Help a budding solar enthusiast please ! MPPT/Charger Related

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next-day-solar asked Jan 28, '21

Help a budding solar enthusiast please ! MPPT/Charger Related

Thank you for your time in advance, Setup is for an Electric van with a fridge unit, with PV power to top up batteries.

- 2 x 115W PV panels in series (40A inline fuse) after MPPT before DC BUS

- 1 x MPPT 100/30

(both those elements wired to bus bar) 150A

Connected to Bus bar is

1 x Fridge unit, which draws 50/70amps when under load, with built in 100A / 12V charger system, to charge batteries. Fused at 125A as shown.

1 x Smart Shunt (all negative loads pass through)

2 x212AH 12V Lithium batteries in parallel

Question

- When the 12V charger (rated to 100A) charges the batteries, the charging current is flowing through the whole circuit. What stops the 40A fuse on the + line of the MPPT not blowing, or will it blow ? as current passes through the MPPT as it will happily give voltage of the battery etc.

What can I do, is there a risk of 'back current' or similar, as I am worried, that when I will blow the fuse before the MPPT when I turn the charger on.

I have done a similar setup before where I have not fused the MPPT, but I understand its best practise to do so.

Thank you

Venus OS fuses

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circuitschematics.jpg (82.1 KiB)

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Answer 1 · 2021-01-28T20:27:37Z

wkirby answered · Jan 28, 2021 at 08:27 PM

Charge power form another charger will not backfeed the MPPT. Multiple chargers can exist in the system.
The 40A fuse is a good idea to protect the wiring.

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next-day-solar commented · Jan 29, 2021 at 08:30 AM

Thank you for helping, what is the 'rationale' or reason' behind the 40A fuse not blowing when I am charging the battery at 100A and that current is flowing around the circuit ?

Current flows to the lowest point of resistance, but surely when the battery is full, it may then flow around the MPPT circuit ?

Thank you in advance

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wkirby ♦♦ next-day-solar commented · Jan 29, 2021 at 02:09 PM

Current can only flow in one direction, out of the MPPT. Current cannot flow in to the MPPT at its output.
Imagine it as if there is a diode blocking reverse flow.
If the 100A charge was switched off and the MPPT was charging, you would not expect energy from the MPPT to go in to the 100A charger either.

Think of it another way, what would happen at night when the MPPT is not charging, the battery is capable of delivering more than 1000A. You would not expect 1000A to go in to the MPPT.
Likewise, if you switch your 100A charger off, you would not expect 1000A to go from the battery into the 100A charger either.

The 40A fuse is there in case of a fault with the MPPT causing current to flow in to it.

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next-day-solar commented · Jan 29, 2021 at 02:45 PM

Thank you very much, super helpful.

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