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diy solar

Charging 48v from 12v

Barbar0ssa

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Mar 3, 2021
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I am planning on putting 500 or 600w of solar on a shipping container with an Mpp Solar 1012LV-M and 4 Coslight 150ah lifepo4 cells.

I am also converting a 36v golf cart to 48v (16 of the same cells) and will be parking in the container.

The obvious way to charge it is an ac-dc charger mounted permanently in the golf cart allowing it to charge from anywhere.

Is there a way to go dc-dc (big smart boost converter - something like this mppt boost charge controller)?

Can a battery be an input to an mppt controller?
 
You do realize that your golf cart is going to have 4 times the storage capacity of the container system, right?

600 watts of solar is not a whole lot to work with. Assuming you get a real 5 sun hours, you get only 3 KWH a day. The battery in the golf cart will store more than twice that. What all do you plan to run off the solar power in the container? How much will the golf cart be driven? Is this just to keep it topped up, or do you expect to charge it after a full day running around? If you try to charge the golf cart at night, it will kill the container battery while only putting 20% or so back in the golf cart.

An MPPT controller could be "confused" if the source is a battery. As it tries to search for maximum power, the current will just keep climbing. If the algorithm is not smart enough, it could fry the input. If you really must do this, I would suggest a simple CC-CV boost converter. Here is one on Amazon that should do the job. (edit. Turns out this converter is limited also to 10 amps input, so 120 watts from 12 volts in)
Dial it for the desired absorb voltage with the output open, and then connect it and adjust the maximum charge current. This one can push up to 10 amps, but you (edit... MUST! set it below 2.5 amps out) 10 amps out would be pulling over 40 amps in. (edit.. that will fry the converter) You should also add a low voltage cut off so it does not kill the 12 volt battery bank in the container.

I think you would do much better to just add another pair of 250 to 300 watt solar panels in series to another charge controller setup for 48 volt charging directly to the golf cart. Maybe skip the container system all together and then have a 48 volt inverter on the golf cart for when you need power. You could also run a 48 to 12 volt buck converter to keep a 12 volt bank charged to run 12 volt LED lighting in the container. It is more efficient to drop voltage than it is to boost. The higher voltage is less current so you can use thinner wire and still have less loss.
 
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That is a great reply, thanks! My primary goal is to learn how everything works so your post was really helpful, especially about the mppt behavior with the battery.

I am ultimately working on 3 solar set-ups primarily to learn. I am going to have a small batteryless system operating a pond aerator while the sun shines, a reasonably small 12v system on the container primarily for led lights, a fan, and maybe charge cordless tools. Then I will take what I've learned and build a 48v system sized to handle the well pump, gas furnace, and sump pumps on the house itself.

So if we concede that it is an inefficient idea at this scale, but explore it just based on "what would happen" - If I use a boost converter is it always drawing or does the current pretty much drop off when the 48v bank is full? (I don't want to overcharge so I assume I would set the voltage to something like 3.3v * 16 = 52.8) Can that just be always on?
 
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