Power niggles

Peter Rosenthal’s article (June, p132) perpetuates an incorrect assumption made by many motor caravanners when discussing power consumption by vehicle services or connected appliances.

In the final paragraph on page 132, Peter states that: “If your starter motor needs 1,200W to crank it and your battery is fully charged at 13V, it will draw 92 amps. If the battery voltage drops to 11V it will draw 109 amps.

“Components have fixed power requirements so, if the voltage drops, the current increases to compensate.”

The only fixed value is the load resistance presented by the starter motor.

The resistance of the starter motor is determined by the equation: resistance = voltage squared divided by power so, in this case, resistance = 13 x 13 / 1,200 = 0.14 ohms.

The resistance is the only fixed value in the equation as it is inherent to the design of the starter motor.

So, taking a step back, the current drawn by the starter motor when supplied with 13V is 13 / 0.14 = 93 amps.

Therefore, the current drawn by the starter motor when supplied with 11V is 11 / 0.14 = 78 amps, which will allow it to absorb 11 x 78 = 858W.

Which is why, if the battery is low, the engine cranks more slowly ( for any pedants, I’m aware that a motor presents a more complex load than a simple resistance but for these purposes can be regarded as such).

The power consumption value in Watts of any device is only correct at the voltage for which the appliance is rated: lower volts = lower power motors or dimmer lights.

Please accept this correction in the spirit intended.

Articles written by Peter Rosenthal are always informative and an easy read and I thought long and hard before writing something that could detract from an otherwise excellent article.

Tony, by email