Adapter for 12 volt DC > 4 volt DC
We use a Jetpack (Verizon internet connection device)in the RV and usually leave it turned on all the time and being recharged by the RV 12 volt system. The Jetpack lithium battery (18 months old) sw...
Forum Discussion
The Jetpack has an internal power supply with its own regulator. The battery is certainly not connected directly to the 5V or whatever input. I would guess that the electronics operate at 3.3V or possibly a lower voltage (for the digital parts, at least). The analog/RF parts might have multiple supply voltages generated internally.
In general, any device with a Li-ion battery is not going to have the external power supply connected directly to the battery, as these batteries require some care and control in charging and discharging to prevent fires and other problems.
As for a supply for the Jetpack, there are many DC voltage converters and regulators available, and a bunch of reasonable suggestions have been made. The old school approach would be to hook up something like a 7804 (fixed 4V) or an LM317 (adjustable) voltage regulator, although these devices aren't particularly efficient as they're linear voltage regulators. You'd need to have an adequate heat sink for them.
In general, any device with a Li-ion battery is not going to have the external power supply connected directly to the battery, as these batteries require some care and control in charging and discharging to prevent fires and other problems.
As for a supply for the Jetpack, there are many DC voltage converters and regulators available, and a bunch of reasonable suggestions have been made. The old school approach would be to hook up something like a 7804 (fixed 4V) or an LM317 (adjustable) voltage regulator, although these devices aren't particularly efficient as they're linear voltage regulators. You'd need to have an adequate heat sink for them.
