Cell phones in remote places with flaky power;
If you use one of these
You probably lose your cell phones and internet the second the power fails.
So this project.
Yes it's just laying there, but that will run a cell booster for 8 hours and swapping out the 18650 cells from the spring battery holder is 8 hours more. This compares to 5 hours with a UPS and a truck battery.
Now what it is;
the small board with a green light ( full charge indicator ) is a combo battery charger and power booster. I feed the 4.5 volt supply straight into it and it charges the battery pack to 4.2 volts.
The black supply is the original power supply for the booster, wired straight through, so when there is electricity it is supplying all the power to the cell booster ( about 5.1 volts ).
Now the slight trick. The booster board puts out 5.3 volts, and so would be the power source all the time, and discharge the batteries, so I added a 3 amp fast recovery diode in series with the booster output as I am too lazy to tweak the regulator to exactly 5 volts. End result is 4.8 volts. Since everything has a rectifier output the only thing determining the power source is the relative voltages, and I tested the booster and it's happy over the supply range.
So no drop outs, the supply is drawing apx. 1 amp from the 18650 pack which has 8 amp hours of storage, and this takes very little space.
I have a similar circuit on the cordless phone base but just floating a single supply as it only draws 150 ma so the charger can handle both the load and charging the battery pack.
A couple of charge - boost boards.
depending on if one amp or two amp loads.
The smaller is fine for a 5 volt cordless phone base ( or a 5.5 if you change a resistor on the regulator circuit, be aware it's surface mount, don't breath too hard ) . The booster is better with the 2 amp board.
are a good idea too especially if left unmonitored. Battery protection for over charge, over discharge .. they suggest one per 18650 cell, but on a low demand circuit having one on a 4 cell 3.7 volt pack should be fine.