Are parallel batteries worthwhile?

[Kuncicky]

A friend advised me to parallel a couple of 12-volt batteries for my starter battery (big diesel engine), saying that it would double the amperage. Is this really true? Seems to be that I have heard that there is a downside to paralleling batteries, but I'm not sure what it might be. Any thoughts from the experts??


Bill

 

[TowLou]

Going in parallel will double the amps at 12v. If you went in series you would double the volts with the same amps.

That is using the same type batteries.

What motor and what batteries are you working with now?

 

[twistedtree]

Unless your starter battery is under sized, doubling won’t really add any benefit.

 

[Comodave]

I have 4 start batteries, 2 for each engine in parallel. It will help if you have trouble starting an engine. But if the engines start readily and you have the proper size battery then a second battery isn’t needed.

 

[Bacchus]

A friend advised me to parallel a couple of 12-volt batteries for my starter battery (big diesel engine), saying that it would double the amperage. Is this really true? Seems to be that I have heard that there is a downside to paralleling batteries, but I'm not sure what it might be. Any thoughts from the experts??


Bill

What your friend advises is factual... doubles amps
I dont know what the downside could be but please enlighten us.
Whether it is needed or unnecessary depends on batty MCA and engine reqmts to comment intelligently

 

[Max1]

A good tip for hooking up parallel batteries is to parallel the batteries first. Then make sure you put the starter (+) lead on one battery and the Starter (-) on the other.
This makes both batteries share the load.

If you hook up the starter (+) and (-) to the same battery and hook up the other in parallel, the first battery works harder than the second.

 

[Bacchus]

I have 4 start batteries, 2 for each engine in parallel. It will help if you have trouble starting an engine. But if the engines start readily and you have the proper size battery then a second battery isn’t needed.

Dave
Is there a proven benefit / rationale for having separate start banks for twins?
I never understood the many that are set up this way.
It seems like one oversized bank sufficient to start one engine and that p,us the alt output more than sufficient to start the second.
A parallel kelnsw with house bank would seem to cover any unusual circumstances w/o need for a redundant start bank.
Open to thoughts as my prior twin had one start bank and currently a single so not familiar w 2 start banks.

 

[Mac2]

A good tip for hooking up parallel batteries is to parallel the batteries first. Then make sure you put the starter (+) lead on one battery and the Starter (-) on the other.
This makes both batteries share the load.

If you hook up the starter (+) and (-) to the same battery and hook up the other in parallel, the first battery works harder than the second.

Excellent point. Let me throw in another wrench. Two 6 volt batteries last twice as long as one 12 volt battery-something to do with the work formula.

 

[psneeld]

Excellent point. Let me throw in another wrench. Two 6 volt batteries last twice as long as one 12 volt battery-something to do with the work formula.

That is not an absolute...lots of variables.

Depends on amp hr ratings and typical amp draw.

Takes a lot of study to design the right system based on use.

If the 6V are deep cycle and you are using them primarily for starting, that is not the best application for them.

 

[Mac2]

That is not an absolute...lots of variables.

Depends on amp hr ratings and typical amp draw.

Takes a lot of study to design the right system based on use.

If the 6V are deep cycle and you are using them primarily for starting, that is not the best application for them.[/QUOTE

Great point. I should have clarified for house batteries.

 

[Comodave]

Dave
Is there a proven benefit / rationale for having separate start banks for twins?
I never understood the many that are set up this way.
It seems like one oversized bank sufficient to start one engine and that p,us the alt output more than sufficient to start the second.
A parallel kelnsw with house bank would seem to cover any unusual circumstances w/o need for a redundant start bank.
Open to thoughts as my prior twin had one start bank and currently a single so not familiar w 2 start banks.

This is the first boat that has 2 start batteries for each engine. But Formula goes to extremes for some things. I haven’t looked up what Cummins requires for start battery size but what is there works so I will just go with it. They do not have a crossover for dead start batteries so that may be part of why there is 2.

 

[DonW28]

The older I get the smaller the batteries I like to carry onto the boat. I can still handle the 27s and 31s. 8Ds are out the window for me. So it is either parallel the smaller ones or pay a kid to tote them. If there is a downside to parallel batteries I'd say it is the cost of the cables to originally parallel them. But in boat dollars and back pain it is worth it.


Don

 

[Porgy]

I’d spec the correct size start battery then also have the ability to parallel in the house bank if the start dies or need an extra oomph.

Arthritic and destroyed rotator cuffs but I can still heft an 8D or four if need be. However, got an LFP for my motorcycle that’s a quarter of the weight of the AGM. They are looking more and more attractive.

 

[rslifkin]

Dave
Is there a proven benefit / rationale for having separate start banks for twins?
I never understood the many that are set up this way.
It seems like one oversized bank sufficient to start one engine and that p,us the alt output more than sufficient to start the second.
A parallel kelnsw with house bank would seem to cover any unusual circumstances w/o need for a redundant start bank.
Open to thoughts as my prior twin had one start bank and currently a single so not familiar w 2 start banks.

As long as you have 2 available sources of start power for each engine, it's adequate regardless of where that power comes from. The only thing I dislike is starting from the house bank in normal ops.

That said, I do have my boat set up with 2 start banks (shared by 2 engines and the generator). Each of the 3 can start off S1 or S2. Normally the port engine starts off S1, stbd and generator share S2 and the house bank is independent.

I don't have massive 8d start batteries or anything, so having 2 isn't a big deal. And I figure that way in a real "oh crap" situation at anchor, even if a start battery fails suddenly, at least one engine will start right away instead of having none until I go down and throw a switch.

 

[Bacchus]

Excellent point. Let me throw in another wrench. Two 6 volt batteries last twice as long as one 12 volt battery-something to do with the work formula.

That's a misleading comparison at best.
I assume by "last twice as long" you mean has more AHs... and that CAN be true. Some 12V common group sizes, say a GP 31, have about half the AHs as a comparable 6V GC batty... roughly 100AH vs 200AH but it takes 2 6V battys to deliver 200 AH at 12V and it takes 2 12V battys to deliver 200 AH at 12V.
When comparing you need to compare at the same V your equipment/ system requires.
And for electrical comparison it is better to look at Power Delivered
Power (Watts) = V (volts) X I (Amps)
6V X 200A = 1200W. and. 12V X 100A = 1200W
Both are capable of delivering roughly 1200W Each
There really is no free lunch here!

 

[rslifkin]

For deep cycle use, the 6V stuff is often better quality than most available 12v stuff, so they really do last longer in a house bank. But for a starting battery, it's unlikely to make a significant difference.

 

[ranger58sb]

Seems to be that I have heard that there is a downside to paralleling batteries, but I'm not sure what it might be. Any thoughts from the experts??

Your engine specs will tell you minimun cranking amps required; might be two batteries was a way to get there easily.

I've read its not great to parallel more than 4 12V batteries. Dunno why.

I have 4 start batteries, 2 for each engine in parallel. It will help if you have trouble starting an engine. But if the engines start readily and you have the proper size battery then a second battery isn’t needed.

This is the first boat that has 2 start batteries for each engine. But Formula goes to extremes for some things. I haven’t looked up what Cummins requires for start battery size but what is there works so I will just go with it. They do not have a crossover for dead start batteries so that may be part of why there is 2.

Does Formula also use each engine start bank for approx half of the house loads? That's not uncommon... and in that case, adding capacity to each battery bank could also be useful for time at anchor.

-Chris

 

[MYTraveler]

A friend advised me to parallel a couple of 12-volt batteries . . . saying that it would double the amperage.

I would say your friend is only half right. The available amperage would double, but if your single battery is providing all of the amperage demanded by your starter, the consumed amperage will not double.

 

[rslifkin]

I would say your friend is only half right. The available amperage would double, but if your single battery is providing all of the amperage demanded by your starter, the consumed amperage will not double.

However, in the real world, you almost never have a big enough starting battery not to see a significant voltage dip during cranking, so while it'll be far from double, starting performance will almost always improve some with a larger battery (assuming the cables are big enough).

 

[Kuncicky]

Thanks, guys. Those responses answer the original question very well, and I am very appreciative. Probably the most valuable pointers which I got were to make sure that the two batteries are the same, and put negative cable on one and positive cable on the other. I had not realized either of those points.


Bill

 

[Slowmo]

We also have paralleled start batteries for a simple reason. The original 8D is just too cumbersome so we now have paralleled Group 24s.

 

[DDW]

Excellent point. Let me throw in another wrench. Two 6 volt batteries last twice as long as one 12 volt battery-something to do with the work formula.

For momentary high amperage use (starting for example) you want low internal resistance, almost anything has enough energy (AH). 6V batteries of a similar size as 12V have about 1/2 the internal resistance compared to 12V. However with two 6V in series, you have twice the spec'd internal resistance, whereas with two 12V in parallel you have half the spec'd internal resistance. Two 12V in series is almost always better for this type of use.

As an example, Lifeline GPL-4CT are their golf cart sized 6V, having a spec'd internal resistance of 1.79 mOhms. GPL-31T are their G31 12V battery, having a spec'd internal resistance of 3.62 mOhms. They are similar in energy content. The two 6V will have a combined series resistance of 3.58 mOhms resistance, while the two 12V will have a combined parallel resistance of 1.81 mOhms. The voltage drop during high current draw will be 1/2 as much.

 

[Comodave]

Does Formula also use each engine start bank for approx half of the house loads? That's not uncommon... and in that case, adding capacity to each battery bank could also be useful for time at anchor.

-Chris

No, they are only used for starting. We, unfortunately, have one house battery. Understand Formula builds a party boat and they really don’t expect you to hang out for days on the hook, or you just run the genset. I took out the overboard discharge pump and that gave me room to add a second house battery, but I won’t do that until after we launch. We have done enough work on Black Dog already this winter.

 

[Comodave]

Our battery situation is kinda strange. We have 10 batteries on board as of now. The 4 dedicated start batteries. 4 thruster batteries, 2 forward and 2 aft for the 24 volt thrusters, 1 genset start battery and 1 group 31 house battery. I plan on adding a second group 31 to the house bank.

 

[rslifkin]

Our battery situation is kinda strange. We have 10 batteries on board as of now. The 4 dedicated start batteries. 4 thruster batteries, 2 forward and 2 aft for the 24 volt thrusters, 1 genset start battery and 1 group 31 house battery. I plan on adding a second group 31 to the house bank.

Have you given any thought to combining the second start bank and the stern thruster bank into one? Or ditching the dedicated generator battery and letting it share with the engines? That would free up more space for house batteries, giving you more choices of how and how much to expand the house bank.

 

[Comodave]

Have you given any thought to combining the second start bank and the stern thruster bank into one? Or ditching the dedicated generator battery and letting it share with the engines? That would free up more space for house batteries, giving you more choices of how and how much to expand the house bank.

We will be fine with the 2 house batteries, I think. Can’t combine the start and thruster banks because the start are 12 volt and the thruster is 24 volt. The access is really really tight in the stern. I have it all set except the second house battery and I want to take a break for a bit before I redo the house batteries. It has been a nonstop winter of working on the boat, 2 thrusters, added a swim platform extension, Seadek, new canvas, new upholstery, new carpet in the cabin, new electronics and about 20+ miscellaneous projects. So we are going on a small vacation before we get to launch the boat to relax a bit.

 

[friz]

........What motor and what batteries are you working with now?

This is a big (biggest?) diesel. The OP never has come back to ID his "big diesel". As stated in 20 odd posts that would determine if "paralleling" is necessary.

 

[NEtrawler58]

We will be fine with the 2 house batteries, I think. Can’t combine the start and thruster banks because the start are 12 volt and the thruster is 24 volt. The access is really really tight in the stern. I have it all set except the second house battery and I want to take a break for a bit before I redo the house batteries. It has been a nonstop winter of working on the boat, 2 thrusters, added a swim platform extension, Seadek, new canvas, new upholstery, new carpet in the cabin, new electronics and about 20+ miscellaneous projects. So we are going on a small vacation before we get to launch the boat to relax a bit.

That's a strange choice of setup IMO with such a small house capacity. The 24V thrusters do limit your options. But it seems like overkill on the start batteries and the generator. I am sure you are correct, that Formula did not build the boat with cruising in mind.

I also have 2 group 31 house batteries. It's not terrible, but I don't get much more than an overnight before I need to recharge with the generator if the fridge is running. If you have the room, I would add even more or larger house batteries.

 

[Kuncicky]

This is a big (biggest?) diesel. The OP never has come back to ID his "big diesel". As stated in 20 odd posts that would determine if "paralleling" is necessary.

Um, two old Perkins 4-365's. Start one, then start the other. One starts easily, the other is more of a slowpoke before it will fire off. So seems to me that having enough amps there is nice.


Bill

 

[Tom Callan]

A good tip for hooking up parallel batteries is to parallel the batteries first. Then make sure you put the starter (+) lead on one battery and the Starter (-) on the other.
This makes both batteries share the load.

If you hook up the starter (+) and (-) to the same battery and hook up the other in parallel, the first battery works harder than the second.

I don’t see how putting the negative placement on the opposite battery would make any difference since they are now connected in parallel, please explain.