Revisiting the Tuna Power issue, I recently purchased a 6AH LIFEPO4 battery that claimed it was a drop-in replacement for an SLA battery. Of course, the charging circuit of the Tuna Power is designed only for the SLA, but it won't hurt or overcharge the LIFEPO4 lithium battery for two reasons:?
1) The LIFEPO4 has a BMS battery management system that will cut off the charge once it has determined it is FULL or ~14.6 volts.
2) The charge voltage of the UC3906N as hardware programmed is actually only sufficient to change the LIFEPO4 battery to approximately 66% at 13.8 volts. So overcharging isn't actually likely.
The charging input voltage is asking for an 18-20 volt wall cube which is the same as a solar panel with peak power at 18 volts and an open circuit voltage of ~20 volts. I have a 20-watt solar panel that outputs a 1-amp peak in the ideal sun with afternoon temps in the "cool" range. This seems ideal. Any more and the 1N4004 Rectifier diode would need to be upgraded. A 1.5 - 2-amp suggestion? At one amp that is a good idea anyway just for the margin of cooling safety.
So for a long weekend of camping and Tuna operating, the solar panel could extend the operating time of the 6 AH battery and a conventional LIFEPO4 changer could be used on the disconnected battery after the fun is packed up for the weekend. It seems that the T.P.S. hasn't been passed over in technology quite yet!
Stay tuned for a video on it, of course!
David Knapp