Batteries for an emergency kit: match the device, not the pack

Make a small battery list.

Read the device's manual or battery compartment. Record the exact size, chemistry, voltage and number of cells it needs. AA and AAA are different sizes. A cylindrical cell that looks similar is not automatically compatible; never substitute a lithium-ion cell simply because it fits.

For each device, write down:

• The exact battery requirement from its instructions.
• How many cells make one complete replacement set.
• Where that set is kept and which charging equipment, if any, it needs.

A light that needs three AAA cells cannot use a twenty-pack of AA batteries. Standardising sizes when replacing worn-out equipment may simplify spares, but it is not a reason to discard working devices.

Do not choose cells for medical equipment, smoke alarms or other safety-critical devices from a general shopping list. Follow the exact equipment or provider requirements.

Single-use alkaline or rechargeable NiMH?

Single-use alkaline cells are straightforward spares for devices that accept them. Replace the whole set with fresh cells of the specified type. Do not mix old and new cells, different makes or different chemistries in one device. Never try to recharge ordinary alkaline cells.

NiMH rechargeables can be useful in regularly used compatible equipment. Typical NiMH AA cells are labelled 1.2V, while alkaline AA cells are 1.5V. That difference is a compatibility check, not a reason to guess how your device will behave. Follow its manual.

A suitable NiMH charger is separate equipment. It needs a usable power source; a charger alone cannot replenish batteries during an outage. Keep rechargeable cells ready in advance and follow their own charging and storage instructions. Do not copy one brand's maintenance interval onto every device or chemistry.

Buy usable sets, not the biggest pack.

Count complete replacement sets before comparing packs. A low unit price is not useful if the cells do not fit, your charger cannot charge them or the household will never use them.

Equivalent cells from a suitable ordinary retailer are fine when they meet your device's instructions. There are no independently tested runtime rankings or current prices here.

Check AAA spares and chargers separately rather than substituting an AA pack or an unsuitable charger. A torch, radio and battery pack only help when the complete setup works.

Store complete sets safely.

Keep spares in original packaging or suitable protective holders, away from coins, keys and other metal objects. Loose terminals can short-circuit. Store batteries cool and dry, away from heat, children and pets; refrigeration is not necessary.

Check polarity when fitting cells. Remove exhausted cells promptly. For equipment stored unused for a long period, follow its instructions about removing batteries while keeping a ready-to-fit set alongside it. Do not dismantle, puncture, crush or burn cells, and do not use damaged batteries. Follow local collection and recycling arrangements for the exact type.

Test the whole setup.

Try the light and radio, not just a battery tester or charger display. Can you find the controls in the dark? Can you hear local broadcasts where you keep the radio? Does everyone know where spare cells are?

Choose a household review reminder that includes batteries, cables and changing device needs. A manufacturer's shelf-life claim does not establish a device's runtime, prove that a cell cannot leak or make forgotten equipment ready to use.

For the rest of the plan, see preparing for a power interruption. Check what you already have, replace the actual gaps and practise using it.