12v from 24v series batteries

[Rocksteadee]

Mate has asked me about wiring for his 12, 24, 240 wiring system.

2 options:
1. Stand alone 12v battery. My preferred option as it gives redundancy.

2. 12v tapped off 24v series start battery’s. Reduces weight and space.
However is there a problem with over charging the uppermost battery for the 24 series connected and under charging the lower 12v battery in the same series as they would both be taking the same current whilst charging?

 

[halcyon]

View attachment 90171
Mate has asked me about wiring for his 12, 24, 240 wiring system.

2 options:
1. Stand alone 12v battery. My preferred option as it gives redundancy.

2. 12v tapped off 24v series start battery’s. Reduces weight and space.
However is there a problem with over charging the uppermost battery for the 24 series connected and under charging the lower 12v battery in the same series as they would both be taking the same current whilst charging?

Taking a center tap from a 24 volts system will always kill one of the batteries, it is possible, but needs mains charging via a special battery charger.

You are draining more current from one battery, but charge current is common to both, charge volt split between batteries means one is at a lower voltage so limiting current acceptance.

Brian

 

[starfire]

Taking a center tap from a 24 volts system will always kill one of the batteries, it is possible, but needs mains charging via a special battery charger.

You are draining more current from one battery, but charge current is common to both, charge volt split between batteries means one is at a lower voltage so limiting current acceptance.

Brian

I would use a suitably rated 24V to 12V converter.
Depending on the loads.
Keep a separate 12V battery for the generator.

That's how it's done on most modern boats, with several converters for different services.

 

[tico]

Starfire is correct...... Tapping 12v from 24 will lead to an imbalance in cell charge, not corrected until an 'equalising' charge is applied.
Use a DCC converter

 

[noelex]

If I understand correctly, the 12v system is used to start the generator. Unfortunately the high current draw involved in starting even a small engine means this cannot be done with a 24v to 12v DC converter.

Centre tapping a 24v system is not good for long term battery health so the only sensible option is a seperate 12v battery.

Having a 12v system available on a 24v boat is useful for the small number of systems that will not run on 24v, for example power to NMEA 2000 bus. This is ideally done with a 24v to 12v DC converter.

 

[halcyon]

I would use a suitably rated 24V to 12V converter.
Depending on the loads.
Keep a separate 12V battery for the generator.

That's how it's done on most modern boats, with several converters for different services.

That's how we have been doing it 35 years, that's the industry standard.

Back in the late 1990's I did a design exercise to produce a system that allowed for 12 volt center tapping a 24 volt system and charged batteries correctly to maintain full life. We got as far as testing it on boats, but it needed a transformer system, but with the growth of switch mode I did not consider a marketing viability.

Brian

 

[Rocksteadee]

Cheers guys. Pretty much as I thought. Now updated with option 3

 

[jrudge]

Personally I would do a hybrid.

1. Dropper for all boat 12v

2. Centre Tap for the gen start.

The load will be very short and will not lead ( in my view) to a material imbalance.

The charger I assume will sense the drop and then the battery outside of the tap will be overcharged for a short while.

Batteries on boats tend to last 3-4 years in my experience and unless the gen is being started day in day out or needs lots of goes to start I would be surprised if it made a material difference. My Onan starts with I guess a few seconds cranking - if that.

Others of course may have a different view.

 

[Portofino]

We have two twin banks of pairs for each on a 24 v boat , so no centre bat as such .How ever a sep small dedicated gen bat with its own trickle charger .
A little 60 Ah for a 400 cc diesel Yanmar engine .
There is next to it a push down ( spring loaded ) switch which takes 12 v from somewhere in an emergency.Not sure which bank it nicks 12 from tbh .
The geny is a good starter and as JRudge infers can’t ever see a few secs draw off one of the pairs is gonna make a material difference in the bigger scheme of bat longevity.
I have used it once ....the robbing 12 from the 24 in 5 years btw .....more out of curiosity.

Having a small 12v separately is handy in the sense you can use it for a dingy / paddle board inflator or AN other ad hoc 12 temporary supply

 

[halcyon]

We have two twin banks of pairs for each on a 24 v boat , so no centre bat as such .How ever a sep small dedicated gen bat with its own trickle charger .
A little 60 Ah for a 400 cc diesel Yanmar engine .
There is next to it a push down ( spring loaded ) switch which takes 12 v from somewhere in an emergency.Not sure which bank it nicks 12 from tbh .
The geny is a good starter and as JRudge infers can’t ever see a few secs draw off one of the pairs is gonna make a material difference in the bigger scheme of bat longevity.
I have used it once ....the robbing 12 from the 24 in 5 years btw .....more out of curiosity.

Having a small 12v separately is handy in the sense you can use it for a dingy / paddle board inflator or AN other ad hoc 12 temporary supply

You have one advantage giving advise, you are not selling something, some of us have to back up what we say with a warranty.

Me I would use a separate 12 volt battery and a 12 volt charger powered from the 24 volt battery bank.

Brian

 

[Portofino]

You have one advantage giving advise, you are not selling something, some of us have to back up what we say with a warranty.

Me I would use a separate 12 volt battery and a 12 volt charger powered from the 24 volt battery bank.

Brian

It’s what we have a separate 12 v for the geny + it’s charger . On a 24 v boat .
The 24 v banks , std engine + domestic have there own smart charger .

So it’s all been up sold if I understand your point correctly?

Even more so as they ordered and fitted this little x over switch to power the genys 12 v starter motor from somewhere,
So if disconnect it’s 12 v bat or it’s discharged......I can still start it .Bear in mind the std banks are 24 v .

Also got a cross over switch on the dash to use domestic bank if the starter bank is duff .

So what else could be procured ?

 

[starfire]

My thinking on this.

Keep the generator battery separate, you never know when you might want to start it to charge the main banks.!

Most generator batteries are only charged from the generator alternator itself.
That's solved by using a small trickle charger.

I have been on enough boats where a centre tap was used for 12V, "it's only a small load" Result, one boiled battery, the other permanently flat.

One was a 12v passerelle on a 24v boat

 

[vas]

I've got the t-shirt (stealing 12V out of the engine bank of 24V to run the generator) Two new starter batteries later, I bought a third smaller one just for the generator and I've got the extra safety that the generator will start no matter what!
my generator hasn't got an alternator, so got a 3euro pcb trickle charger thingy which charges this 80Ah battery from the service bank (which is topped up with 600W solar)

Re 24->12V dropper, if you get one, CAREFUL not to get the one outputting 12V cause with the N2K bus and a few more bits and bobs (and the VHF) output is always 11.4-11.5V
There's a version outputting 13.something sort of charging current and that's much better as you end up with 12.something always.

cheers

V.

 

[jrudge]

Thanks vas. I am honestly surprised it makes a difference but now I know !

wonder if you could start a generator from the small lead acid ones used in the likes of Williams Ribs.

 

[Portofino]

I've got the t-shirt (stealing 12V out of the engine bank of 24V to run the generator) Two new starter batteries later, I bought a third smaller one just for the generator and I've got the extra safety that the generator will start no matter what!


cheers

V.

I once in the winter had a hesitant start .Put it down to cold relatively and a bit of time since the last visit .
Went round to Antibes , not a long trip under an hr .
Arrived and the fuel pontoon was on limited opening hrs and it was a 2 hr wait .No problem Some went off into town , both ridges on , a few lights , music and the sun came out .
Fuel guy rolls up , it takes 40 mins of phaffing to be ready to go ......so nearer 3 hrs with no shore power .
Turn the key ....nothing not even dead mans solinoid click .
Press the x over and this time a slow crank but not as fast as normal and within a few mins getting slower it won’t crank any more .No shore power about .
Bloody marvellous!

Fortunately the tiny 60ah started up the geny straight away and I turned the charger on .
I could see by the amps gauge the charger was stuffing a lot in both banks ie both pretty low .

I took a punt after 40 mins saw the bulk was over on saloon gauge so turned it off and the engines cranked sufficiently quick enough to fire up .

When we got back to La Nap I replaced the starter bank 2x 180:ah .

Suppose in hindsight towards the end of that season Oct ish I guess the crank speed did reduce .
But the thing is the bats just slowly decay in terms of holding charge ....the CCA .They may be sitting at 14 v or what ever buts it’s the cold cranking amps to spin 12.8L of cold engine that’s indeterminable in normal use .
With a car you can jump it .......not sure how that works practically on a boat ?

So now I know via the geny I can find some power in the short term .

 

[vas]

Jeremy,

tbh, I always had issues starting the generator, so I was doing often long crank sessions (not good for batteries)
Took it two seasons to kill BOTH 180Ah LA starter batteries

Doubt you can start a decent 3-4cyl diesel generator from the tiny jetrib battery. their CCAs are silly low. TBH, I'm amazed they do start the jetribs
I also bought a battery tester, among the best 50euro spent on the boat. I regularly go through the batteries and check their CCA and state of charge individually (or in pairs for the 6V Trojans)

 

[kashurst]

can you post a link/source for your battery tester Vas please?

 

[vas]

can you post a link/source for your battery tester Vas please?

tried yesterday when I was about to post and failed miserably!
not on ebay apparently, and don't remember the brand name (I'm working from home today) will post in the evening or tomorrow when I get there.
it's got a basic setup where you configure the CCA of the battery and go through a process of 4button presses (iirc) where it checks and finally tests the battery. on screen display shows current voltage and after the test the CCA measured and that's the important bit. 12V batteries only (a problem with the 6V Trojans)

V.

 

[jrudge]

Jeremy,

tbh, I always had issues starting the generator, so I was doing often long crank sessions (not good for batteries)
Took it two seasons to kill BOTH 180Ah LA starter batteries

Doubt you can start a decent 3-4cyl diesel generator from the tiny jetrib battery. their CCAs are silly low. TBH, I'm amazed they do start the jetribs
I also bought a battery tester, among the best 50euro spent on the boat. I regularly go through the batteries and check their CCA and state of charge individually (or in pairs for the 6V Trojans)

I borrowed a digital tester last year as I had a battery problem ( as opposed to the large load testers). I was sceptical but it identified the dud battery correctly in a few seconds.

 

[vas]

not sure of the terminology, mine is a tiny and light thing say 80X150X20mm with two short cables with croc clips at the end.