12v from 24v battery bank

[tesswoodnutt]

We are currently re-engining. Old engines were 24v, with supply for radar and windlass taken from engine batteries (24v) House electrics are all 12v, and were taken from a seperate bank of two.
New engines are 12v so we need one battery for each. We plan to have a second alternator on one engine to charge the 24v bank (2 batteries). My question is should I have another bank for 12v house use or can I take a centre tapping (ie 12v) from the 24v bank? Obviously the loading has to be taken into consideration but how would this arrangement effect battery life etc? Saving on batteries saves on weight but at what cost?

 

[bdsweeting]

I don't see why you shouldn't use the 24v bank.

Why not have 24v alternators from the start and split the output into the engine start batteries, 12v, hang on I think I'm confusing myself here but perhaps you get my drift...

 

[absit_omen]

We have 24v system for all but instruments. These are 12v and we use a 24 to 12v converter.

 

[yachtbits]

DO NOT take centre tapping from 24v bank. It will cause and in-balance in the bank and end in tears!

Best options are,

2 alternators and 2 seperate battery banks, 1 x 12v and 1 x 24v

or

use 24v alternator and battery bank with to 12v converters.

kev

 

[William_H]

24Volt is a vastly better system especially for starting and anchor winch. The current is half while the voltage drop in wiring has less significance anyway.
However if you now have 12 volt starters on new engines and you don't feel inclined to run them on 24 volts. (thats not as daft as you might imagine although you would do it with some concern) then you are stuck with a primarily 12 volt system.
If you are stuck with a 24 vollt anchor winch and radar then it would be too expensive to replace with 12 volt gear.

A 24 volt system with converter to 12 volt with or without a 12 volt battery is the usual and best way to go. Definitely don't try tapping off half of thhe 24 volt battery it just gets too unbalanced.

However I think it would be disastrous to try to operate 1 or 2 12 volt starters using a converter with a 12 volt battery. that is unless the converter was disconnected from the 24 volt supply before trying to start the engines (12volt) because the voltage sag of the 12v battery due to the high start current would overload even the bigest converter. An even bigger problem when you start the second engine with one running and charging the system. If you did try that system the converter would not be as good at recharging the engine start battery as an alternator. ie much less current capacity hence slower recharge. You can have an alternator charging a battery which is used to start a second engine without any problems.
So my suggestion althouigh messy is to have a 24 volt alternator for a 24 volt bank and 12 volt alternator and battery for engine starting.

You could have a 24to12 converter and use the 24volt as a domestic bank without another 12 volt domestic battery. You could keep 12 volt battery for engine start only and use another 12 volt battery for domestic using a Voltage sensing Relay to provide charge from the engine battery system.
The tidiest but most expensive is to go all 24 volt with a converter for radios etc. hope this all makes sense olewill

 

[penultimate]

Agree with Yachtbits that centre tapping a 24v battery bank will lead to premature battery failure through overcharging. I have a 24v system and 24v/12v dropper for instrument supplies (up to 10 amps). The dropper is a bit wasteful because there ia a current drain even on on load; but the benefit is a rock steady 12V supply.
The big problem comes if you need to supply higher 12V power. I have recently installed an anchor winch and bought a 12V model because I couldn't obtain a 24V one except at silly prices. I have fitted a 3 pole change over switch (only 2 poles are in fact required, but I couldn't find one with sufficient current carrying capacity). This enables me to switch one battery bank between series and parallel so that I can operate the 12v anchor winch and switch back to 24v for re-charging from the 24v alternator or for engine starting.

 

[PCUK]

If the 24 volt batteries have exposed contacts on the tops, cut out the centre one and you then have two perfectly good 12 volt batteries in one case

 

[JerryHawkins]

This may be just what you need...

I think you need a "battery equaliser" from Merlin Equipment. Have a look at the PDF on their web site: http://www.merlinequipment.com/downloads/Battery%20Management%20Info%20Pack.pdf
Towards the bottom of this file you'll see this device. No idea how much it costs but it would seem to fit your requirements of high power 12v (for your 12v windlass) use from your 24v bank. Seems you can get up to 90A at 12v with a max of 100A.

Cheers,

Jerry

 

[Bodach na mara]

Re: This may be just what you need...

My Westerly Renown was built with the Watermota engine which uses a 24 volt alternator and battery system. It did this because there are no heater plugs on the engine. It has a 12 volt started and when you hit that with 24 volts it spins the engine fast enough to start without fail no matter how cold it is. You must be careful not to activate the starter for more than 15 seconds however.

The draw-back of this system is that most of the gear on the boat is 12 volt. Where possible, I have renewed with 24 volt versions as the current is halved which means lighter cable can be used. I now have the fridge and heating system as 24 volt. For the rest, I use a 24 volt to 12 volt DC to DC converter form Alphatronics. Theirs was the one with the lowest stand-by current that I could find. One model (from another company) drew a whopping 250 milliamps when delivering zero current output.

Before I fitted the converter, I used a centre tap on the battery bank and, as someone else said, it ended in tears. One battery was flat (the one I was using) and the other was fully charged and would not allow the charging curent through. Swapping round the batteries was the temproary solution but was hard on the back, lifting them out to re-position. The DC-DC convertor is the way to go.

 

[penultimate]

Re: This may be just what you need...

Interested in the Merlin Battery Equaliser which I was not aware of when I designed my 24V/12V change over switch arrangement. The illustration on the last page of the Merlin catalogue appears to show a 12V/24V convertor, not the equaliser; and I am puzzled by the wiring schematic which appears to depend on separated 12V and 24V earths. The largest Equaliser is even bigger and heavier than my change over switch, and looks more expensive than my £63 change over switch.