12V and 24v Series + parallel config

[jacpet]

Hey,

im really lost on this and would love some input

i am running an 8 cell, 24 volt LIFEPO4 280 amp battery on my sailboat with a JKBMS that has an active balancer that can do about 2 amps.

originally i wanted to use a DC-DC converter and have a 12 volt setup for small things that i can't run on the 24 volt system.

but then an american captain told me in the states it's common to run in parallel and series to have both 24 volt and 12 volt without a DC-DC converter

my thought is to have 2x4 cells in parallel (no bms) for 12 volt and at the same time run all 8 in series for 24 volt. the bms would be on the 24 volt side and all charging and heavy loads (above 10 amps) would go through the BMS 24 volt system.

the question is can i do this, are there some unintended consequences, do i risk shorting out the battery?
also if there is anyone who knows what a setup like this would be called since i can't find anyone doing it, even though it seems like a great idea more efficient, and i get to save a bit of money on a DC to DC.

 

[littleharbor2]

Can't have your cake and eat it too.

 

[chrisski]

Please include a diagram of what you want.

I think you mean make two 12 volt batteries and put in series for 24 volts Then pull 12 volts off each battery.

This would work, but the risk would be unbalancing the batteries, which is not good for charging or battery life.

 

[jacpet]

Something like this, sorry for the poor drawing. im hoping the active balancer will be able to keep the balance in the long run, since the 12 volt loads will be so small

 

[mikefitz]

do i risk shorting out the battery?

As you have described, yes, you will have issues with battery charging , battery balance and most likely, damage to the battery and/or loads.
As its a sailboat , safery and security of the electrical system is critical, especially if the 12v supply feeds radios and navigation equipment.
I suggest a DC to DC charger, 24v to 12v, feeding an auxiliary AGM 12v battery. Thus if the 24 volt system fails or sufferers BMS protection shutdown, you still have emergency power at 12 volts.
Your diagram shorts out 4 cells! The BMS won't help!
Please dont proceed down this path.