Mini Jump Starter

Mini Multi-function Jump Starter

Also power and charge cell phones or even computers

You accidentally left lights on in your car, and now the battery doesn't have enough power to start the car. You could call for a jump start, but often that will take well over an hour even in an urban area, and can be expensive if you don't have an auto club membership. You could carry jumper cables with you, but you would still have to find someone willing and able to give you a jump start. Or you could carry an emergency jump start battery.

Until recently jump start batteries were too big and heavy to be practical accessories, but that's now changed. The same advanced lithium ion technology that's given notebook (laptop) computers such long battery life is now available in small and light (mini) jump starters that are practical to carry with you. While they don't have the total capacity of a regular car battery, they are able to deliver sufficiently high current more than long enough to start a normal car. (See Note below) The latest models also have the ability to power and recharge not only things like cell phones but also even computers, and there's even a powerful flashlight.

One of the best current units is sold on Amazon with 1 year warranty by BESTEK and by Bolt Power. It comes with a variety of computer power tips, including one that works for late model Lenovo ThinkPad T-series machines. The USB connector will charge cell phones, but even though rated at 2.1A, it only charges the Nexus 5 Android at slow "USB" rate. (For faster charging see Best USB Car Charger) The included zippered carrying case fits neatly under a car seat. Recommended.

Update: Used it to easily jump start two cars, so I know it works.

Notes

1. When jump starting:
   1. Make sure connections are clean and solid.
   2. After turning unit on, wait a few seconds before trying to start the car.
2. Larger engines need higher starting current. Figure 150A for 4 cylinders, 200A for 6 cylinders, 250A for 8 cylinders, 300A for truck diesel. (These are base values, not peak values.)
3. Watch out for unbranded (unknown) units with dubious quality, warranty and support.
4. Check to see if the correct power tip for your particular computer is included
5. Top up charge every 3-6 months. 
6. The mAh claim is the sum of the mAh ratings of the 3.7V battery cells, but that's not a valid calculation. Real capacity of typical units is 40-50 Wh (watt-hours).
7. If you must call for a jump start and don't have an auto club membership, try Allstate Roadside Assistance at 877-469-7650, charged to credit or debit card.