Using solar mppt charge controller as a DC to DC charger

[ofer]

thinking out of the box again, not sure it's always a good thing.

My main engine runs 32V system. i have a fairly substantial 12V system also. when the main is running I'd like to feed the 32-37V into the solar input of the solar charger and then feed my 12V system in order to charge it.

Am I way off?

 

[dhays]

I have no idea if it would work or not. All I understand is that my MPPT controller can take up to a 100v input and put out 30amps 12v nominal out the other end. I have no idea what magic happens inside its little blue box.

The other option would be to find a step down convert to take your 32v input and then send a 12v output to a 12v DC-DC charger.

 

[DavidM]

It should work, as long as it is an MPPT controller and its voltage rating exceedes the alternator output.

David

 

[SteveK]

What David said, both the maximum output voltage and amps must be below the max spec of the MPPT. It should then work. I cannot confirm.
That is same as not adding more panels to one controller than it can handle.
Do post your result as this may help others with more than a 12V system.

 

[O C Diver]

How would you limit the amperage input to avoid cooking the controller?

Ted

 

[ofer]

@Ted not sure. that is one question i have

 

[O C Diver]

@Ted not sure. that is one question i have

Victron makes a 24 VDC to 12 VDC battery charger (Orion), that has a maximum input of 35 VDC. It's offered in atleast 2 output amperages. Before reinventing the wheel, I would send Victron an email explaining that you want to use their product to charge a 12 VDC bank from a 32 VDC bank. I'm guessing this was part of their original design instead of limiting the input to 30 VDC.

Ted

 

[DavidM]

How would you limit the amperage input to avoid cooking the controller?

Ted

With a DC to DC charger

 

[Delta Riverat]

If you know what the max output of the MPPT is then it's easy to estimate the input current. These things are very efficient switching converters and the power out = power in minus the small loss.

If you are 12 volts out and 36 volts in the input current will be about a third of the output current.

 

[ofer]

I checked into it a bit and it looks like the MPPT controller will limit the incoming amps according to its capacity.

my 32V system will produce up to 38 volts. i am not sure Victron will approve that.

 

[ofer]

If you are 12 volts out and 36 volts in the input current will be about a third of the output current.

this is exactly what i am trying to accomplish without frying the mppt controller

 

[Delta Riverat]

What are the ratings of your solar charger?

 

[ofer]

What are the ratings of your solar charger?

i am getting a 100A one

up to 100V to 12V etc.

 

[Delta Riverat]

I don't see a problem then. With 100A into your 12 volt bank the draw from the 32 volt bank should be about 40 amps. Just don't connect both the solar panels and the 32 volt bank to the charger input at the same time.

 

[mcarthur]

I'm using an MPPT to trickle charge our 12v battery from our 48v battery. Works fine as I limit the amperage to the MPPT size. I'm not sure about connecting an alternator through it though - voltage wise should be fine, and amperage should be controllable by the MPPT. The two questions I would have is whether the alternator will degrade properly when the MPPT limits the amps, and secondly whether the alternator goes bang when the MPPT says "I've had enough" and stops charging.

 

[ofer]

@mcarthur.
not sure i understand your comments specifically about the alternator.

also how do you limit the amps to the mppt?

.

 

[mcarthur]

Let me show my ignorance of alternators!
Essentially trawler alternators are usually designed to, and are, connected to the starter battery for a diesel or sometimes directly to the house battery. Externally regulators can take into account LiFEPO4 chemistry for those batteries.
What I was meaning for you, is that I'm not sure what the alternator's internal or external regulator would make of the MPPT controller data (state of charge from the alternators POV). For example, if the MPPT turned off charging suddenly would the alternator fare the same as if a LiFEPO4 BMS stopped charging?
Steve has his usual good information but it's not talking directly about what you are wanting.

 

[ofer]

@mcarthur. my alt is internally regulated. i have 16 8v batts for the 32V system. i think that's is enough absorption not to cause issues. ill install and see. small investment if it doesn't work.

 

[twistedtree]

I think it will work, but you may want to take steps to turn off the MPPT when the engine is off. Otherwise it will always be holding the 12V battery at float, drawing power from the house bank. This is what you get with a DC/DC charger, so might be what you want. But if you want it to only run when the engine is running, you will need to somehow wire the ignition to the enable/disable of the MPPT to turn it on and off. Note that some DC/DC devices do this automatically, only turning on when the input bank (house bank) voltage elevates because it's getting charged by some external source which is the alternator in your case.

Is the MPPT significantly cheaper than a made-for-purpose DC/DC charger?

 

[dhays]

@twistedtree I think the issue is that he is trying to go from a 32v alt to a 12v bank. Hence the MPPT which are designed to take a higher voltage current and output a 12v nominal current.

There are some 24v-12v DC -DC chargers but their max input voltage is right about at the 37v that the alt is putting out.

However, I just wired up a normally open relay to the same circuit that powers the start battery solenoid on my boat. That relay then closes a loop that is connected to my Orion XS DC -DC charger Hi-Lo input. So, the charger is deactivated until the Engine circuit is energized to connect the start battery to the starter. It was a lot easy to use that circuit as the relay trigger than it was trying to get a wire from the ignition switch.

 

[twistedtree]

@twistedtree I think the issue is that he is trying to go from a 32v alt to a 12v bank. Hence the MPPT which are designed to take a higher voltage current and output a 12v nominal current.

There are some 24v-12v DC -DC chargers but their max input voltage is right about at the 37v that the alt is putting out.

However, I just wired up a normally open relay to the same circuit that powers the start battery solenoid on my boat. That relay then closes a loop that is connected to my Orion XS DC -DC charger Hi-Lo input. So, the charger is deactivated until the Engine circuit is energized to connect the start battery to the starter. It was a lot easy to use that circuit as the relay trigger than it was trying to get a wire from the ignition switch.

Yes, good point about 32V being an uncommon input, though I haven't checked what might be available.

And what you have done is exactly the sort of thing I talking about to turn the MPPT on/off with the engine.

 

[SteveK]

thinking out of the box again, not sure it's always a good thing.

My main engine runs 32V system. i have a fairly substantial 12V system also. when the main is running I'd like to feed the 32-37V into the solar input of the solar charger and then feed my 12V system in order to charge it.

Am I way off?

The ALT is charging a 32V bank? As TT said turn off the connection to MPPT. as in remove the jumper between MPPT and 32V bank. I suppose a fet battery isolator may work, ALT to input and then 32V bank and MPPT.