Charging a 12Vdc battery from a 48Vdc battery bank

[SolarMusher]

Hi,
I'm looking for a way to charge a 12Vdc battery to start a 14kw Kohler propane generator directly from the 48Vdc battery bank/2.5kwh PV system. This kind of gen is intended to be used for grid back up but is used here in an off grid system. The control board is a constant load that draws this little 12Vdc starting battery down. I've tried to keep it charged with a small 12V/50W panel and controller but too much trees and not enough sun.
So, I'm looking at a small Morningstar SunSaver MPPT which is rated 75Voc 15A 12/24V to charge and float this 12V battery from the main 48Vdc bank.
Could this be done without smoking anything?
Any thoughts/advices greatly appreciated,
Erik

[TomW]

Erik;

If you have an inverter running anyway maybe a simple 12 volt wall wart could keep it charged with a trickle of energy?

Those are dirt cheap and common.

Just another angle.

I am not familiar with the MorningStar but somehow I think a battery bank as an MPPT source might be a Bad Thing (TM)?

Tom

[firerescue712]

I used the Morningstar MPPT15L between from the 24v bank to charge the 12v bank.  It worked flawlessly.  I do not see any problems with doing the same from a 48v bank.  It will only place about 15 amps into the 12v battery. 

[SolarMusher]

Erik;

If you have an inverter running anyway maybe a simple 12 volt wall wart could keep it charged with a trickle of energy?

Those are dirt cheap and common.

Just another angle.

I am not familiar with the MorningStar but somehow I think a battery bank as an MPPT source might be a Bad Thing (TM)?

Tom

Hey Tom,
The Inverter is off when this guy leaves his cottage for one or two weeks, so no AC available at this time. That the problem of these !$?%& back up generator, they need permanent AC source to keep their own battery charged and supply the control board. Even a Generac off grid Ecogen is built in the same way... In winter here, it's hard to let a system unattended with snow covering panels.
I agree, I was also afraid that MPPT could be an issue.

I used the Morningstar MPPT15L between from the 24v bank to charge the 12v bank.  It worked flawlessly.  I do not see any problems with doing the same from a 48v bank.  It will only place about 15 amps into the 12v battery

Hi Fr,
Very interesting,
If it work for you, it should work for him.
has it worked for a long time without problem?
Erik

[firerescue712]

I had that set up for over a year without any problems.  I have since changed out the system. 

[TomW]

I had that set up for over a year without any problems.  I have since changed out the system.

712;

Interesting. Learn something new most every day. I guess old dogs can learn new tricks!

I would have been afraid to risk the equipment trying it.

Only MorningStar equipment I have is a relay driver mothballed for "someday".

Tom

[Rybren]

I haven't tried it, but this method was also recommended to me by one of the mods on Wind-Sun.

[SolarMusher]

Hi Rybren,
Are you talking about the little Sunsaver MPPT?
Do you have a link to this?
Thanks,
Erik

[Rybren]

Erik,

I willl look for the post(s). IIRC, I believe that the recommendation was made by BB.  I don't believe that he mentioned the Sunsaver specifically, but has, on a number of occasions suggested that a small MPPT controller could be used to charge a 12V battery from a higher voltage bank. 

[SolarMusher]

It would be great, thanks Rybren 
Erik

[Westbranch]

Yes that is one of the little tricks that will work, just that most people don think about CC'ing from a battery. 
Just have to ensure the CC can accept the voltage and that one looks good as EQ should not be much over 63V and the CC has maxV at 75 .
E

[Vic]

This is a clever idea,  using an MPPT CC to do the voltage conversion.

BUT,  personally, I would place a power resistor in series with the input to the MS MPPT CC.  PV are not infinite current sources,  as are batteries (in essence).  This resistor would of course be in addition to circuitbreakers/fuses on the input and output of the MS CC.

Also, the starting battery's voltage will probably dip below that speced for the MS CC while the genset is cranking.  The MS CC may reboot during this -- probably OK (?) ... dunno,  have no experience with the MS MPPTs

Good Luck,  Erik ...   Mush!   Mush ...!    Vic

[firerescue712]

Since the MPPT15L was limited to 15 amps charging, I used the Rogue 3024 for a few months to double the charge rate.  It worked fine for a few months.  Then, the display would go crazy.  It seemed to be working fine, though.  Marc, at Rogue, said it had not been tested in this configuration.  I believe if it was powered down every few days for a few hours, it would have worked fine.  I left it running 24/7.  As with anything electrical, breakers or fuses are a must.  I always try to think outside the box.  I reconfigured a  MNE250STM Epanel to run both the 12v and 24v systems in to.  I replaced the AC breakers with DC breakers (took a little modification).  I even had the battery switches in the Epanel. 

[SolarMusher]

Yes that is one of the little tricks that will work, just that most people don think about CC'ing from a battery. 
Just have to ensure the CC can accept the voltage and that one looks good as EQ should not be much over 63V and the CC has maxV at 75 .
E

Hey Eric,
That was my thought too. Do you think the max power point tracking could be an issue?

BUT,  personally, I would place a power resistor in series with the input to the MS MPPT CC

Hello Vic,
Hope you are fine!
You're above my little musher knowledges  , could you explain a little more...
Are you talking of a small varistor? Where can I find it?
Thanks,
Erik

[dgd]

Erik,

I think he is referring to a power resistor in front of the charge controller to act as a current limiting device to protect the controller.  ie   48V bank ---> resistor ----> mppt charge controller  ---> 12 battery
ohms law  input 50v,  MPPT15L amps, say 5  so   50 = r * 5  so r = 10 ohms, watts = 250w = ebay.. #310579430342, better #230866078104   

A varistor is generally used to protect (?) a device/circuit from high voltage transient spikes eg  MN SPD

Although if I was doing this I would be happy to depend on a quality (MN Outback) DC breaker between the 48v bank and the
MPPT115L. 

dgd