24v on 12v system??

[vasant]

Hi
This is more an academic question i understand buying a DC converter is the clean way to go , BUT
If i have a 24V appliance on my boat could i connect the + to the service battery and the MINUS to the starter when connecting the other two poles in series while all other applinces are served from each battery on 12v?
Just wondering what can go wrong....

 

[oldmanofthehills]

To enable the batteries to be charged by the alternator, they must have a common earth (negative) with each positive fed from common alternator output either by splitter diode or VCR.

So there is no way to connect them in series as they are already in semi-parallel

 

[chriscallender]

Hi
This is more an academic question i understand buying a DC converter is the clean way to go , BUT
If i have a 24V appliance on my boat could i connect the + to the service battery and the MINUS to the starter when connecting the other two poles in series while all other applinces are served from each battery on 12v?
Just wondering what can go wrong....

That would only work if currently the starter wiring and the service wiring are completly independent of each other. For instance, if the negiative of both systems is currently connected to an anode or other common ground point (even indirectly - for example through the engine - it might not be completely obvious ) then you come along and connect the two batteries in series you have just shorted out one of the batteries because you will connect the positive terminal of one of them to the negative terminal of the other which is actally the negative of both.

The other issue is charging - if you want to charge them off a 12v alternator they need to be in parallel, or one at a time for charging. So you need some switching scheme that allows you to get rid of the series connection.

If the 24v appliance doesn't need much power then a non grounded small 3rd battery in series with the 12v house battery would be easier than messing with the starter battery. But you'd still need to switch it to be paralleled for charging and forget powering your 24v appliance while charging battery #3.

But then if the 24v appliance doesn't need much power a DC-DC converter is cheap as chips, so definitely not worth the bother.

https://www.amazon.co.uk/Youmile-Converter-Adjustable-Voltage-Regulator/dp/B0C5HXZ6HT

It all depends how much juice you need at 24v. Bur regardless, it is almost always more trouble than it is worth unless the whole system is 24v, so as you say its an academic question. Even powering a diesel heater glowplug off 24v would be straigtforward with a DC-DC boost converter from 12v.

Chris

 

[oldmanofthehills]

That would only work if currently the starter wiring and the service wiring are completly independent of each other. For instance, if the negiative of both systems is currently connected to an anode or other common ground point (even indirectly - for example through the engine - it might not be completely obvious ) then you come along and connect the two batteries in series you have just shorted out one of the batteries because you will connect the positive terminal of one of them to the negative terminal of the other which is actally the negative of both.

The other issue is charging - if you want to charge them off a 12v alternator they need to be in parallel, or one at a time for charging. So you need some switching scheme that allows you to get rid of the series connection.

If the 24v appliance doesn't need much power then a non grounded small 3rd battery in series with the 12v house battery would be easier than messing with the starter battery. But you'd still need to switch it to be paralleled for charging and forget powering your 24v appliance while charging battery #3.

But then if the 24v appliance doesn't need much power a DC-DC converter is cheap as chips, so definitely not worth the bother.

https://www.amazon.co.uk/Youmile-Converter-Adjustable-Voltage-Regulator/dp/B0C5HXZ6HT

It all depends how much juice you need at 24v. Bur regardless, it is almost always more trouble than it is worth unless the whole system is 24v, so as you say its an academic question. Even powering a diesel heater glowplug off 24v would be straigtforward with a DC-DC boost converter from 12v.

Chris

Heaven, what a receipe for disaster. Though I think one could arrange such switching to be applied at the point of using the 24V, there is clear risk that if switching fails one battery is shorted out and destroyed, or if effective and forgotten one battery fails to charge and engine cannot be started from it - which means no alternator and no way to get electric back except jump pack.

Of course a complex auto control system could be devised and all wiring modified, but it would be invasive, possibly expensive, and probably beyond the capability of the OP. What is more it leaves the 24V system use capable of flattening the engine starter battery thus resulting in the earlier mention failure situation

DC to DC converters from aux battery by comparison are cheap and easy

 

[Refueler]

Simple answer is NO.

 

[chriscallender]

Heaven, what a receipe for disaster. Though I think one could arrange such switching to be applied at the point of using the 24V, there is clear risk that if switching fails one battery is shorted out and destroyed, or if effective and forgotten one battery fails to charge and engine cannot be started from it - which means no alternator and no way to get electric back except jump pack.

Of course a complex auto control system could be devised and all wiring modified, but it would be invasive, possibly expensive, and probably beyond the capability of the OP. What is more it leaves the 24V system use capable of flattening the engine starter battery thus resulting in the earlier mention failure situation

DC to DC converters from aux battery by comparison are cheap and easy

Indeed. The OP asked an academic question so I tried to answer it. I agree with you, there's plenty to go wrong. It might have made sense 30 or more like 40 years ago, but it would be a silly way to get a 24v applicance to work in 2025 when you can get DC-DC boost converters for a few quid. Don't do this old school approach!

 

[Refueler]

Indeed. The OP asked an academic question so I tried to answer it. I agree with you, there's plenty to go wrong. It might have made sense 30 or more like 40 years ago, but it would be a silly way to get a 24v applicance to work in 2025 when you can get DC-DC boost converters for a few quid. Don't do this old school approach!

Old School approach ??

Its just plain wrong ... and would create more problems than the single its supposed to solve.