Using an MPPT controller to charge from an AC to DC power supply: I need a sanity check here guys


New Member
May 29, 2020
When I built my electrical system in my school bus, I built an 8s lifepo4 pack and got a 60a MPPT controller(thanks for your videos Mr. Prowse) and everything was great. But then, I wanted to connect to shore power. I realized I should have bought a charging-inverter on aliexpress because of the amazing all-in-one features, but I didn't even know they existed. Ah well.

So now I had the job of building a 110v charging system. I scored a 110v to 48v 1000w DC power supply on ebay for $1. Alright, now I just need to go from DC to DC. Now here is the weird part.

I looked for HOURS for a lithium compatible charger that could charge between two arbitrary voltages(48v input to ~24v output) and they basically don't exist(or are like $800), at least not for "house" batteries. I ended up with charger designed for hobbies/RC that works very well and gives up to about 800w, except for the limitation that the firmware just isn't designed for a permanent installation. When the battery is full, it's full, and the system shuts off. No way around it. Also the fan is tiny and loud and I can't swap it for a noctua like I did with everything else.

And then it hit me, there are hundreds and hundreds of solutions designed to take arbitary input power and work continuously...for solar. So why don't I just buy another MPPT, 40a this time, and install it between my PSU and battery. This one looks nice and I honestly like that the cooling is passive because the duty cycle will generally be low.

This has been posted before, like here, and people always say you will blow out the supply because it will overdraw it. Except that, and here's the mind blowing part, you can just set the current limit to less than the supplys rating. Duh. And a 40a would put me just under the 1000w max at 24v so it can't even blow the supply if I set it to maximum or it forgets its settings. The "MPPT" part of the charger will be useless as it will always be at 100%, but I'll be using the DC to DC voltage conversion part of the charger which is what I really need.

I'm just not buying people saying things about the wave form. This is a clean, switching power supply with big capacitors. It's a flat 48v.

Talk me out of this one. Does Prowse use bench supplies to test his controllers?