Battery cable size - link

[zoidberg]

I'm a bit hesitant to ask, considering how swiftly a simple 'lecky query can degenerate. But.... A question about battery cables' routing and connections.....

I have 3 battery groups Starboard, Port and Start. The negative leads all go to an accessible Power Post conveniently close to the designated 'ground' bolt on the engine block.

The Start battery has 50mm^/345amp cable, positive and negative, and offers CCA of 815amps nominal. The Port 'services' battery group uses 25mm^/170amp cable, with CCA of 2x845amps nominal, and this is close adjacent to the Start battery.

I'm thinking of attaching the 25mm^ Port 'services' negative cable to the Start battery negative terminal, to which is also attached the 50mm^ Start negative cable leading to the negative Power Post and on to the designated engine stud.

A: Is this OK, or should the two different-size negative cables run independently to the Power Post?


Next, I want to provide an 'Emergency Boost' to the Start battery, if necessary, by fitting a Positive cable from the Port 'services' group to the Start battery positive terminal nearby - with an Isolator On/Off switch in the middle. That positive battery terminal is connected to the Start solenoid direct by its 50mm^ cable.

B: Does this 'Emergency Boost' link cable need to be also 50mm^, or can it be 25mm^.


Cube fuses are directly attached to P&S 'services' battery groups appropriate to 25mm^/170amp cable. No fuse is presently attached to the Start battery cables.

Thoughts?

 

[VicS]

I'm a bit hesitant to ask, considering how swiftly a simple 'lecky query can degenerate. But.... A question about battery cables' routing and connections.....

I have 3 battery groups Starboard, Port and Start. The negative leads all go to an accessible Power Post conveniently close to the designated 'ground' bolt on the engine block.

The Start battery has 50mm^/345amp cable, positive and negative, and offers CCA of 815amps nominal. The Port 'services' battery group uses 25mm^/170amp cable, with CCA of 2x845amps nominal, and this is close adjacent to the Start battery.

I'm thinking of attaching the 25mm^ Port 'services' negative cable to the Start battery negative terminal, to which is also attached the 50mm^ Start negative cable leading to the negative Power Post and on to the designated engine stud.

A: Is this OK, or should the two different-size negative cables run independently to the Power Post?


Next, I want to provide an 'Emergency Boost' to the Start battery, if necessary, by fitting a Positive cable from the Port 'services' group to the Start battery positive terminal nearby - with an Isolator On/Off switch in the middle. That positive battery terminal is connected to the Start solenoid direct by its 50mm^ cable.

B: Does this 'Emergency Boost' link cable need to be also 50mm^, or can it be 25mm^.


Cube fuses are directly attached to P&S 'services' battery groups appropriate to 25mm^/170amp cable. No fuse is presently attached to the Start battery cables.

Thoughts?

If I was going to use a services battery for emergency starting would wire it with the same gauge of cable as the exiting starter battery, both positive and negative. Ie in this case 50mm².

BTW it is better to put the emergency link on the switched side of the isolators, rather than the battery side so that a duff battery can be left isolated bit it sounds like you dont have an isolator for the start battery.. I would also therefore fit one. Charging arrangements might also have to be taken into account.

Must dash, shopping to do !

 

[lw395]

Here's a little calculator to play with.
Alas it's in RedNeckUnits:
http://nepsi.com/resources/calculators/short-time-current-rating-of-conductor.htm

You will see that for fat cables, the allowable current for say 10 seconds of cranking can be a whole lot more than the steady-state.
I doubt you'll find many vehicles where the starter wiring is continuously rated for the CCA current or even the nominal power of the starter motor.
I'm not sure where one should draw the line.

However, since you a re not spec'ing a production run of Bavarias, there will be very little criticism coming your way if you use thicker cable than necessary.

 

[rogerthebodger]

If I was going to use a services battery for emergency starting would wire it with the same gauge of cable as the exiting starter battery, both positive and negative. Ie in this case 50mm².

BTW it is better to put the emergency link on the switched side of the isolators, rather than the battery side so that a duff battery can be left isolated bit it sounds like you dont have an isolator for the start battery.. I would also therefore fit one. Charging arrangements might also have to be taken into account.

Must dash, shopping to do !

I agree with Vic use the same size cable size. It not just the current carrying capacity its also the potential volt drop during engine cranking.

Are you going to keep your port and starboard domestic banks separate with separate isolators if so you emergency bridging is more complicated.

I also have 2 seperate domestic banks and seperate engine start battery with seperate isolators but use the 2 domestics as a single bank. I keep then separate as the age of the 2 banks are quite different so wish to be able to keep then seperate when and if one bank goes down.

Also consider the current requirements fro say a windlass of bow thruster. mine take similar and much greater current than engine starting for a longer duration.

 

[zoidberg]

That's helpful of you, 'lw395'. Thanks....


I have played about with the calculator. Making some 'blind' assumptions...... Assuming the 50mm^/AWG 1 cable and the CCA815 full monty from the Start battery, normally, then...

That cranking time of 27 seconds seems OK, if......


Should I need to add a second battery giving a further CCA845, at CCA1660 the calculation of cranking time becomes an unacceptable 6.7 seconds.

Perhaps more reasonably, should I need to supplement a very weak Start battery with additional 'grunt' from the second battery - guessing CCA1000 - then it looks like the 50mm^ cable could cope with the heating for 18.5 seconds.

I can't see me cranking for longer than, say, 10 seconds without giving the system a rest, to cool, and to investigate 'why no joy'. That seems to endorse the argument for makingthe 'Emergency Link' from 50mm^ cable. However, would the On/Off switch cope?

Any observations? Any better ideas....?

 

[PaulRainbow]

Ordinarily, all cables that can be used in the starting of the engine should be rated for, erm, starting the engine. So, the domestic bank would have the same size negatives as the starter battery and the positive cable to the emergency switch would also be the same size.

In your case, a proper job would be to fit 50mm negative cable between the port services battery and the engine battery, 50mm positive cable to the port services isolator switch, plus 50mm cable from the load side of the isolator to the emergency switch. You then need a fuse suitable for 50mm cable, or the max current draw of the starter. No point fitting bigger cable without uprating the fuse. If any part of the emergency circuit is 25mm it may as well all be 25mm.

Unless you have an unusually large engine or it's a mile from the batteries, 50mm cable is over rated, it's commonly 35mm on most modest cruisers. 25mm might be a little under rated if you were using it for the main engine battery, but if it's just for a jump start you would likely be ok. If you leave the 170a fuse in place it's perfectly safe, the worst things that would happen is the engine wouldn't start or the fuse would blow, i doubt either would happen.

Fit the emergency switch cables to the load terminals of the service and engine isolators, not the battery terminals. You can test the ability of the services bank to start the engine by switch the engine isolator off, the emergency switch on and starting the engine.

 

[PaulRainbow]

Any observations?

Too much thinking, guessing, assuming and calculating

 

[lw395]

I think the CCA of the battery is no guide to the actual current involved.
I think if you put 800A through the starter motor it will catch fire before the cables.

I'm told that starter motors are actually designed to be wired with some resistance in the cables to limit the inrush current. Actual data is a bit scarce though.
Things tend to be optimised for a best guess of typical use, not some theoretical lab condition of a zero impedance system.

Realistically, is there significant expense or difficulty in erring on the fat side?

100A is a fair round number for starter current. How long is the total cable run, what will the volts at the motor be at 100A?

 

[PaulRainbow]

How long is the total cable run, what will the volts at the motor be at 100A?

Most of the cable is 50mm, so it's not the total run, it's the length of 25mm cable.

The "start assist" feature of the Victron VSR uses a 120a relay and 10mm cables to "jump start" the engine.

 

[Balbas]

This thread has been very timely for me. In response to an annoying earth fault which is giving me starting problems, I was about to buy 5m of 70mm cable in order to make up a 3 metre run from the engine start battery and a 2m run from the domestics.

I was working on the old formula 'P for Plenty' and thinking I may as well 'go large'. Based on the replies above I shall save myself about £20 and stick to 50mm.

 

[lw395]

Most of the cable is 50mm, so it's not the total run, it's the length of 25mm cable.

The "start assist" feature of the Victron VSR uses a 120a relay and 10mm cables to "jump start" the engine.

Each metre of 25sqmm cable has double the resistance of a metre of 50sqmm.

'A 'start assist' is not always the full shilling, nor does it need to be. Most likely the 'start' battery is not completely dead, and paralleling it with the house battery will make everything good enough. Without needing more than 120A from the house side.
If you see it as 'a get out of jail' card when your start battery is on its last legs, it will work. If you completely destroy the start battery, it might not. Just closing that switch can often warm the start battery a little, tipping things into the zone where the engine starts. Smart people then fix the fundamental problem.

At the core of it, I like to trust the start battery to start the engine. I will buy new batteries to make this so. Everything else is complication, excuses and prone to circular arguments. I know people who are uncomfortable if there's not at least one engine with a pull start.

 

[zoidberg]

Finding the engine won't start won't be the end of the world. I'm of a vintage that apprenticed on boats with 'dicky' engines and batteries, and have sailed onto/off moorings and pontoons as far apart as Lerwick and Brest, on more occasions than I care to count.

The last old engine I had, a marinised Kubota tractor unit with a B-I-G flywheel, had decompressors. Not so, this one.

I'm not able to guess what the needed starting current is likely to be, so I'm grateful for the above guidance. Initial advice suggested 35mm^ would suffice, but that forking out a couple of extra £s for 50mm^ cable in the start circuit would do no harm, should the start-battery voltage be a bit depressed one morning. So I did. And I'll take the guidance here that it'll do no harm to provide 50mm^ where there's a possibility of providing hefty 'boost' power in a selectable link, with an isolator switch of some form.

I could use one of the 300A Durite Battery Isolators from 12VoltPlanet I mentioned before, or one of the several Key-types or the Perko I-2-Both-Off switches I have going spare.

Or even jump leads.....