48 volt bank to power 12v loads?

Re: 48 volt bank to power 12v loads?

Search Google for: "dc to dc power converter 48/12 vdc"

And you should find quite a few... One of the best (at least low power paper specs.) seems to be this 15 amp output unit (no price listed--may be scary).

What I like about it is the 65 volt maximum input voltage (and 35 volt low)... Most of the others are 60 volts peak (may be an issue or not for your system)--and others that are only 50 volt peak (not acceptable).

The converters are probably around 85% efficient. Probably beats a good Inverter/12 volt converter combination.

Normally, wholesale pricing should be in the $0.50 per watt; Retail would be 2-4x the cost (for any standard function/quantity production converter).

Vicor is a very good/high quality mfg. of "power bricks/converters"... They where always to expensive for my applications--but perhaps you can track down one for your use (telecom spares, scrap, over-runs, etc.). When I lasted looked at them a couple decades ago--they did not stock product--but built to order (automated line).

Digikey.com has a nice 100W DC to DC converter with the following specs for $180 (even has remote sense for output votlage--very nice option--I wish the solar charge controllers had this too):

Voltage - Input 18 ~ 75VDC
Output 12 VDC @ 8.3A

I wonder if you could take a, for example, a Morning Star 15 amp MPPT charge controller--connect it to the 48 volt bank and throw a storage battery (AGM would be nice) on the output. Seems like it should work OK at a reasonable price (~$260 + batter cost).

-Bill

Re: 48 volt bank to power 12v loads?

If your main array is already running on a MPPT type controller, then you should not connect another MPPT controller in parallel to the same solar array.

Next, the Sun Saver has a maximum of 75 volts on the solar input (as I recall). If you have a Vmp=65 volts array--Voc would probably be too high (cold weather, no load, etc.).

I would check what the "standby current" of the MorningStar 15 amp MPPT controller with the battery bank on standby--it may end up the same or less vs just using a 48/12 converter anyway.

If you decide to place the MorningStar MPPT directly on the 48 volt battery--definitely add some sort of fuse/breaker to protect the controller and wiring:

• 15 volts * 15 amps * 1/42 volts min * 1.25 safety factor = 7 amp or next size up (8-10 amp fuse and wiring). I assume the MorningStar is rated for at least 15 amp continuous when used with solar panels--So 12 awg wire and 15a*1.25=20a maximum fusing for "safe" operation

One other issue to watch for--some MPPT controllers place a shunt resistor in negative side of the solar panel input--so you cannot have a common ground between Vin and Vout for the charge controller (Xantrex XW MPPT controller is the one I recall as having this negative shunt resistor). So for those types of controllers, you will not be able run a common negative ground between the 48 volt and 12 volt battery banks (one of the banks will have to be floating with respect to ground--probably the 12 volt slave bank).

I don't remember the MorningStar as having a negative sense resistor. From the PDF manual it appears that any grounding combination is OK:

NOTE: The SS-MPPT is a negative ground controller. Any combination of negative connections can be earth grounded as required. Grounding is recommended, but not required for correct operation.

One recommendation on grounding--Be careful that you home run the 48 volt grounding separate from 12 volt loads/grounding... You don't want (the probably large) 48 volt return currents going through the MorningStar MPPT solar/battery negative connections.

-Bill

PS: I have never done anything like this type of connection--But Solar Guppy has suggested this configuration before (if I recall correctly)--so hopefully he can confirm/correct my assumptions.