Electronic – Need help designing an airplane legal battery pack

The general strategy I would use would be to always directly power your device from the internal batteries. The only question then would be when to use car power to charge the batteries, and likewise for the solar.

For the car power, you need a way of detecting when it is present and the alternator is running, and then to switch in the charger. In this case, I would use a comparator circuit to detect whether the external car voltage is above or below 13 V. Most car voltage regulators are set to around 13.6 - 13.8 volts when the alternator is running, which is more than a 12 V lead-acid battery will produce on its own. Even if you do erroneously take some power from the battery if it happened to exceed 13 V, the battery would have to be very full for this to happen and you would stop automatically as soon as it got discharged a little.

As for the car power charging circuit, you can probably get away with a Schottky diode, P channel FET and resistor in series. Size the resistor so that the batteries are not charged too aggresively, and then have something watching their state of charge. When your internal batteries get full, this system disconnects the charger until they get a little discharged again.

The solar panel is a little easier. You can size it so that even in full sun it can't supply enough current into a 12 V load to charge the batteries too fast. You can therefore just connect it to the batteries with a FET as a switch and a Schottky diode. You can leave the FET on whenever the batteries aren't fully charged.

It doesn't work that way. One of these two issues will get you:

1. You "spilt" the original batteries into smaller batteries, maintaining the overall battery capacity. The smaller batteries now have lower capacity and lower maximum allowed charging current. If a original battery can be charged at 1 mAh/minute, then a battery resulting from a 1:2 split could only be charged at ½ mAh/minute.

   With the batteries split to conserve overall capacity, a larger fraction of the whole volume would be taken up by interrconnects, so the overall battery pack would now be larger and a little heavier.

2. You use more of the same size batteries. Each can now be charged at the original rate, but the battery pack has become proportionally heavier and larger.

   The cell phone maker has already picked a tradeoff between size and weight on one side and longevity of charge on the other. For the same battery technology, you can't cheat this tradeoff. By adding more batteries, you increase longevity of charge, but you can't get away from the proportionally larger size and weight of the resulting battery pack.

If it were really so simple, they'd be doing it.