Battery Backup Resistor(from ex.1.29 Art of Electronics 2nd Ed.)

Thread Starter


Joined Jun 7, 2010
I've attached a schematic copied from exercise 1.29 "Art Of Electronics" 2nd Edition. The problem is to create a 10mA trickle charge for the battery. The answer is to use the resistor shown in my schematic (the exercise does not show the resistor.) I have an answer page that I found from the Harvard syllabus and it shows the resistor being 300Ω?! By my interpretation of Ohm's law the resistor value should be 1500Ω (1.5kΩ.)

What am I missing?

There were no values for the diodes in the book. I just used what seemed reasonable from the Eagle library.


Last edited:


Joined Nov 30, 2010
You'll have about 14.3V minus (Voltage of the battery) across the resistor.
If the battery is really 12 volts, 300 ohms would allow 7.6666 ma to charge the battery.
1500 ohms would allow 1.533333 ma to charge the battery.
Rechargables are usually higher than the labeled voltage, so reality would have even less charging current.
How much current is required to maintain the charge on the unknown size battery?

One thing I think you are missing is how you arrived at 1500 ohms. Without that, I am not able to say what you did "wrong". I can only describe how I would arrive at a resistance.

Thread Starter


Joined Jun 7, 2010
You answered my question. I keep forgettng that two voltage sources "nullify" each other when on the same circuit and need to be addressed accordingly. As I said, there were no values for the diodes in the exercise so the figure you came up with is probably more accurate than what is in the exercise. 15-12 Volts leaves 3 V and when divided by .01A gives 300Ω. So there you are. I was dividing 15V/.01A and getting 1500Ω.

FWIW: the 12V battery is probably a car battery or gel cell which are used by Ham Radio people to power their rigs. The trickle charge circuit is very common in that case.