Is it possible to build a curcuit that can charge a 12v battery from a 48v battery bank with minimal losses but still be capable of delivering up to 100amps @ 12v nom? To me the current control seems to be the killer, the voltage from a 48v bank while charging is up around 60v, trying to limit current flow to a stable 13.8v would produce a lot of heat or there a better way?

T1 Terry

100 amps. Wow hats alot of power.

Use a BUCK converter, or even better a push-pull, using a PWM controller like a tl494. do some current sensing on the output using a high power shunt, feedback to the control circuit.

You will need 2 control loops, one for voltage and one for current.
It seem like what your after is a CC-CV charger.

TonyB wrote:100 amps. Wow hats alot of power.

Use a BUCK converter, or even better a push-pull, using a PWM controller like a tl494. do some current sensing on the output using a high power shunt, feedback to the control circuit.

You will need 2 control loops, one for voltage and one for current.
It seem like what your after is a CC-CV charger.

I have very limited electronics knowledge so some of that went over my head. My thoughts were to connect up the house batteries for a 48v bank and use it to maintain 13.8v in the start batteries so all 12v dependant accessories can run from there. Would such an item be expensive to make?

T1 Terry

Are you 'making' 240v AC from your 48v DC? If not there are plenty of options to do so.

Add an off the shelf 240v smart charger.

Or perhaps smarter and more efficient; use your HV BZ solar controller between the 48 volts battery string and 12 volt battery. Set the 12 v float voltage set point as required.

It's good for 100 Voc.

This one 68volts and 60A

http://www.atensolar.com/m5_view_item.html?m5:item=1388

It's the limited current flow that's the problem, I could use the PL20 but it's limited to 20A, the battery bank has a few more than that The PL20 just trying to control a 40amp input at a peak 20v gets it so hot the LCD display disappears and it strts to smell a little . I thought of using an inverter/charger system but the losses are very high, doesn't really matter if you are doing it from the start batteries to house batteries while driving because the difference in fuel economy to cover the losses is hardly noticeable. but the losses from a house system that's only recharged from solar need to be far less.
The reason I am looking at possibly converting the house system to 48v is that connecting the house batteries to a 12v configuration has real problems with balancing them, AGM's in particular. Flooded cells don't have as much of a problem but AGM batteries have very low internal resistance so 5a or 50a can be pulled out with very little drop in terminal voltage so unless the cabling resistance is perfectly matched across all the batteries in the bank the one with the least cabling resistance will be flattened first, then a chain down the line. The battery capacity is greatly reduced (Peukert’s theory) and discharging below the magic 50% capacity happens quite unexpectedly, only the very expensive AGM batteries can handle that sort of treatment but recharging becomes an even bigger problem.
If the nom voltage of the bank is raised the cabling resistance becomes a much smaller factor in the calculations. I believe 72v is the magic figure but at a charging voltage heading toward 90vdc it's a little too dangerous for a mobile application. Getting connected across 24v with wet hands causes enough excitement, 72v could well be fatal and if you have ever seen the results of a loose/dirty battery terminal on a 24v starting system can you imagine the carnage at 72v when an inverter load hits

T1 Terry