Is it OK to daisy chain battery negative?

[Rivers & creeks]

The engine start battery has just failed so I have a new one to fit. The negative cable is very poor so I thought why not just use a short link cable to the house batteries right next to it? Does each battery need its own negative link? Linking to the other batteries also have the benefit of removing another cable from the spaghetti in the engine room

 

[sailorman]

The engine start battery has just failed so I have a new one to fit. The negative cable is very poor so I thought why not just use a short link cable to the house batteries right next to it? Does each battery need its own negative link? Linking to the other batteries also have the benefit of removing another cable from the spaghetti in the engine room

yes,my engine & domestic share neg`s

 

[DavidGrieves]

Yeah most use a common negative bus bar. Just ensure the cable is big enough to handle the starting loads.

 

[Appledore]

+1

 

[lw395]

Since the start battery is highest current, it should ideally have the best return path.
Less connections is better, but a daisy chain will work fine if done properly with fat cable.
It does mean using fatter more awkward cable than would be needed for house wiring, so may not be ideal.
In practice, if all connections are clean and tight it will probably be fine, but if it does not crank fast I would check the volts dropped on both feed and return.

 

[Spuddy]

I installed a second battery next to the engine start and got a made up link from an autoelectrician to daisy chain the negatives.

 

[sailorman]

I installed a second battery next to the engine start and got a made up link from an autoelectrician to daisy chain the negatives.

yes then

 

[SJF]

Is it OK to daisy chain battery negative?

As long as you don't have any kind of shunt for a battery monitor fitted (such as for a BM-1) in the negative wire and the existing negative wiring is of a suitable size then yes.

 

[Chalker]

Are you sure that the service batteries' negative has a heavy enough cable to the engine for the starter supply?

 

[Rivers & creeks]

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?

 

[nigelmercier]

Are you sure that the service batteries' negative has a heavy enough cable to the engine for the starter supply?

... AND for its own load

 

[VicS]

The engine start battery has just failed so I have a new one to fit. The negative cable is very poor so I thought why not just use a short link cable to the house batteries right next to it? Does each battery need its own negative link? Linking to the other batteries also have the benefit of removing another cable from the spaghetti in the engine room

The batteries would normally have the negatives connected but there must be a heavy cable all the way from the batteries to the negative connection on the engine that is capable of carrying the starter motor current.

I fear you may be removing a vital cable.

 

[MrCramp]

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?

This site is one of the best I have come across for explaining about boat electrics, and much more.
http://www.tb-training.co.uk/index.htm

 

[sailinglegend420]

As long as you don't have any kind of shunt for a battery monitor fitted (such as for a BM-1) in the negative wire and the existing negative wiring is of a suitable size then yes.

With a battery monitor installed the bus bar downstream of the shunt becomes the new battery post for the service bank, so the starter battery negative must also be connected to the bus bar, not to a service battery negative terminal. This error is probably one of the major causes of battery monitors not reading the charging amps. This will often happen when the starter bank has priority and is charged first because the charger positive and negative cables will be connected to the starter battery. When the split charging system supplies positive volts to the service bank the charger will supply the negative arm via the starter battery post and then to a service battery negative stud. For the battery monitor to measure the service battery charge the starter battery negative must go through the new shunt.

 

[nigelmercier]

With a battery monitor installed the bus bar downstream of the shunt becomes the new battery post for the service bank...

Indeed, the only connections that should be made to the battery negative are to the battery monitor and the shunt.

 

[Norman_E]

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?

I take it you're joking, but just in case, the current in any circuit is the same on positive and negative.

Just one thought for the original poster. On my boat, and I suspect many others there are three rotary battery isolator switches. Two are on the positive cables from the domestic and start batteries respectively, and the other isolates all of the negatives. The domestic bank are daisy chained and then wired to the isolator, but the start battery has its own cable to the terminal on the isolator. I think that this could be because the combined load of both starter and domestics could exceed the total capacity of a single cable, though all of them are pretty massive.

 

[nigelmercier]

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?

If you are serious, I strongly suggest you engage the services of a boat electrician

 

[SJF]

With a battery monitor installed the bus bar downstream of the shunt becomes the new battery post for the service bank, so the starter battery negative must also be connected to the bus bar, not to a service battery negative terminal. This error is probably one of the major causes of battery monitors not reading the charging amps.

And is exactly the reason I said:

Originally Posted by SJF View Post
As long as you don't have any kind of shunt for a battery monitor fitted (such as for a BM-1) in the negative wire and the existing negative wiring is of a suitable size then yes.

To make the OP aware of the potential problem he may cause to any battery monitor he has installed.

 

[Rivers & creeks]

Thanks for all the replies. Both the domestic bank and starter battery have their own independent negative cables going to the earthing point. It seems that by daisy chaining I will be relying on the cable from the domestic bank and a large load on both sets of batteries could overload that cable. Mind you, the only time the starter battery is used is when the key is turning the starter motor so that shouldn't be a problem.

My comments which caused such incredulity were based on two replies that called the negative cable the "return". My approach with things I don't understand is to ask first, then do what I can, then ask about the next step. The trouble is that in revealing ignorance you sometimes get hit with the "goodness, if you dont even know that you shouldn't be doing anything!" That's probably not what was intended but I wasn't suggesting rewiring the boat, just asking whether it was wise to daisy chain two battery negatives. Working as a teacher I have learned to value questions that ask for clarification, rather than just responding that if they don't know they should employ someone who does - oh wait a minute they have - me

Anyway, the conclusion seems to be that I need to check the domestic negative to see if it's up to the job of looking after both sets of batteries and only then to link the domestic and starter.

 

[CreakyDecks]

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!