Lithium 18650 battery bank , reduce voltage ?

[KD0CAC John]

I am collecting old 18650's to start to experiment with using on medium size and then maybe something like home-brew Tesla power wall .
The mid size is for ham radio gear , most of that runs on 13.8vdc & to maintain high RF output , many rigs should have a steady voltage close to 13.8 .
Then putting together a 18650 block , so using 4 in series & maintaining , lets say 13.6 , a fully charged 4.2 x 4 = 16.8
Image result for 18650 state of charge chart - not sure if the copy/paste is going to work ?
But keeping the battery block from discharging too much to extend battery life and trading off with that to maintain useful voltage .
The main question is controlling voltage high point [ 13.8 ] WITHOUT dissipating capacity [ using resistor ] ?
I am guessing some type of DC-DC auto regulator .
A side note , I have been using solar for decades with Trojan L16 x4 , 5000 watt inverter / charger and now around 1,200-1,500 watts in solar panels , the FLD - flooded lead acid batteries last over 12+ yrs. and need replacing , and now about double the cost $1,000 - $1,400 .

[KD0CAC John]

Was hoping to shorten the shopping , there are lots of DC-DC converter out there , so many that trying to find something that fits gets lost in all the options and seems like what I am looking for is not all that common , I am not stuck on but have a good inverter now 12v .
So with building a lithium battery starting with 4 in series gets [ depending on state of charge ] 16.8 to just above 12v .
And the min. draw I would like would be 25-35 amps at 13.8vdc .
Then latter on my full-time RVing , up to 4-5kw peak and min. continuous 1,500 - 2,000 watts .
Also helping a friend that is looking at similar , but ready to start at a 24v battery system , but has 12v [ 13.8 ] loads - regular RV stuff , plus ham radio - this is where the 13.8 min. 20amps is ?

[Inverted18650]

You can use an LM338 (a few in parallel) to build your own voltage regulator with the current draw you need. I recommend a 24V pack and buck it down to your needs. If you don't want to build it. you can buy the "Uxcell" branded high powered DC-DC buck boxes from Amazon.I have no real complaints about them. They make a 24-to-12VDC  @100Amp box that sells for about $110USD. They have them 2-/40/60/85A as well. To be clear, I am not promoting the brand, just saying I haven't had any issues and I use them to run a small solar bank straight into my Turnigy Reaktor Quadcore 1200W battery charger to ensure smooth & steady power.

Building your own is the best route because you can tailor it to your exact needs. There are a bunch of great schematics available, the best of which is directly on the datasheet for the LM338, and itll be cheaper than buying a prefabbed box.

Cheers.