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

Light24bulbs

New Member
Joined
May 29, 2020
Messages
1
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. https://www.amazon.com/gp/product/B...rd_w=44xm7&pd_rd_wg=lIyaw&ref_=pd_gw_unk&th=1

This has been posted before, like here, https://diysolarforum.com/threads/t...oller-function-with-bench-power-supply.11183/ 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?
 

pollenface

Solar Addict
Joined
Nov 14, 2020
Messages
457
Location
Perth, Australia
I recently connected a 24v 5a power supply to a 40a mppt and the power supply voltage kept getting pulled down to ZERO.

But in another instance, I used to connect an 18v 1.5a supply to the mppt input on my vehicles dc-dc charger while it was parked in the garage to keep the batteries maintained and it worked perfectly.

If it's possible for you to set the max current of the mppt or use a device to limit the current between the power supply and the mppt it might work (I don't anything handy to try).
 
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Synopsis

New Member
Joined
Jul 21, 2021
Messages
6
I use a Victron 75/15 with a AC power DC power supply at 24V, attached to the solar input, to charge my 12V banks - have done for years - essentially works as a DC/DC converter. Main thing is that the DC power supply needs to be at least about 4 volts higher than the voltage you are aiming to charge at.

I have zero problems with it. Just make sure you don't pull more amps from the DC power supply than it's rated at - so for example the maths looks like this:-

Voltage MPPT at Absorb - 14
Voltage DC Power Supply - 24
Maximum Current MPPT - 15
Maximum Current DC Power Supply - 10

Voltage DC Power Supply/VoltageMPPTAbsorb*MaximumCurrentDCPowerSupply=17.14 - this is the theoretical current your DC power supply can supply to the battery once it's down converted.

Since 17.14 Amps is much higher than the maximum mppt output (15 Amps), I'm good to go. If I wasn't, I could always set the current output of my charge controller to match.

This is not a 'sanctioned' use of victron charge controllers, but I've never had an issue with it.
 

NPhil

Freezing in the dark non-enthusiast
Joined
Apr 23, 2021
Messages
130
I've used a cheap Chinese DC-DC boost converter (with current limiting) to charge from the 12V system to the 24V (house) system by connecting it, through a Schottkey diode, just in case, to the solar input of my Epever Tracer AN 40A controller. I set the output voltage to 30 because it's about what I was used to seeing from the panels, and limited it to 5 amps (the converter is supposed to be good for 20, but I doubt it is) output, which evidently doesn't go over 15 (the size of the fuse on that cable) from the input. It seems to work without any spitzensparks.
 
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