I know. I know that mixing these into one pack is a bad idea. I do read warnings about it, but I have not found a good explanation. I do want to know the details of what would happen during charge and discharge cycles if I added a 12V SLA to a pack made up of 16 LiFePO4 15Ah cylindrical cells.
Lead acid will add more weight. New lead acid lasts about 100 cycles vs life 3000 or whatever high number they have.
Ah of lead acid are measured by C1/20 meaning if your application dras the lead acid faster you won't get the ah hours stated on the lead battery.
A charger will charge until it reaches the chargers set voltage and will not help you with balancing your pack for you
If you want to use then maybe read, watch more videos and meet local or not local people doing this things already.
If you put in another serie of voltage the average voltage of all series connected will go down and one can get more life of your battery thus using only a lower percentage of full charge.
For your information I used a lifepo4 cell as a security, as a buffer for my 20 series li-ion battery so that when my battery was charging it would eat the voltage at the higher level so that the rest of my battery didn't go above each cells voltage threshold. (lifepo4 charges at 3,75v then goes down to 3,45 after sitting)
I replaced my buffer with 1s65ah lipo for the same reason and after a while of use, I guess, I heard a pop and one of the lipo cells broke and I had to remove it and remember that they aren't as safe as one cell type. There are safe lipo types also but I don't know so better not use it to.
What I did learn. Better use only one type of cell.