Charging a 12Vdc battery from a 48Vdc battery bank

[Westbranch]

Do you think the max power point tracking could be an issue?

thinking it through, it will get a very small voltage range, say ~48v to 60v.  with a LARGE amp supply.  As said, you will want to have a CB in line to protect the CC ~15a.

Just looked at the MS site and that CC states it will work for 36 or 24 V to 12v, so you might want to contact MS and see what they say about 48V.  It may be the issue of the Voc of a 36v panel in cold climate??

[Rybren]

Erik,

Glad these other guys chimed in.  I could only find one comment (although I know that I've seen more) from BB about using a MPPT controller in this general fashion. He was talking about using one to go from 24V to 12V here...

"The MPPT current limit capability gives us the ability to do "interesting things" with the charge controller... One option that is used quite a bit--You have a 24 volt battery bank and a 12 volt load--Get a small 15 amp MPPT charge controller and a 12 volt battery bank. Connect the MPPT controller from the 24 volt battery bank (to solar panel input) and the charge controller's output to the 12 volt battery bank. You now have a very efficient step down DC to DC charge controller for your 12 volt DC loads."

[Vic]

All,

dgd has it about right.   A power restisor in the range of 5 - 10 Ohms at 50 or so watts would help to protect the CC against high fault currents.

But with the circuit breakers/fuses mentioned earlier,  there should be no danger of fire.

So if one is willing to risk the possible death of the CC due to high fault currents before any protective device operates,   the resistor would not be necessary.  Most OCPDs protect wires/cables,  not  other hardware,  as you know.

I know nothing about the MS MPPT CCs,  so I do not know how it may behave when the 12 V starting battery's voltage drops during cranking.

Perhaps am too conservative.  But hate to see hardware get toasted.

Good Luck,   Vic

[SolarMusher]

Fisrt, thanks to all for your invaluable help!
I just realize that I'm talking with electronic fanatics  .
Still a few questions for you guys:
Would this power resistor draw 50Whr to 250Whr permanently? I mean, even when the SunSaver is in Float?
The last time I've used this kind of resistor, it was to divert load when I mounted my first wind turbine and it was using 100% of the current that was generated from the turbine. Is it the same in this case?
I was thinking, maybe a little 1A to keep this battery on float and something like 1A max to supply the Kohler control board, so 20/25Whr total.
The 48V bank is 800Ah, so a 500W/day load (with a 2.54kw/hr PV array) wouldn't be an issue when the cottage is left unattended, but more than this wouldn't be realistic with snow in winter. Of course, the SS would be mounted with 15A breakers in and out.
Erik

[Vic]

Hi Erik,

1.  I have not checked the cost of the SS 15 MPPT,  but the resistor would be in the circuit as an attempt to help protect the SS from large fault currents that MIGHT possibly exist when something goes wrong.  It would just reduce the maximum current that the SS could "see".   It would probably reduce the maximum charge current into the 12 V starting battery,  a bit,  as well.  This would depend upon the resistance of the resistor,  the headroom of the SS CC and the voltage of the starting batt.  The power dissipated by the series resistor would be the current through the resistor is the voltage drop across the resistor times the current flowing through it.  When the start batt is fully charged,  this current should be very small,  and drop across the resistor is small.

2.  The resistor would dissipate only the heat (and power)  resulting from the current flowing into the input of the SS CC.  When the statrting battery is fully charged,  this current is very small,  and therefore there would be a very small drain on the 48 V bank.  This current should be much less than that of an inverter,  which is another way do the job of tending the starting battery.

3.  All that would be required by the Codes that I know of,  is for  input and output Fuses or Breakers,  sized to protect the wiring to/from the SS CC.

As was suggested earlier,  you might contact MS Tech support to ask about their feelings for such a use of their CC.

It is possible that the designers of this SS CC "knew" that the maximum current available from the PV input source IS limited to the Isc X safety factor of any reasonable string that might be connected to the CC.   A battery could have hundreads of times this amount of current available,   although,  the larger any fault current (from the battery),  the more quickly the OCPD would act.

If the suggested resistor had a high power rating,  one would not need to be concerned about the heating in the resistor due to the current flowing into the SS CC,  and so on.  Keep warm,  Vic

[SolarMusher]

Hello Vic,
Sorry for the delay but I was very busy with dogs/custommers for these past two days  .
I've another idea that could be better/safe. I had bought a PPT48-10A Rxx (SolarConverter) that can step down at fixed/adjustable voltage (17V), I would like to give it a try with a cheap PWM SunSaver 12V/6A or 10A (-50$) mounted inside the generator.  The PPT48-10A has to be mounted with 10A breaker/fuse on its input. This could colve eventual resistor or MMPT issues, I think...
My 24V fridge and freezer have worked flawlessly on one of these units for the last 5 years, the SS cc should work the same way.
What do you think about that?
Erik

[Westbranch]

comparing the 2 CC options I agree with using an MPPT unit, as it would do he  better job of 'maintaining' the battery charge levels.

Too bad you can't eliminate the battery but I sure don't think 10A will be able to start that gen.

What is the price difference between the 2 options (48 vs 12 v)?

[Vic]

Hi Erik,

Had previously just looked at the Morningstar site for info on the MPPT 15.

Just now went to the wind-sun store to find the current pricing.  It is a bit expensive,  but a cute small MPPT CC.

The Wind - Sun blurb mentions the very use that you are looking for,  with no disclaimers regarding the issue that concerned me.  So appears that my initial concern about the huge current available on the input of the SS CC  is most probably unfounded -- that is,  seems probable that Morningstar is not concerned about this  issue  for this (your) application:

http://www.solar-electric.com/mosumpsochco.html

Seems to me that the DC-DC converter into a PWM CC which has an RTS connected should be fine.  It is possible that an MPPT CC might have better precision in choosing the exact 12 volt battery charge voltage,  but this level of precision may not be too important for a starting battery.

And Wb,  would bet that the peak starting current on that 12 V battery could easily be in the order of 40-50A for a short time ... depends mostly on the starter motor.  IMHO the DC-DC converter would stall at the very best.  It is possible that the DC-DC would just limit the current when the starter  begins cranking,  and recover nicely when the durrent demands are reduced to within the output spec of that converter.  ... All just opinions.  Good Luck,  Vic

[Westbranch]

Vic, I agree, I was thinking of battery amps being up to as much as 100a,  my Quad battery is rated at 12Ah, 10 hr rate and probably way smaller than the starter battery , more like a car battery.