DC 24v to dc 12v battery charger with galvanic isolation

[fuss]

Does anyone know of an input 24v battery charger with output 12v, around 10 amps and 3 stage charging and galvanic isolation?
What I want to do is charge my dedicated 12v starting battery from my 24v house bank.
I dont need many amps as not much comes out of the start battery when starting.
Galvanic isolation is important as my boat is aluminium.

If it could recognise when the house bank was at say more than 26v and then charge the 12v start battery until the house bank voltage dropped to maybe less than 25.5v then this would be even better. However I can probably detect this using a 24v vsr.

 

[Seanick]

I charge my 12v domestic from my 24v engine batts.
I use a 24 to 12v converter, rated at about 65 amps.
Its not a 'dropper' but a digital converter. Its two units back to back which are probably the smaller 30amp units.

It was supplied by Furneax Riddall in Portsmouth. Not sure if its on thier website but if not give them a ring and ask for Giles.

Not sure what the issue with 'galvanic isolation' is. All your electrics should be completely isolated from the hull anyway.
Galvanic isolators are for the 240v ring main, if installed.

 

[kesh]

would it be a really bad idea to charge two 12 v in series from a nominal 24 v source? i can't think of any reason why not in theory.

 

[fuss]

Not sure what the issue with 'galvanic isolation' is. All your electrics should be completely isolated from the hull anyway.
Galvanic isolators are for the 240v ring main, if installed.

Thanks for the information.
The galvanic isolation refers to the earth. as with an isolating transformer the 24v creates the 12v magnetically with no direct 24v to 12v earth connection. this means that the 12v produced has its own seperate earth that is in no way connected to the 24v earth.

As my boat is aluminium I am always careful.

Mastervolt for example have 2 types of 24-12v chargers. The mastervolt mac which is not galvanically isolated and the mastervolt magic that is isolated and costs alot more. Before someone asky why I dont just buy a mastervolt magic, I find the cost of around 500 a bit steep.

 

[VicS]

would it be a really bad idea to charge two 12 v in series from a nominal 24 v source? i can't think of any reason why not in theory.

You can connect two 12 volt batteries in series and charge them from a 24volt charger just as if they were a 24 volt battery ... in effect they are when in series.

BUT If you are thinking about a pair of 12 volt batteries normally connected in parallel and used as a 12 volt bank then the switching required to swap them from parallel to series gets a bit complicated I think you will find but technically possible. A 24 to 12 volt charger may be the simpler way.

 

[kesh]

You can connect two 12 volt batteries in series and charge them from a 24volt charger just as if they were a 24 volt battery ... in effect they are when in series.

BUT If you are thinking about a pair of 12 volt batteries normally connected in parallel and used as a 12 volt bank then the switching required to swap them from parallel to series gets a bit complicated I think you will find but technically possible. A 24 to 12 volt charger may be the simpler way.

Those switches could get very hot if you messed up the switching sequence .

 

[VicS]

What I want to do is charge my dedicated 12v starting battery from my 24v house bank.

Just to clarify

you have a strange set up with 12 volt starting but 24 volt domestics ? The opposite way, as Nick has, would be more common I would have thought.
(Although I am aware that some Watermotor engines had 12 volt starter powered from a 24 volt battery)

So you have an engine with a 12 volt starter but a 24 volt alternator that charges the house bank rather than the starter battery ?

You say you need a 24 to 12 volt multi stage charger with an isolated output. I am wondering why you do not have a common negative for the two systems ??

Why bother with a multi-stage charger for the engine start battery. Its normally the house bank where one is interested in optimising the charging The starter batter does not need it surely?

 

[VicS]

Those switches could get very hot if you messed up the switching sequence

yes I don't think you'd want to do it with manual switches. Heavy duty relays (solenoids) with break before make contacts would be required.

 

[kesh]

yes I don't think you'd want to do it with manual switches. Heavy duty relays (solenoids) with break before make contacts would be required.

i made a slightly similar relay switched system around an ammeter so it would measure current flow out of the battery during use and into the battery when being charged. i could have bought a zero centred ammeter, but enjoyed the challenge.

 

[Seanick]

http://www.furneauxriddall.com/shop/acatalog/DC-DC_Converters.html


Cheap enough, even for the 60A.

 

[VicS]

Cheap enough, even for the 60A.

But all non isolated. No good if he really does need an isolated one.

 

[William_H]

12v 24v

I agree with Vic that you would be far better off to use 24v on the starter and so have a standard engine/service batteries with the option of starting the engine on the service batteries or both engine and service . Of course galvanic isolation is still going to be tricky but presumably you have that sorted at the moment. 24v is used for high current services so that current is reduced for the same power and wiring volt drop is less significant. The engine starter is the highest current device you will have so the benefits would be most felt for engine starting.
You might find another starter rated at 24v or in fact find that the 12v starter goes well on 24v.
The reason for this is that the stater motor draws a lot of current when stationary or low speed but current reduces dramatically as it speeds up. So it speeds up faster on 24v so total current is not necessarily much more on higher voltage. If running a 12v starter on 24v you might want to be wary if doing prolonged cranking. good luck olewill

 

[fuss]

Just to clarify

you have a strange set up with 12 volt starting but 24 volt domestics ? The opposite way, as Nick has, would be more common I would have thought.
(Although I am aware that some Watermotor engines had 12 volt starter powered from a 24 volt battery)

So you have an engine with a 12 volt starter but a 24 volt alternator that charges the house bank rather than the starter battery ?

You say you need a 24 to 12 volt multi stage charger with an isolated output. I am wondering why you do not have a common negative for the two systems ??

Why bother with a multi-stage charger for the engine start battery. Its normally the house bank where one is interested in optimising the charging The starter batter does not need it surely?

Yes, that is correct I have 24v domestics charged with a big 24v alt. I have a 12v engine and a 12v AC generator which are both started using the same dedicated (starting only) 12v battery.
The AC generator has a very small 12v alternator/generator that charges at maybe 5 amps.
There is is a small 12v, 15a AC battery charger that can run off the inverter.

The 12v starting battery is charged by either the small alternator on the AC generator or the AC 12v charger through the inverter. This has worked fine since i bought my boat.

I also did not like this at first and also wondered about the complication of having 12v starting. However the AC generator is 12v starting and so it is simpler when they both have a common (starting only) battery.

So why do I want to now charge my 12v battery through the 24v battery....
Whenever the 24v battery is being charged, I want the 12v battery to be charged as well at lets say 15a with a small charger that uses the 24v battery as input. I want this to happen automatically which I thought could be achieved using a 24v VSR.

the advantages are as follows.....
When the main engine runs the 12v battery gets charged. When the main engine is stopped the 12v charging stops as the 24v voltage drops to under 26v.
When the solar panel charges the 24v battery then the 12v battery is also charged if the voltage goes above 26v.
This is a big advantage for when I leave the boat for some extended periods.

Why dont I have a common negative for the 24v battery and the 12v battery..... The 24v and the 12v systems both were designed with separate negative bus bars.

 

[kesh]

24v domestic is useful at reducing voltage drop. it is occasionally found in narrowboats where the long cable lengths can cause quite a large amount of drop. and as twin alternators are pretty common, and narrowboat engines don't need 24v cranking, the 12v cranking, 24v domestic set up in principle is a good one.

 

[TrueBlue]

This is getting awfully complicated, and beginning to lose the plot.
Point to consider:-

A voltage converter is not suitable for battery charging as it will put out its rated voltage "forever" and cook your batteries.
Starting engines take relatively little power which can be replaced quickly; that is a low output charger will put the energy back quickly.

Rather than question why you do have a mixed system (I do and understand the issues), here are some suggestions:-

If money is not an issue then lookey here for what that nice Mr Sterling has to offer However its quite expensive and overkill.

What about a cheap Sterling Pro budget mains battery charger running from your inverter, will keep the starter battery topped up but will have a standing load on your main battery bank.

Start the engine via a 12v tap on your 24v bank - not usually recommended. I have 12v taps, but then I use NiCd batteries....

 

[ff_nick]

Does anyone know of an input 24v battery charger with output 12v, around 10 amps and 3 stage charging and galvanic isolation?
What I want to do is charge my dedicated 12v starting battery from my 24v house bank.
I dont need many amps as not much comes out of the start battery when starting.
Galvanic isolation is important as my boat is aluminium.

If it could recognise when the house bank was at say more than 26v and then charge the 12v start battery until the house bank voltage dropped to maybe less than 25.5v then this would be even better. However I can probably detect this using a 24v vsr.

Mastervolt also do an isolated 24-12v job with a 3 stage charger mode, probably expensive.

I don't think the set up on your boat is so unusual, I've seen it before on a large MS - 12v starter for 6 cyl Perkins and generator charged by small main engine and generator alternators, 24v house bank running off the main alternator and mains charger.

A centre tap off the 24v volt bank is probably a bad idea for routine engine starting, but a excellent cheap solution for an alternative back up should the starter battery fail.

 

[VicS]

Thanks for clarifying that, Fuss
I now understand what you have and why you want to keep it that way

I was going to do as True Blue has and suggest a Sterling battery to battery charger,
Sterling battery to battery chargers.
They appear to have the auto cut in / cut out feature that you require , so the VSR wont be necessary but are a much larger current rating than you ask for. They are multi-stage, which you want, although IMHO that's unnecessary for charging the starter battery.
I am not sure if the output is isolated or not you would have to check that.
BUT I wonder if it is necessary to keep the 12 and 24 volt systems isolated. Why not interconnect the negative bus bars?
You obviously have an AC power system. I'd expect the "earth" of that to be bonded to the DC negative as I think is required by ISO 13297. presumably you have it bonded to just the 24 volt system. I think both DC systems should be interconnected and earthed to the AC system. If they are not you could have an big potential difference between them and that sound very undesirable with an aluminium hull. Maybe I've missed something.

Another suggestion: A second alternator on the engine ... a 12 volt one. I reckon that would be the best solution but of course may not be possible. (as described in effect by the post above)

 

[fuss]

Thanks Vic - the Sterling battery to battery chargers look perfect. I will do some checking on whether they are isolated.
I also just found an adverc battery to battery charger that does the same thing. they both look like very good devices.
http://www.adverc.co.uk/product/671

BUT I wonder if it is necessary to keep the 12 and 24 volt systems isolated. Why not interconnect the negative bus bars?
You obviously have an AC power system. I'd expect the "earth" of that to be bonded to the DC negative as I think is required by ISO 13297. presumably you have it bonded to just the 24 volt system. I think both DC systems should be interconnected and earthed to the AC system. If they are not you could have an big potential difference between them and that sound very undesirable with an aluminium hull. Maybe I've missed something.

When I bought my boat it had some problems with power in the hull, which i fixed by ensuring no dc earthed to the hull. All earths return to the respective 24v negative bus or 12v negative bus and if necessary the device is isolated from the hull. The anodes now have lasted 4 yrs so I am nervous about changing things. I seem to remember that I tried connecting the two negative buses a long time ago and it caused a galvanic problem, but this might not be true as there were many problems at the time.
(the ac earth is connected to the 12v earth and as this is separate from the 24v earth it is not bonded to the 24v earth.)

The motor has special functions like a negative earth relay to isolate the motor from the 12v electrics under certain circumstances.

So a question.....
If i did try connecting the 12v and 24v earths, what test should I do to ensure I have not introduced a problem. The only test I know for sure is the test of time to see what happens to the anodes... this is a test that I do not like.

 

[VicS]

I cant suggest any tests but i cannot think why interconnecting the negative busbars would cause any problems.

You might like to read ISO 13297 SEE HERE

I think you will find that the AC earth should be connected to the hull at just one point and that it should also be connected to (each of) the DC systems. What you must not do is use the hull as a negative return like they do in cars. Hence only one earthing point on the hull. I think that is the key to avoiding problems with the hull. One connection to it for safety ( with the AC system) reasons but no other connection , deliberate or inadvertent.

An earthing relay is sometimes used with engines coupled to saildrives. All the engine electrics have both +ve and -ve connections and are isolated from the engine (two wire sensors for warning lights and gauges, insulated negative for the alternator etc) but a relay connects the engine block to the earth/dc negative during (preheating and) starting. I am sure there must be starter motors with an insulated negatives but they seem to use the ordinary non insulated type with an earthing relay.

You must also be careful than any electrical fittings mounted on the hull do not use one of the fixing screws as a convenient parking place for the negative ... no fittings intended for cars!

Dammit I've steered clear of discussions on aluminium hulls (and steel ones) in the past now I'm in the middle of this one.

Primarily for steel it has to be acknowledged but there a lot of info on electrics on metal hulls in the technical pages of the SMARTGAUGE WEBSITE Written by a highly respected electrical engineer who lives on a steel narrow boat I believe.

.

 

[TrueBlue]

Thanks Vic - the Sterling battery to battery chargers look perfect. I will do some checking on whether they are isolated.
I also just found an Adverc battery to battery charger that does the same thing. they both look like very good devices.
http://www.adverc.co.uk/product/671

Adverc stuff used to be very much more expensive than Sterling's, so I didn't bother to check there first. Your item above fits the bill, both from the price and utility points of view.
Adverc have a lower public profile than Sterling, but I have found them very helpful and prepared to design - well tailor - the offerings for one's specific needs. They did an oddball setup for my unpopular batteries (different charge / discharge characteristics) and were very patient while I got up to speed. Their solution works well, so I'm happy about their ethos.

Give them a bell about your isolation issues. I do agree about a common earthing point for all systems - your galvanic isolator should protect the hull from shore power issues, but you've had plenty of help on that issue..

Sterling is good for higher powered solutions