How to size an On/Off switch

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cbitterfield asked Sep 12, '23

How to size an On/Off switch

I have some conflicting information from a couple of dealers and I would like to get some ground truth on switch selection.

I now have a variety of switches that are on/off or (1, 2 1+2, off) for batteries.

All of my battery bus wires are 4/0.

My major load is a Victron MultiPlus II (2 x 120V, 3000W) + a camper set of loads.

In the Battery compartment I have 4 x 400AH batteries that can max deliver 200-225A instanteously.

I see that Victron Only sells a 275A (On/Off) switch.

The battery compartment is 25' away from the batteries. +-

The batteries are arranged as 2 x 400AH (Bank 1) and 2 x 400AH (Bank2). Each battery will be fused with 250A at the terminal. Each bank is in Parallel (12x800AH).

I see that the victron switch for instance is rated for 275A Continuous, with 455A 1minute and 1250A cranking.

The multiplus II side is protected by a BMS500 (and I might put a switch there for safety/convenience to shut off power while working. (lesson learned previously)

The multiplus II is fused at 400A with a mega fuse in the Lynx Distributor. Assuming:

ISO 8820 Rating / Blow Time 110% / 4 Hours - Min. 135% / 10 Minutes - Max. 200% / 5 Seconds - Max.

I have 3 battery compartment switches to choose from (250A, 300A, 600A)

There could be up to 50As (DC load from the Camper) + Whatever the Max the MultiPlus II can draw.

3000W = 250A (@12V)

Battery Bank 1 -> Switch(1+2) -> Battery Bus -> Switch(on/off) -> BMS 500 (+Lynx Distributor)
Battery Bank 2 -> Switch(1+2)

This is the switch I picked: (I also have a 600A supersized one)

Rated current, voltage:12-24V,
Contact resistance:50m
Insulation resistance:100M
Dielectric strength:1500VAC/1min
Electric life:10000 times
Screw specification:M10
350A Continuous
600A Intermittent
1000A 1 Minute
2000A 15 Seconds

Question 1:

How do I select the on/off switch for each of these:

Switch 1: Battery Side

Switch 2: BMS 500 side

How is the math done?

Question 2:

Is 300A (Continuous Rating sufficient for this?

Question 3:

Does the number of AH(batteries 400AH-1600AH) impact the size of the switch considering each battery is independently fused at 250A (Battery Max rating 200A instantaneously)

fuses switch megafuse

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Answer 1 · 2023-09-12T15:38:56Z

matthias-nagel answered · Sep 12, 2023 at 03:38 PM

I am not hundred percent sure if I fully understood how all components are arranged. I drawing might be helpful, but I'll try anyway.

The math is actually really simple. However, please be aware that there might be local regulations which demand a certain size for an on/off switch despite what physics dictate. For example, in Germany the main on-off switch for a battery bank must be a certified, so-called "load isolating switch" acc. to VDE 0660-107 and also provide a certain minimum breaking capacity one must not fall short of even if all the components in the system are not able to produce that load. This being said, let's go back to physics, but be warned that this might not be enough to stay within the "legal range".

In principle, the on-off switch must be able to contiguously bear and safely switch off the maximum current which can occur on the wire before any fuse or MCB cuts off the power.

If I understood your design correctly, you have four batteries arranged as 2x2, i.e. two in series each and then the strings connected in parallel. Each string is fused with 250A, hence the two parallel strings can deliver 500A in combination. On the load side you have the MultiPlus which is fused at 400A. Now imagine there is an (accidental) short-circuit after the on-off-switch (seen from the batteries) but before the MultiPlus. Then the batteries will happily deliver the 500A before the fuses blow. This means that your on-off switch must also be able to handle 500A. In that case a 300A switch wouldn't be sufficient.

However, you could also put one switch into each string. As each string can only deliver 250A before the fuse blows, a 300A switch is OK. Moreover, you get the additional benefit to be able to switch on/off each string individually.

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cbitterfield commented · Sep 12, 2023 at 05:18 PM

I am not sure how that works like that. If each battery can deliver 200a (x4) that's 800A. The BMS will shutdown anything higher than that. The fuse can't last more than 5 seconds on a direct short. (Say all 800A). The rating on these switches is 30 seconds or more at over 1000A. The Multiplus and all DC loads all have their own fuses from 120A to 400A. (also with 5 seconds until they blow). The batteries each (on them) have a 250A to prevent over loading. This is the logic that is escaping me.

Battery1: 400AH Battery -> 250A Fuse (MRBF) -> Bank 1
Battery2: 400AH Battery -> 250A Fuse (MRBF) -> Bank 1
Battery3: 400AH Battery -> 250A Fuse (MRBF) -> Bank 2
Battery4: 400AH Battery -> 250A Fuse (MRBF) -> Bank 2


Bank 1 -> Switch (1+2) -> Battery Bus -> BMS500 -> Lynx Distribution -> Multiplus II, DC Loads, Solar Chargers(180A-200A), DC/DC Charger (120A)
Bank 2 -> Switch (1+2) -> Battery Bus -> 

MultiPlusII (Fuse 400A)
DC Loads (Fuse 50A) -> Camper Distribution All separately Fused
SolarCharger (Fuse 200A, possibly 250A) -> 2 x "SmartSolar MPPT 150/100 Tr VE.Can"
DC/DC Chargers (Fuse 125A) -> 2 x 60A DC/DC Chargers to Alternator

Each Battery has a BMS that will shutdown at 225 Amps (Rating us 200A)

Each of the switches has a rating for continuous, cranking, intermittent, and maximum. 
Fuses Blow in 5 seconds +-
Batteries need to be fused within 7" of the battery according to what I read.

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matthias-nagel cbitterfield commented · Sep 12, 2023 at 05:46 PM

> I am not sure how that works like that. If each battery can deliver 200a (x4) that's 800A

The question is how are the batteries connected?

If the batteries are all connected in parallel, then, yes, the current adds up. If the batteries are all connected in series, then the voltage adds up, but the total current remains the same. Even if 200A flows through each battery, the current of the string is 200A, too.

My understanding was that you have a 2x2 arrangement, i.e. 2 parallel strings of 2 batteries in series. In that case my math is correct. Of course, if my assumption is wrong, so is the conclusion.

That is why I asked for a drawing. A drawing is much more unambiguous then a textual description as it clearly shows what is connected to what.

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matthias-nagel cbitterfield commented · Sep 12, 2023 at 05:59 PM

Each Battery has a BMS that will shutdown at 225 Amps (Rating us 200A)

That is an interesting argument, but not sound. That is not how electrical safety works. If you trusted the BMS to shutdown the batteries faster than the fuses blow, you would not require fuses at all (because the BMS does the job).

Imagine you have a full short-circuit which results in a extremely high current (only limited by the batteries internal resistance), then the BMS might break and the fuses are in place as a last line of defence. The current limit of the BMS is supposed to throttle the current under normal operating conditions, i.e. if a lot of load is in place. The current limit of the BMS is not supposed to handle the event of a full short-circuit.

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Answer 2 · 2023-09-12T16:27:48Z

Kevin Windrem answered · Sep 12, 2023 at 04:27 PM

Additionally, the Multiplus presents a HUGE inrush current when the DC is switched on. This inrush could damage the switch. It is best to add a recharge circuit to avoid this inrush. A recharge circuit can be as simple as a resistor and a small switch across the main switch contacts. You close the small switch and the resistor charges the capacitors inside the Multi with limited current. I also put an LED (with resistor) across the main resistor. The LED lights when the small switch is closed but dims and eventually goes out when the caps are charged and it is safe to close the main switch. There are also automatic precharge circuits that use a contractor in place of the main switch that closes when the caps in the Multi have been charged. the On/Off switch then becomes a small, low current switch feeding the control circuit.

Be advised that any battery monitor bases its state of charge on a preset battery capacity. Opening some of the battery switches will change the capacity of the battery bank and throw off the monitor's SOC calculations.

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Answer 3 · 2023-09-13T04:41:47Z

kevgermany answered · Sep 13, 2023 at 04:41 AM

I faced a similar 4 battery problem with 4 large parallel batteries. Solution was four Victron isolators. One per battery at the batteries. Overkill better than trying to push things to the limit.

These switches should not be switched under load. And should be treated as normally on, except for maintenance. So installing at the batteries shouldn't be an inconvenience. Thes fuses are for emergencies.

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