question

greg-nelson avatar image
greg-nelson asked

AC/DC conversion for BlueSolar MPPT 250/70-Tr input

My batteries and my Victron MPPT units have arrived, but there's been a delay in the solar panel installation and the grid backup inverter/charger is on backorder. I want to get the rest of my system prepared, and in particularly charge my 48V (16S) LiFePO4 batteries.

I've seen a few discussions of the possibility of using an AC/DC brick as an input in place of solar, and they usually say "better to just use a charger" but I'm reluctant to trust a crappy generic charger that can't talk to the battery BMS. I trust my Victron products. But I'm finding that's extremely difficult when I need 58.4V + 5V headroom, because people just don't make a lot of 72V or 80V or 144V power supplies (that aren't more expensive than buying a whole new inverter-charger). It's also hard to find smart inverter-chargers for 48V systems.

Rectified, 120VAC is about 170VDC, well within the Victron's 250V input range. My question is, will there be any negative effect on the Victron device from excessive ripple, and what is excessive ripple? If the input runs from 100VDC to 170VDC (35Vpp ripple) that's well within the bounds between battery voltage headroom and max input, but I worry that it will seriously mess up the MPPT algorithm. With the SmartSolar MPPT, I can limit the battery charge current to a desired level (for example, 10A) to reduce the ripple from the rectifier, but it would be crazy to wind up buying $300 worth of capacitors; at that point I could buy something like an EG4 chargeverter (assuming I could find one).

Any suggestions for a practical AC/DC input for this application?

battery chargingMPPT SmartSolar
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5 Answers
Michelle Konzack avatar image
Michelle Konzack answered ·

I am not sure, that you know, what are you doing!


For 300US$ you can get already nice LiFePO4 chargers and this whithout the risk you get killed!

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greg-nelson avatar image
greg-nelson answered ·

Certainly, for $300US I can get a very nice charger. But since this is just temporary, I wanted to know if for $50US I could get a temporary solution that would work correctly.

I used to work designing electronics including high-voltage power supplies, so I'm clear on the proper safety precautions for working with both 120VAC and 200VDC. I was just hoping to understand how the MPPT algorithm would deal with ripple on the input side, and how much ripple would be "too much" for the algorithm to work correctly.

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k-k avatar image
k-k answered ·

@Greg-Nelson

Remember: AFAIK Mppt's are testing Isc every 10min or so. So in my opinion you NEED a shortcircuit simulation/damper. Maybe in form of a resistor/incandescent light. Ideally with an resistance that fit your PV-panel(-strings)

(if at hand and for safety use a isolating transformer too)

If you do so, please inform us how well its going after ducking the ripple near to zero.

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greg-nelson avatar image
greg-nelson answered ·

Interesting. I did not realize they were actually testing Isc, I thought they only went to short circuit after the batteries were charged, to shut down the panels and put them in a "safe" state.

With the holidays it will be a week or so before I get the parts. I was planning on simply putting a DC circuit breaker (of which I have a whole bunch sitting around for a later part of the project) inline so that if the MPPT decided it was time to short the "panels" it would just trip the 10A breaker before it drew very much current from the 20A AC circuit. However, given what you say I might want the inline resistor as well. It happens that I have a nice 500W 30 ohm resistor sitting around...

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Michelle Konzack avatar image
Michelle Konzack answered ·

Short circuiting if Solarpaneks happen only in PWM chargers, NOT in MPPT!


Isc isvnot tested and if the batteries become full, hence the MPPT reduce the current, the PV voltage is ramping up

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