BU-104: Getting to Know the Battery

Recognize the strength of the battery and learn how to live with its weakness.

Nature offers many ways to produce power. Most result through combustion, mechanical movement and photosynthesis, as in a solar cell. Electrical energy generation of the battery develops by an electrochemical reaction between two metals of different affinities. When exposed to acids, a voltage develops between the metals as part of ion transfer; closing the circuit induces a current. In 1800, inventor Alessandro Volta discovered that the voltage potential became stronger the farther apart the affinity numbers moved.

The simplest manifestation of a battery is a lemon. Driving a zinc-plated nail and a copper coin into a lemon creates a voltage, but this quasi battery does not deliver much power. The current delivery system is weak and any electrical load causes the voltage to collapse. The energy does not come from the lemon itself but from the chemical change in dissolving zinc into the acid or lemon juice. Figure 1 illustrates the lemon battery.

Lemon Battery Figure 1: Lemon battery
The experiment is often used for educational purposes. The electrodes are zinc in the form of a galvanized nail and copper in a coin. The lemon juice acts as electrolyte to induce a chemical reaction. 

Standard potentialof zinc                = –0.76
Standard potentialof copper           =   0.34
Cell potential with conducting path  =  1.10V

Elements with the greatest negative electrode potential serve as cathodes; those with the highest positive potential assist as anodes. The difference between the electrodes provides the terminal voltage. For a rechargeable battery to be practical, the chemical reactions between the elements must be reversible. To complicate matters further, chemical reactions between compounds cannot consume the active chemicals, and this limits the pool of suitable electrodes.

Multiplying the voltage by the current provides power. Power is measured in watts in honor of James Watt, the 18th-century developer of the steam engine. The amount of energy a battery can store is expressed in watt-hours (Wh).

All energy sources have limitations, and power must be harnessed carefully so as not to cause an overload. An analogy is a bicycle rider (Figure 2) who chooses the best gear ratio to transfer power into propulsion. On a flat road, a high gear provides high speed with moderate pedal torque simulating high voltage. Climbing a hill, the pedal torque increases while the speed decreases. This, in our analogy, results in a lower voltage and higher current. The pedal force the rider exerts relates to torque in newton meter (Nm); the endurance before exhaustion defines energy in watt-hours (Wh).

Bicycle Figure 2: Analogy of a bicycle rider.
Energy is the product of power and time, measured in watt-hours (Wh); power is the flow of energy at any one time, measured in watts.

A battery is rated in ampere-hours (Ah). This specifies how much charge a pack can hold. Like fluid in a container, the energy can be dispensed slowly over a long period of time or rapidly in a short time. The amount of liquid a container holds is analogous to the energy in a battery; how quickly the liquid is dispensed is analogous to power.

The physical dimensions are specified by volume in liters (l) and kilograms (kg). Adding dimension and weight provides specific energy in Wh/kg, power density in W/l and specific power in W/kg. Most batteries are rated in Wh/kg, revealing how much energy a given weight can generate. Wh/l denotes watt-hours per liter. (See BU-105: Battery Definition.)

Batteries are custom-fit for a specific use, and manufacturers are well in tune with customer needs. Mobile phone and EV markets are examples of clever adaptations at opposite extremes. While batteries for consumer products emphasize small size, high specific energy and low cost, industrial batteries strive for reliable performance and long life. Safety in all applications is of utmost importance.

Last updated 2016-09-14

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Basics You Should Know
The Battery and You
Batteries as Power Source


On July 6, 2011 at 10:15pm
Paul Peter wrote:

Can I get some litreature/ details about the Lithium ion Yitrium based rechargaeable power batteries and its chargers

On May 6, 2012 at 12:57am
Syed Muhammad Tahir Hussain wrote:

Why does a lead acid battery less accept charging current rather than discharging current.?
Example :
Charging : we can battery charge at normal current at about 10~25 % capacity rate and some time we can charge at 100% capacity rate (its called rapid charging). Battery plates active mass disintegrate at high charging current.

Discharging: We can battery discharge 3~5 times capacity rate. A 30 AH battery can be discharged at 150 Amp or more.

On October 1, 2012 at 12:24pm
E'Trigan wrote:

I do not understand why one battery is used over another. Example: I have a solar light that used a 4v 1.0Ah SLA (small 1.5 inch cube) so can I substitute a 9v off the shelf? Does more volt capability fry the LED bulbs? Could I substitute a 4v 2.5Ah? Very confusing…

On October 26, 2012 at 12:50pm
M J Bunce wrote:

A 9V battery would most likely damage the LEDs and the solar cell will be too low voltage to charge it. A 2.5 Ah battery would work, but only if the solar cell produced enough current to charge it, particularly on a dull day. If your light is no longer lasting long enough, the battery may be dying. Replace with a new 1.0Ah SLA and assume the manufacturer of the light has matched the solar cell to the battery by design.

On December 5, 2012 at 11:59am
john sutton wrote:

You said “When charging an SLA”. What is a SLA? I looked through many pages of the site and could not find this acronym defined.
Please respond to my email with an answer.

On January 28, 2013 at 2:01pm
mdg wrote:

SLA = Sealed Lead Acid

On January 29, 2013 at 11:25pm
nishan wrote:

What is the ideal charging rate(From alternator) .Is it max 14.3 v. if exceed what will happen.

On June 1, 2013 at 6:26pm
Charles Due wrote:

I had alternator putting out close to 18v and it boiled the battery, what a mess.

On July 9, 2013 at 3:43pm
hind wrote:

hello ...what is the name of the manufacturing process of making the p.v.c insulators in flooded lead acid batteries ??

On January 2, 2014 at 10:26pm
libin wrote:

add some picutres and explain… it will be more interesting

On January 3, 2014 at 8:07am
Charles Due wrote:

I have no pics but it was a mess. Never heard of 18V from alternator till last year.

On April 27, 2014 at 8:42am
mehmood ali wrote:

please send me about battery charging circuit and charging volts and charging time

On July 4, 2014 at 3:29pm
Ricardo C. Il Grande wrote:

Dear friends, as an industry consukltant here in Venezuela, I am receiving a lot of questions on how to improve battery life, . I have received dozens of 4-to-8AH AGM, 6 and 12 V batteries, basically for UPS and emergency light service. So I am documenting myself through your website, before I start the actual work. I have opened some of the rubber covers for the cells, and noticed they looked quite dry inside. Is there any need to add water to AGMs, or not at all? If so, how much? Many of those batteries have been standing on an emergency light for months or even years.
Best regards, Ricardo

On February 20, 2015 at 3:23pm
Steve wrote:

I believe that your power density should be listed as (W/L) not (Wh/L) as that is an energy density.

On August 20, 2015 at 10:48am
Wayne Little wrote:

Very simply which small battery is most cost efficient? The old regular stand-by’s, alkaline or the more expensive one’s above alkaline (all availabile in the big box stores). Thank you!.

On April 6, 2016 at 6:58am
Caitlin wrote:

recommend a very very small clarification to: “A battery is rated in ampere/hours (Ah)”


with a hyphen, not a slash, so eliminate any misconceptions that the units are ampere(divided by)hours

excellent website!!!! keep it up smile

On April 6, 2016 at 7:39pm
Charles wrote:

Thanks for the data, I will keep that in mind. I usually do not make many comments. 
So keep your SMILE ON ! !
Not enough of that going on   smile  smile

On April 30, 2016 at 1:02pm
Camilo Martin wrote:

I’m probably asking something stupid, but what about how many amps can you safely draw from a battery? I mean, say you have something that draws 1.5 volts at 10 amps (just an example). How many AAs would you need to have in parallel? (And, surely this changes over time like the voltage, right?)

On May 1, 2016 at 8:13am
Charles Due wrote:

I have come to understand over the years,always use distilled water in the standard car battery.

On August 26, 2016 at 1:46pm
Marvin Baden wrote:

I just purchased a radio with a Lithium battery that requires a charging voltage of 5V/500MA. Since no charging adapter was provided, is it OK to use a charging adapter (i-phone) that is rated at 5V/1A? I’m am presently charging it using the USB jack on my computer which apparently know what amperage to apply.

On September 5, 2016 at 1:30pm
Charles Duex wrote:

My neighbor used a new lawn mower battery to run his car, I thought he was joking till I seen it for myself. It lasted close to a year.  $50.00 or standard battery, $100.00 up

On September 11, 2016 at 12:43pm
Nicolas wrote:

I see two errors in this page.

“The length of time the battery can deliver energy is expressed in watt-hours (Wh).”
This implies that watt-hours is a unit of time when it’s actually a unit of energy.

“A battery is rated in ampere-hours (Ah). This specifies how much current a pack can deliver in an hour.”
No, that would be amperes-per-hour (A/h). The sentence should read “This specifies how much charge a pack can hold.”