Are parallel batteries worthwhile?

[Comodave]

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.

Absolutely hook the positive lead on one battery and the negative lead on the other battery. Otherwise the resistance in the paralleling cables will make the second battery in effect disappear. You want one lead going into one end of the bank and the other lead going into the other end of the bank.

 

[Bacchus]

Absolutely hook the positive lead on one battery and the negative lead on the other battery. Otherwise the resistance in the paralleling cables will make the second battery in effect disappear. You want one lead going into one end of the bank and the other lead going into the other end of the bank.

I agree. It will work either way but best to balance multi batty banks.
Just search about balanced batty bank wiring and you will get several sites with diagrams. Same goes for load or charging. If too many connections on a terminal use opposite ends for charging and mirror image for load.
The other way to balance is using buss bars and equal length cables to all
battys.

https://www.windynation.com/jzv/inf/how-configure-battery-bank

 

[wkearney99]

A point to consider is wire gauge. Adding capacity is great but does little good if the wiring's not done properly.

I've just recently had pretty much all of my 12vdc battery, charger, inverter and starting wiring re-done. Lots of accumulated 'cruft' from previous owners random attempts to "solve things".

Anything that pulls a lot of current depends on not having undue voltage drop due to poor connections, improperly sized or excessive length wires-- or all the above.

We had plenty of all of them.

Now we've rejiggered things, including moving the bow thruster batteries to the bow, moving the start batteries and using proper gauge wire between them and their respective engines (and likewise the paralleling system).

I'm sure previous owners thought something was being accomplished but whoever did the work should be flayed alive with left-over hunks of bad wiring. That and paper cuts from the PAGES of invoices...

 

[rwaldrop13]

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

I have similar situation since I have a 24volt boat and 24volt starters. But when I pay someone to haul the existing 8D's out I'm replacing with 6volt golf cart batteries that I can handle alone.

 

[foxbat67]

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

I can not agree with that. Connecting two (of the same rating and age) will increase the amp rating

 

[NEtrawler58]

I can not agree with that. Connecting two (of the same rating and age) will increase the amp rating

While you are correct, how much current do you need to start your engine? If it takes a lot of the capacity of 2 large batteries in parallel, maybe you have other issues.

 

[RickyD]

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.

IN my boat, I have two 8D AGMs in parallel to start one generator and the port engine. I have six L16 six volts as a house bank and to start the starboard engine. I have a second generator on its own Grp 31 AGM that can charge all three battery banks and a 30 amp charger connected to all three banks. The house bank is charged by the main generator and its own 130 amp charger. I also have a combiner that needs servicing. Its on the list but down quite a ways.

 

[BigJimB]

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.

Consider battery failure and you've your answer. I've actually 4 "banks". 2ea 12v 8D mains, 1ea Grp 24 genny, and one large house w/ 6'vs in series and parallel. Then have all combiners to charge from alternators and chargers. This also allows engine amps to augment house depth with cut-offs high enough to start engines.

 

[NEtrawler58]

Consider battery failure and you've your answer. I've actually 4 "banks". 2ea 12v 8D mains, 1ea Grp 24 genny, and one large house w/ 6'vs in series and parallel. Then have all combiners to charge from alternators and chargers. This also allows engine amps to augment house depth with cut-offs high enough to start engines.

In my life with many cars, boats, motorcycles, etc., I've never had a battery fail. Sure they get weak with age and you generally have plenty of warning to replace them, but is an outright failure of a relatively healthy battery common? I'm always willing to learn something new here. Also, how would you guard against that with a single engine? I have the option of paralleling my start and house banks if needed, but would that help if there was a failed battery in the mix?

 

[rslifkin]

Outright failure without warning does happen, but it's not terribly common. What's more common is a battery being degraded (but not yet noticed) and then when you ask just a little more than normal from it, it fails to deliver.

 

[RickyD]

In my life with many cars, boats, motorcycles, etc., I've never had a battery fail. Sure they get weak with age and you generally have plenty of warning to replace them, but is an outright failure of a relatively healthy battery common? I'm always willing to learn something new here. Also, how would you guard against that with a single engine? I have the option of paralleling my start and house banks if needed, but would that help if there was a failed battery in the mix?

I did have an 8D battery fail. It was my port engine start battery. It started to fail and the starter was pulling so many amps the cable post melted off. Luckily I have the starboard engine starting from the house bank and was not effected. I came home on the one engine.

 

[oak_box]

Interesting thread.

One downside of parallel batteries is this: If one battery fails (bad cell), the other battery will attempt to discharge directly into it, which can potentially heat up the battery cable enough to start a fire.

That having been said, I currently have two old 8D's. When the time comes to pay someone to haul them off, I will mostly likely be replacing the 8D's with two paralleled group 31's.

Though, I have to ask: Is a single group 31 battery in good condition sufficient for starting a Ford Lehman 120?

The boat has two start batteries (8D's), a single generator battery (group 31), and no house battery - the house connects to either of the start batteries.

When I replace the 8D's, I might see if I can find a place for a house bank - and will then "get the joy" of determining how hard it is to separate the house from the starter wiring...

At that point, I'll also get the joy of contemplating lithium vs. AGM, and how to charge the house bank when the engines are running... Maybe an MTTP charger with the two engines as inputs?

 

[PNW Drifter]

I like extra capacity for the starter bank. No start, and I ain't goin home!


If I encounter hard starting, at least I have several go rounds before the battery is drained.

My starter bank is 2x 27s. One 27 can start my AD41B under normal circumstances.

House bank is 2x 6V in series - and can start my AD41B, - but can be also paralleled into the starter bank if needed.

 

[EngNate]

The biggest advantage to parallel batteries, opposed to one larger one, will be shown by your lower back after installing them. I like to use GP 31s.

 

[Trawlerman]

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

You have had a lot of good and pertinent info which I will not repeat. However saying "it will double the amperage" is likely not exactly meaningful just stated that way. Yes, it will double the amperage potential capability and also will double the amp/hour capability -- but this may not be of much value to you depending on how hard the engine is to start. Here is why:

1. At a steady 12 volts, the amount of current that will flow into the starter is determined by (a) the resistance of the windings, and (b) the back EMF when the starter armature is spinning, and this is pretty much fixed in the starter design. Since current equals voltage divided by resistance, this current draw on cranking will not change regardless of 1 or 2 batteries UNLESS the battery amperage capability is so undersized that there is significant voltage "sag" due to the internal resistance of the battery in play (i.e. being too small amperage capacity). One adequate battery could be achieved by putting two inadequate batteries in parallel, but likely is not the ideal approach.
2. As regards the greater amp/hour capacity, which again this is true, a normally behaving engine will start in a matter of seconds. Even if you say it takes 30 seconds (.008333 hours) of cranking (my Perkins need only max 2 to 3 seconds), --- at a current draw of say 300 amps, this will still only use 300 amps x .008333 hours = 2.5 amp/hours. This is why start batteries are optimized for short term maximum amperage delivery capability, not long term delivery. As such, the extended amp hours are not very useful.

Not trying to discourage paralleling batteries, just pointing out that it might not be necessary or valuable, unless you have a VERY hard to start engine that needs a heck of a lot of cranking time.

 

[Bacchus]

Consider battery failure and you've your answer. I've actually 4 "banks". 2ea 12v 8D mains, 1ea Grp 24 genny, and one large house w/ 6'vs in series and parallel. Then have all combiners to charge from alternators and chargers. This also allows engine amps to augment house depth with cut-offs high enough to start engines.

I just wonder how many layers of redundancy is reqd though.
I have a start / thruster bank for my single eng, a house bank and a separate gen batty to run charger. Would anyone add a redundant start bank for a single in addition to the above 2 back-ups? I guess my 3rd level of back-up is my towing policy but I've not needed that in 30+ yrs. (Knock on wood)
I would think and decent start bank ( say a pair of GP31s) should be capable of starting one eng and then the alt is supplementing assisting for the 2nd start or the house as a boost / back-up, or 3rd level gen running charger?
If one does little or no checks or PM no level of redundancy will be adequate but with some diligence, monitoring & maintaining reduce the odds of a sudden catastrophic failure with no warning

 

[rwidman]

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.

In theory, perhaps, but not in practice unless the cables connecting the batteries together are undersized or the batteries are some distance apart.

 

[rwidman]

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

If your engine starts easily with the existing battery, there is no need to add an additional battery. If it's marginal, you may be able to simply replace the existing battery with one that has a greater capacity.

Two identical batteries wired in parallel will have twice the amperage capacity of the single battery. There is no real downside to wiring batteries in parallel. It's done all the time.

 

[NEtrawler58]

In theory, perhaps, but not in practice unless the cables connecting the batteries together are undersized or the batteries are some distance apart.

I was thinking of this and I agree with you. If 2 batteries are right next to each other and have a good connection through heavy short cables, it shouldn't make a difference where the leads from the starter connect.

 

[koliver]

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

You started this thread by asking about paralleling batteries to start a pair of "big diesel" engines.
Then you tell us that these engines are Perkins 4.365s.
Most of the answers you have elicited relate to your original premise, and should prove helpful. IMHO those are actually pretty small engines, so the total CCA that you will need ( reference your owner's manual) will not be huge. If you don't have a single battery that delivers the required CCAs, you can either change up to one that does, or parallel in a second battery that, alone would not be enough, but in combination amounts to the required CCAs.

You don't need to double the required CCAs to service the second start, or even a third start, as others have pointed out, once the first engine is running, you have now added its alternator's output to the available CCAs for starting the second engine. Also, as pointed out above, if your engines require only a couple of revolutions to get going, the available capacity of the start battery has not been diminished by more than a very small fraction, so the second engine will experience as much of a starting jolt as the first one received.
If your second start is on a battery that isn't up to the job, I would suspect that battery has reached the end of its useful life, rather than suspecting that it needs a brother

 

[Kuncicky]

You started this thread by asking about paralleling batteries to start a pair of "big diesel" engines.
Then you tell us that these engines are Perkins 4.365s.
Most of the answers you have elicited relate to your original premise, and should prove helpful. IMHO those are actually pretty small engines, so the total CCA that you will need ( reference your owner's manual) will not be huge.

You don't need to double the required CCAs to service the second start, or even a third start, as others have pointed out, once the first engine is running, you have now added its alternator's output to the available CCAs for starting the second engine. Also, as pointed out above, if your engines require only a couple of revolutions to get going, the available capacity of the start battery has not been diminished by more than a very small fraction, so the second engine will experience as much of a starting jolt as the first one received.

Thanks. I didn't realize that they were considered pretty small engines, but will certainly take your word for that. I don't have much experience with diesel engines -- these are my first ever.


And thanks also for the information regarding the need for more than one battery.


Bill

 

[SteveK]

One starts easily, the other is more of a slowpoke before it will fire off.

Is that the same engine regardless of which engine is started first?
Is it always the same engine being the slowpoke even when started first?

 

[NEtrawler58]

I'll add my 2 cents for what it's worth (hope at least 2 cents).


I have a single diesel that always starts at the click of the ignition. Never even hear it crank. I have 2 parallel start batteries but 1 would easily start the engine. However, that bank also powers the thrusters and starts the genny, so I'm glad I have 2. Be sure what the total load on your bank is before deciding what you need. It may be doing more than starting your engine(s).


Secondly, although a running engine alternator helps replenish battery power, it doesn't significantly help with CCA which is measured in hundreds of amps for a short duration. An alternator output cannot take the place of a start battery. Given enough time it will recharge the start bank depending on how much it was depleted, but it's incorrect to say that you don't need the CCA capacity for the second engine because an alternator is running.

 

[rslifkin]

I'll add my 2 cents for what it's worth (hope at least 2 cents).


I have a single diesel that always starts at the click of the ignition. Never even hear it crank. I have 2 parallel start batteries but 1 would easily start the engine. However, that bank also powers the thrusters and starts the genny, so I'm glad I have 2. Be sure what the total load on your bank is before deciding what you need. It may be doing more than starting your engine(s).


Secondly, although a running engine alternator helps replenish battery power, it doesn't significantly help with CCA which is measured in hundreds of amps for a short duration. An alternator output cannot take the place of a start battery. Given enough time it will recharge the start bank depending on how much it was depleted, But it's incorrect to say that you don't need the CCA capacity for the second engine because an alternator is running.

To add to that, you also don't want to only account for what it needs for a normal start. You want to account for how much it'll need to crank when you have a re-prime the fuel system on the coldest boating day of the year (or other worst-case cranking scenario).

 

[Kuncicky]

Is that the same engine regardless of which engine is started first?
Is it always the same engine being the slowpoke even when started first?

Yep. Always the port engine. It chugs for awhile before starting. The starboard engine whirls, and then starts. Every time. My guess would be that for whatever reason, the port engine is just not getting the proper voltage, so maybe corrosion in the battery cable.


Bill

 

[SteveK]

Yep. Always the port engine. It chugs for awhile before starting. The starboard engine whirls, and then starts. Every time. My guess would be that for whatever reason, the port engine is just not getting the proper voltage, so maybe corrosion in the battery cable.
Bill

Start with that, eliminate any corrosion. May have to replace the cable.
Eliminate battery and cable as the problem.

 

[backinblue]

Start with that, eliminate any corrosion. May have to replace the cable.
Eliminate battery and cable as the problem.

Maybe I am misunderstanding something, but if the engine cranks ok but fails to start, why would you think you have a battery-related issue? Unless you mean by chugging that the engine is turning over much slower than the other.

 

[SteveK]

Maybe I am misunderstanding something, but if the engine cranks ok but fails to start, why would you think you have a battery-related issue? Unless you mean by chugging that the engine is turning over much slower than the other.

this thread has been about battery capacity. so find out if it is battery/cable and move on from there.

 

[backinblue]

this thread has been about battery capacity. so find out if it is battery/cable and move on from there.

I get that, however he is reporting that one engine starts quickly while the other engine cranks for a while and is a rough start. Unless I understood wrong, that doesn't sound like a battery issue. Always good to check regardless, but it sounds like that's not the issue.

 

[rslifkin]

I get that, however he is reporting that one engine starts quickly while the other engine cranks for a while and is a rough start. Unless I understood wrong, that doesn't sound like a battery issue. Always good to check regardless, but it sounds like that's not the issue.

It may or may not be a power problem. Diesels are compression ignition, so if one is cranking slower it makes sense that it would crank longer before building enough heat to fire. But if it's cranking just as strongly, then I agree, it's not a battery problem.