SloopJonB

Multiple Battery Grounding.

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I have to re-do my battery connections and selector switch and need a piece of advice from the brain trust.

I have two batteries - 1 starting and one deep cycle. Currently only the starting battery is connected, grounded to the engine block and the cable runs are ridiculously long. There is a positive cable - also stupid long - for the second battery but no ground. I plan to install a new selector switch close to the batteries - less than 3' away and shorten the positive cables appropriately when I connect them to it.

My question is - can I ground the second battery to the first with a short jumper cable rather than running 2 negative cables to the engine?

Electrical theory ain't my thing and I don't know if this will have some sort of negative -_- effect on the batteries.

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1 hour ago, SloopJonB said:

I have to re-do my battery connections and selector switch and need a piece of advice from the brain trust.

I have two batteries - 1 starting and one deep cycle. Currently only the starting battery is connected, grounded to the engine block and the cable runs are ridiculously long. There is a positive cable - also stupid long - for the second battery but no ground. I plan to install a new selector switch close to the batteries - less than 3' away and shorten the positive cables appropriately when I connect them to it.

My question is - can I ground the second battery to the first with a short jumper cable rather than running 2 negative cables to the engine?

Electrical theory ain't my thing and I don't know if this will have some sort of negative -_- effect on the batteries.

Yes, that's how most dual battery systems are wired.

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5 hours ago, SloopJonB said:

I have to re-do my battery connections and selector switch and need a piece of advice from the brain trust.

I have two batteries - 1 starting and one deep cycle. Currently only the starting battery is connected, grounded to the engine block and the cable runs are ridiculously long. There is a positive cable - also stupid long - for the second battery but no ground. I plan to install a new selector switch close to the batteries - less than 3' away and shorten the positive cables appropriately when I connect them to it.

My question is - can I ground the second battery to the first with a short jumper cable rather than running 2 negative cables to the engine?

Electrical theory ain't my thing and I don't know if this will have some sort of negative -_- effect on the batteries.

Should be fine, unless you have a battery gauge with a shunt in the negative to your domestic battery. In that case then you have to make sure that the wire splits to go to the engine battery before the shunt not after otherwise it will read incorrectly.

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Why don't you put in a grounding bus bar near both batteries, ground each battery to that, then have one stupid long run from the bus bar to the engine ground?

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2 hours ago, ryley said:

Why don't you put in a grounding bus bar near both batteries, ground each battery to that, then have one stupid long run from the bus bar to the engine ground?

This. A 4 terminal bus bar will do what you want and give you a spare ground stud for future connection.

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2 hours ago, ryley said:

Why don't you put in a grounding bus bar near both batteries, ground each battery to that, then have one stupid long run from the bus bar to the engine ground?

It takes a pretty big bus bar to handle the 5/16" lug of a battery cable. They are available but certainly don't try to put a battery cable lug under the #10 screws of the smaller bus bars.

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4 minutes ago, F_L said:

It takes a pretty big bus bar to handle the 5/16" lug of a battery cable. They are available but certainly don't try to put a battery cable lug under the #10 screws of the smaller bus bars.

Absolutely correct. and it doesn't take into account that you need to make sure that the stupid long run has to be properly sized for both of the batteries on a common bar. But the bar isn't really that hard to find - any of the bluesea powerbars will do. 

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3 hours ago, ryley said:

Why don't you put in a grounding bus bar near both batteries, ground each battery to that, then have one stupid long run from the bus bar to the engine ground?

Interesting idea but the whole point of the change is to get rid of the stupid long cables. What I have in mind will make all the cables about the same length as a typical car setup.

Running the grounds to a buss bar would actually make them longer than "jumpering" them.

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Question from an electrical ignoramus. If all circuits are correctly grounded back to the batteries, why do the batteries themselves need to be grounded at all?

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I’m an ABYC electrical tech. Your instinct is correct: Shorter runs are better. The size of the cables for DC electricity is determined by the load (how many amps are required by what device) and the combined length of both ground and hot cables: From battery to switch to circuit breaker to device to ground (bus bar) to battery. It’s called “common ground” because all DC grounds together. More than three wires should not be stacked on top of each other and wherever possible, only one per connection (terminal) because there is resistance with stacks of terminals and resistance causes heat and overheated wires have more resistance, melting insulation, causing shorts and burning boats to the waterline. Bus bars allow even current and multiple connections without this added resistance, making things not only much safer but also giving each device the amperage it needs without a fuss.

a simple system has the engine block, both house and engine battery grounds and a cable to a small common ground bus bar, all going to the common ground bus. Small ground wires in this picture go to a smaller ground bus that is then connected to the main common ground bus. The idea is to always minimize stacking of terminals and maximizing the efficiency of your electricity. 
Sometimes we simply have to have long runs but where possible, it’s always best to minimize them. Yellow called safety and is preferred to be used to indicate DC ground and avoid confusing a black wire as a DC ground or AC hot, either of which are black. That’s not so important with battery cables but I like the congruent look.

As the previous contributor mentioned, if there is a shunt, batteries ideally ground to a bus bar, then the shunt, then another bus bar for the loads (including battery charger ground). The second picture shows this.

Best wishes!

* Note: I was in the middle of wiring the first battery bank and had not yet secured the cables. That needs to be done every 14” 

1DFFAB3F-87B8-42C1-B848-75FA11A259DA.jpeg

5E26996A-3A3A-4831-A792-41470062AEFA.jpeg

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34 minutes ago, Guvacine said:

Question from an electrical ignoramus. If all circuits are correctly grounded back to the batteries, why do the batteries themselves need to be grounded at all?

Good question: The battery ground terminal provides the ground. In a boat, we connect to a common ground because that is what all devices then use as the ground. The batteries supply that and all their ground terminals are joined, allowing every battery the same electrical level of ground. Think of it as the devices being sailors. Now they are all on one boat, working together.

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1 hour ago, SloopJonB said:

Running the grounds to a buss bar would actually make them longer than "jumpering" them.

Not by much. I dunno but I suspect that future you might thank you for putting in a bus. Jumpering the battery negatives should be fine if the system is going to stay very simple. Forgive my sloppy paint "skills" here a "Cunningham's Law" sketch:

image.png.d15c9af51067605bb1ee2ff6c155ca73.png

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If you take all the grounds on the engine - alternator, instruments etc. off the block and run them to a buss bar and connect it to battery ground then your electrical system is isolated from the boat is it not?

It would seem that would eliminate "self induced" stray current galvanics - leaving only what is in the water in say a hot marina.

Correct?

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7 minutes ago, weightless said:

Not by much. I dunno but I suspect that future you might thank you for putting in a bus. Jumpering the battery negatives should be fine if the system is going to stay very simple. Forgive my sloppy paint "skills" here a "Cunningham's Law" sketch:

image.png.d15c9af51067605bb1ee2ff6c155ca73.png

It will be a very simple system. Here's a pic of how it stands right now. You can see the distance between the ground lugs whereas a buss bar would have to be mounted an the bulkhead on the left, high out of sight in that pic.

It would probably quadruple the length of the cable VS a jumper. It would go from around 1' to several feet - at least 3 or 4 I suspect.

In that pic you can see the yards of excess cabling - only about 20% of the red cable is visible. :o

 

Bilge Pump 4.JPG

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2 minutes ago, SloopJonB said:

In that pic you can see the yards of excess cabling - only about 20% of the red cable is visible. 

Well, I did invoke Cunningham's law, but what red cables are missing? The only excess cable is the very short run from the battery negative to the negative bus bar that should be right next to them. That bus just keeps you from putting excess connections onto your battery posts and cleans things up -- also nice if you want a shunt. But whatever, you do you.

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5 hours ago, weightless said:

Well, I did invoke Cunningham's law, but what red cables are missing? The only excess cable is the very short run from the battery negative to the negative bus bar that should be right next to them. That bus just keeps you from putting excess connections onto your battery posts and cleans things up -- also nice if you want a shunt. But whatever, you do you.

I think you misunderstood my comment - it did not refer to your diagram, rather to the pic of my current setup.

 

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34 minutes ago, SloopJonB said:

I think you misunderstood my comment - it did not refer to your diagram, rather to the pic of my current setup.

I think you are correct. Sorry about that.

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8 hours ago, Guvacine said:

Question from an electrical ignoramus. If all circuits are correctly grounded back to the batteries, why do the batteries themselves need to be grounded at all?

It's should be noted, calling a DC neg circuit a ground is not the best nomenclature.  In many instances on a boat the DC neg and case grounds or equipment grounds are indeed connected however not always.  When thinking of the DC electrical system on the boat you should think in terms of positive voltage and distribution, negative voltage and distribution and equipment grounds as three separate entities.  The positive and negative circut do their own thing a ground buss does it's thing.  Virtually all new equipment will spell out how they want it terminated.

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1 hour ago, SASSAFRASS said:

It's should be noted, calling a DC neg circuit a ground is not the best nomenclature.  In many instances on a boat the DC neg and case grounds or equipment grounds are indeed connected however not always.  When thinking of the DC electrical system on the boat you should think in terms of positive voltage and distribution, negative voltage and distribution and equipment grounds as three separate entities.  The positive and negative circut do their own thing a ground buss does it's thing.  Virtually all new equipment will spell out how they want it terminated.

I would agree if you used isolated power supply for all your 12v electronics so the ground/negative is actually not common. Std design on ships.

This means mister lightning cant see a ground from your wind wand or mast head light.

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Mister lightning can jump from a cloud 5000' feet to the ground. If your boat is in the way, it don't care if your mast head light is grounded.

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Although it can be a pretty lengthy topic, in reference to your typical sailboat it's more of a safety concern.  A grounding provision outside of RF concerns is one of safety, generally providing a path for current in the event of a failure of some sort, and for that reason a good idea not to mix the terminology.  For a super simple boat not something you will get into much but once you start adding equipment it comes into play.

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9 hours ago, Zonker said:

Mister lightning can jump from a cloud 5000' feet to the ground. If your boat is in the way, it don't care if your mast head light is grounded.

How your boat is configured has everything to do with it which is why mast height has nothing to do with it and some boats get killed by lightning over and over and some never get touched whilst in the same anchorage/dock area.

A boat that is properly engineering can support a direct hit to the mast without blowing any electronics up.

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7 hours ago, Sailabout said:

A boat that is properly engineering can support a direct hit to the mast without blowing any electronics up.

Really?  Know of boats that were directly hit that didn't have electronics damage?  I thought the EMP associated with a direct hit inevitably killed all the tiny little transistors in the integrated circuits.

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7 hours ago, Sailabout said:

 

A boat that is properly engineering can support a direct hit to the mast without blowing any electronics up.

I wish that were true, but have a pretty hard time biting on that. We got hit and seeing some of the crazy damage makes me pretty skeptical. As Zonker said I don't think the small electronics can take the abuse even with a rock solid setup.  I've seen way too many electronics get fried on ships from hot work just in the vicinity.  I did just get a quote from Sertec on a setup for our boat $7k ouch!!  Pretty hard sell but still thinking about it they seem to be the closest thing to the real deal outside of all the snake oil salesmen.  

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Back to Sloop,. Based on your pic I don't see a need for a neg buss.  If you had a bunch of stuff close to the batteries and wanted to eliminate runs to the panel it makes sense to add a neg buss but that doesn't look like the case.  Since you only have two lugs per jump I would just Daisy chain the neg run.  For the long run to the panel I would up size.  Pretty firm believer in the small cost of larger cable being well worth it.  It's a pain to run any of it but if you go big in long distribution runs you are setup for later on. Short local jumpers are easy to change.

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5 hours ago, Zonker said:

Really?  Know of boats that were directly hit that didn't have electronics damage?  I thought the EMP associated with a direct hit inevitably killed all the tiny little transistors in the integrated circuits.

you would think but after seeing a couple hit and no damage I learnt otherwise.

They did have a full kit from the furry stick on top of the mast ( burnt to a crisp)to heavy cable from mast base to grounding plate.

I did race on several boats that were huge lightning magnets as in lighting anywhere around and the B&G wand would blow up at a minimum.

 

Re commercial stuff, I was on a drilling rig that got a hit and it got 2 of the 3 DGPS antennas trashed and burnt the cable down to the lightning arrestors and thats all that happened.
I know radars sometimes get zapped and destroyed on commercial vessels but never heard of complete destruction like yachts get.
Is that because yachts arent faraday cages or they isolate the negative and earth the cases of all electronic gear so each unit in its own faraday cage or both?

Lightning doesnt have rules, thats one thing i have come to learn.

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Commercial vessels, typically being steel or aluminum, are much better grounds and do act like Faraday cages to some degree.  Probably more because the charge runs on the surface of the entire ship to the ocean and is quickly dissipated.

The commercial airliners have their electronics boxes in mesh cages I believe and may have optical interfaces so no copper to bridge the gap.

And lightning does come in various sizes too - bolts can vary a lot in power.

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11 hours ago, Zonker said:

Commercial vessels, typically being steel or aluminum, are much better grounds and do act like Faraday cages to some degree.  Probably more because the charge runs on the surface of the entire ship to the ocean and is quickly dissipated.

The commercial airliners have their electronics boxes in mesh cages I believe and may have optical interfaces so no copper to bridge the gap.

And lightning does come in various sizes too - bolts can vary a lot in power.

I have a buddy that added 12 12 islolated power supply for his race yacht and moored where lots get hit and he never had a problem over 20 years.

B&G have finally said in very small print we recommend 12 12 islolated power supplies for electronics.
That breaks the circuit that says your wind wand has an earth via your depth transducer or anything else with a ground.
Anchor light to thru hulls?

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 Some friends were delivering a boat down the east coast a few years ago, couple hundred miles offshore.  About halfway down they got hit by a couple weather systems that turned into a loong storm.  Second night of the bad weather, they got their go kits out as it was real bad.  For unknown reasons, one of the guy stuck his keys, wallet and cell phone into the microwave in the galley right by the companionway.  It was the handiest cabinet that he thought would stay closed, he said, and he stuck them in there because he got a hasty call to get on deck to fool with the sea anchor or somesuch.  He forgot about the phone and they were hit by a huge lightning strike later in the evening.  

Everything on the boat was either scrambled - the chartplotters and GPS'es were giving divergent information - or dead, like the VHF and most of the electrics on the boat.  They used the phone to get a fix and then navigated in safely using paper charts - after a couple days of bad fights in which one of the three crew wanted to keep using the Zeus chartplotter.  

Lesson:  Store your handheld electronics in a faraday cage if you can.  Grounding ain't. 

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21 hours ago, Lex Teredo said:

 Some friends were delivering a boat down the east coast a few years ago, couple hundred miles offshore.  About halfway down they got hit by a couple weather systems that turned into a loong storm.  Second night of the bad weather, they got their go kits out as it was real bad.  For unknown reasons, one of the guy stuck his keys, wallet and cell phone into the microwave in the galley right by the companionway.  It was the handiest cabinet that he thought would stay closed, he said, and he stuck them in there because he got a hasty call to get on deck to fool with the sea anchor or somesuch.  He forgot about the phone and they were hit by a huge lightning strike later in the evening.  

Everything on the boat was either scrambled - the chartplotters and GPS'es were giving divergent information - or dead, like the VHF and most of the electrics on the boat.  They used the phone to get a fix and then navigated in safely using paper charts - after a couple days of bad fights in which one of the three crew wanted to keep using the Zeus chartplotter.  

Lesson:  Store your handheld electronics in a faraday cage if you can.  Grounding ain't. 

yep lots of cruising people have done that and got the same result

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