Apple engineers have begun contacting iPhone 4S owners about a known "issue" with its battery.

Ever since the iPhone 4S hit stores two weeks ago, owners have been complaining of inexplicable drops in the iPhone 4S' battery, even during standby. According to The Guardian, one owner experienced a 10 percent drop every hour.

"I then got a call from a senior [Apple] engineer who said he had read my post and was 'reaching out' to users for data and admitted this was an issue (and that they aren't close to finding a fix!) and asked lots of questions about my usage and then asked if he could install the file below and that he would call back the day after to retrieve the info," he told The Guardian.

When Apple CEO Tim Cook launched the iPhone 4S, he promised a longer battery life. And it does, conditionally. In PCMag's review of a Sprint iPhone 4S, lead mobile analyst Sascha Segan said the iPhone 4S got 7 hours, 33 minutes of 3G talk time, versus 6 hours, 17 minutes on a Verizon iPhone 4.

But he added a caveat: "It delivers on Apple's promise of longer battery life - as long as you have strong wireless signal."

A recent teardown by iFixit revealed that the battery in the iPhone 4S is slightly more powerful than the one in the iPhone 4, though that only seems to improve talk time. Users get an extra .05 WHrs of juice compared with the battery that was in the iPhone 4. But is that enough to compensate for power consumed by the iPhone 4S' dual-core A5 processor?

It's a mystery to us all. In its spec sheet Apple said the iPhone 4S gets 200 hours of juice on standby, compared to 300 hours on the iPhone 4. Those in Apple's 117-page discussion thread said they've seen some improvements after turning off iCloud or deactivating the Notification Center; features that put the "S" in the iPhone 4S.

Meanwhile, Sprint iPhone 4S owners have also been complaining about slow data speeds. See Sprint: iPhone 4S Isn't Crushing Our Network for more.