Simple LifePo swap sense check

[lustyd]

Thanks Pete - yes, it was the 400A that was spooking me but I see now I should be sizing for what I will be using (with a margin), not the mythical power of the LifePo beast!

You can size either way. The electrical system on the boat is in two halves to my mind, the consumption end has a device consuming power, and you need to provide it with a cable sufficiently large to supply it's needed power, and a fuse to protect either the cable (or the device since that will be a smaller fuse, but generally for the cable). These go backwards towards supply, usually to a switch panel where you'll add up the consumptions of things and size the next cable back and it's matching protection.
In theory you can do this all the way to the battery, using the smallest cable possible all the way back. Racers in carbon boats might do this to save weight, and some may do it because copper is surprisingly expensive, especially in the stranded tinned cable variety.
The other half of the system starts at the battery and works forward towards consumption. This is distribution and sized to current and future requirements. In theory you could size to the battery capability and be set for life, assuming no upgrades. You could also size to expected load (don't forget chargers going in, of course) and you could add a bit.
To my mind this is where we fill in the blanks with a bit of love. You only have a few sizes to choose from - 35, 50, 70, 95 being common so taking into account cost, capacity, growth, size and use of boat it ends up being a relatively easy choice. One gotcha here is that you should check the ratings of busbars and switches are adequate for the cable, and that they have suitably sized bolts for the lugs. You ideally don't want an M4 bolt for 70mm lugs and you don't want an M8 bolt for 4mm lugs if you can help it, although crimps do exist to enable this. Also never forget that larger cables won't bend through small radius' and fighting cables while doing boat yoga in a locker can be exhausting. The short runs to the switches and distribution should be easy and straight enough though.

 

[stranded]

You can size either way. The electrical system on the boat is in two halves to my mind, the consumption end has a device consuming power, and you need to provide it with a cable sufficiently large to supply it's needed power, and a fuse to protect either the cable (or the device since that will be a smaller fuse, but generally for the cable). These go backwards towards supply, usually to a switch panel where you'll add up the consumptions of things and size the next cable back and it's matching protection.
In theory you can do this all the way to the battery, using the smallest cable possible all the way back. Racers in carbon boats might do this to save weight, and some may do it because copper is surprisingly expensive, especially in the stranded tinned cable variety.
The other half of the system starts at the battery and works forward towards consumption. This is distribution and sized to current and future requirements. In theory you could size to the battery capability and be set for life, assuming no upgrades. You could also size to expected load (don't forget chargers going in, of course) and you could add a bit.
To my mind this is where we fill in the blanks with a bit of love. You only have a few sizes to choose from - 35, 50, 70, 95 being common so taking into account cost, capacity, growth, size and use of boat it ends up being a relatively easy choice. One gotcha here is that you should check the ratings of busbars and switches are adequate for the cable, and that they have suitably sized bolts for the lugs. You ideally don't want an M4 bolt for 70mm lugs and you don't want an M8 bolt for 4mm lugs if you can help it, although crimps do exist to enable this. Also never forget that larger cables won't bend through small radius' and fighting cables while doing boat yoga in a locker can be exhausting. The short runs to the switches and distribution should be easy and straight enough though.

Cheers Lusty. It is the difficulty of getting 70mm cable to a securely fixed class t fuse within 200mm of +ve without undue bending that is exercising me. If it were possible to find a terminal fixed class t fuse holder I’d go with that, but I can’t find such a thing. So I think it’s going to be a choice between >200mm to the fuse or cable sized for now. Taking full account of what you and others are saying, my inclination is to size for now and then during the coming season learn the capabilities of the system - particularly how much solar I can harvest when battery chemistry doesn’t throttle it - and then tweak for a ‘forever’ setup next winter. I’ll have bigger cable, spare 300A and 400A class ts, and cutting and crimping tools onboard so I can uprate sooner if needs be.

 

[geem]

Cheers Lusty. It is the difficulty of getting 70mm cable to a securely fixed class t fuse within 200mm of +ve without undue bending that is exercising me. If it were possible to find a terminal fixed class t fuse holder I’d go with that, but I can’t find such a thing. So I think it’s going to be a choice between >200mm to the fuse or cable sized for now. Taking full account of what you and others are saying, my inclination is to size for now and then during the coming season learn the capabilities of the system - particularly how much solar I can harvest when battery chemistry doesn’t throttle it - and then tweak for a ‘forever’ setup next winter. I’ll have bigger cable, spare 300A and 400A class ts, and cutting and crimping tools onboard so I can uprate sooner if needs be.

You can run parallel smaller cables if you are struggling with 70mm2.
Also, if you are over 200mm cable length from the battery +ve to the fuse, add a sleeve to this section of cable. This is an acceptable solution.
As I said in an earlier post. NH00 fuses and just as good as class T and way cheaper. Have a look at RS components website or local electrical factors. They are an industrial fuse and very common in Europe unlike class T which is a USA standard

 

[PaulRainbow]

Cheers Lusty. It is the difficulty of getting 70mm cable to a securely fixed class t fuse within 200mm of +ve without undue bending that is exercising me. If it were possible to find a terminal fixed class t fuse holder I’d go with that, but I can’t find such a thing. So I think it’s going to be a choice between >200mm to the fuse or cable sized for now. Taking full account of what you and others are saying, my inclination is to size for now and then during the coming season learn the capabilities of the system - particularly how much solar I can harvest when battery chemistry doesn’t throttle it - and then tweak for a ‘forever’ setup next winter. I’ll have bigger cable, spare 300A and 400A class ts, and cutting and crimping tools onboard so I can uprate sooner if needs be.

You could use 35mm and fuse accordingly, or 50mm and fuse accordingly. You can use anything up to the max battery or BMS current, but you are limited to which you choose. I'd be inclined to go (in your case) for 50mm cable and 300a fuse, as long as the BMS is rated for that.

 

[geem]

You could use 35mm and fuse accordingly, or 50mm and fuse accordingly. You can use anything up to the max battery or BMS current, but you are limited to which you choose. I'd be inclined to go (in your case) for 50mm cable and 300a fuse, as long as the BMS is rated for that.

I sized my cables for the rating of my BMS. 200A. My maximum load will come from the inverter. This won't go above 150A as that's the setting of my breaker. My individual battery fuses are sized to suit the two different sized batteries. 225A fuse on the 280Ah battery and 160Ah fuse on the 105Ah battery. I can run both batteries separately if I had a failure on one battery.
Being a 24v boat makes all this easier. Cables are 35mm2 generally

 

[stranded]

You can run parallel smaller cables if you are struggling with 70mm2.
Also, if you are over 200mm cable length from the battery +ve to the fuse, add a sleeve to this section of cable. This is an acceptable solution.
As I said in an earlier post. NH00 fuses and just as good as class T and way cheaper. Have a look at RS components website or local electrical factors. They are an industrial fuse and very common in Europe unlike class T which is a USA standard

Thanks for the suggestions Geem - really appreciate it. I’ll definitely look at paralleling the cable. With the fuses, the trouble is, for someone starting out on a project like this for the first time, looking down a list of fuses in somewhere like RS just boggles the mind. Can’t find any that are described as NH00 - NH2 etc. yes, but then all sorts of different specs, tags etc. I simply don’t know enough to make a choice from a menu of ‘everything’, for all sorts of purposes, which may or may not be suitable for my purpose. So I have instead to rely on trusted retailers like 12v planet and assume that what they sell will suit my purpose, and they also sell a nice holder that I know will match the fuse…. To acquire the sort of expertise I would need to second guess the reputable sellers for all the things that I need to do on the boat this (or any other) winter, I would do better to just sell the boat and spend my life in the library! One day, as I absorb all the invaluable info. I get from here, I will feel able to go ‘off piste’, but for now, to get the job done, I need to go for the most accessible solutions, even if that costs a bit more.

 

[lustyd]

It is the difficulty of getting 70mm cable to a securely fixed class t fuse within 200mm of +ve without undue bending that is exercising me.

70mm is definitely overkill, as the others have said, smaller cables are fine for what you've described. I only went with 70 because it was easy for me to do so

 

[geem]

Thanks for the suggestions Geem - really appreciate it. I’ll definitely look at paralleling the cable. With the fuses, the trouble is, for someone starting out on a project like this for the first time, looking down a list of fuses in somewhere like RS just boggles the mind. Can’t find any that are described as NH00 - NH2 etc. yes, but then all sorts of different specs, tags etc. I simply don’t know enough to make a choice from a menu of ‘everything’, for all sorts of purposes, which may or may not be suitable for my purpose. So I have instead to rely on trusted retailers like 12v planet and assume that what they sell will suit my purpose, and they also sell a nice holder that I know will match the fuse…. To acquire the sort of expertise I would need to second guess the reputable sellers for all the things that I need to do on the boat this (or any other) winter, I would do better to just sell the boat and spend my life in the library! One day, as I absorb all the invaluable info. I get from here, I will feel able to go ‘off piste’, but for now, to get the job done, I need to go for the most accessible solutions, even if that costs a bit more.

You need NH00 fuses gG class. Suitable for DC applications. They are about £8 each. You eill also need a fuse holder. My fuses are Siba and my fuse holder are Siemens

 

[stranded]

Btw. what is the purpose of the sheathing for >200mm to the fuse - is it just to protect against physical damage?

 

[PaulRainbow]

Btw. what is the purpose of the sheathing for >200mm to the fuse - is it just to protect against physical damage?

Yes.

 

[PaulRainbow]

I sized my cables for the rating of my BMS. 200A. My maximum load will come from the inverter. This won't go above 150A as that's the setting of my breaker. My individual battery fuses are sized to suit the two different sized batteries. 225A fuse on the 280Ah battery and 160Ah fuse on the 105Ah battery. I can run both batteries separately if I had a failure on one battery.
Being a 24v boat makes all this easier. Cables are 35mm2 generally

Same here.

 

[stranded]

Right. I’ve got my battery and class t fuse. The existing battery cables are either 50 or 70mm2 (casing label suggests 50mm2 “1-50”, but suppose that might mean something else, whereas external diameter of 16.5mm suggests might also be 70mm2, or fat insulation. We’ll see when I cut it.

So about to order dc to dc charger and while I tend to default to Victron, the Sterling BB1240 has a reverse charge facility which means that when the service battery is well charged it will top up the start battery. This seems like an easy way to introduce solar top up to the start battery - never needed it but seems no harm? Any downsides to this?