On Sun, 13 Jul 2014 03:41:38 -0700 (PDT), Michael Mud
wrote:

I went out to buy a charger for one of my digital devices.

I'll assume your unspecified digital "device" uses a Lithium Ion
battery. It would be helpful if you disclosed the nature of this
mystery "device". Hopefully, it's not a charger for a stolen cell
phone as many aftermarket chargers are sold for this purpose.

All they could sell me was a FAST CHARGER. What is the difference
between that and a regular charger?

A fast charger charges quickly. A regular charger charges slowly.

Both chargers have some method of determining EOC (end of charge)
which is helpful for preventing the battery from burning down your
house or melting your mystery "device".
http://batteryuniversity.com/learn/article/how_to_measure_state_of_charge
A regular charger just slowly charges to the EOC point, and quits. A
fast charger usually is a 3 stage charger, with 3 stage charging.
http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries

Does a Fast charger shorten battery life? TIA.

Only if it screws up and overcharges the battery. I recently had an
RF balance charger do exactly that to a quadcopter battery.
Fortunately, all fast chargers use switching power supplies, which go
to zero output when they screw up, thus saving your battery from a
premature demise.

Leaving the battery at full charge, and running it hot (like inside
your mystery "device") will kill a LiIon battery.
http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries
http://www.mpoweruk.com/lithium_failures.htm
Newer laptops have a setting where you can charge to about 50% should
you need to leave the charger plugged in 24x7. However, cellphones,
smartphones, and media players usually do not have this feature.

However, I wouldn't worry much about battery life in your device. The
average lifetime of a common cell phone is about 18 months. 30 months
for smartphones. About 4 years for laptops. Your "device" will be a
candidate for eWaste long before the battery dies. That's one reason
that Apple embalmed the battery inside their "devices" and made them
almost impossible to replace.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558