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Old 25-01-2018, 10:28   #1
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Charge at 24V, consume at 12V

Just looking for HowTo advice, don't want to get into "the why" background here, take as given that the concept is sound, or just treat it as a design challenge or thought experiment.

All charge sources are 24V

There are some 24V consumer loads, but they will only be running while a charge source is active.

I want something like a VSR/ACR that will connect the two halves of the bank in serial when a charge voltage is sensed, then return it to the default parallel state when voltage drops.

Kind of like an inverse of Yandina's Trollbridge functionality.

The charging buss(es) will be separated out from the loads buss.

Ignore the 12V consumer load circuits for now, assume they are switched over to a separate power supply while the bank is charging.
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Old 25-01-2018, 11:22   #2
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Re: Charge at 24V, consume at 12V

making and breaking the battery primary connections that way, assuming something like a high-current capable relay to do the actual switching, you've got to realize that even a Group24 battery can put out over 3000 amps in a crowbar short, so that's the kind of current that relay might be seeing. Even if it is just handling routine loads, you're asking for contacts that can switch the full capacity of the house bank.

Not impossible but other than theatrical lighting or Dr. Frankenstein's lab...just not commonly found.

I can't think of a robust and reliable way to handle that task, which would be any simpler or cheaper than down-converting the 24V to 12V in the first place. Especially since that eliminates the need for high amperage rated physical contacts.

As a design challenge and thought experiment, I keep coming back to the thought of solenoid-thrown knife switches and big sparks and interrupting the entire power supply as it gets switched back and forth, and while the picture is interesting...it just doesn't seem viable to me.

Like Elon Musk using a Tesla as a rocket testing payload. Interesting concept, but a waste of a perfectly good car.
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Old 25-01-2018, 18:10   #3
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Re: Charge at 24V, consume at 12V

Well, there are some very robust contactors out there, e.g. https://www.bluesea.com/products/770..._-_24V_DC_500A , even more so for relays used in electric propulsion

and as I mentioned, loads won't be on the circuit at switching time,

so in fact, I can specify there will be no live currents present at all, on the relevant circuits at that time.

_____
As further food for thought, here's a HowTo for a (I'm not sure how) relevant scenario

http://www.gonefcon.com/trucktcom/parallel_sw.htm

Apparently there is a similar use case in the world of racing cars
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Old 25-01-2018, 18:50   #4
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Re: Charge at 24V, consume at 12V

You can find series/parallel switches alive and well in the heavy duty truck industry where they are used to connect two 12v batteries in series to drive a heavy duty 24v starter motor, and then switch back to 12v for routine operations and consumers.

Normally these switches are controlled by a mechanical key switch or a starter button, but they could easily be controlled by a voltage sensing relay.

Might work for you....

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Old 25-01-2018, 19:57   #5
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Re: Charge at 24V, consume at 12V

Yes, that's where the linked HowTo came from.

Didn't realize they were usually manually switched, that may make it easier to adapt as you say.

A big truck starter would pull well over 600A in the cold so maybe the contactors would be robust enough. If not, then maybe just use theirs to open/close bigger ones.
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Old 26-01-2018, 09:08   #6
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Re: Charge at 24V, consume at 12V

Quote:
Originally Posted by john61ct View Post

All charge sources are 24V

There are some 24V consumer loads, but they will only be running while a charge source is active.

I want something like a VSR/ACR that will connect the two halves of the bank in serial when a charge voltage is sensed, then return it to the default parallel state when voltage drops.

Kind of like an inverse of Yandina's Trollbridge functionality.

The charging buss(es) will be separated out from the loads buss.

Ignore the 12V consumer load circuits for now, assume they are switched over to a separate power supply while the bank is charging.
Online you can get a 24v to 12v converter that is commercial, industrial, military, and marine compatible. We have been using this converter for years. The converter converts to 12 volts to a house battery and does the switching all automatic. The converter has no switch, no lights, connected directly to 24v and 12v. all automatic, worry free.
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Old 26-01-2018, 10:00   #7
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Re: Charge at 24V, consume at 12V

John, it wouldn't have to be manually switched. After all, many vehicles have used manual starter buttons ("push to start") and the same button that energizes the starter motor relay, can energize the relay that is switching the batteries into 24V configuration.

I see a couple of gotchas in there: While the batteries are being switched, what is keeping up power to the alternator, lights, etc. and presumably the engine systems, unless it is an older dumb diesel with a mechanical fuel pump that doesn't need electricity to run. (Faithful dumb beasts!)

Similarly, I guess the "push" button needs to be powered while there's no power during the switching? Or else it would just chatter? Little gotchas like that.

Folks say this is common in the trucking industry, OK, I can believe that. Did you notice the web site is named "gonef" which is Yoiddish for THIEF ? And he mentions needing to dress the relay points, which indicates whatever relays he is using are under-rated, but he doesn't mention what 500A rated relays he is using. Or maybe he's just over-rating some relays.

It can be done, yes that is a way to do it. But industrial quality relays that won't burn their points aren't going to be cheap. And there's the transient power losses to deal with. It just seems...well...like the real application is a "sound and light" show.(G)

Seabreeze? Got a name and price for that converter to be found online?
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Old 26-01-2018, 17:03   #8
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Re: Charge at 24V, consume at 12V

A bit confusing if we don't know what you really want to achieve or improve with this approach.

Is the basic problem that all charging needs to happen at 24V?

Then there is "some other power supply" to run the 12V devices while charging. A DC-DC converter maybe?

At some point it was stated there is no current when switching, then truck starter contactors were thrown in for massive currents... what is it now?

Just a thought experiment, ok, not really a problem then...
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Old 26-01-2018, 17:22   #9
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Re: Charge at 24V, consume at 12V

What about leaving the batteries in series and half the loads are on the 0/12v battery the other half the loads are on the 12/24v battery. Could cause some confusion sometimes but no switching required. Just a thought experiment
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Old 26-01-2018, 17:46   #10
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Re: Charge at 24V, consume at 12V

Yes I'm very familiar with DC-DC converters, but they are costly at high currents, both up front and in energy wasted. Victron Orion series, Samlex is OK, plenty of industrial suppliers.

Quote:
Originally Posted by hellosailor View Post
While the batteries are being switched, what is keeping up power to the alternator, lights, etc. and presumably the engine systems,
As stated, all loads are presumed offline wrt the charged bank, likely switched over to a reserve bank, but don't want to sidetrack with details like that for now.


Quote:
Originally Posted by hellosailor View Post
he mentions needing to dress the relay points, which indicates whatever relays he is using are under-rated, but he doesn't mention what 500A rated relays he is using. Or maybe he's just over-rating some relays.
I wasn't spec'ing those specific products, just that seemed a clear diagram of how the category works in general.
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Old 26-01-2018, 18:31   #11
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Re: Charge at 24V, consume at 12V

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Originally Posted by foufou View Post
What about leaving the batteries in series and half the loads are on the 0/12v battery the other half the loads are on the 12/24v battery. Could cause some confusion sometimes but no switching required. Just a thought experiment
Give it a (thoughtful) try and see what happens. Just not indoors, please, because you're shorting out the "upper" 12V battery in such a configuration.
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Old 26-01-2018, 21:26   #12
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Re: Charge at 24V, consume at 12V

Quote:
Originally Posted by john61ct View Post
Just looking for HowTo advice, don't want to get into "the why" background here, take as given that the concept is sound, or just treat it as a design challenge or thought experiment.

All charge sources are 24V

There are some 24V consumer loads, but they will only be running while a charge source is active.

I want something like a VSR/ACR that will connect the two halves of the bank in serial when a charge voltage is sensed, then return it to the default parallel state when voltage drops.

Kind of like an inverse of Yandina's Trollbridge functionality.

The charging buss(es) will be separated out from the loads buss.

Ignore the 12V consumer load circuits for now, assume they are switched over to a separate power supply while the bank is charging.
Since you say the 24 volt loads will only be operating during charging, you could use a 24V to 12V convertor for charging, instead of having to deal with all sorts of issues of switching the batteries and loads. It would be helpful to know the maximum ratings you are looking for. For less than $500 you can buy a convertor that will output 80A continuous at 12V. But you will lose some power since the conversion is not 100% efficient.
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Old 27-01-2018, 10:21   #13
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Re: Charge at 24V, consume at 12V

Try looking at In Power Mobile power solutions
They sell solid state contactors

UltraSwitch Dual Electronic Battery Disconnects
The industry standard!
Withstands harsh vehicle environments
75 amp to 200 amp capacities
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Old 27-01-2018, 12:07   #14
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Re: Charge at 24V, consume at 12V

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Originally Posted by waterman46 View Post
could use a 24V to 12V convertor for charging
As I've said I'm up to speed on DCDC conversion, and while it may be an alternative solution, I'd like to keep this thread to the OP request.

Thanks
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Old 27-01-2018, 12:16   #15
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Re: Charge at 24V, consume at 12V

Quote:
Originally Posted by Techtool View Post
In Power Mobile power solutions
Is this who you mean? http://www.inpowerdirect.com/solidstatecontactors.php

The Blue Sea ones I posted seem more robust, but where lower currents are present, cheaper options are a good thing.
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