question

quantumquest avatar image
quantumquest asked

Why doesn't Victron specify the resistance of the products it sells?

In the Wiring Unlimited guide, Victron explains how voltage drop is important and why it should be estimated to size cables correctly. They also give some example resistances of common elements (shunt 0.10mΩ, fuse 0.35mΩ, page 10) and that's nice. But why don't they give this value in the datasheet of the items they sell? Are we supposed to guess?

What's the resistance of the Battery Protect, for example? Someone asked in 2019, but Victron didn't reply. Someone suggested it could be about 5.6 mΩ, which seems huge.

And what about the Battery Switch? Should it count as 3 connections or does the switch connection have a much higher resistance?

Battery Protect
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Alexandra avatar image
Alexandra answered ·

@QuantumQuest

Wiring and switch gear is not Victron product. The inverter and mppts etc are.

It is installation related. And any good installer will know less resistance is better.

Voltage drop and heat are produced by high resistance they are things you don't want. It is inefficient and causes potential for other problems. Resistance is affected by heat as well. So you as the installer needs to get a good balance between the right sized cable, protective gear for install conditoons and what is considered an acceptable loss. Voltage drop over 3% is not it.

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quantumquest avatar image quantumquest commented ·

I meant the Battery Switch they do sell: https://www.victronenergy.com/dc-distribution-systems/battery-switch-on-off

I understand what you say but how am I supposed to estimate the right size of the cables when there are elements in the circuit that have a unknown and potentially large resistance, like the Battery Protect (https://www.victronenergy.com/battery_protect)?

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Alexandra avatar image Alexandra ♦ quantumquest commented ·

Follow the guidelines in the install manual for the products connecting. It gives min spec. Anything above that is is also good. Your crimps need to be on point as well, and terminal torque. That also affects resistance in circuits.

Cable is sized by amperage expected withiut volt drop in the circuit, not sure why guestimates are needed?

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quantumquest avatar image quantumquest Alexandra ♦ commented ·

Because devices in the path of the current add resistance (in addition to the resistance of the cable), so they increase the voltage drop. For example they say that a 150 A fuse adds 0.35 mΩ, which is the equivalent of adding about 30 cm of a 16 mm² cable.

If the Battery Protect really does add 5.6 mΩ that is the equivalent of adding 5 meters of a 16 mm² cable!

It's on page 10 of their wiring guide: https://www.victronenergy.com/upload/documents/The_Wiring_Unlimited_book/43562-Wiring_Unlimited-pdf-en.pdf#page=13

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kevgermany avatar image kevgermany ♦♦ quantumquest commented ·

Maybe you are over thinking this. Taking the fuse as an example.

At 100A, voltage drop will be 0.35*100/1000V. =0.035V. That's negligible.

If there's a significant voltage drop, something is very wrong.


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quantumquest avatar image quantumquest kevgermany ♦♦ commented ·

I know a single fuse is not big deal but 0.035 V is still about 10 % the recommended maximum voltage drop in a 12 V system.

All I'm asking is the resistance for other devices. If the Battery Protect is 5.6 mΩ it's not negligible.


I also know a high resistance in the Battery Protect is not exactly the same problem as a high voltage drop in a cable because the device should be designed to handle the heat correctly. But it still causes a voltage drop that may have consequences, and it wastes energy.

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kevgermany avatar image kevgermany ♦♦ quantumquest commented ·
General rule of thumb is 3% or less. So yes, at 12V.

But this is very high current. Maybe consider a 24V or 48V system if you're drawing that much power.


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