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Old 26th November 2009, 04:29 PM   #1
gootee is offline gootee  United States
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Default 24V Add-On Question

I am relatively ignorant about automotive electrical systems and need some general guidance. (I'm sorry if this is slightly off-topic for this forum.)

Assume that I have a medium-sized SUV with a 12-volt electrical system and a 150-Amp alternator. Assume that I need to operate a piece of equipment that needs 24V at about 35 Amps. How could I do that? Is there already a standard, available implementation?

One idea was to add a second 12V battery, in series with the original battery. In that case, what else would need to be changed and/or added? Are there standard ways of doing that? e.g. Could I replace the alternator with a 24V model? Would I then need to add some electronics to control the charging of the individual batteries?

Another way might be to add a second 12V alternator with a floating ground, for the second battery. But I don't think I can easily add another alternator.

Another idea was to add a 12V-to-12V converter that had an isolated (floating) output, to use for charging the second battery. (Maybe such a converter could even be used in place of a second battery?)

Ideally, I want to draw as little extra current from the alternator as possible, above the 35 Amps needed for the 24V equipment.

Thanks for any help or insight anyone can provide.

Last edited by gootee; 26th November 2009 at 04:37 PM. Reason: Additional text
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Old 26th November 2009, 04:49 PM   #2
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What is this for? What kind of load,constant,or intermittent?

A suitably large DC-DC converter should work. To output 24V at 35A it's going to draw over 70A from the 12V side.Total power will remain the same (~840W) plus a little more to cover the losses in the DC-DC converter (they aren't 100% efficient). Let's call it about 80A being drawn from the 12V side (~960W).

I know I've seen such devices being sold,but I can't recall where/what for at the moment.
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Old 26th November 2009, 04:56 PM   #3
gootee is offline gootee  United States
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Originally Posted by DigitalJunkie View Post
What is this for? What kind of load,constant,or intermittent?

A suitably large DC-DC converter should work. To output 24V at 35A it's going to draw over 70A from the 12V side.Total power will remain the same (~840W) plus a little more to cover the losses in the DC-DC converter (they aren't 100% efficient). Let's call it about 80A being drawn from the 12V side (~960W).

I know I've seen such devices being sold,but I can't recall where/what for at the moment.
Thanks for the reply. But I was hoping to be able to avoid drawing 70 Amps from the 12V alternator. That was the reason for possibly adding a second battery to get 24V.

The load will be continuous, not intermittent.

Maybe I should just replace the alternator with a 24V model, and add a hefty 24V-to-12V converter for the rest of the vehicle. I guess that then I might also need to change the starter to a 24V model. Or do you think that I could just leave the 12V starter as is, on the original battery?
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Old 26th November 2009, 05:04 PM   #4
SS64 is offline SS64  United States
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Ok here's what you could do... The alternator might be already capeable of the 28 volt charge rate needed to maintain a 24 volt system... In that case it would require a change in the voltage regulator.. But remember your amperage would drop way over half at that voltage... Say a stock GM alternator found on many Gm products through 2005 are rated at 140 amps at 14.4 volts.. you could only expect maybe 40-50 amps at 28 volts reliably... Now to make a system that could be used in everyday driveing all the alternator needs is a rotor and regulator change to a 24 volt type which is basically just a higher ohm coil... I would suggest modifiying the alternator to an external regulator type so that the voltage could be adjusted as needed.... as to the electrical system of the vehicle nothing needs to be changed.. You simply only draw off of one 12 volt battery to run the vehicles system... So basically you have 2 12 volt batteries hooked in series.. so basically + - to + - the negative post of one battery goes to the Positive of the other and therefore the positive post of battery one is the +24 volts and the negative comes from battery 2's negative post.. now to run your vehicles system you simply take the + post from battery 2 and run you vehilce but remember DO NOT try to use the positive post from battery one because that will yield 24 volts to the system...
If you need me to draw a pic I can do so.. and it would help to know exacly what vehicle you want to do this to...
Let me know If I need to clarify anything!!!
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Old 26th November 2009, 05:27 PM   #5
gootee is offline gootee  United States
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Quote:
Originally Posted by SS64 View Post
Ok here's what you could do... The alternator might be already capeable of the 28 volt charge rate needed to maintain a 24 volt system... In that case it would require a change in the voltage regulator.. But remember your amperage would drop way over half at that voltage... Say a stock GM alternator found on many Gm products through 2005 are rated at 140 amps at 14.4 volts.. you could only expect maybe 40-50 amps at 28 volts reliably... Now to make a system that could be used in everyday driveing all the alternator needs is a rotor and regulator change to a 24 volt type which is basically just a higher ohm coil... I would suggest modifiying the alternator to an external regulator type so that the voltage could be adjusted as needed.... as to the electrical system of the vehicle nothing needs to be changed.. You simply only draw off of one 12 volt battery to run the vehicles system... So basically you have 2 12 volt batteries hooked in series.. so basically + - to + - the negative post of one battery goes to the Positive of the other and therefore the positive post of battery one is the +24 volts and the negative comes from battery 2's negative post.. now to run your vehicles system you simply take the + post from battery 2 and run you vehilce but remember DO NOT try to use the positive post from battery one because that will yield 24 volts to the system...
If you need me to draw a pic I can do so.. and it would help to know exacly what vehicle you want to do this to...
Let me know If I need to clarify anything!!!
Thanks. No picture necessary. That all sounds good except that I would probably want to replace the alternator with a 24V model, for more current capacity, since drawing 35 Amps from a 40-50 Amp alternator probably wouldn't leave enough excess capacity. What are some standard high-current 24V alternator current ratings?

Wouldn't there be a problem with the batteries having uneven loads?
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Old 26th November 2009, 06:11 PM   #6
SS64 is offline SS64  United States
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The only issue you might run into is that one battery wont have as long of a life as the other.. the alternator should be able to keep up both with no issues..Besides when the vehicle is off and nothing is loading the batteries .. they tend to equalize by themselves to a point... I have done this numerous times with 18 volt systems and have never had a lick of trouble..
The usual 24 volt alternator varies in amperage from 30 to as much as 100 amps in 24 volt systems. If you redesign the mounting system you might be able to fit a Leece Neville style alternator on your application.. From my experiance they are the most reliable...and you can find as high as 160 amps ++ at 24 volts in those type.. The normal GM or Ford applications you can usually squeeze about 60-80 amps at 24 volts but they must be built right... Remeber you must compensate for temerature under the hood and several other environmental issues in the alternator's operating environment...
The key to this whole system is useing good batteries.. I wouldnt use regular Lead Acid type.. I'd use Absorbed Glass Matt or Gell type batteries to prolong the systems life...
If you would like a schematic I can send you one on how to build a voltage regulator for you alternator.. it will allow the system..when modified.. to vary from 13 to 30 volts respectively..
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Old 26th November 2009, 06:44 PM   #7
gootee is offline gootee  United States
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Quote:
Originally Posted by SS64 View Post
The only issue you might run into is that one battery wont have as long of a life as the other.. the alternator should be able to keep up both with no issues..Besides when the vehicle is off and nothing is loading the batteries .. they tend to equalize by themselves to a point... I have done this numerous times with 18 volt systems and have never had a lick of trouble..
The usual 24 volt alternator varies in amperage from 30 to as much as 100 amps in 24 volt systems. If you redesign the mounting system you might be able to fit a Leece Neville style alternator on your application.. From my experiance they are the most reliable...and you can find as high as 160 amps ++ at 24 volts in those type.. The normal GM or Ford applications you can usually squeeze about 60-80 amps at 24 volts but they must be built right... Remeber you must compensate for temerature under the hood and several other environmental issues in the alternator's operating environment...
The key to this whole system is useing good batteries.. I wouldnt use regular Lead Acid type.. I'd use Absorbed Glass Matt or Gell type batteries to prolong the systems life...
If you would like a schematic I can send you one on how to build a voltage regulator for you alternator.. it will allow the system..when modified.. to vary from 13 to 30 volts respectively..
Thank you. I will probably try to use a commercial regulator. Don't most 24V automotive alternators include the (28V) regulator? Or is it usually external?

I am still worried about the 12-volt system's current draw, and that battery's charging (and lifespan) etc. I am glad to hear that you think it will be alright (except possibly for shortened lifespan of one battery), but am still not sure, especially if worst-case current-draw was present continuously from the 12-volt part of the system, since the 24V alternators seem to typically not provide as much current as the 12V ones. There will probably be some added-on 12V equipment running, too.

I think that the ideal way might be to use two 12V alternators, with an isolated one for the second battery. I will have to find out if this vehicle will allow for that.

I will definitely use two good batteries, such as Optima 'blue top' gel, or whatever I can determine to be suitable.
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Old 26th November 2009, 07:19 PM   #8
gootee is offline gootee  United States
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See the discussion here, about tapping 12V from between two 12V batteries in series:

24 volt system, Can I tap 12v off the middle of the battery pack? - LR4x4 - The Land Rover Forum - Page 2
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Old 26th November 2009, 07:21 PM   #9
SS64 is offline SS64  United States
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The cheapest and most reliable regulators tend to be made by Transpo if the alternator you get requires an external regulator I would buy a regulator from Transpo's Voyager line..
But yes most do include a internal regulator..
The system should be fine but if your want you could leave the vehicles original alternator alone and add a second to accommodate the 24 volt system. Therefore both systems would be maintained without doing all kinds of crazy isolation.. Doing 2 12 volt alternators you might run on to a whole can of worms..not to mention if something went wrong you"d have one hell of a electrical fire on you hands!
And besides if you wanted to do a isolated alternator you would have to find one that doesn't use the housing a ground..
Optima's are good batteries but I like Deka and Odyssey better...
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Old 27th November 2009, 12:40 AM   #10
gootee is offline gootee  United States
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Looking at the link in my last post, it appears that it would _not_ be a good idea to simply tap the 12V supply from between the two series 12V batteries, when using a 24V alternator, since the 12V system would probably typically be drawing significant current. That would probably result in one battery (the one supplying the 12V systems) always being undercharged and/or the other battery always being overcharged, since they probably wouldn't be able to equalize fast-enough if the 12V systems routinely drew significant current.

So, if it's not easy-enough to add another alternator, so that neither two 12V alternators (one having isolated ground) nor one 12V and one 24V alternator can be used, the only other choices appear to be: 1) use a 24V-to-12V converter to supply the 12V systems, or 2) Use two large, equal resistances in series from 24V to ground to provide a "12V reference" (between the two resistors), which would be used with a comparator circuit to control a relay to switch the charging lead from the 24V point (both batteries) to the 12V point (first battery only), when the 12V-supplying battery needed charging.

For option 2, above, what else would need to be switched by the relay? Sorry, I'm not familiar-enough with alternator systems, yet. Are there some that essentially autosense the voltage? Or would I need two regulators (12V and 24V) and a switched sense line, or how should it be wired?

Option 1 sounds easier, albeit maybe more expensive. I might be able to run the starter from 24V, or replace it with a 24-Volt model. For the rest of the 12-Volt systems, does anybody know of any really-hefty 24V-to-12V converters available, maybe 50 to 100 Amps?

Last edited by gootee; 27th November 2009 at 12:54 AM. Reason: Added text
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