Jumper packs contain a sealed lead acid battery. These batteries nose their charge in about 30-days. If you let them go longer without charging, you permanently damage the plates in the battery. You can buy replacement batteries online. Just open it up and find the size and amp hours. But it may be cheaper to buy a new unit. Read this post on how to buy one. There's all kinds of confusing numbers and some of them are just baloney.

http://ricksfreeautorepairadvice.com/jum...

You can try to charge it.. It may or may not hold a charge. These types of emergency chargers are know for failing. Once you let the charger sit for an extended period of time without a charge, the batteries will go bad..

Chances are the internal battery is dead. You can try recharging it, but that's a long shot if it's sat for years.

You can probably dismantle it and replace the battery with a new one.

Try charging it. If it doesn't, maybe you can take it apart and replace the battery. Do you actually think that batteries last for years in a shed? They don't.

We have something similar to this: http://www.amazon.co.uk/PORTABLE-STARTER-COMPRESSOR-BATTERY-BOOSTER/dp/B00IDCMLFE/ref=pd_cp_auto_1

I dug it out of the shed after years of neglect, but the voltmeter charge has fallen into the red area and does not move when I test it with the green button. Is it done for, or can I safely charge it up (if I find the cables)?