• Zero-cycle battery: You’ll see that some manufacturers will advertise that their battery is a “Zero-Cycle Battery”. This means that the battery is not a refurbished battery, and has not been through any recharging cycles previously.

• Warranty: A lithium-ion battery should usually be able to go through 500 cycles--so it should have a life expectancy of 12-18 months. You want to be sure that the battery has a warranty for that period. The IMILITIS Compatible Replacement iPhone 6 Battery pledges that it has a real 12-month warranty, that is good in the US, United Kingdom, Australia, Japan, Germany, Switzerland, and Brazil.

• Thankful not to have to charge the phone so often: Most customers are overjoyed at not having to recharge their phone several times a day. Some say that they were limited to only 1 hour of talking a day before they had to put their phone in the recharger! So a new battery that can hold its charge for a whole day is a big plus.

• Battery ratings: You’ll see that a battery’s capacity will be given as 1800 or 2200 mAh. “mAh” stands for “milliampere-hour”, which is a unit of electric charge. A 2200 mAh battery can give you 2200 milliamperes of current for an entire hour. But that figure is a little misleading: the battery’s lifetime will also depend on its discharge rate, as well as its temperature. Your battery will draw more current when the screen is brighter, so it will discharge more quickly. But, a battery with a higher mAh rating will generally last longer on a full charge. One of the highest ratings is the Aslanka High Capacity Battery, at 3250 mAh!

• Chip to prevent over-charging: Some companies have a built-in chip in their replacement battery. The chip prevents overcharging and overheating of the battery. Some also have protection from short-circuiting as well.

• Take care when replacing the battery: Installing the new battery yourself requires a good deal of care. Many battery manufacturers supply you with the tools to change the battery, including the pentalobe (five-tipped-star) screwdriver, the tool (that looks like a guitar pick) to help you pry open the case, and adhesive strips to secure the battery in place. Properly done, the entire replacement can be completed in less than 15 minutes--but don’t rush it!

• Check compatibility: It’s important to note that a battery for an iPhone 6 may not be compatible with other iPhone models. For example, the 3250 mAh Aslanka Battery is okay for an iPhone 6S Plus--but not the iPhone 6 Plus (and certainly not the iPhone 7 Plus)! The secret is to match the battery’s Model Number to your phone: You’ll see that the IMILITIS battery is good for the A1549, A1586, and A1589 models. That means it’s good for iPhone 6 versions by Verizon, China Mobile, CDMA, and GSM. On the other hand, the Nohon replacement battery is good for the iPhone 6s, for models A1633, A1688, and A1700.

• Reading battery capacity/battery health: Some manufacturers note that your iPhone will display a misleading percentage when notifying you about how much charge is left, or about your battery health. This is because the iPhone app will work properly only with the original iPhone battery. If you look at the battery’s capacity by means of a PC, it should display the correct percentage.

Aslanka is a maker of replacement batteries for iPhones and Galaxy devices. Their products come with instructions, a screen protector, and repair toolkit.

Yontex is based in Kowloon, Hong Kong. They make screen protectors and cases for cellular phones: iPhones, iPads, and Androids. They also make replacement batteries for a wide range of mobile devices.

Imilitis manufactures iPhone replacement batteries, phone cases, Apple watches, and earbuds. They are based in Chicago, Illinois.

Nohon makes replacement batteries suitable for the iPhone 6, together with a repair tool kit and instructions.

Ombar makes replacement batteries for laptops and cellular phones, as well as computer peripherals such as keyboards and mice. They make replacement batteries for iPhone 4, 4s, 5, 5s, 6, 6s, 6P, 6sP, and 7, as well as for the Samsung Galaxy.