Paralleling 12V with 24V?

[BrianSmith]

I have a 12V house bank, and a 12V genset start battery, and a 24V main engine start battery (only one engine). The 24V "battery" is actually 2 12V's in series. The genset battery and the two main engine batteries are all right next to each other in the same battery box. The house bank is on the opposite side of the engine room (8 - 10 feet away, if you're measuring cable runs).

I have a parallel switch that allows me to parallet the 12V house bank with the 12V genset start battery, in case the start battery ever dies.

Currently, I'm having some issues getting my main engine start batteries fully charged. Not sure what the issue is, but let's leave that alone for now, please. I want to know if I can move the cables from the 12V genset start battery to the posts of one of the 12V main engine start batteries - and then use the parallel switch to add the capacity of the house bank to the main engine start bank? (You know - without a massive shower of sparks, and batteries exploding, and all kinds of bad stuff.)

I'm in the Bahamas, a few days from Nassau, and I'm trying to put together a "Plan B" if my main engine start batteries get too low for a start. I need only a couple to get me to Nassau!

 

[twistedtree]

Probably not a good idea. It could be done with a bunch of restrictions and great care, but not advisable via an internet description. And it might not help get your main started anyway.


If the batteries are all the same size, I think a better and safer approach would be to remove the two main start batteries and swap them with two batteries from elsewhere. That would also get you started in a pinch, and recharge the original start batteries from a different charger.

 

[BrianSmith]

Probably not a good idea. It could be done with a bunch of restrictions and great care, but not advisable via an internet description. And it might not help get your main started anyway.


If the batteries are all the same size, I think a better and safer approach would be to remove the two main start batteries and swap them with two batteries from elsewhere. That would also get you started in a pinch, and recharge the original start batteries from a different charger.

All three of the battery banks are made of different types of batteries. House bank is 6V golfcarts, genset start is a typical car sized 12V, main engine starts are Group 31 (I think - bigger than a typical car battery) 12V.

 

[twistedtree]

All three of the battery banks are made of different types of batteries. House bank is 6V golfcarts, genset start is a typical car sized 12V, main engine starts are Group 31 (I think - bigger than a typical car battery) 12V.

Yeh, time is probably better spent on solving the charging and/or battery problem.

 

[alfamike]

Batteries - 12 V & 24 volt

You need to be very careful as you can NOT normally put a 12 volt battery in parallel to 24 volts without messing things up real bad & having lots of sparks & arc welding. the 24 volts will over power the 12 volts & back feed through that 12 v battery with no resistance in the load, so that is very bad thing.

You could, if careful, put a 12 volt battery in parallel with another 12 volt battery that is in series with other batteries. -- but one missed or wrong connection will be a very bad thing.

I use a Midtronics tester for battery testing & feel it is the best non destructive test & is the best way to check the batteries that you have & next best is a carbon pile load tester & volt meter combo like a VAT-40.

Another check is to see what the OCV ( Open Circuit voltage ) is of each individual battery is after they have the surface charge removed.

You want 12.65 volts - after the surface charge is removed. - often the batteries must sit for 24 hours & have a load on it for a few minutes to get that surface charge effect removed to get a true idea of the battery condition & not get a false positive.

Also - while your working on your battery banks - you should standardize all your batteries for main engine start & gen set start to same type if you can. Group 31's are the industry standard & best technology for flooded lead acid batteries at this time, so go with that.
I understand you probably can't do that with the GC batteries for the house bank, these must be deep cycle batteries & are different design, but you can upgrade to 31's for everything else.


Good luck.


Alfa Mike

 

[High Wire]

It is possible on a temporary basis for little gain IF you know what your doing but I wouldn’t do it. Really easy to do wrong and you don’t want an exploding battery right?
Fix the problem first.

 

[BrianSmith]

Alfa Mike,
I don't have a battery tester, which is a problem. I will look at your suggestions -thanks! (BTW, we started strong this a.m. and are chugging merrily north towards Nassau.)

 

[sdowney717]

you can not parallel a single 12v battery to a 24v battery bank. You could put them in SERIES for 36 vdc to the starter. Which might get it going, BUT depleted batteries have a lot of internal resistance, meaning still not enough current may flow to spin the engine

If you can not charge the 24 volt bank, maybe the 12 volt batteries of that bank are going bad, one or both of them.

 

[FF]

The hassle of a 24v engine and alt. charging system with more common 12v items needing to be fed has been going on since the computers became common on larger engines.

Many OTR trucks and coaches use 2 series 31 for starting and a Vannier equalizer to create 60-100A of 12v DC.

This setup is seamless and does no harm to the start batts,as center taping would.

On bus type MH a 24V charger is used constantly to feed the starts and 12V is created by the Vannier taking juice from 1 start batt at a time is used to charge the house set..

Other folks simply use a large 12V charger directly to the house batts , and don't bother with the starts as they can easily sit for a month or two .

IF the air police requires even tiny engines to run "cleaner" this may be the future of new boats and many re powers.

 

[BrianSmith]

<snip>...

I use a Midtronics tester for battery testing & feel it is the best non destructive test & is the best way to check the batteries that you have <snip>

Alfa Mike

Alfa Mike, I looked online, and Midtronics makes a lot of battery testers. Which one do you have / suggest? Thanks.

 

[foggysail]

The hassle of a 24v engine and alt. charging system with more common 12v items needing to be fed has been going on since the computers became common on larger engines.

Many OTR trucks and coaches use 2 series 31 for starting and a Vannier equalizer to create 60-100A of 12v DC.

This setup is seamless and does no harm to the start batts,as center taping would.

On bus type MH a 24V charger is used constantly to feed the starts and 12V is created by the Vannier taking juice from 1 start batt at a time is used to charge the house set..

Other folks simply use a large 12V charger directly to the house batts , and don't bother with the starts as they can easily sit for a month or two .

IF the air police requires even tiny engines to run "cleaner" this may be the future of new boats and many re powers.

I have no clue what a Vannier equalizer is but I am familiar with trucks that use 24V for starting. This though has zero to do with placing a 24V in parallel with a 12V. The method commonly used with trucks employs power relays to switch off the 12 circuits by isolating the batteries which are temporarily placed in series WITH NO 12V LEFT IN PARALLEL with relay configured 24V configuration. Maybe this is what you were trying to imply and I misunderstood.

 

[DavidM]

Yes, you can do it. But realize that you are only paralleling one of the two 12V batteries in series. If it is the other battery that is bad or not getting charged for some reason, or maybe both, it won't help a bit.


As others have noted, you would be much better served to track down why your series 12V batteries are not getting charged fully. Are they only charged by the propulsion engine's 24V alternator? If so then look at the alternator output. Is there another source of charging for the two 12V batteries in series?


David

 

[BrianSmith]

I must not have been clear with my original post. I was looking ONLY for a way to get my main engine started, with its 24V starter, with assistance from my 12V house bank, as a last resort. I was not looking for a long term solution to the problem. Just a way to start the main, if I happen to be 50 miles from any assistance. BTW, I have purchased a very basic load tester (all I could find in Nassau) and it appears that one of the two 12V Group 31's is failing. The gauge goes to the bottom of the "Weak" zone on the gauge as soon as the load is induced. Time for new batteries, if Nigel Caulder can't tell me how to revive a weak AGM. Those batteries are only 2 1/2 years old - disappointing.

 

[ranger58sb]

I have a 12V house bank, and a 12V genset start battery, and a 24V main engine start battery (only one engine). The 24V "battery" is actually 2 12V's in series. The genset battery and the two main engine batteries are all right next to each other in the same battery box. The house bank is on the opposite side of the engine room (8 - 10 feet away, if you're measuring cable runs).

I have a parallel switch that allows me to parallet the 12V house bank with the 12V genset start battery, in case the start battery ever dies.

Currently, I'm having some issues getting my main engine start batteries fully charged. Not sure what the issue is, but let's leave that alone for now, please. I want to know if I can move the cables from the 12V genset start battery to the posts of one of the 12V main engine start batteries - and then use the parallel switch to add the capacity of the house bank to the main engine start bank? (You know - without a massive shower of sparks, and batteries exploding, and all kinds of bad stuff.)

I must not have been clear with my original post. I was looking ONLY for a way to get my main engine started, with its 24V starter, with assistance from my 12V house bank, as a last resort. I was not looking for a long term solution to the problem. Just a way to start the main, if I happen to be 50 miles from any assistance. BTW, I have purchased a very basic load tester (all I could find in Nassau) and it appears that one of the two 12V Group 31's is failing. The gauge goes to the bottom of the "Weak" zone on the gauge as soon as the load is induced. Time for new batteries, if Nigel Caulder can't tell me how to revive a weak AGM. Those batteries are only 2 1/2 years old - disappointing.

You mean the bad battery is one of the main engine start batteries from the 24V bank?

If so... one option is maybe to make that bad battery your 12V genset start battery, then combine the other two to become the 24V engine start bank. Probably no matter the size difference, for a short-term fix. And the bad battery may still be OK for starting your genset.

But if not... you have that parallel switch to to get the generator going...

And/or...

What charges your engine start bank? Only the alternator? Or maybe also a battery charger somewhere on the boat, that you can feed from the genset? In this instance, starting the generator, charging the start bank for a while... could be enough to start your mains.

-Chris

 

[BrianSmith]

You mean the bad battery is one of the main engine start batteries from the 24V bank?

If so... one option is maybe to make that bad battery your 12V genset start battery, then combine the other two to become the 24V engine start bank. Probably no matter the size difference, for a short-term fix. And the bad battery may still be OK for starting your genset.

But if not... you have that parallel switch to to get the generator going...

And/or...

What charges your engine start bank? Only the alternator? Or maybe also a battery charger somewhere on the boat, that you can feed from the genset? In this instance, starting the generator, charging the start bank for a while... could be enough to start your mains.

-Chris

Yes, the bad battery is one of the two Group 31 12V's that makes the 24V starting bank. Unfortunately, the genset start battery is not a Group 31, and there isn't space to change it to one. I agree that having three identical batteries all in the same place, any two of which could be used to make a 24V bank, would be ideal.

The two Group 31's are charged by the main engine alternator, and by a wall mounted 24V charger that runs off the house 120V system, which is powered by the genset. Unfortunately, when all this started, I discovered IT isn't outputting the charge that it should, so it's been shipped back for repairs. When it's back, and I have it as a backup, then I'm probably pretty well covered. But with only one main engine, I like to be a little better than "pretty well covered."

Thanks for all the input, TF'ers!