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oscarmax avatar image
oscarmax asked

Which wire for PV to Smart Solar 75/15 to Battery?

Please excuse my grammar I have suffered a brain injury

My setup will be 2 x80w mono solar panels with Victron Smart Solar MPPT 75/15 to 12 v battery, this should produce in theory 9 amp maximum.

I have fitted a new Smart Solar 75/15, I going to fit my new Smart Battery Sense, the original factory wiring from my old PWM control to battery is 4 metres long in 2.5mm twin core wiring, based on 9 amp this setup will have a voltage loss of 4.08%.

I have read somewhere that the Smart Solar will automatically compensate for this loss or do I need to change the wiring to 4.5mm.

wiring
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3 Answers
JohnC avatar image
JohnC answered ·

Hi Oscar. Your grammar is fine.

At the nominal 12V, 160W of panel is closer to 13A. Over 4m distance, and both ways, no point mucking about.

Your 75/15 has terminals specced for 6mm2 wire for good reason. That's what I'd use, perhaps with a 20A fuse on the +ve one to protect them.

And oh, 'auto wire' often is specced including the plastic insulation, so be wary of that. And if you have a solar shop or electrician with some twin-core outdoor 'solar wire', 6mm2 is a stock size, highly insulated, and rugged as hell..

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oscarmax avatar image
oscarmax answered ·

John I am a bit confused my 80w mono solar panels are only rated @ 4.49amp each, 2 x 4.49= 8.98amp?

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nigelfxs avatar image nigelfxs commented ·

Hi Oscar

The Smart Solar MPPT 75/15 transforms the 18V @ 9A (approximately) from the solar panels to the lower battery voltage (e.g. 12.3V @ 13A). This boost in output current is the main advantage of using an "MPPT" type of solar charger compared to the cheaper "PWM" type of charger.

I agree with John that you should use thicker wire between the MPPT and the battery to reduce your losses.

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JohnC avatar image JohnC ♦ commented ·

Your panel ratings are for the panels, not the batts. So a single 80W panel @ 4.49A = 17.8V. That should match the panel 'Vmp' rating. Does it?

To stretch your confusion, I'd recommend you wire those panels in series rather than parallel, giving you 4.49A @ twice the Vmp, 35.6V. Still 160W, hey. They'll work better in low light that way, so overall better performance.

Your 75/15 is a good match, and 6mm2 wire to the batts would make for a perfect installation.

It's the A on either side of the mppt that varies, depending on the V there. Keep asking if you need more help understanding this. The beauty of mppt at work..

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oscarmax avatar image oscarmax JohnC ♦ commented ·

Thankyou for the explanation yes I going to wire up in series, looks like I need some 6mm2 wire.

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klim8skeptic avatar image klim8skeptic ♦ oscarmax commented ·

Hi Oscar,

Just check your figures before you buy any more cable Voltage drop calculator.

The smart battery sense is used to compensate for voltage drop between the mppt and battery. It also gives the mppt the battery temperature for the mppt to do temp compensation.

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oscarmax avatar image oscarmax klim8skeptic ♦ commented ·

Fitted the Smart battery sense to my caravan this morning, set up the network, now it is measuring battery temperature and voltage.

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oscarmax avatar image oscarmax klim8skeptic ♦ commented ·

I just had a good look in the caravan they have used a 4 metre length of 2mm2 thinwall wire, I think this is rated at 25 amps, will the smart battery sense compensate for the 4.52% voltage drop.

It looks like a nightmare to run a new section of wire

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oscarmax avatar image
oscarmax answered ·

I have some 4,5mm2 and 6mm2 wire, the new proposed position should be within 8-9 inches of the battery and battery sense.

However relocating the MPPT under the caravan upholstered seat may affect the Bluetooth function..

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spirou avatar image spirou commented ·

It won't. I can connect to all bt gear in my van (under the bench) from inside the house 10m away.

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oscarmax avatar image oscarmax spirou commented ·

Thankyou for you quick resopnce

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