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mcdees84 asked

Adding second mppt - need help

Current system, 4x390 watt Canadian bifacial panels. Specs = 46.8VOC; 38.3 VMP; 10.74 ISC; 10.19 IMP. Wired in series to a 250/100 MPPT.

A tree branch fell and shattered one of my panels so I contacted my guy to get another. He had 2 in stock and offered great deal so I bought both. So I have 5 panels with above specs

I can’t safely put all 5 of these into the 250/100 but wondering if it’s even worth it to get a second mppt (100/30 or 100/50) for a single panel. Other option would be run a string of 3 panels to the 250/100 and then 2 panels to a 150/35 or 150/45. My concern with both options is I won’t have enough pv to actually get the mppt’s to reach close to the max charge output, I.e., the 250/100 will never get to 100 amps on 3 panels or the 100/30 or 100/50 will not do reach those amps on 1 panel.

I’m not entirely sure at what wattage these mppts reach their max charging capability. These are on my rv and I boondock a lot. Would love to get to about 130/140 charging amps as that would offset my ac and residential fridge with little to no battery loss during the day.

Any help as to whether this would be worth it to get another mppt and at what wattage I get the full charging power would be greatly appreciated.

MPPT SmartSolar
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kevgermany answered ·

You don't say what voltage your batteries are.

MPPT output in watts is charging voltage times current. As the MPPTs are over 90% efficient and panels rarely have enough light for full power, you can calculate roughly as follows:

Panel power/Charging voltage * 90% is output current.

You'll have to buy a second MPPT. Don't forget that the MPPT won't start charging until the panels give 5V more than the current battery voltage. So if you have a 48V battery pack, you'll need at least two panels in each string. Also do the low temp calcs, two panels with 46Voc is close to the limit for a 100V controller. There's an online Victron solar calculator that will size the MPPT for you.

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mcdees84 answered ·

Sorry. 12v system and lithium - so I'm not worried about the 5+vbat. I did the online calculator for both 1 panel and 2 panels in series. They are 100/35 and 150/35 respectively.

Here is what I'm trying to decide and could use some help on...put the 5th panel on the roof and buy an extra MPPT if its worth it, or just stick it in storage as a back up in case another gets broken.

So I guess I'll clarify based on your above formula and you can tell me if I'm correct.

390/14 volts = 27.85 * 90% efficiency = 25.07 amps would be the max I could get out of 1 panel? I used 14 volts as a ballpark since its a lithium battery bank.

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seb71 answered ·

My concern with both options is I won’t have enough pv to actually get the mppt’s to reach close to the max charge output

With not enough PV panels you won't use an MPPT to the maximum, but this also means that the MPPT will run cooler (and potentially last longer).

On the other hand, for an RV (with very limited mounting surface for PV panels), I would go for under-paneling rather than over-paneling, to make sure I get the most from the PV panels in perfect weather conditions.

For a house system with plenty of room to put PV panels, I would prefer the opposite (over-paneling), to make sure I get the most energy I can in bad conditions (low/not favourable Sun position, clouds) in a certain budget.


I can’t safely put all 5 of these into the 250/100 but wondering if it’s even worth it to get a second mppt (100/30 or 100/50) for a single panel. Other option would be run a string of 3 panels to the 250/100 and then 2 panels to a 150/35 or 150/45.

If the 4 panels connected to the 250/100 are all oriented the same (facing the same way - on the RV roof, I suppose), go for the first option (a second 100/30 MPPT for a single panel), because this smaller MPPT is cheaper than a 150/60 MPPT. Unless you plan for future expansions/repurposing (for instance if later you might switch to a 48V system), then get the 150/60 Smart Solar and connect 2 PV panels on it even for your current 12V battery.

Make sure you get a SmartSolar, not a BlueSolar.

Then, if you do not also have a GX device and if your existing MPPT is SmartSolar, use VE.Smart Networking.

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mcdees84 answered ·

@kevgermany and @Seb71, thanks for your responses! I think I’m in agreement with you @Seb71 about just doing the single panel on a 100/30. My other 4 panels are mounted north to south on the rv. This panel will have to mount east to west. Plus, I’m currently getting 95-100 amps from those 4 panels through the 250/100. I fear dropping it to 3 would make my investment in the 250/100 somewhat obsolete.

If I do this, I’ll get the 100/30 smart solar and connect them via V.E. Connect.

In your opinions, is my above math somewhat close to what I could actually expect out of a single panel into a 100/30 - about 20-25 amps?

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seb71 avatar image seb71 commented ·

what I could actually expect out of a single panel into a 100/30 - about 20-25 amps?

The maximum would be about 27A into the battery in perfect conditions (clear sky, the Sun rays perpendicular to the PV panel and a cold day).


If your other 4 PV panels give you 100A into the battery, a single identical panel should give you 100A/4 = 25A into the battery in the same conditions (although if 100A, the 250/100 might be limiting the output, so the single PV panel can give you more than 1/4).

Assuming the battery (and any loads) can take that total of about 125A.

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mcdees84 answered ·

One final question as I’m moving forward with the additional mppt. Any reason I cannot wire both mppts to a single appropriately sized fuse on a lynx distributor if space on the distributor is at a premium?

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mcdees84 avatar image mcdees84 commented ·
I might be rethinking this. The fuse wouldn’t be my concern but the need for the wire to handle a combined 130 amps. I don’t think I’ll go that route as I don’t think I can go bigger than the 2awg on the 250/100 and I doubt the terminals are anywhere near big enough on a 100/30 to accommodate the proper wire gauge.
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seb71 avatar image seb71 mcdees84 commented ·

The MPPT-busbar wires should be sized for the currents those MPPTs can output into the battery. So wires and fuse for 100A for one MPPT and wires and fuse for 30A for the other MPPT.

So there is no need to use AWG 2 for the 100/30 wires. You will not have 130A through those wires.

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Any reason I cannot wire both mppts to a single appropriately sized fuse on a lynx distributor if space on the distributor is at a premium?

The reason to not wire both is different wire gauges, with different size fuses. If there is no other way, use extra fuses/fuse holders outside of Lynx distributor. The smaller gauge wire for the 100/30 MPPT (AWG 6 / 16mm^2) won't be protected by a 125A (or so) fuse. The wire will melt well before the fuse would blow.

Also you should avoid 2 (and especially more than 2) wire lugs on a single bolt.

More flexibility with wire lugs for low current devices (such as a SmartShunt wire lug) - it's not that problematic to use more such wire lugs on the same bolt (sized for much larger currents).

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The wires (and fuse) between the busbar and the battery are the one which should be sized for the combined maximum possible current. Usually this is dictated by the inverter current (this is usually the largest current through those wires).

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