I have a rapid charger and a regular USB charger. I let my Playbook get so low, it wouldn't turn on or charge. Since the charging system is software controlled, the playbook can get so low the software won't run to charge it. I had to "stack" charge it, which means putting the charger on for 10-15 seconds, taking it off, putting it right back on for 10-15 seconds, taking it off, and repeating this process about 50-100 times. The charger will charge for about 10-15 seconds before it shuts down because the Playbook software isn't taking over the charging process. Until the playbook gets enough charge to run the charging circuit, it won't take a prolonged charge. By repeating this process of "stacking" all of these little charges together, you are able to build up enough charger for the Playbook to run the charging circuit software to fully charge itself.
Another time, I let my Playbook get too low and hooked it up to my desktop and was told I needed to download some software to use it. I said "ok" and it began downloading the software which would take about an hour. The problem was the USB connection to the desktop would not provide enough power to run the Playbook in order to download the software AND charge the playbook at the same time. The playbook would just run out of power and shut down in the middle of the process. I had to hook up the rapid charger and the USB cable to do this; and it worked fine.
Hope these help.