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Batteries
14 questions By Tony R. Kuphaldt
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Question 10 of 14
Some brands of dry-cell batteries come equipped with a built-in charge indicator in the form of a thermal strip along one side. Pressing two white dots closes a circuit, which heats up the strip and indicates battery charge by changing color.
Compare the accuracy of a charge indicator of this general design against using a voltmeter to measure open-circuit battery voltage. Which method of measurement is a more accurate indication of battery charge, and why?
Reveal answerThe thermal strip charge indicator is actually a more accurate indication of battery charge than an open-circuit voltage test.
Challenge question: in the absence of such a “charge indicator” on the side of a battery, how could you perform an accurate assessment of battery charge?
Notes:This question is practical because I’ve seen students measure the open-circuit voltage of a battery and declare it “good” when it is really past the end of its useful life. This is not to say that a plain voltmeter is useless for determining battery charge. Obviously if an open-circuit voltage test yields abnormally low voltage, we know the battery is dead. The question is, under what condition(s) is a “good” voltage measurement a reliable indicator of adequate battery charge?
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Question 11 of 14
What does it mean if a battery, or a bank of batteries, receives a floating charge? What does it mean if a battery, or a bank of batteries, receives an equalizing charge?
Reveal answerA “floating” charge is a very low charge rate, just enough to maintain a state of full charge despite normal load fluctuations on the power bus. An “equalizing” charge is a state of overcharging, done to equalize the chemistry in all cells of a battery.
Follow-up question: qualitatively speaking, at what voltage should the power supply be set to in order to “float” a battery bank, versus in order to “equalize” a battery bank?
Notes:Being able to identify these types of charge strategies is important, because many battery-powered systems with automatic chargers are either designed to charge in a particular way, or have the ability to switch between different charging modes. Failure to properly charge a battery, or a battery bank, will lead to decreased performance and/or decreased life.
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Question 12 of 14
An important equation in predicting the capacity of a lead-acid cell is called Peukert’s formula:
In t = C Where,
I = Discharge current, in amps
n = Constant, particular to the cell (typically between 1.1 and 1.8)
t = Time of discharge
C = Constant, particular to the battery (varies with the amp-hour capacity)
Explain how Peukert’s formula relates to the simpler “amp-hour” formula.
Also, calculate the discharge time for a battery with an n value of 1.2 and a C value of 200, given a discharge current of 25 amps.
Reveal answerThe “amp-hour” formula is linear, while Peukert’s formula is not (at least for values of n not equal to unity).
The discharge time for the example problem is 4.2 hours.
Notes:Ask one of your students to write Peukert’s equation on the whiteboard in front of class, along with the standard amp-hour formula. Viewed side-by-side, there is but one difference between the two equations, and it is the exponent n.
sometimes battery with small capacity (Ah) has a high Cranking Amperes (CA). for example Motorcycle battery 4 Ah has 140 CA. Automotive battery 35 Ah has 350 CA. 35 Ah is more than 8 times of 4 Ah but the CA just only 350 CA