The math of MacBook Pro battery life and trickle charging


Batteries are rated in one of two ways. Watt Hours which means for how many hours can they supply 1 watt of power. The new MacBook Pro 2016 has a 76 Watt-hour battery
Put another way if you load Battery Doctor and see you are drawing 18 watts then you will have roughly 76/18=4 hours of life. 
Another common measure is amp-hours. This assumes you know the voltage that you are pull out. For a MacBook Pro 2016, it says it has a 6773 milliamp hour or 6.7Ahour. By the way that means that the output voltage for the battery is 76/6.7 since V x A = W so it is a 11V battery internally. 
Now for the reason for the low battery life. Apple says the MacBook Pro should last 10 hours or they think average battery usage is 7.6 watts. 
Today when I started my MacBook Pro it was running at 4 watts so the mystery is why yesterday it was 18 and today it is 4.8. Sounds like a background process or Safari going crazy to me. 
As a final aside the older MacBook Pro 2015 had a 99.5WHr battery. The FAA maximum for a battery is 100WHr for safety reasons. That means that for their 10 hour use they were expecting 25% greater power usage at 9.95 watts. So the new mbp 2016 is supposed to be much more efficient. 
One interesting thing is that such a lower power consumption, you can use a tiny 10 watt, 2 amp USB charger to keep your laptop powered up. U can even use a car charger to freshen the battery. That's one of the nice things about USB C. In fact even a lowly 1 amp 5 watt USB outlet is pretty good with today's low power consumption laptops. I now need a USB A to USB C cord when going to meetings as it's lighter. And your little iPhone charger even at 10 watts should be able to charge a battery in eight hours or over night. 
The huge 87 watt charger you get with your MBP 2016 should be able to recharge a completely dead 76 watt hour battery in less than an hour. Even with power losses which would make it longer, that's pretty impressive. 
So on to figuring how why the laptop is drawing so much! But this USB c does add lots of flexibility. 

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