270ah DIY LiFePO4 build

[Pete7]

Btw I am fully persuaded on the merits of using lead acid only for non critical loads. My plotter, sounder, autopilot, and nav lights will remain on the lead acid system. I haven't decided about radar yet, it's a big consumer but then again often used whilst motoring, much the same as the autopilot.

Or you could use a hybrid bank so that the LFP takes all the load and a parallel FLA is there just in case. There is one further advantage with this, if you don't want to float the LFP when not on board you can throw the big switch and its isolated. Any solar then charges the FLA which keeps the bilge pump and any alarms still functioning. The FLA on float will be about 13.5v or so and the isolated LFP say 13.3v. When you connect them back up the voltages are very similar so there isn't a problem I have noticed so far.

 

[Poey50]

Or you could use a hybrid bank so that the LFP takes all the load and a parallel FLA is there just in case. There is one further advantage with this, if you don't want to float the LFP when not on board you can throw the big switch and its isolated. Any solar then charges the FLA which keeps the bilge pump and any alarms still functioning. The FLA on float will be about 13.5v or so and the isolated LFP say 13.3v. When you connect them back up the voltages are very similar so there isn't a problem I have noticed so far.

Do you use this system? It does seem the simplest solution to the problem of sudden disconnect. The Nordkyn site describes it under 'Alternative 1' in the article below. The lead acid doesn't add much to the overall capacity until LFP is quite heavily discharged and Nordkyn does raise some concern that the LFP alone may not then get the lead acid back up quickly enough to avoid sulphating but your 'big switch' isolation of the LFP presumably takes care of that. I prefer dual-bus set-ups but this certainly has something to offer for drop-ins since it addresses my 3 questions to test for BMS drop-out as follows.
1. How will you know it's going to happen? YOU WON'T.
2. What will the effects be? NONE. ALTERNATOR DIODES AND MARINE ELECTRONICS ARE PROTECTED.
3. How will you mitigate the effects? UNNECESSARY.

Electrical Design For a Marine Lithium Battery Bank | Nordkyn Design

 

[Poey50]

How to lose your boat with Lithium

Save money by using an ANL fuse in place of an expensive Class T fuse for your LFP installation. Taken from another forum. The current just arced across the gap.

What should have been used ... Class T Fuse Block with Insulating Cover - 225 to 400A - Blue Sea Systems

 

[Zing]

How to lose your boat with Lithium

Save money by using an ANL fuse in place of an expensive Class T fuse for your LFP installation. Taken from another forum. The current just arced across the gap.



What should have been used ... Class T Fuse Block with Insulating Cover - 225 to 400A - Blue Sea Systems

NH are an alternative also, more common this side of the pond. Class T is a UL rating, dominant in the US.

 

[Kelpie]

David, have you come across a reliable figure for the maximum short circuit current for prismatic LFP? I've read that it could be around 7-10C. For my 4S pack that means a 2700A current which is well within the interrupt rating of most ANL fuses when used at 12v.

OTOH Cactus Nav will sell me a proper class T for about £40 which is half of what I thought I was going to have to spend, so I'll just go with that. With ANL or breakers downstream.

 

[Poey50]

David, have you come across a reliable figure for the maximum short circuit current for prismatic LFP? I've read that it could be around 7-10C. For my 4S pack that means a 2700A current which is well within the interrupt rating of most ANL fuses when used at 12v.

OTOH Cactus Nav will sell me a proper class T for about £40 which is half of what I thought I was going to have to spend, so I'll just go with that. With ANL or breakers downstream.

I'm waiting for an order from Cactus, too. It's not something I want to save money on.

 

[Kelpie]

I'm waiting for an order from Cactus, too. It's not something I want to save money on.

I'm swithering about the size of fuse I need. Been studying Blue Sea's graphs a lot this morning.

My inverter suggests a 500A fuse (twice rated input current). But surely that's only for surges, and the Blue Sea class T can handle a 200% load for one minute, whereas the inverter will only need to draw that for 5s. So I'm thinking of scaling back the fuse to 250A or less.
Protecting the system at 250A also allows me to use things like Blue Sea's powerpost and bus bars. It becomes increasingly difficult and expensive to source suitable connectors once you go above 250-300A. Which is strange really as they are all using M10 stainless studs. I suppose it comes down to the insulation?

Given the huge cost differences involved I am also considering a single class T straight after the battery, and a smaller ANL for the inverter itself. I can carry a bucketload of spares for the ANL. If I am understanding things correctly, by the time the ANL reaches its interrupt current, the class T will have blown.

I've seen installations (e.g. recently watched Cadoha's Youtube about this) where MBRF fuses are used on each battery, with a class T further down the line. So surely there is nothing inherently wrong with having different types and ratings of fuses in series?

 

[Poey50]

I'm swithering about the size of fuse I need. Been studying Blue Sea's graphs a lot this morning.

My inverter suggests a 500A fuse (twice rated input current). But surely that's only for surges, and the Blue Sea class T can handle a 200% load for one minute, whereas the inverter will only need to draw that for 5s. So I'm thinking of scaling back the fuse to 250A or less.
Protecting the system at 250A also allows me to use things like Blue Sea's powerpost and bus bars. It becomes increasingly difficult and expensive to source suitable connectors once you go above 250-300A. Which is strange really as they are all using M10 stainless studs. I suppose it comes down to the insulation?

Given the huge cost differences involved I am also considering a single class T straight after the battery, and a smaller ANL for the inverter itself. I can carry a bucketload of spares for the ANL. If I am understanding things correctly, by the time the ANL reaches its interrupt current, the class T will have blown.

I've seen installations (e.g. recently watched Cadoha's Youtube about this) where MBRF fuses are used on each battery, with a class T further down the line. So surely there is nothing inherently wrong with having different types and ratings of fuses in series?

I'm very much a fractional C person, don't have an inverter and so haven't researched the high C issues. However, your penultimate paragraph sounds right.

 

[Pete7]

Do you use this system? It does seem the simplest solution to the problem of sudden disconnect. The Nordkyn site describes it under 'Alternative 1' in the article below. The lead acid doesn't add much to the overall capacity until LFP is quite heavily discharged and Nordkyn does raise some concern that the LFP alone may not then get the lead acid back up quickly enough to avoid sulphating but your 'big switch' isolation of the LFP presumably takes care of that. I prefer dual-bus set-ups but this certainly has something to offer for drop-ins since it addresses my 3 questions to test for BMS drop-out as follows.
1. How will you know it's going to happen? YOU WON'T.
2. What will the effects be? NONE. ALTERNATOR DIODES AND MARINE ELECTRONICS ARE PROTECTED.
3. How will you mitigate the effects? UNNECESSARY.

Electrical Design For a Marine Lithium Battery Bank | Nordkyn Design

Yes, been using it for two months so far, working well. Of course now we can run mains electric items on a small yacht we are finding more things to use which means we may need to expand the bank. 120Ah of LFP and 85Ah of FLA currently with space for more in the future.

We have a large ANL fuse between the batteries and the rest of the wiring with more fuses downstream to match individual charging and domestics. A large class T fuse looks sensible instead of the ANL or in addition. Am also planning to use a contactor instead of a battery master switch just for convenience to connect and disconnect the LFP from everything else.

 

[Poey50]

Yes, been using it for two months so far, working well. Of course now we can run mains electric items on a small yacht we are finding more things to use which means we may need to expand the bank. 120Ah of LFP and 85Ah of FLA currently with space for more in the future.

We have a large ANL fuse between the batteries and the rest of the wiring with more fuses downstream to match individual charging and domestics. A large class T fuse looks sensible instead of the ANL or in addition. Am also planning to use a contactor instead of a battery master switch just for convenience to connect and disconnect the LFP from everything else.

Very interesting, thanks. I'm curious to know how you start the engine with your set-up.

 

[Zing]

I'm swithering about the size of fuse I need. Been studying Blue Sea's graphs a lot this morning.

My inverter suggests a 500A fuse (twice rated input current). But surely that's only for surges, and the Blue Sea class T can handle a 200% load for one minute, whereas the inverter will only need to draw that for 5s. So I'm thinking of scaling back the fuse to 250A or less.
Protecting the system at 250A also allows me to use things like Blue Sea's powerpost and bus bars. It becomes increasingly difficult and expensive to source suitable connectors once you go above 250-300A. Which is strange really as they are all using M10 stainless studs. I suppose it comes down to the insulation?

Given the huge cost differences involved I am also considering a single class T straight after the battery, and a smaller ANL for the inverter itself. I can carry a bucketload of spares for the ANL. If I am understanding things correctly, by the time the ANL reaches its interrupt current, the class T will have blown.

I've seen installations (e.g. recently watched Cadoha's Youtube about this) where MBRF fuses are used on each battery, with a class T further down the line. So surely there is nothing inherently wrong with having different types and ratings of fuses in series?

A dead short will produce truly massive currents, so the interrupt rating is key. Reckon on 10 to 20 x the battery rating. I have a 1,000 A installation with a 10x rated battery, so the standard 20,000A NH fuse I have only covers it.

I shorted a 18650 briefly with a screwdriver. The sparks were impressive. These things scare me now.

 

[TernVI]

Fusing big currents for 12V is surprisingly difficult.
The resistance of the battery and the wiring can mean that it's very difficult to actually get enough short current to blow the fuse quickly and cleanly. If you've done any welding you'll know that a 200A arc is quite dramatic and usually takes more than 12V to strike. Getting 200A through a casual contact of two bits of metal with just 12V behind them is actually quite unlikely. What's more likely is that you'll get a few dozen amps and a lot of heat without blowing the fuse.

What can happen is something along the lines of you pull 200A through a 100A fuse, the fuse heats up (along with everything else) and the current drops to say 100A before the fuse blows properly. The fuse then gets very hot and things around it melt.
Voltages higher than 12 are often considered safer, because the fuse will blow more reliably.
Things like 12V starter motors are rarely fused, because there is a conflict between reliably blowing the fuse before anything else catches fire and nuisance fuse blowing. With things like inverters there is an old joke borrowed from high powered audio, the semiconductors blow to protect the fuses.

The fuses are a last resort or there to keep pedants happy. What you need is proper insulation and installation so that it's beyond 'unlikely' that the fuses will be put to the test.

You might do better with some other form of overload protection.
You should understand what the BMS will do in a marginal situation where the fuse may blow slowly.

If you're hoping it will work going down to 90% discharged or further, it gets tougher still and makes the 'can discharge much further than lead acid' claims quite sketchy.

 

[noelex]

Fuses with a high interrupt rating are needed for lithium batteries.

T class fuses are excellent in this regard with 20,000 A IC rating. However, keep in mind they are not IP rated.

 

[Poey50]

Fuses with a high interrupt rating are needed for lithium batteries.

T class fuses are excellent in this regard with 20,000 A IC rating. However, keep in mind they are not IP rated.

Rod Collins in his MarineHowTo paper gives this note "Class T fuses do not have an ignition protection rating. As near as I can tell, from speaking with Blue Sea Systems, as well as Cooper Bussmann, they have not been specifically tested for this. This only means that they’ve not been tested, not that they would necessarily be unsafe."

 

[TernVI]

Have any of the youtube chancers actually ever tested a 200A fuse in a 12V system?

 

[starfire]

I wonder if the din nh fuses in rating gg would do, sand filled, with a 25kA rating at DC.

I know bavaria & others fitted them as battery protection fuses

 

[Poey50]

I understand that one of the factors that influenced the recommendation of Class T main fuse in the ABYC technical advice (TE-13) was the suitability, not just of the fuse, but also of the fuse-holder for marine installations.

Edit: I see Blue Sea Class T are being advertised here as a new design that can be used in environments needing ignition protection. Blue Sea Systems 5502100 Ignition Protected Class-T Fuse Block 225-400A

 

[Pete7]

Very interesting, thanks. I'm curious to know how you start the engine with your set-up.

There is an AGM battery to start the engine, well two batteries actually, I am also using up a spare 85Ah FLA.

This might make it clearer.

 

[Pete7]

Rod Collins in his MarineHowTo paper gives this note "Class T fuses do not have an ignition protection rating. As near as I can tell, from speaking with Blue Sea Systems, as well as Cooper Bussmann, they have not been specifically tested for this. This only means that they’ve not been tested, not that they would necessarily be unsafe."

What is it about an ignition circuit that makes a class T fuse a potential problem? Not withstanding they haven't actually been tested for this.

I don't have any fuses on the battery / starter / alternator cables and can't remember a vehicle ever having them either.

 

[Poey50]

What is it about an ignition circuit that makes a class T fuse a potential problem? Not withstanding they haven't actually been tested for this.

I don't have any fuses on the battery / starter / alternator cables and can't remember a vehicle ever having them either.

Ignition protection refers to the avoidance of sparks and is relevant to environments with potentially explosive fuel-air mixtures. So an engine compartment with a petrol engine, for example but, on a boat also LPG leaks, petrol fumes from spare cans for outboard, hydrogen from overcharging lead acid are possibilities. But Blue Sea Class T are now ignition protected.