18650 DIY powerwall 48v


SLS BATTERY AUCTIONS are now live at www.batteryhookup.com EV, Lithium, LifePo4, Inverters & More

OffGridInTheCity

Active member
Joined
Dec 15, 2018
Messages
2,294
Slight clarification. The OP asked about a panel config rated for 21.7a - e.g. 2p. But the charge controller unit has a 18a max input. I'm not saying the panels would produce more than 21.7a but that they could produce up to 21.7a if requested by the MPPT algorithm. Even 19a is 87% of 21.7a and is higher than the18a max the unit gives.

I'm guessing that the built-in MPPT algorithm will try to maximize power but is not so smart as to be 'aware' of the unit's physical limits. Thus, I'm speculating that I personally wouldn't parallel my strings knowing the amps could exceed the max input amps explicitly given for a unit.
 
Last edited:

Wolf

Active member
Joined
Sep 25, 2018
Messages
1,826
The way I look at this is, you can buy a car that has an engine that redlines at 7000 RPM and if you constantly run this engine at 7500 RPM with some spurts to 8000 RPM you are asking for trouble. That engine will probably not run very long and you will have to replace it.
That engine, however, will run for a long time at 3500 RPM with an occasional spurt to 7000 RPM.
The manufacturer, of whatever equipment they make, have set limits to what it is capable of for a reason. They don't want their equipment to be destroyed potentially causing you harm or bodily injury.

As far as Solar Charge controllers if they say max input is 18A then that's it. They may handle 21.5A but for how long?
and at 450V those mosfets can turn into fuses real quick.
I still suggest 3s or maybe 6s and however many controllers you need. The other issue is if you have such a long series string you are left with the christmas tree light dilemma if one panel fails you loose the whole string.
Additionally there is the shading. Half a panel in a long string being shaded can really screw with your output. I know there are bypass diodes and all that to help eliminate some of that but it still has an affect.

I personally would not be fond of such high DC voltages, I'd rather have a lower DC voltage with more Amps. It is essentially the same as Watts are Watts no matter how you slice it.
I just know that the higher the voltage the easier it is to jump a gap, and once DC jumps its hard to stop it. Also the breakers that are required for safety shutoffs are certainly more expensive.
I use ABB S202 breakers which are some of the best in the world. The ABB Max rating for that MCB are 125V DC and 440V AC.
So you see this breaker can safely cut the AC voltage up to 440V with its internal arc chamber however only 125V for DC.
I would leave high voltage to the Solar farm professionals and keep this a DIY project where no one could get hurt.
Just my opinion. :)
1674183251034.png

Wolf
 
Last edited:

cak

Member
Joined
Mar 14, 2021
Messages
166
I'm guessing that the built-in MPPT algorithm will try to maximize power but is not so smart as to be 'aware' of the unit's physical limits
In my experience the opposite of this is true. Any decent quality MPPT should know it's max current and adjust the MPPT curve to try and not exceed it's max. In my case with Victron it knows it's max and I can manually adjust it(I haven't tested if I can override it's design limits) which is helpful because I might have other parts of the system like the solar input breaker that have a limit lower than the MPPT. I don't hear people doing this in systems with batteries but in grid coupled systems it is actually very common to "overprovision" the solar capacity so that you get to max out your inverter(which uses an MPPT internally before doing the DC-AC conversion) more of the time in less than ideal solar conditions. For example since Net Metering rules in the USA limit such systems to 40kw AC if folks really want to maximize production they might put in 50kw DC of panel capacity but for legal and technical reasons the inverter/MPPT must still not exceed the 40kw AC limit so in full sun not all of the 50kw DC is used.
 
Top