charging a 24v starter bank with a 12v alternator

[gdufus]

I don't see how, but some mechanics on the islands told me it would be no problem. as some beers had been opened, I never questioned it, although I sincerely question it. The issue kept ringing in my head and I would like your input on it

[tazmann]

Interesting quesion
There may be a step up voltage converter to do it but I have not seen one. I know you can charge one of the batteries with 12 volt but to do them all the cable between + and - would have to be disconected or it would cause a direct short .
Tom

[gdufus]

I thought the same thing, and voltage conveters usually don't bring the charge of 26.something volt, and definatelly not the amps...
Anyway, if anyone know of anything, please inform me :-)

[Chase_B]

The only way I can think of doing that would be a series of 12 volt batterys that are connected to a control box in a way that as each battery reach full charge the control box would switch to monitor/charge the next battery in line,
And when the starter was engaged the control box would draw from all the batteries, this analogy derived from assuming the rest of the electrical system is either 12 volt or the starter is connected to a generating system that provides the needed voltage/amperage for auxilary systems if there are any

[GTS225]

Seems to me that the only way would be to have some manual disconnect switches between the two batts, and a manual switch in the alternator field wire.
An alternator needs a voltage "sample" in order to excite the field windings. If it doesn't get that, there isn't any output, no matter how fast you spin it.
If you could put a disconnect in the cable between your "A" negative and "B" positive, you can isolate the two from each other, eliminating the shorting effect. You would then have to be able to turn off or on the field exciting voltage at will.
Biggest hazard would be not getting things turned off or on in the proper manner. If one would sit down and draw it out, it could most likely be done with a single toggle controlling relays/solenoids wired in the proper configuration.

I do know that one can get a battery disconnect with a second (smaller)set of contacts in it. They are commonly used in drag race cars for main battery disconnect when the battery gets moved to the trunk. NHRA requires that all electrical gets shut down when this rear-mounted switch gets turned off. The alternator excitation voltage gets run through it on it's way back to the battery. If it wasn't, the engine would continue to run after the main was thrown. (I have one in my drag car, and it does work properly.)
The problem with them for this application is that both sets of contacts open at the same time.

Oops.....Chase and I were writing at the same time, and his is also a possibility. The solar pv panel world has some interesting electronics out there, and one of "thier" charge controllers just might be the ticket.

This does bring something to mind, though. Is it just the starter that is 24vdc, or is the rest of the electrics also 24v?

Roger

[gdufus]

yeah, I'm glad I'm getting confirmation here, it is simply not possible, at least not "idiot-proof" to do it (manual disconnecting the bridge is what I do now, paralel-connecting with jumper cables and charge simultaniously)

Yes, it's just the starters, the complete rest is 12v, incl some items that won't run on 24 (bilgepumps, LED lightning, etc.)

[powerabout]

Series parallel switch
pommy trucks had them to start their god awful diesels with their god awful electrical systems.
Google it
for sure Cole Hersee and Bosch make them

[gdufus]

Thanks, not that it helped directly, I found these switches but my engines (Detroit Diesels, 8v71ti's) are sure to pull more than the specifications of these switches allow for. scrolling through the results however I came across another forum (sorry boatdesign) and there was a guy who wrote this:

--------------------------------------------------------------------------------

As the other contributors have said, series/parallel switching of batteries is VERY dangerous. You have heavy conductors and a heavy duty switch. You have two poles to switch. If everything doesn't work just right in the timing, you seriously short out one or both batteries and cause an explosion.

Check out the technique in the link below. It only requires a single pole single throw switch, and is inherently safe. Using this technique, you can have batteries of different age, different size, different style however you should not mix wet cell batteries with AGM or GEL cell types.

http://www.yandina.com/24VoltBattery.htm

This link works (at least still today, September 6th 2011) and it looks like they're on to something, going to try it this week, will keep everyone posted...

Cheers

[CDK]

Except for a 2nd alternator, fully isolated from engine ground (easier said than done), or a non-existent 14V to isolated 14V/50Amps converter, the yandina solution is the only viable option.

BUT.... it was intended for another purpose. Your Detroit starter motor needs an immense current to start spinning, so you need a relay with at least 250 Amps inrush current capacity. A 12V starter solenoid intended for a large car engine can do the job.

[tazmann]

Come to think of it I remember working on a truck, been 25 or 30 years ago but it had 6 volt batteries, wired up for 12 volt so 2 banks at 12 volt, it started on 24 volt so they had a pile of solenoids, it all operated off key switch. I remember thinking what a mess and why but it did work.
Tom

[Injuneer]

12 to 24 V DC converter?
12 to 120 VAC inverter and a 24 V battery charger?
Or am I missing something...

[gdufus]

missing something injuneer, converters don't deliver charge current, and battery chargers require 110 / 220, and I like to charge the starting bank off the running engine, which has, you guessed right, a 12v alternator. I am in Panama with this boat, so resources are limited, and a 24v alternator is not at hand... :-)

[gdufus]

CDK, I was thinking of a manual battery switch, the ones I have here are capable of switching 500A, and I have manual start buttons installed in the engine rooms as well, so I can manually switch the battery to Series, start the engine, and manually switch back to parallel... I think.... I hope.... we'll see if it sparks... at least the batteries are in the generator room, seperated by a bulkhead, if the blow up, at least we're safe.... :-)

[Injuneer]

Originally Posted by gdufus

missing something injuneer, converters don't deliver charge current,

Why not? Just as an example, solar MPPT controllers contain a DC/DC converter that delivers a charge current to the batteries.

and battery chargers require 110 / 220, and I like to charge the starting bank off the running engine, which has, you guessed right, a 12v alternator.

That's why I said use a 12 VDC to 120 VAC inverter and plug the battery charger into the 120 VAC output of the inverter.

What am I missing?

[gdufus]

a solar panel delivers per definition a larger current than the battery it is meant to feed, i.e. a 12v solar panel will deliver appr. 16v , for a 24v system I'd have to get a 24v solar panel, or connect 2 12v versions in series. again, investment. as for a battery charger on 110, again, I am in Panama, a 24v battery charger I could find up til now only from a professional car-parts shop, and it was a charger for professional use, which was good... less good was it's price, 450 dollar. :-(