How we test rechargeable batteries
By Sarah Ingrams
Find out how we test rechargeable batteries to reveal which last the longest. Only the best rechargeable batteries become Best Buys.
Our independent rechargeable battery reviews tell you what you need to know about a rechargeable battery before you buy a pack. This means you will only buy the best rechargeable batteries - the ones that last the longest and stay charged between uses. And you can avoid wasting money on the worst.
Every rechargeable battery will power your device to begin with, but that doesn't mean they're all equally good. We can help you find the rechargeable batteries to suit your devices and avoid ones that last a frustratingly short time before they need recharging, power your devices for a shorter time after multiple recharges or leak away their charge while not in use.
We test every rechargeable battery in the Which? test lab using a strict set of criteria to sort the outstanding models from the short-lived.
Our reviews answer the most crucial questions about rechargeable batteries:
- How long will the batteries last?
- Do the batteries last for as long when recharged multiple times?
- How long will they stay charged when I’m not using them?
- Do rechargeable batteries live up to their capacity claims?
- Which batteries charge fastest?
- Should I buy them?
Find out which models give top-class results in our rechargeable batteries reviews. If you’re looking for disposable batteries, take a look at our batteries reviews. To find out more about our testing, read on.
How long will the rechargeable batteries last?
Frequent battery changes - in your portable radio while you’re gardening, or in your games console's handset in the middle of battling baddies - are frustrating, and this is where you’ll notice the biggest difference between the best and worst batteries. So we test rechargeable batteries in high-drain conditions to reveal which keep even the most power-hungry devices running for longest on a single discharge.
Using rechargeable batteries in lower-power devices will mean it takes longer for the batteries to pay for themselves. This is because they take longer to run down between each recharge, so it will take more time to make a saving, compared with buying disposable batteries.
The best AA rechargeable batteries last nearly two and a half hours longer than the worst in the most power-hungry devices – see for yourself in our Best Buy rechargeable battery reviews.
Do the batteries last for as long when recharged multiple times?
A long battery life on a single discharge is great initially – but not if your batteries can’t maintain this when you use them repeatedly.
After 42% of Which? members told us (online survey of 1,306 people, March 2015) that longevity (maintaining performance over multiple charges) is the most important technical factor when choosing rechargeable batteries, we decided to test it. We'd also found that the top problem reported with rechargeable batteries was older batteries not lasting as long between charges as new ones.
So we recharge and discharge each rechargeable battery hundreds of times in some of the toughest conditions you could use them in at home, such as a bright LED pocket torch, to reveal which batteries last the distance.
400 hoursThe best AA rechargeable batteries give more battery life than this before they power your devices for noticeably less time
We measure each battery’s capacity throughout to find out for how long they can reach at least 80% of their initial capacity, and for how long they can reach 60% of their initial capacity (the point at which you’ll notice you have to recharge them more often).
The best AA rechargeable battery we've tested gives more than 400 hours of battery life while its capacity remains above 60%. But the worst battery manages just over 100. Because our tests replicate the most power-hungry devices, your batteries should last much longer in lower-power gadgets such as cameras, clocks and remote controls.
How long will rechargeable batteries stay charged?
Being organised and charging your batteries in advance is no help if they’ve lost their charge by the time you want to use them. All rechargeable batteries ‘leak away’ their charge over time, so we test this by fully charging eight batteries from each brand on test and then leaving them unused.
We measure the amount of power remaining in four of the batteries after 25 days, and the amount of power remaining in the other four batteries after 50 days. The worst batteries we’ve found lost 35% of their charge after 50 days, but many retain more than 85%.
See our Best Buy rechargeable batteries to find out which won't leak away their charge.
35%The amount of charge the worst rechargeable batteries lose in 50 days
Do rechargeable batteries live up to their capacity claims?
All rechargeable batteries have their capacity marked on the packaging. This is measured in milliamp hours (mAh). In general, the higher the capacity rating, the longer the batteries should keep your devices powered.
But batteries that don’t deliver on their stated capacity make it harder for you to predict which will last longest. So we measure the capacity of each battery in our lifetime tests and compare it with the capacity stated on the packet.
All disposable batteries we tested achieve more than 90% of their minimum stated capacity. The worst manage 92%. Some achieve more than 100%. You can find out which - see our best and worst rechargeable batteries.
Which rechargeable batteries charge the fastest?
Waiting around for rechargeable batteries to charge can be frustrating if you haven’t planned ahead. So we measure how long each set of batteries takes to charge using our smart charger on the soft-charge (ie not super-fast) setting.
The fastest AAAs are ready to use more than one and three-quarter hours sooner than the slowest. Charging time isn't factored into the total test score because it depends directly on the capacity of the rechargeable batteries you choose; smaller-capacity batteries will charge faster.
Should I buy them?
The assessments listed above go into making the final overall score for each rechargeable battery we test. But certain assessments are more important than others and so carry different weights. You told us that it was important to be able to use your rechargeable batteries repeatedly without them lasting for a shorter time, and we agree, so a larger proportion of our test score is based on this.
Our overall ratings ignore price and are based on:
- 30% Overall long-term use
- 30% Percentage of stated capacity achieved
- 25% Charge retention
- 15% Battery life on a single charge
Top-scoring rechargeable batteries with scores that stand out earn our Best Buy recommendation. Those that score 40% or less are highlighted as Don’t Buy batteries to avoid.
Now you know the lengths we go to in order to make sure you only buy the best rechargeable batteries, check out our Best Buy rechargeable batteries.