smartbank relay wiring thickness

[Nev]

Hi,

 

Another request for help. I am swapping out the diode split charge controller on my boat for a smartbank advance (I already have the smartgauge)all just about installed but now I need to connect the two battery banks (85 ammp starter and 220 amps of leisure via two batteries) to the two respective teminals on the swithching relay. (It is the one that comes with the smartbank advances - 180 amps)

 

The original cables that connected the diode split charge 'controller' seem undersize... 8mm diameter inc insulation.... they are crimped up I expect them to be 6mm?

 

In ordere to calculate the right cables I have read the info on the smartgauge web site on cable size... smartgauge cable info

 

the calc is volt drop (I am using .05 x1000 / current in amps (I am using 220 as the maximum from the leisure battery to the starter if I use emergency start mode ?) divided by the cable run lenght in metres total for my install will be 1 metre

 

cable size[mm2]=18*metres*amps/(V*1000)

 

this gives me 18*1*220 / (.05*1000) = 79.2mm

 

Obviously wrong !! the only variable I am adding I doubt is the amps. I have assumed 220 as the maximum but this must be wrong to give me such a large cable size. Can someone put me right so i can go and get the cables made up !!

 

Thanks in advance,

[Paul C]

You don't have 220 amps of leisure batteries.....you have 220 amp HOURS of leisure battery CAPACITY

 

The actual current which will flow will be less than 220 amps, I imagine. To be sure, you'd need to look at the way your leisure batteries are charged, for example how many alternators do you have? Do you have shoreline (mains power), and if so how large is your battery charger? Any solar or wind? etc

 

Also you could go a little less conservative on the volt drop. 0.05V volt drop at 220A current does indeed need a thick cable, as your calculations have revealed. If you were happy with 0.05V volt drop at say, 10A, but conceded that you may get 0.1 or 0.2 at higher currents, then you don't need so thick wires. You get to choose the voltage drop though.

[Keith M]

Hi,

 

Another request for help. I am swapping out the diode split charge controller on my boat for a smartbank advance (I already have the smartgauge)all just about installed but now I need to connect the two battery banks (85 ammp starter and 220 amps of leisure via two batteries) to the two respective teminals on the swithching relay. (It is the one that comes with the smartbank advances - 180 amps)

 

The original cables that connected the diode split charge 'controller' seem undersize... 8mm diameter inc insulation.... they are crimped up I expect them to be 6mm?

 

In ordere to calculate the right cables I have read the info on the smartgauge web site on cable size... smartgauge cable info

 

the calc is volt drop (I am using .05 x1000 / current in amps (I am using 220 as the maximum from the leisure battery to the starter if I use emergency start mode ?) divided by the cable run lenght in metres total for my install will be 1 metre

 

cable size[mm2]=18*metres*amps/(V*1000)

 

this gives me 18*1*220 / (.05*1000) = 79.2mm

 

Obviously wrong !! the only variable I am adding I doubt is the amps. I have assumed 220 as the maximum but this must be wrong to give me such a large cable size. Can someone put me right so i can go and get the cables made up !!

 

Thanks in advance,

 

 

The initial current flow between the two battery will easily around 200 amps for a short time

Starting current can also be a similar amount of current while the engine is cranking

And the volt drop I would work to is 3%

 

Hope the above helps

 

keith

[Nev]

Paul & Keith,

 

Thank you for taking the time to provide replies with useful information. In the end I went to Merlin who very helpfully provided the cable information ...AWG6 13mm2. I have Awg5 made up which I understand to be thicker which they said would be fine,

 

Sorted, just got to tidy up the wiring and test it all out,

 

Thanks again,

 

Nev

[Roger Crown]

Hi Nev,

13mm2 sounds way too small.

To determine the correct cable size for the relay supplied with the Smartbank Advanced, we need to know the maximum output of each charging source. Battery bank size is not relevant. (if it was,the supplied 180A relay would be useless)

 

I hope this helps

Regards

Roger

[Nev]

Hi Nev,

13mm2 sounds way too small.

To determine the correct cable size for the relay supplied with the Smartbank Advanced, we need to know the maximum output of each charging source. Battery bank size is not relevant. (if it was,the supplied 180A relay would be useless)

 

I hope this helps

Regards

Roger

Roger,

 

It is a 70amp alternator.... I am a little worried now I have put the boat back with the original AWG5 cables !

 

Nev

[Paul C]

Roger,

 

It is a 70amp alternator.... I am a little worried now I have put the boat back with the original AWG5 cables !

 

Nev

 

Don't worry, 13mm2 doesn't sound too small to me. Now that you've given us the alternator size, a quick rerun of the calculation shows all is okay.

[Roger Crown]

Roger,

 

It is a 70amp alternator.... I am a little worried now I have put the boat back with the original AWG5 cables !

 

Nev

Hi Nev

I wouldn't use anything less than 25mm2 (with that size alternator, usually bigger), its not just about the current carrying capacity of the cable. The less voltage drop you introduce into the equation the better charging you will get.

Regards

Roger

[Nev]

Hi Nev,

13mm2 sounds way too small.

To determine the correct cable size for the relay supplied with the Smartbank Advanced, we need to know the maximum output of each charging source. Battery bank size is not relevant. (if it was,the supplied 180A relay would be useless)

 

I hope this helps

Regards

Roger

 

Roger,

 

My concern was when using the emergency start the leisure batteries are connected to the starter via the 180 amp relay.... When I look at the size of the positive cable on the engine batter to the starter Motor it is much bigger (AMG2)

 

Getting more confused

 

Nev

[ditchcrawler]

Roger,

 

My concern was when using the emergency start the leisure batteries are connected to the starter via the 180 amp relay.... When I look at the size of the positive cable on the engine batter to the starter Motor it is much bigger (AMG2)

 

Getting more confused

 

Nev

I think most of them missed that bit Nev.

[Keeping Up]

Also, do you have a large inverter? If you are using the inverter on a high-powered load at a time when the domestic batteries are very low (eg when you have only just started the engine) then you can also get quite a high current flowing back from the starter battery which tries to help the poor domestics.

 

(Edited for spilling mistakes)

Edited February 15, 2013 by Keeping Up

[Nev]

Also, do you have a large inverter? If you are using the inverter on a high-powered load at a time when the domestic batteries are very low (eg when you have only just started the engine) then you can also get quite a high current flowing back from the starter battery which tries to help the poor domestics.

 

(Edited for spilling mistakes)

 

Allan,

 

Thanks for the post, I did not know it allowed the starter to help the domestics via the relay. I presume this is based on setting for the Smart bank related to level of charge.

 

Nev

[Keeping Up]

Allan,

 

Thanks for the post, I did not know it allowed the starter to help the domestics via the relay. I presume this is based on setting for the Smart bank related to level of charge.

 

Nev

 

 

It is an unintentional side-effect of just having a relay (there's an advantage of splitter diodes over relays). I don't know the details of the Smartbank, but I would assume that if it is set up properly it should help to prevent this from happening when you don't want it to. All the same it is worth investigating, to see what is the maximum extent to which it can happen.

 

ETA having now read the Smartbank's technical description, I see that it would almost certainly not allow the starter battery to feed the domestics to any significant degree if at all. So I don't think you need worry about it Nev - my post should just stand as a warning for anyone who is reading this thread in the context of relay wiring rather than of Smartbank wiring.

Edited February 16, 2013 by Keeping Up

[Keith M]

Roger,

 

My concern was when using the emergency start the leisure batteries are connected to the starter via the 180 amp relay.... When I look at the size of the positive cable on the engine batter to the starter Motor it is much bigger (AMG2)

 

Getting more confused

 

Nev

 

Hi Nev

 

I would follow the advice that was given by Crown narrow boats he is a very good sparks

I install this type of system into vans etc and we always use a minimum of 35 mm

I hope that clears up your confusion

That cable must be replaced or it may cause a fire if it over heats

 

Keith

[Nev]

As an aside in my reading on this I have come across this table that helped me a great deal in converting the respective sizes (MM and MM2) plus the AMG sizes

 

A nice lookup table

 

The cable calculator I missed from the pinned link above !

 

Edited to get the link right (and now to add the link the the cable calculator I should have used in the first place !)

Edited February 16, 2013 by Nev

[Iain_S]

Also, do you have a large inverter? If you are using the inverter on a high-powered load at a time when the domestic batteries are very low (eg when you have only just started the engine) then you can also get quite a high current flowing back from the starter battery which tries to help the poor domestics.

 

(Edited for spilling mistakes)

 

We had that situation on Copperkins for a time, when the Hoover was used, and the alternator couldn't keep up. The relay would drop out for the minute and a half I'd set on the Smartbank, then come back in again. While the relay was in the disconnect position, the cabin bank wasn't getting a charge, which made things worse ... Not a problem after a few hours cruising, but now cured by connecting the alternator to the cabin bank, rather than the starter battery. The relay still drops out, but now stays out until the cabin bank has been charged back up a bit. (And current through the relay should be much reduced in normal operations)

 

Iain