If you can charge via the discharge port of the battery (which is pretty common these days) then all you need is a splitter cable so that the battery has two power legs. One would go into the controller permanently while the other would just hang around to be plugged into a charger.
As battery side connections are typically female, you'd just need... looking... something like this
As for "Like I said - already tried that and fried a battery" that appears to have been a response buried in a deep thread, so it's pretty easy to miss. Regardless, if you set this ablaze, either your battery isn't (wasn't, I suppose) actually ok to charge through the discharge port or the charge plug was wired in some incompatible fashion.
If the battery you currently uses can be unplugged from the controller and then be immediately plugged into your charger, a Y split will work. I've wired several batteries in this exact method; including doing this approach for dual paralleled batteries. It works when done correctly.
Going to try this: I will update after installation.
https://www.amazon.com/DIY-Lithium-Batteries-Build-Battery-ebook/dp/B06XRKD15B
This book is super helpful!
Take your time. If you learn properly you will save both time and money in the long run :)
The battery pack would be used for a radio that runs on 7.4 volts (so yes to being in series) and draws a max of about 2-3 amps from what I know. I would pull the 18650s out of the radio for charging in a dedicated charger I have for them since I don't know how to make a charging circuit for them. The radio will automatically shuts off it the voltage gets low enough so that should in theory keep my from discharging to low.
As far as connecting the circuit I was planning on using something like this and then soldering wires from the contact to the BMS (If I understand what you are saying) and then from the BMS to the actual output of the battery pack.
I got this one https://www.amazon.com/gp/product/B08P5SFV4D/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
It works well like i mentioned before. It goes as low as 9% or so before my 40 quart setpower fridge errors out with low voltage cut off.
I charge the battery with a 100w renogy flex panel on the roof of my truck topper. What sucks is that no matter what the generator bank cant charge at anything over 55 watts. I am still pretty happy with the generator overall tho.
Buy something like this power supply and connect the + and - to the + and - of the fridge, ideally with a properly rated connect. XT60s are a cheap and popular option, but you need to know how to solder.
Can I be a pain and ask one of you if I can wire a DuraPro 7.4V 5500mAh battery GSP0931134 in series to get 14v? Will that still result in a 5Ah battery?
they are generic power tool batteries. https://www.amazon.com/gp/product/B07KW5KGJ3/ref=ppx_yo_dt_b_asin_title_o07_s00?ie=UTF8&psc=1
this is were i got them
So your pack max discharge will still be 3C but since you're saying you would have a each series in 5p, that would mean you would have 5 cells at 2.9Ah =14.5Ah so your max discharge would be 3 times that in amps, so 3x14.5 = 43.5 Amps continuous(make sure it says 3C continuous). If you have 6 series you're going to have 6 x 3.7 V =22.2V. Your pack would be 14.5 Ah at 22.2V for a total of 321.9Wh
Is this the controller you're looking at? http://vedder.se/2015/01/vesc-open-source-esc/
I think you'll be fine as far as the battery pack, you can expect 16-20 miles of range with conservative use. My battery pack is 3C and when I'm at full throttle sometimes I notice some power loss, so that's something to consider. The higher the C rating the better it will hold up even as the charge starts to get low. As long as you have the know how programming that, by all means. Be aware of voltage sag with lower C rated batteries, it can be tricky. I've always been interested in doing an RPI/ESP8266/Arduino battery monitoring thing, I'm definitely interested in helping you out with that if you follow through with that. I have some voltage sensors for my Arduino and what not.
Edit: they also have these things called voltage buzzer or beepers, but you just hook it up to your battery pack with a standard balance cable and once your battery pack reaches below a set voltage it beeps, and they're dirt cheap, I'll find you a link.
Edit 2: https://www.amazon.com/Battery-Tester-Voltage-Buzzer-Alarm/dp/B005GJCJOA