question

csc avatar image
csc asked

Use smartsolar as a dc dc converter

Hi there


Here's the situation : I have an electric vehicle with 72v batteries inside. What would happen if I disconnect my solar panels from the 250 / 60 smartsolar and plug the 72v instead ?! I guess it's gonna work like a dc dc step down charger with my 56.8v lifepo4 battery !!!???!!!


And what would happen if this 72vdc battery is plugged IN PARRALLEL with 220vdc solar panels (6 trina vertex 400 in series) to the smartsolar's PV input ?


Even weirder : the fact is that my 72vdc vehicle can be directly charged by a 54vdc voltage. So how fried am I if I have PV + 72v battery on the smartsolar pv input... + my EV controller (not directly the 72v battery, this controller charges through the motor used as an induction) on the smartsolar's 48v battery output ?!?!?!?


Of course I see some diodes, fuses etc potentially added but let's discuss the questions above first, please.


I imagine a situation in winter when I don't move from home and would be happy to reduce daily manipulations on the system. The other huge advantage would be that my vehicle can be used to charge 10kWh outside of my offgrid facility and come back to feed my system in case of super bad weather. Big battery on wheels !

dc system
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9 Answers
csc avatar image
csc answered ·

Ok, ok, I guess I'll have to eventually fry my mppt for this test. If anybody including Victron staff, could interfere before that mess, eternal thanks in advance.

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Alexandra avatar image Alexandra ♦ commented ·

@Csc

The mppt has an input current limit of 50A.

You connect it to a source that can supply more than that, the magic smoke will come out.

Also warranty void. It is definitely not a sanctioned use of the equipment.

Better to buy an Orion or sometbing that suits your idea better for b2b charging.


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csc avatar image csc Alexandra ♦ commented ·
Yep, I may be wrong but nothing from victron takes 84v to turn it into 56,8. Any advice ?


And the smartsolar 250 60's limit is 30a, the 50a you're talking about are probably for bigger versions.


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Alexandra avatar image Alexandra ♦ csc commented ·

@Csc

The mppt will short the incoming (pv) input to prevent charging and overcharging under certain circumstances. If you have a way for it to limit the input current and be able to protect against bad things from that side. It will work. But when things go wrong (as mentioned by several members here) it is going to be bad, cacasding to possibly very bad.

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csc avatar image
csc answered ·

Up ! Let's simplify. Can I plug a battery nightly in the pv input, after 100% unplugging the pv array ? Please don't let me fry my Smartsolar !!!!

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klim8skeptic avatar image
klim8skeptic answered ·

I can smell the smoke from here.

Your mppt is supposed to connect PV panels to a battery.

KISS.

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csc avatar image
csc answered ·

I like the KISS principle but still, what's the difference between a dc current from panels and a dc current coming from an accurately fused battery ?

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nickdb avatar image nickdb ♦♦ commented ·
Go read about how an mppt actually works, the lightbulb may start to illuminate then.
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JohnC avatar image JohnC ♦ commented ·
@Csc Not much difference. But there's a limit to short circuit panel current (Isc) in the mppt specs. Can you be sure you won't exceed it?
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csc avatar image csc JohnC ♦ commented ·

Thought about a 30a fuse doubled by a 32a dc circuit breaker (Schneider C32n 4p, 2x2p in series to cut + and - simultaneously)

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csc avatar image csc csc commented ·
And a limitation by programming the battery's bms (bluetooth, easy)
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nickdb avatar image
nickdb answered ·

I guess it's gonna work..

Keep guessing, just please make sure you take a video while you do, it could be entertaining.

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csc avatar image
csc answered ·

Sure ! I might do it as soon as somebody tells me exactly how the mess will happen ! Entertainment powaaa

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klim8skeptic avatar image
klim8skeptic answered ·

I like the KISS principle but still, what's the difference between a dc current from panels and a dc current coming from an accurately fused battery ?

You actually make a very valid, but fatally flawed point.

Solar PV panels have a known Isc point.

Batteries have different max peak currents relative to their rated ah capacity.

Simple.

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csc avatar image csc commented ·
Then the bluetooth bms could do the job, I can set the output to 30 amps in a matter of seconds
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klim8skeptic avatar image klim8skeptic ♦ csc commented ·

matter of seconds

Hmmm. I had this happen this morning. 3 years off grid.

A week of crap weather, 40% battery soc, 90 seconds of microwave to heat up breakfast.

30 seconds in I have a low batt warning from the multiplus. I am sitting in front of the computer (and monitoring tablet) and I could not detect the fault in the time before the multiplus shut down.

It turns out that the multi shut down due to low voltage, the battery was healthy though. Need to upgrade my dc cabling

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csc avatar image csc klim8skeptic ♦ commented ·
Probably any owner of an EV may be interested by the idea of a backup battery on wheels, making you able to go beg for juice at neighbour's door !


And yes, cables are important. I went 50mm² for the battery ones


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csc avatar image csc klim8skeptic ♦ commented ·
I had a similar problem with a perfectly fine battery because of a "dc" 1p circuit breaker getting too warm. Now I use 2p or even 4p breakers with poles connected in series and the problem disappeared
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csc avatar image
csc answered ·

Any advice from a Victron staff hero ?

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kevgermany avatar image kevgermany ♦♦ commented ·
Victron are not likely to approve incorrect non-warranted use of their equipment.
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csc avatar image csc kevgermany ♦♦ commented ·
Boring (with all respect due). It's a very important subject, many evs could turn into a smartgridproof solution.
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csc avatar image
csc answered ·

Alternative option : of course I'd better find an appropriate dc dc converter, did not find anything in the Victron range. What would you use knowing that it's a 56.8v max system based on lifepo4, not the 54.2v that you'll find in most converters ?

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