diy solar

diy solar

Using a MPPT as a battery charger?

Of course they're going to say that... and all batteries MUST be the same age, brand and capacity, and only so many in series/parallel, etc.

If they said yes, they'd have to explain the nuances and then take liability for idiots doing it wrong.
 
sunshine_eggo said:
I believe you have it backwards.

PWM can't effectively regulate current. They work by shorting the source to the battery, i.e., shorting a 24V battery to a 12V battery.
Click to expand...
Just going on what I have read in the past.
People have done it with cheap PWM chargers, without issues. But with MPPT, there were issues using LiFePo4 as a power source.
And no, I can't provide any links. I'm just going from memory. It was just things that I have read and seen on YouTube videos.
 
BradCagle said:
A little back story. The company I work for has a IOT system that runs on solar. Basically a 20a EPEver MPPT, 200w panel, and 100ah LiFePO4
I wanted the guys in the shop to be able to maintain/charge these units before they go out to a new site. I experimented hooking up POE power supplies that have 54vdc out to the MPPT. I went through several, some were rated 2.5a, and some 5a.

They all had the same problem, it would initially work but the MPPT would pull it down, then PSU shuts off, then comes back in a cycle. Also managed to totally kill a few of the PSUs :)
Click to expand...
POEs put out. .3 to .6 amps per pair max at 50ish volts no where near even 1 amp. so if you connected 2 of the cat cable to the input of your mppt you get 30 watts so of course it overloaded. Screenshot_20220614-221540_Chrome.jpgScreenshot_20220614-222231_Chrome.jpg
 
The thing you have to watch out for is to not have a power supply that would exceed the Victron MPPT input current spec which would normally be the Isc of the PV panel.

Also, the power supply will have to be 5V over battery voltage for the MPPT to start.
 
brb58 said:
The thing you have to watch out for is to not have a power supply that would exceed the Victron MPPT input current spec which would normally be the Isc of the PV panel.

Also, the power supply will have to be 5V over battery voltage for the MPPT to start.
Click to expand...

Victron publishes max PV input current. The aforementioned fuse on the PV connection would suffice. A 24V battery should always be 5V or more above the 12V.
 
sunshine_eggo said:
Victron publishes max PV input current. The aforementioned fuse on the PV connection would suffice. A 24V battery should always be 5V or more above the 12V.
Click to expand...
And the question that remains is why does the max input current spec exist? Can the controller try to sink a short circuit current from the PV. If it does, are these fuses being put on to protect going to be blowing constantly.
 
brb58 said:
And the question that remains is why does the max input current spec exist? Can the controller try to sink a short circuit current from the PV. If it does, are these fuses being put on to protect going to be blowing constantly.
Click to expand...


page 60:

A higher short circuit current may damage the solar charger in case of reverse polarity connection of the PV array.
 
Last edited:
sunshine_eggo said:

page 60:

A higher short circuit current may damage the solar charger in case of reverse polarity connection of the PV array.
Click to expand...
There is also info about Victron will short its PV input to protect itself: Notes I am not sure other SCC manufacturers use this method (Crowbar the PV input) as Victron.

MPPT Solar Charger Error Codes [Victron Energy]

www.victronenergy.com www.victronenergy.com

Err 38, Err 39 - PV Input shutdown​

To protect the battery from over-charging the panel input is shorted.
Possible reasons for this error to occur:
  • The Battery voltage (12/24/48V) is set, or auto-detected, incorrectly. Use VictronConnect to disable auto-detect and set the Battery Voltage to a fixed voltage.
  • There is another device connected to the battery, which is configured to a higher voltage. For example a MultiPlus, configured to equalise at 17 Volts, while in the MPPT this is not configured.
  • The battery is disconnected using a manual switch. Ideally the charger should be switched off before disconnecting the battery, this avoids a voltage overshoot on the charger output. If necessary the voltage trip-level for the PV Short protection can be increased by raising the Equalization voltage set-point (note: equalization does not have to be enabled in this case).
  • The battery is disconnected using a Lithium charge relay connected to the “allow-to-charge” output of a BMS. Consider wiring this signal to the Remote terminal of the charger instead. This shuts down the charger gracefully without creating a voltage overshoot.
Error recovery:
  • Error 38: First disconnect the solar panels and disconnect the battery. Wait for 3 minutes, then reconnect the battery first and next the panels.
  • Error 39: The charger will automatically resume operation once the battery voltage drops below its maximum voltage setting (normally Equalisation or Absorption voltages), for 250V versions or float voltage for the other units. It can also takes a minute to reset the fault.
If the error persists the charge controller is probably faulty.
 
BradCagle said:
I didn't use POE itself nor did I use cat5 cable. I used the 54vdc power bricks that we use to power POE switches. Some of these power bricks I used do 54vdc at 5amps

like this: https://www.amazon.com/COOLM-100-240V-Switching-Transformer-Interface/dp/B07G21P3KD
Click to expand...
never seen a POE with an external brick. all our stuff is integrated into the switch or a power inserter for a single line...
learned new stuff all the time

that 20a mttp at 13.8v is 276w or 6.1 amps at 45v so it would still over load a 5 amp brick
 
GSXR1000 said:
never seen a POE with an external brick. all our stuff is integrated into the switch or a power inserter for a single line...
learned new stuff all the time

that 20a mttp at 13.8v is 276w or 6.1 amps at 45v so it would still over load a 5 amp brick
Click to expand...

Yeah, we have some with built in power supplies, and some that have bricks. The ones with bricks are meant to be located on the edge where a group a cameras would connect.

Anyhow, I have a 10a 48v PSU I'm bringing up to the office tomorrow to see if the controller continues to drag the voltage down. Might do a video so you guys can see what I'm talking about.
 
Aright I just tried it with this 10a 48v psu. Still drags the voltage down. I see what it's doing now, it's searching for MPP all the way down to the battery voltage, once it directly connects to the battery it releases. Now on this PSU it only did it once, but I think that's because the battery is fully charged. I'll let it drain, and get some video next.

IMG_20220616_082359790.jpg
 
Now try it with a fully charged 24 volt lthium battery on the input please.
 
mikefitz said:
Now try it with a fully charged 24 volt lthium battery on the input please.
Click to expand...

The battery in this unit is 24v, so would need a 36-48, but I get where you going with that.

Essentially it would be like connecting a 48v battery directly to a 24v battery for a brief period of time. Is that a problem? I don't know.

I think the thing here is, all this might be fine on some MPPT's, and not so much on others. YMMV
 
Alright, here's the video. I wanted to let it run longer, but started smelling components burning so I shut it down.

 
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