ifixit iPhone battery: better than cheap competition
20 - 30 minuten
I had previously replaced the battery in my iPhone 3GS with a cheaper battery with the exact same model number (616-0435) as what ifixit sells, and even looking at them side by side looked identical (APN, VPN, voltage etc). The cheaper one (~$7) unfortunately caused problems slowly until eventually the phone was rebooting frequently and not charging the battery. Also in debugging the issue I tried to restore / reflash the iOS firmware, however this failed with a dreaded #29 error message. The message from my online research relates to a battery which is not compatible with the phone.
Long story short, I took the gamble with ifixit despite the battery appearing to be the exact same model as the cheap one causing me problems. Luckily it came fully charged and I was able to reflash with a jailbroken iOS without any error. It has been holding a charge since without issue.
Beware of replacing parts to your iPhone with cheap alternatives from sources that do not have a strong support focus such as ifixit. The battery in particular is worth paying a premium for to ensure higher quality.