Do domestic batteries have to be close to each other to be in a bank?

[Thresher]

My existing 70 amp/hour car battery that I use as my domestic battery isn't enough. I want to replace it with a proper leisure battery of the same size that will fit into the space provided near the engine. I also want to have a second larger leisure battery and have them as a battery bank but the only place that I can put the larger battery is under a bunk about 6 feet away. Will that distance cause any problems?

 

[sailaboutvic]

The rule is keep them next to each other ,
keep all the leads the same size and length.
after saying that , my time as a liveaboard I seen batteries put from one end of the boat to the other ,
we too at one time has six batteries spread all over the place ,
yes they may not be balance or charged at the same rate but I not come across anyone who had problems.

possibly they won’t last as long as them who are close to each other .

 

[Pinnacle]

Location should not be a problem as long as you size the cables correctly (think big fat ones) and you wire them up correctly. By that I mean you ensure the load is delivered across equal lengths of cable. Might you be able to put both of the new domestic batteries in the under-bunk location?

 

[AntarcticPilot]

@sailaboutvic has it right. There is an optimum way to wire batteries in parallel that ensures that the load on the batteries is balanced. If you imagine the batteries to be arranged like the rungs of a ladder with positive terminals on the left and negative on the right, the positive connection should be from the top left terminal and the negative from the bottom right. Ideally the cables making the parallel connections should all have the same resistance - easiest to ensure if they're all the same length.

But as Vic says, that's an ideal situation that may not be practical in all situations. Other hookups may not be ideal, but providing you use really heavy duty interconnects to minimize the resistance, it should be ok.


Incidentally, why don't we use aluminium wire on boats? It has a higher conductivity by weight than copper, so conductors made of it are thicker than equivalent copper ones but lighter. I understand that it's used in the National Grid - the cables on pylons are aluminium with a steel core for strength.

 

[Tranona]

Best not to mix batteries of different types and ages in one bank. You don't say what size boat you have and your charging arrangements, but if you are charging from an engine alternator better to keep the current battery as a start battery and use the new one (larger size?) for your house. Charge them through a split charging system.

 

[NormanS]

Like the OP, I wanted to add a battery to my domestic bank. It's away on the other side of the boat, BUT it's connected in the correct way using professional size welding cables. Absolutely fine.

 

[jamie N]

Incidentally, why don't we use aluminium wire on boats? It has a higher conductivity by weight than copper, so conductors made of it are thicker than equivalent copper ones but lighter. I understand that it's used in the National Grid - the cables on pylons are aluminium with a steel core for strength.

Aluminium conductors are awful; truly horrendous! They become brittle very swiftly; they're almost impossible to make a soldered joint on and deteriorate at an alarming rate. Many of us will have seen BT OpenReach vans working to replace the aluminium cables, albeit with fibre, but the 'ally' cables were a shagged, cheap option back in the day that BT chose. Awful stuff.
I've only ever been exposed to them in an ROV setting, where they were the briiliant choice of umbilical by an accountant who couldn't spell enjinear.

 

[AntarcticPilot]

Aluminium conductors are awful; truly horrendous! They become brittle very swiftly; they're almost impossible to make a soldered joint on and deteriorate at an alarming rate. Many of us will have seen BT OpenReach vans working to replace the aluminium cables, albeit with fibre, but the 'ally' cables were a shagged, cheap option back in the day that BT chose. Awful stuff.
I've only ever been exposed to them in an ROV setting, where they were the briiliant choice of umbilical by an accountant who couldn't spell enjinear.

That makes sense; I was curious because I know aluminium is used for the National Grid interconnects. But that's a specialized application where weight matters. I hadn't thought about the problem of soldering, either. I did know that the aluminium had to be if high purity, so it's reasonable that it has poor mechanical properties, too.

 

[LittleSister]

Can anyone explain exactly why it is 'less than ideal', assuming adequate sized connecting cables, to have -

a) batteries in different locations; or

b) different capacity batteries?

I am not arguing against it, just trying to understand why it it so, and what to bear in mind if going down this route (as I am contemplating).

On the face of it, batteries will accept the different charges they need from what is available , and will contribute what they can to power any drain.

 

[PCUK]

They are only together for convenience. As long as the cables are 70mm or more (preferably flexible welding cable) length is not a problem on a small boat.

 

[William_H]

To OP stick with the car battery for engine start. Then buy deep cycle batteries for the additional domestic supply. Parallel only for charging. (VSR is ideal or juwst a manual switch)
As said parallel the batteries with robust cable. Something heavy enough to take current for engine start if you need it in an emergency. Take the negative of the paralleled batteries from one bank and the positive from the other bank. With this arrangement charging and discharging even at high currents should be equal. ol'will

 

[Thresher]

Can anyone explain exactly why it is 'less than ideal', assuming adequate sized connecting cables, to have -

a) batteries in different locations; or

b) different capacity batteries?

I am not arguing against it, just trying to understand why it it so, and what to bear in mind if going down this route (as I am contemplating).

On the face of it, batteries will accept the different charges they need from what is available , and will contribute what they can to power any drain.

Good questions and I have a further one.

Why do the cables connecting domestic batteries have to be so thick?
I have two separate switches for my engine and domestic so I can't accidently use the engine battery for domestic and vice versa.
I have a car battery for the engine, massive power draw, thick cables, I get that.
The greatest draw that I will have on my domestic battery is probably the autopilot, do I really need 70mm2 between domestic batteries?

 

[sailaboutvic]

Over the years there been much written here on batteries some who knows their stuff others who just make it up as their go along , which confuses thos who are looking for answers .
over my 40 plus years of owning sailing boats many as long term live aboard , I try many combinations, the best was using lithium Lifepo4 they are simply the best for live a board sailors and worth every penny ,
there safer ,lighter,give you more power and hold up the voltage much better then any other batteries and will charge at what every you can give them .
but many here are weekend sailors with the odd long trip and it just don’t justify spending thousand on lithium although there are ways of building a lithium bank without breaking the bank.

life before lithium for us was cheap heavy duty batteries or lorry batteries and hands up I broke every rule there was ,
batteries where spread al over the boat, they where brought at different time, as one collapsed it was replace with a new one what ever I could find at the time , some time of different Amperages .
as I kept adding battery on our cruise I could only add what every I could buy at the time so there been times different size cables length usually what I found in my locker possibly what I picked up from other cruisers or took out of a bin , yes us long term cruising are know to hit the marina bins .
evey three or four years I was having to replace a battery , I don’t consider this too bad of a deal , we was using them 24/7 , 7 days a week , paying around £100 a batteries in todays money .
so I do have to wonder the rules on keeping batteries bank close together, using same size cables length, same ampages Batteries and same type batteries , especially for short time use like the weekend sailor .

 

[Thresher]

Over the years there been much written here on batteries some who knows their stuff others who just make it up as their go along , which confuses thos who are looking for answers .
over my 40 plus years of owning sailing boats many as long term live aboard , I try many combinations, the best was using lithium Lifepo4 they are simply the best for live a board sailors and worth every penny ,
there safer ,lighter,give you more power and hold up the voltage much better then any other batteries and will charge at what every you can give them .
but many here are weekend sailors with the odd long trip and it just don’t justify spending thousand on lithium although there are ways of building a lithium bank without breaking the bank.

life before lithium for us was cheap heavy duty batteries or lorry batteries and hands up I broke every rule there was ,
batteries where spread al over the boat, they where brought at different time, as one collapsed it was replace with a new one what ever I could find at the time , some time of different Amperages .
as I kept adding battery on our cruise I could only add what every I could buy at the time so there been times different size cables length usually what I found in my locker possibly what I picked up from other cruisers or took out of a bin , yes us long term cruising are know to hit the marina bins .
evey three or four years I was having to replace a battery , I don’t consider this too bad of a deal , we was using them 24/7 , 7 days a week , paying around £100 a batteries in todays money .
so I do have to wonder the rules on keeping batteries bank close together, using same size cables length, same ampages Batteries and same type batteries , especially for short time use like the weekend sailor .

Ha! I like your style. I am intending to be a liveaboard by the end of the year but had dismissed the idea of lithium batteries because of the complications of charging them from my existing alternator. How do you charge them?

 

[Sandy]

Good questions and I have a further one.

Why do the cables connecting domestic batteries have to be so thick?
I have two separate switches for my engine and domestic so I can't accidently use the engine battery for domestic and vice versa.
I have a car battery for the engine, massive power draw, thick cables, I get that.
The greatest draw that I will have on my domestic battery is probably the autopilot, do I really need 70mm2 between domestic batteries?

Redundancy...

Just in case one dark and stormy night on a lee shore you can't start the engine and in an act of desperation switch to using the domestic bank, but from what you write that is not an option.

 

[Thresher]

Redundancy...

Just in case one dark and stormy night on a lee shore you can't start the engine and in an act of desperation switch to using the domestic bank, but from what you write that is not an option.

So, no reason for the thick cables then? I carry heavy jump leads on board for the desperate situation you describe.

 

[sailaboutvic]

Ha! I like your style. I am intending to be a liveaboard by the end of the year but had dismissed the idea of lithium batteries because of the complications of charging them from my existing alternator. How do you charge them?

There a very cheap and easy way to charge them without changing alternator or adding exterior regulator, use a B2B so your alternator charge your LA starter battery and that in turn charges your lithium , in the same way you won’t have to change your shore charger.

 

[Thresher]

There a very cheap and easy way to charge them without changing alternator or adding exterior regulator, use a B2B so your alternator charge your LA starter battery and that in turn charges your lithium , in the same way you won’t have to change your shore charger.

I like it. Looks like I have some reading to do.

 

[sailaboutvic]

I like it. Looks like I have some reading to do.

If you really want to find out more,
Phil brooke little @ 12v boating forum or Rod Collins, these are people who really know there stuff.

 

[Thresher]

Can anyone explain exactly why it is 'less than ideal', assuming adequate sized connecting cables, to have -

a) batteries in different locations; or

b) different capacity batteries?

I am not arguing against it, just trying to understand why it it so, and what to bear in mind if going down this route (as I am contemplating).

On the face of it, batteries will accept the different charges they need from what is available , and will contribute what they can to power any drain.

Littlesister, I will try to answer your questions about different battery capacities. I don't think that capacities do matter, as in one being 70 ah and another being 140ah.
What I think does matter is having two batteries of different age or condition.
If you have one good battery and one poor battery and they are connected together in a bank, the poor battery will constantly take a charge from the good battery until the good battery is reduced to the state of the poor battery.