Rechargeable batteries

A rechargeable battery is a battery that, unlike a single-use dry cell, can be recharged and reused many times over its working life. Once its stored charge runs low through normal use, it can be connected to a charger, refilled with charge, and put back into service — ready to power the device once more.

Rechargeable versus dry cell. A dry cell (of the kind used in a torch or a remote control) is designed for one-way use only: it produces electric current until its internal chemicals are used up, and then it is thrown away. A rechargeable battery is built differently, so that the process that produces current can also be run in reverse by a charger, restoring the battery's ability to supply current again. This single difference — being reusable instead of disposable — is what makes rechargeable batteries so much more practical for devices people use every single day.

Where they are used. Rechargeable batteries are found in an enormous range of everyday devices that people reuse constantly: laptops, mobile phones, digital cameras, power inverters used during power cuts, and electric vehicles. In each case, recharging the same battery again and again — instead of buying and discarding a fresh one every time — makes owning and using the device far more convenient and far cheaper over time.

Why lithium-ion dominates. The most common type of rechargeable battery in use today is the lithium-ion (Li-ion) battery. Li-ion batteries are popular because they pack a useful amount of stored charge into a small, light package, and because they can be recharged a large number of times before their performance drops noticeably — a good match for portable devices like phones and laptops that are recharged almost every day.

A rechargeable battery works because the chemical reaction that produces its electric current is one that can be pushed backward as well as forward. When the battery is being used, this reaction runs forward and drives current out to power a device. When the battery is plugged into a charger, the charger forces electric current back into the battery, and this drives the same internal reaction in reverse, restoring the battery's chemicals close to their original state and refilling it with charge. A dry cell, by contrast, is built from chemicals whose reaction is essentially one-directional: once it has run its course, trying to force current back in does not usefully reverse it, which is exactly why dry cells are single-use and must not be recharged. This reverse-and-forward cycling cannot go on forever. Every time a rechargeable battery is discharged and then recharged, small, unavoidable side-changes build up inside it, so the battery's chemicals never return exactly to their original condition. Over many hundreds of such cycles, this gradual wear means the battery can hold less and less charge, until it eventually can no longer power the device usefully and must be replaced. Because worn-out rechargeable batteries — especially Li-ion ones — contain materials that should not go into ordinary household waste, they should be handed in for e-waste recycling, so that useful materials can be recovered and environmental harm is kept to a minimum.

Question: A student's mobile phone battery, which used to last a full day on a single charge when the phone was new, now needs to be recharged twice a day even though the student's usage habits have not changed. Explain what has most likely happened to the battery, and what should eventually be done about it.

Step 1. Identify the battery type and its expected behaviour. The phone's battery is a rechargeable battery, most likely lithium-ion, designed to be charged and discharged repeatedly across the phone's working life.

Step 2. Recall what repeated cycling does. Rechargeable batteries gradually wear out with repeated recharging cycles, because their chemicals do not return perfectly to their original state after each cycle, so the amount of charge they can hold slowly decreases.

Step 3. Match the evidence to the cause. Since the student's usage habits have not changed, but the phone now needs recharging twice as often as before, the battery is clearly holding roughly half the charge it used to — exactly the pattern expected from ordinary, gradual wear, not a fault in how the phone is being used.

Step 4. State the conclusion. The battery has gradually worn out from repeated charge-discharge cycles; once it can no longer hold a useful charge, it should be replaced, and the old battery disposed of through proper e-waste or battery recycling rather than ordinary household trash.

Competitive stretch

Question: A shopkeeper sells two kinds of small torches: one uses ordinary dry cells that must be bought new each time they run out, and the other has a built-in rechargeable battery with a small charging cable. A customer who uses their torch daily for several hours asks which is more practical. What should be suggested, and why?

Step 1. Compare how each battery behaves once it runs down. The dry-cell torch needs brand-new cells bought every time the old ones go dead, since a dry cell's reaction cannot be usefully reversed. The rechargeable torch can simply be plugged in and its reaction driven backward by the charger, refilling it with charge.

Step 2. Apply this to frequent, heavy daily use. Several hours of use every day would run dry cells down often, forcing repeated purchases, while the rechargeable battery only needs recharging and keeps working through many such cycles.

Step 3. Conclude, with the honest caveat. The rechargeable torch is the more practical choice for this customer's frequent, heavy usage pattern — though it too will eventually wear out after many cycles and need replacing, just much later than a string of dry cells would.

• Assuming a rechargeable battery lasts forever once it can be recharged — it still gradually wears out with repeated cycles and eventually needs replacing, only after many more uses than a dry cell.
• Confusing "rechargeable" with "no disposal needed at all" — a worn-out rechargeable battery must still be disposed of responsibly, ideally through e-waste recycling.
• Thinking all rechargeable batteries are the same type — lithium-ion (Li-ion) is the most common type today, but "rechargeable" is a broader idea than just this one type.
• Believing an ordinary dry cell can be made rechargeable simply by plugging it into a charger — dry cells are built for one-way use, and their reaction is not designed to be driven usefully backward.

Ask the tutor: Want a side-by-side comparison of how a rechargeable battery, a dry cell and a voltaic cell differ?