Learning the Art of Power Supply [4robot]

I've done some servo stuff, made a little walker who 'most certainly does not even lift' and researched many things in that vein. I've accepted the fact that my little robots will require a battery 'pack' which I've managed to get going alongside another battery line for my 3v stuff and while I initially wanted to solar charge BOTH those, I've had difficulty since I was chaining my servo batteries in Parallel for 3.7v and longer life(sort of). Problem was my setup was a lil 3.7v charge/use unit leading to a booster to push up to 7v. This had the obvious problems, lost much current and couldnt push the servos much.

So today I stacked the 2 lipos in Series to get my 7.3v and fed that direct to the servo controller. Hoo boy, now thats more like it.

I now realize, my charge unit works for 3.7v. So I can make this 'pack' but now I cant charge it while in use. Unless.... do they make the same circuit in 7v for such a platform?

I will still be able to easily charge the separate mcu since it will only be 3v etc.. but how to charge the legs branch/pack?

I also lost my little booster too, since its also specifically for stepping up a 3-5v input. It seems clear to me that while I can call my little 3v section a fine little recharging power supply, that I will likely have to figure out a 7v replacement or devise my own power supply/charge apparatus.

Problem is my knowledge of power supplies is not up to snuff.

Would appreciate any suggestions, I'll go ahead and pre-promise to be careful and safe in my experimentation. I swear I'm not a super noob, I just dont usually build power supplies. Any links suggested will be read!

Edit: was a bit vague, I'm using 2 lipo 3.7V batteries in series, my 3v stuff is fed also from a lipo to a charging board:
( 2A 5V Charge Discharge Integrated Module 3.7V/4.2V PCB Board Module )
and from that outputs ~4-5v to my booster:
( Eiechip dc to dc Step up Boost Converter ).

Since my servo circuit is 7v I cant use any of that stuff to charge it(no boost needed anymore). I look at these units and I'm guessing I can build a similar circuit and I want to learn more about what those circuits typically include.