A lot of applications have rechargeable batteries build in. Often these batteries are easy removed or replaced, to be able to put in a fresh charged battery, or a new one if the old one is broken.
To recharge the battery several options are used. In laptops it is most common to recharge the battery while in place in the laptop, when a grid connection is available.
The standard external power supply is used to charge the battery, using the build in charge circuit in the laptop. This is fine when the laptop is only used for short periods on the battery, running almost all time on the connected external power supply.
If the laptop is used for a longer time on the battery running out of energy happens frequently. People who use the laptop for longer periods on the battery use a second fully charged battery that they exchange when the battery mounted in the laptop is almost discharged. To recharge the batteries the laptop is connected to the external power supply and the second battery has to be placed back to recharge that battery too.
This is possible, but when one has to use the laptop for a longer time while no external power supply is available, one would like to recharge the empty battery while using the other one. This can be done with a special charger with build in charging circuit, and suitable connector to hook-up the laptop battery.
This can be done a little better. Make a battery pack with build-in charge circuit, and connector compatable with the one normally used to power the laptop externally.
This also adds the possibility to charge the battery with another power source, like a solar panel with simple voltage regulator.
Often this option will improve the usability of the battery powered application quit a lot. When only halve the power of the average use of the application is provided by the solar powered charger the total use time will be 3 to 4 times the time possible with one battery. While the first battery is in use, the solar charger can be used as an external power supply, helping to reduce the battery load, such that the run-time can be about twice the normal time. When the first battery is empty, the second one is placed, and the charger can be used on a sunny place to recharge the first battery while using the second battery in the laptop. When the second battery is empty, already 3 times the normal run time is past, and the first battery is charged half way. Placing the first back in the laptop adds another half battery period, and the second battery can be charged to 0,25 of its capacity. Theoretically this way 4 times can be reached; but at least 3,5 times is very realistic.
So as a proof of this principle we will start building a simple solar laptop charger and build battery packs with build-in charge circuit electronics!
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