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Old 03-02-2025, 13:55   #61
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Re: Fuses for Lithium Power Systems

I think a class T would be fine. From what you've described, you don't have a combination of loads that can cause an overload. So a class T sized to protect the wiring in a short (which will still allow a very short duration overload like windlass motor inrush) should do just fine.
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Old 03-02-2025, 13:59   #62
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Re: Fuses for Lithium Power Systems

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Originally Posted by rslifkin View Post
I think a class T would be fine. From what you've described, you don't have a combination of loads that can cause an overload. So a class T sized to protect the wiring in a short (which will still allow a very short duration overload like windlass motor inrush) should do just fine.

That's what I was thinking, but if someone more knowledgeable has a contrary view, I'm all ears.


I have no windlass, thruster, or winches on these circuits -- those are served by an entirely separate, lead-acid battery bank. The only heavy load is the inverter. The charge sources are not capable of producing more than 170A combined. The inverter can't draw more than 250A, and that only briefly, unless there is some kind of internal failure in which case the short circuit protection should do it.


I'm not seeing what is wrong with this plan, but nevertheless I am listening in case of more qualified opinions.
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Old 03-02-2025, 14:07   #63
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Re: Fuses for Lithium Power Systems

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Ignoring the rhetoric and insults, I will try to get at what is useful in this post.


1. Does anyone agree with CR that a Class T fuse won't work here, because it's superfast? This advice seems to conflict with what I've read elsewhere.


2. I have suggested that KISS might mean in my case not to worry too much about protecting the cables with a fuse. I didn't just pull this out of my posterior; I gave reasons for that: 1. devices connected in this circuit can't overload it; 2. BMS has a current limiting functioning (claimed even short circuit protection). I may well be wrong -- I never claimed to be an EE -- but I'd like to know objectively why, so that I can learn in this case.
A class-T fuse is 100% suitable for your boat. You *must* size it to protect the cable, you can always overload it like by a short somewhere, even inside an appliance like a Multiplus.

A class T fuse diagram is attached; as you can see, it won’t blow that quick and not at all at 100% load or maybe after a very long time.

Yes, your BMS should protect but these fuses are mandatory and insurance is void without them. I also think they should be there and be seen as part of the wiring infrastructure.
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Old 03-02-2025, 14:15   #64
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Re: Fuses for Lithium Power Systems

Quote:
Originally Posted by s/v Jedi View Post
A class-T fuse is 100% suitable for your boat. You *must* size it to protect the cable, you can always overload it like by a short somewhere, even inside an appliance like a Multiplus.

A class T fuse diagram is attached; as you can see, it won’t blow that quick and not at all at 100% load or maybe after a very long time.

Yes, your BMS should protect but these fuses are mandatory and insurance is void without them. I also think they should be there and be seen as part of the wiring infrastructure.
OK, the cable has ampacity of 400A, so connecting it to two circuits each through a 400A fuse will not protect the cable.

You don't agree with my logic that it's not needed because of the nature of the loads? And the presence of the current protection in the MOSFET BMS? I can program the current limit in the BMS.

I could downsize the fuses to 200A each and I guess that would be acceptable given the blow curves. Normal (non-surge) loads should not exceed 170A charging or about the same discharging. I don't really see how the Multiplus could overload the cable other than with a short circuit.

I would hate to lose the advantage you yourself pointed out, of a larger fuse operated at a smaller percentage of its nominal capacity, which would bring lower resistance and losses.
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Old 03-02-2025, 14:19   #65
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Re: Fuses for Lithium Power Systems

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Originally Posted by Dockhead View Post
OK, the cable has ampacity of 400A, so connecting it to two circuits each through a 400A fuse will not protect the cable.

You don't agree with my logic that it's not needed because of the nature of the loads? And the presence of the current protection in the MOSFET BMS? I can program the current limit in the BMS.

I could downsize the fuses to 200A each and I guess that would be acceptable given the blow curves. Normal (non-surge) loads should not exceed 170A charging or about the same discharging. I don't really see how the Multiplus could overload the cable other than with a short circuit.

I would hate to lose the advantage you yourself pointed out, of a larger fuse operated at a smaller percentage of its nominal capacity, which would bring lower resistance and losses.
I wouldn't want to run 2x 400A fuses in parallel, as there's a narrow range of fault conditions that could lead to the cable melting before a fuse blows in that case.

If you wanted each battery bank fused individually above 200A, I'd do that, then after the point where the power from both banks is joined, run it through an additional 400A fuse.
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Old 03-02-2025, 15:26   #66
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Re: Fuses for Lithium Power Systems

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Originally Posted by Dockhead View Post
OK, the cable has ampacity of 400A, so connecting it to two circuits each through a 400A fuse will not protect the cable.

You don't agree with my logic that it's not needed because of the nature of the loads? And the presence of the current protection in the MOSFET BMS? I can program the current limit in the BMS.

I could downsize the fuses to 200A each and I guess that would be acceptable given the blow curves. Normal (non-surge) loads should not exceed 170A charging or about the same discharging. I don't really see how the Multiplus could overload the cable other than with a short circuit.

I would hate to lose the advantage you yourself pointed out, of a larger fuse operated at a smaller percentage of its nominal capacity, which would bring lower resistance and losses.
Yes, you have a couple of options:

- use 200A fuses. If both batteries are the same, this should be okay unless you want to run a load higher than 200A continuous on one battery. What cable is used, 4/0 ? Each battery it’s own cable to the busbars? You could go higher than 200A if the cable can do it. If you have a single 4/0 then you can use two 225A fuses.

- use one 400A fuse and connect both batteries to that one fuse. If the cables between fuse and battery are shorter than 72” then you can simply sleeve them and it’s within ABYC. This sleeve: https://www.amazon.com/dp/B07FW5ZTVH

I think you have a remote battery switch for each battery? In that case you should really give each battery its own 4/0 cables to the busbars. What batteries are we talking about?

If you describe what batteries, switches, cables and busbars you have, I can give a more detailed answer
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Old 04-02-2025, 04:11   #67
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Re: Fuses for Lithium Power Systems

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Originally Posted by s/v Jedi View Post
Yes, you have a couple of options:

- use 200A fuses. If both batteries are the same, this should be okay unless you want to run a load higher than 200A continuous on one battery. What cable is used, 4/0 ? Each battery it’s own cable to the busbars? You could go higher than 200A if the cable can do it. If you have a single 4/0 then you can use two 225A fuses.

- use one 400A fuse and connect both batteries to that one fuse. If the cables between fuse and battery are shorter than 72” then you can simply sleeve them and it’s within ABYC. This sleeve: https://www.amazon.com/dp/B07FW5ZTVH

I think you have a remote battery switch for each battery? In that case you should really give each battery its own 4/0 cables to the busbars. What batteries are we talking about?

If you describe what batteries, switches, cables and busbars you have, I can give a more detailed answer
OK, thanks, that's very helpful

I don't think I will have continuous loads more than 200A. The inverter spec is 125A and other loads max 50A. I'm more and more happy I decided to keep the service bank separate (lead-acid bank driving windlass, winches, thruster, per original build design).

So individual 200A fuses it is. Yes, each battery is connected separately to the busbar via fuse, contactor with remote switch (rated 500A), and BMS with MOSFETS (rated 200A continuous). The cabling is a different question but that's not rocket science -- I have the ampacity charts.

Then busbars connected to (1) house distribution panel with its own contactor and breakers; (2) inverter/charger through heavy cable; (3) alternator.

Gear:

1. 2x Heavy enclosed bus bars, M8 bolts, rated 500A continuous
2. 2x Blue Seas 7702 bistable contactors with remote switches, 500A continuous
3. 2x JK inverter BMS with MOSFETs, rated 200A continuous (400A for a few minutes)
4. 2x Blue Seas T-Class fuse holders and fuses (probably 200A now).

Existing cabling to inverter, 155mm2 or something like that. Other cabling TBD.


Per your recommendation, I will add a Blue Seas 9003e manual battery switch in the cable from bus bar to inverter/charger, for isolating the inverter/charger from the DC system, rated 350A continuous.
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Old 04-02-2025, 07:04   #68
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Re: Fuses for Lithium Power Systems

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Originally Posted by Dockhead View Post
OK, thanks, that's very helpful

I don't think I will have continuous loads more than 200A. The inverter spec is 125A and other loads max 50A. I'm more and more happy I decided to keep the service bank separate (lead-acid bank driving windlass, winches, thruster, per original build design).

So individual 200A fuses it is. Yes, each battery is connected separately to the busbar via fuse, contactor with remote switch (rated 500A), and BMS with MOSFETS (rated 200A continuous). The cabling is a different question but that's not rocket science -- I have the ampacity charts.

Then busbars connected to (1) house distribution panel with its own contactor and breakers; (2) inverter/charger through heavy cable; (3) alternator.

Gear:

1. 2x Heavy enclosed bus bars, M8 bolts, rated 500A continuous
2. 2x Blue Seas 7702 bistable contactors with remote switches, 500A continuous
3. 2x JK inverter BMS with MOSFETs, rated 200A continuous (400A for a few minutes)
4. 2x Blue Seas T-Class fuse holders and fuses (probably 200A now).

Existing cabling to inverter, 155mm2 or something like that. Other cabling TBD.


Per your recommendation, I will add a Blue Seas 9003e manual battery switch in the cable from bus bar to inverter/charger, for isolating the inverter/charger from the DC system, rated 350A continuous.
Each battery has a 155mm2 minimum path to the busbars? That is more than a 4/0 cable and you can/should use 400A fuses in that case. If both batteries share a single 155mm2 cable, then I would use 225A fuses. Remember, the fuse must be matched to the wiring, not expected load, which is for the BMS to worry about.
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Old 04-02-2025, 10:11   #69
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Re: Fuses for Lithium Power Systems

Quote:
Originally Posted by s/v Jedi View Post
Each battery has a 155mm2 minimum path to the busbars? That is more than a 4/0 cable and you can/should use 400A fuses in that case. If both batteries share a single 155mm2 cable, then I would use 225A fuses. Remember, the fuse must be matched to the wiring, not expected load, which is for the BMS to worry about.

Cabling to the bus bars is not designed yet. These are very short runs and I can do whatever. But if the fuses are meant to protect the inverter cable (?) then the fuses will need to be no more than 200A.
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Old 04-02-2025, 10:36   #70
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Re: Fuses for Lithium Power Systems

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Cabling to the bus bars is not designed yet. These are very short runs and I can do whatever. But if the fuses are meant to protect the inverter cable (?) then the fuses will need to be no more than 200A.
No!

The fuses protect the complete cable path from battery to busbar and must be as close to the battery as possible. If within 7” you only need cable, if longer than 7” but shorter than 72” you need cable in a sheath like I posted earlier. If you don’t have this yet, you can simply go down in wire size. For example, use 1/0 cable from battery to fuse, from fuse to battery switch and from battery switch to busbar. You can fuse that at 250A or even 300A because the ampacity is 285A for which there is no fuse and you can go one size up.

Then, you have a cable from the busbars to the Multiplus. From the busbar first to a fuse as close to the busbar as possible. I like to use a MRBF fuse there which has a fuse block that bolts directly to the busbar, eliminating a cable. From the fuse you go to the manual switch with capacitor precharge circuit and from that switch to the Multiplus.

That’s how it’s done

You we’re trying to combine everything in one thing but you must see the busbars as the center and create circuits between busbar and batteries as well as between busbars and inverters/chargers/solar/windlass etc.

The fuse is always at the side where the power comes from. So for a battery it is t the battery side. For an inverter it is at the busbar side. For a charger, it is still at the busbar side because the charger is current limited and you must use a cable that can do its maximum current output.

The Multiplus probably has double terminals for cables, allowing two positive and two negative cables of smaller gauge to make it easier to handle. With a 24V unit this may not be an issue. You simply use manufacturer recommendations and voltage drop tables to decide on which cable gauge to use there. Also look at which fuse values are available because again the fuse must protect the cable.
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Old 05-02-2025, 05:11   #71
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Re: Fuses for Lithium Power Systems

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Cabling to the bus bars is not designed yet. These are very short runs and I can do whatever. But if the fuses are meant to protect the inverter cable (?) then the fuses will need to be no more than 200A.
For these very short run requirements we've designed this class T Z-bar link setup, super helpful when working in a very tight space and meets the ABYC 7" requirement.
https://theyachtrigger.com/products/...jmyi1ydhb2BmJF
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