How to reduce current?

My component is a battery, which should not be able to draw more than 150mA from the power source.

You aren't looking for a current divider but rather a current limiter and this is an active device because, in principle, it doesn't restrict current into (or out of) a device until the "limit value" is reached.

So, trying to make one from passive components or diodes isn't going to be effective as a protection against over charging with too high a current.

Well, as others in this thread have already pointed out, a (decent) current limiter requires some active circuitry. A very simple current limiter using only one basic component, a JFET, is the following:

^{simulate this circuit – Schematic created using CircuitLab}

That circuit limits the current into the load to the value of \$I_{Dss}\$ of the JFET. Just select a JFET having an \$I_{Dss}\$ of 150mA and you'll be fine.

There are a couple of drawbacks, though:

• Current limiting is not very stable: due to its simplicity there will be small variations of the current with varying supply voltage.

• Idss is not well specified and has wild tolerance: usually a datasheet will give you a (broad) minimum and maximum value, or sometimes just only a minimum. For example, the J107 has a specified minimum \$I_{Dss}\$ value of 100mA, and no maximum (see image below). So, in the end, you will end up selecting a specific specimen by hand using trial and error.

• Power dissipation: JFETs are usually low power devices (<1W); if the voltage drop across the JFET multiplied by the current is higher than that (assuming a reasonable safety margin), you are out of luck.

There are two ways to provide a current-limited supply to charge a battery.

a) The current limiter way. Use an active current limiter. The simplest of these, if you have the voltage headroom, is an LM317, which maintains 1.2v between its output and adjust terminals. If you connect (for instance) 12ohms between them, it will limit at 100mA. Choose different value resistors to get between 5mA and 1A.

b) The poor-man's limiter, designed for battery charging. As your battery voltage doesn't change quickly, and as power supplies are often adjustable, a resistor of an appropriate value will limit current from a supply to the battery. As the battery voltage rises, the drop across the resistor will fall. This will reduce the current, unless you are monitoring it every few minutes and adjusting the supply up to compensate. Quick to do as a one-off, very tedious if you have to do it more than twice!