Is it OK to daisy chain battery negative?

CreakyDecks said:
What are you planning to connect the old negative cable to? You need a complete circuit (a loop) and the same current will flow at all points in that loop. With a proviso that if part of the loop is formed by another circuit, then the current flowing in that loop will also be added, so your "shared" negative wire could be carrying considerably more than just the starter motor current. I think the best solution would be to replace the existing wire.

Cross posted!
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The domestic side has its own neg cable, so that is forming the loop. I am a bit confused (more ignorance on display) by the helpful comments about additional loads - when starting the engine the only other items running I'll be the engine room fan, fridge and maybe some lights. Probably about 10 amps all in - the starter battery is capable of producing 1000 CCA and whilst the starter motor doesn't use that much will the negative side notice an extra 10 amps?

But all the comments suggesting I be wary means I think I'll beef up one of the negative cables and daisy the two sets of batteries.
 
The concern is that it sounds like you are planning to attach the starter motor's negative terminal to some convenient point on the domestic circuit's negative side, in which case the starter current will flow through that wire on its way back to the battery. There might be a couple of hundred amps flowing in a starter motor in cold weather.
 
The Lazy Kipper said:
Hmmmmmm. Not sure actually, though this would be a good time to find out and upgrade if necessary. I wish I understood electricity - I always thought the power went from the positive side, does the starter take more juice than it needs and send the excess back via the negative, like the fuel return on a diesel engine? :o:o:o
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No; the amperage is the same on both sides. Think of it like water in a pipe - the amps are the rate of flow of the electricity. If so much is leaving the battery on one side, the same amount has to go back on the other side.

In fact, if you have a common negative connection and separate positive connections (as proposed) then it is feasible that under some circumstances more current will flow through the negative side than through any one of the positive connections - think of it as being the outflow from several pipes coming back into one common pipe. However, as the greatest drain is the starter motor, it is unlikely you'd be running that the same time as any other large load.

More technically, Ohm's Law and Kirchoff's Laws lead to this result as well.
 
AntarcticPilot said:
No; the amperage is the same on both sides. Think of it like water in a pipe - the amps are the rate of flow of the electricity. If so much is leaving the battery on one side, the same amount has to go back on the other side.

In fact, if you have a common negative connection and separate positive connections (as proposed) then it is feasible that under some circumstances more current will flow through the negative side than through any one of the positive connections - think of it as being the outflow from several pipes coming back into one common pipe. However, as the greatest drain is the starter motor, it is unlikely you'd be running that the same time as any other large load.

More technically, Ohm's Law and Kirchoff's Laws lead to this result as well.
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careful at the back, kirchhoff's law is applicable, ohm law isnt - the load from the starter motor is not resistive SO you must ensure the earth/return/negative whatever you wish to call it is sized to maintain the CCA of the battery which appears to be 1000A. Read about what happens to electric motors and current (amps) if the voltage is low to find out why.
 
CreakyDecks said:
The concern is that it sounds like you are planning to attach the starter motor's negative terminal to some convenient point on the domestic circuit's negative side, in which case the starter current will flow through that wire on its way back to the battery. There might be a couple of hundred amps flowing in a starter motor in cold weather.
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Currently both sets of batteries have a negative battery cable that run independently to the common earth on the engine which is where the starter motor's earth is. In that sense they are already linked, based on Antartic Pilot's post above by making the link between them a short and direct one, all the return is running along one cable, which as mentioned by others is a concern if that cable isn't up to it. Having said that, the first four replies all said that's how their installation works :confused:

So it seems like two choices:

1. Beef up the domestic negative cable and daisy chain
2. Replace the currently inadequate starter negative cable (appologies for the pun :o)

I prefer option one because it gives me less cable and I know the replacement cable will be more than adequate, is there still merit in option two?

And ref Antartic's post, if the current returning is the same as the one leaving, how has energy has been used up?
 
The Lazy Kipper said:
And ref Antartic's post, if the current returning is the same as the one leaving, how has energy has been used up?
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Brilliant, someone who understands even less than I do about electricity ;-) .

It's the idea of current 'leaving' and 'returning' to the battery that is faulty. Current will flow when there is a circuit with a potential difference (voltage) as part of the circuit. The current will do some work somewhere (heat something up, make something glow, make a magnetic field or whatever) and the store of potential difference in the battery will gradually be used up as the battery discharges. You can get an idea of how much work is being done by the current by measuring the voltage at the battery; at rest your battery will show say 12.6 volts but as soon as you apply a load such as a light or a starter motor the voltage will drop. I thoroughly recommend The 12Volt Bible for Boats as a guide for the electrically illiterate.
 
The Lazy Kipper said:
The domestic side has its own neg cable, so that is forming the loop. I am a bit confused (more ignorance on display) by the helpful comments about additional loads - when starting the engine the only other items running I'll be the engine room fan, fridge and maybe some lights. Probably about 10 amps all in - the starter battery is capable of producing 1000 CCA and whilst the starter motor doesn't use that much will the negative side notice an extra 10 amps?

But all the comments suggesting I be wary means I think I'll beef up one of the negative cables and daisy the two sets of batteries.
Click to expand...

Provided the cable from the negative terminal of the domestic battery to the "earthing point" is heavy enough to carry the starter motor current you will be OK.
The fear was that the cable from the domestic battery may not have been heavy enough.

Just check... all cables that carry the return current from the engine to the starter battery negative must be heavy enough to carry the starter motor current.
 
The Lazy Kipper said:
And ref Antartic's post, if the current returning is the same as the one leaving, how has energy has been used up?
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The same way as a hydraulic system works; as another has said by reducing the potential difference produced by the battery. Simplistically, N electrons leave the battery (actually from the negative pole) and N electrons have to return, otherwise there would be a build up of charge somewhere. Amps simply measures the number of electrons passing along the wire in a given time. What does the work is the voltage DROP across the equipment; like a waterwheel has the same amount of water flowing in and flowing out, but the fall in elevation has done work turning the wheel.

For the pedantic, these are illustrations only; I am not suggesting that the actual mechanisms are anything like this!

And thanks for the correction about Ohm's Law; I had forgotten there would be non-resistive loads in the starter circuit. But for most boaty circuits, it is good enough for government work.
 
AntarcticPilot said:
No; the amperage is the same on both sides. Think of it like water in a pipe - the amps are the rate of flow of the electricity. If so much is leaving the battery on one side, the same amount has to go back on the other side.

In fact, if you have a common negative connection and separate positive connections (as proposed) then it is feasible that under some circumstances more current will flow through the negative side than through any one of the positive connections - think of it as being the outflow from several pipes coming back into one common pipe. However, as the greatest drain is the starter motor, it is unlikely you'd be running that the same time as any other large load.

More technically, Ohm's Law and Kirchoff's Laws lead to this result as well.
Click to expand...


That's how I like to think of it. Amps as amount of flow and volts as pressure pushing it along. In even more simpler terms if the water in a pipe was turning a wheel the amount of water going in has to be the same as the amount coming out but you lose some pressure this would be the volts drop.
 
The Lazy Kipper said:
Currently both sets of batteries have a negative battery cable that run independently to the common earth on the engine which is where the starter motor's earth is. In that sense they are already linked, based on Antartic Pilot's post above by making the link between them a short and direct one, all the return is running along one cable, which as mentioned by others is a concern if that cable isn't up to it. Having said that, the first four replies all said that's how their installation works :confused:

So it seems like two choices:

1. Beef up the domestic negative cable and daisy chain
2. Replace the currently inadequate starter negative cable (appologies for the pun :o)

I prefer option one because it gives me less cable and I know the replacement cable will be more than adequate, is there still merit in option two?

And ref Antartic's post, if the current returning is the same as the one leaving, how has energy has been used up?
Click to expand...
I would size the cable from the domestic battery to take ALL loads from both batteries. Main battery cables are normally 16mm sq as far as I know (depending on length and size of engine) but I would increase to 25mm sq to be on safe side. (pretty chunky - you'll need a hydraulic crimper to get the terminals on)
 
I think this is making sense. So provided either cable from the domestic or the starter batteries is big enough to carry the maximum peak load from the largest draw - i.e. the starter motor - then that is fine. Earlier someone mentioned 200amps as a likely maximum draw for the starter (Thornycroft 245 75hp) - is aiming for a cable capable of 500amps sufficient? I know there are strange peaks in electricity draw, e.g. the bow thruster has a nominal rating of 250amps but the recommended fuse is 400amps because of these surges in load. The cable run from battery to starter via mega fuse and 1,2,both switch is only about 3m (negative side is a shorter run obviously) so cost isn't the issue.
 
ghostlymoron said:
I would size the cable from the domestic battery to take ALL loads from both batteries. Main battery cables are normally 16mm sq as far as I know (depending on length and size of engine) but I would increase to 25mm sq to be on safe side. (pretty chunky - you'll need a hydraulic crimper to get the terminals on)
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16mm is a too small. a 3kw starter motor will draw about 250 amps. for that you will need 35mm sq. 16mm is only rated at 110amps.
 
At the end of the day we have to concede that in a majority of cases - on boats - there is a vital component omitted from starter circuits - the fuse! A cable capable of 500amps should be OK as long as the battery is charged, including a 400amp fuse would be a very good idea. It would protect the starter and wiring from burnout.
 
Marsupial said:
At the end of the day we have to concede that in a majority of cases - on boats - there is a vital component omitted from starter circuits - the fuse! A cable capable of 500amps should be OK as long as the battery is charged, including a 400amp fuse would be a very good idea. It would protect the starter and wiring from burnout.
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You don't want to see one of them go bang.:eek:
 
I fitted a Mega fuse in each of the positive cables before the 1/2/both switch at the recommendation of the electrical surveyor, but I wasn't 100% certain what it protects, each downstream load has its own fuse or circuit breaker so I dis wonder what the Mega fuse protects?
 
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