) just fine...

Second question: What's the best way to control seat heater temperature? Potentiometer? That's all I've been able to come up with, but if there's a better way please feel free to advise.

In our cars that have electrically heated seats, I think all they do is have a Hi/Lo switch; no thermostat. There are likely more than two wires going to the heaters. Power input is controlled by having one element on, the other element on, possibly both elements at the same time...

 

Here's an example of the PWM circuit that Scott mentioned.
Insert the seat heater in place of the motor in the schematic.

 

Most loads are specified to operate at some particular voltage, and then the load will draw whatever current it needs, which means you don't need to regulate the current from the battery, you only need to worry about providing the correct voltage and fuse the circuit appropriately.

Loads like motors and heaters usually tolerate a wide range of voltages, so there should no need to regulate the voltage.

As for controlling the seat temperature, pwm (Pulse Width Modulation), as mentioned ScottWang and Panic Mode is certainly the appropriate technique.

If you don't want to build the control circuit itself, see if you can find out what BMW does about it and then buy the necessary controller from a BMW dealer, if such a thing exists.

It is likely that, like many heaters, it is only on/off.

 

The heated seats in my car have no user control of temperature. As Dick says, just on/off.

BTW, you should make certain that the vehicle mods don't invalidate your insurance.

 

So I don't have to worry about getting the whole 600 cold cranking amps flowing through the seat and burning something up?

 

So I don't have to worry about getting the whole 600 cold cranking amps flowing through the seat and burning something up?

Do you have to worry about 600 cold cranking amps going through the ignition module, the car radio, the headlights, the seat warmer, the power windows, ...?

Note Ohm's law, which basically states that at a constant voltage (~14V in a car), the current is determined by the internal resistance of whatever you are connecting... The lower the internal resistance, the higher the current. Think about the internal resistance of the starter motor vs the internal resistance of the dome light lamp.

 

car seats with heater normally just have just one or two levels (on off, or at best high or low heat when on).
DC motors can draw a lot (20+ Amp) when stalling. some have built in protection, others could use a bit of help.
if motor does not shut off when stalling, it will be burning hot in 2-3 seconds.

 

Good stuff, thanks. Yeah I'm not the most experienced when it comes to wiring. This is all rather new to me.

 

So I don't have to worry about getting the whole 600 cold cranking amps flowing through the seat and burning something up?

Apparently Volvo did sometime in the 90s.

 

So I don't have to worry about getting the whole 600 cold cranking amps flowing through the seat and burning something up?

I did that once by routing a wire where it got pinched between the seat and the frame. (I wasn't smart enough to put the fuse right next to the battery 50 years ago.) If you do the wiring right, the fuse will protect the wiring and the motors will protect themselves.