AC DC PWM power supply and MPPT to charge battery

[thexformer]

Hi experts,

A new comer here. I have a little bit experience of building small solar system before but this time I am trying to build a bigger system with possibly multiple charge option.

Core components of my setup would be roughly as below:

1. MPPT solar charger 20A 12/24v
2. 8s Lifepo4 batteries of 105Ah
3. BMS of 100A rating
4. AC DC PWM power supply 48v 8.3A
5. Solar panels of 54V 300W

My question is, can I charge the battery if I feed the PWM power supply to MPPT controller? Provided that I will use the inputs separately ( won't use both solar and AC simultaneously) ?

I searched in this forum for a concrete answer but it seems it's a mixed answer. From those answers, my concern is that if MPPT charger tries to pull more current than the supply can provide? If that's the case, then may be the power supply will burn which makes a fire hazard. If someone has real life experience, then would appreciate if they can share the expressive or anyone can explain the logic that what should be impact hence caution or design change etc.

Thanks again in advance. I am already benefited from this forum hugely. And sorry if I have asked a duplicate question.

 

[Mike Jordan]

Your power supply should look no different to your SCC, than a solar panel. Can your SCC handle the output of your power supply?

 

[Pappion]

CAUTION!!! Many SCC are positive grounded (pos in to pos out is hardwired). Many DC-DC converters are Negative grounded. It's Short City, no regulation thru SCC.
Of course, some SCC are negative grounded, and some power supplies are floating. Best to grab your ohm meter and start checking.

 

[Whinny]

Just did it testing the voltage drop across 65' of #12 landscape wire......
Just don't overcurrent the controler. I have done it with MPPT as well...

 

[Bud Martin]

Just did it testing the voltage drop across 65' of #12 landscape wire......
Just don't overcurrent the controler. I have done it with MPPT as well...

4.96VDC Vdrops on 65' of wire?
The meter and the probes are set to DCA, not DCV, so that cannot be right, it is not Vdrops reading.

 

[Whinny]

That was last week! I can't remember that long ago anymore...
I did both, that's the amps in to the battery comparing it to the amp reading on the PWM controller. That controller toggles panel voltage, batt voltage, current and WH, it's new to me replacing a non waterproof one. I think the vdrop was around .2 -65' round trip, panel to controller I decided it was inconsequential for my use.

 

[Pappion]

How does this help the OP?

 

[Whinny]

edit.....

 

[thexformer]

Your power supply should look no different to your SCC, than a solar panel. Can your SCC handle the output of your power supply?

Thank you Mike for your answer. My own assessment is also aligned with this answer without thinking much technicalities. Regarding the Power capacity, power supply would supply maximum 400W and the SCC is rated 580W @24v. Should it be okay or should I dimension the power supply more capable than the SCC rating. This is my confusion and the reason for posting in here.

My understanding is, power supply can supply maximum 400w in case the load demands for it. So MPPT should limit the current based on its own capacity and if that's lower than 400w then power supply doesn't have to supply even 400w full. However, in this case, danger is the SCC will run on its full capacity the whole time. I don't know if that's a right treatment for the costly controller.

On the other hand, if SCC is rated higher then probably it would draw more current than the power supply can provide hence trouble for the power supply. The difference with a solar panel in this case, a solar panel is hard limited by it's current generation/sun, no matter what, you can't draw more current through a solar panel than its designed for. In case of a Power supply, it's rating is based on the wire size of the primary/secondary winding which can be overloaded in a situation like short circuit. SCC is aware of the battery requirement but unaware of the power source, hence SCC won't stop at the sources limit rather it will maximize the charging current of its battery.

Let me know if I am thinking wrong. Without real life testing, I can't really confirm. So it's better to learn from the experts before learning in the hard way

 

[thexformer]

CAUTION!!! Many SCC are positive grounded (pos in to pos out is hardwired). Many DC-DC converters are Negative grounded. It's Short City, no regulation thru SCC.
Of course, some SCC are negative grounded, and some power supplies are floating. Best to grab your ohm meter and start checking.

Thanks Pappion for your answer. But most of this answer is above my pay grade ? Kindly educate me a bit more if you are not impatient to educate a noob. If you kindly explain how the ohm measurement should be used that would be a great help.

 

[Pappion]

It's better to find it in SCC and power supply documents. But not always possible.

SCC positive ground have very low ohms between Batt+ and Solar+.
SCC negative ground have very low ohms between Batt- and Solar-. This is a $ bragging feature.

Power supplies outputs are isolated (floating) or non-isolated. Isolated is a $ bragging feature.
DC-DC converter power supplies are typically non-isolated negative ground. With very low ohms between the two grounds.
AC-DC power supplies may have an output grounded to AC ground, be it - or +.

 

[Pappion]

Your power supply should look no different to your SCC, than a solar panel. Can your SCC handle the output of your power supply?

It is acceptable to have more solar than the SCC can handle. It's self limiting, if designed right.
His power supply is well below SCC current rating. Got links on your power supply?
I'm not sure what kind of current limit it has and how it will handle max current over time.
MPPT might sense the voltage drop and cut back the demand.
I might emulate a solar panel by putting a resistor inline. 0.1 ohm (10W) at 8.3A gives 0.83v drop. Trial and error to find best value.

 

[thexformer]

Thanks again guys for your valuable replies.

These are the products I am planning to use which are on the way now..

AC DC power Supply
Victron 100/20 MPPT

The power supply is a AliExpress product so there is not much detailed information which might indicate the mechanism for current limit.

 

[Pappion]

Victron is good kit. My 100/50 MPPT is negative ground. Yank their chain for answers.
Power supply is not good. Should get one exceeding SCC current or claims to have CC (constant current mode).
Not having experience doing this, I would likely use an inline resistor and fuses.

 

[Pappion]

I overlooked something. Power instead of current.
PS 48v * 8.3A = 398W
SCC 4s14.6v * 20A = 292W, so 292w / 48v = 6A OK
SCC 8s 29.2v *20A = 584W, 12A exceeds the supply output

My reasoning for the resistor:
Solar Panels have aluminum interconnects, built in resistance + wire resistance, insignificant capacitance.
Power supply has large output capacitors, very low source resistance.
Power supply has a voltage feedback loop.
SCC has an MPPT feedback loop, expecting some resistance. But may have caps on it's input.
Adding some resistance might prevent the loops from interacting and appear more like a solar panel.