have dual batts, need 24VDC output

any of you EE's know of a simple way to pull an independent 24VDC output from dual batteries, without affecting the rest of the system?

i haven't sketched it out yet, but i'm figuring there must be a way to do this using diodes etc. just wanted to see if anyone knew of a standard circuit to do this.

the story behind this is i want to speed up the ballscrew lift actuator on my rear sundeck. what effect do you think this would have on the motor life? not that i care all that much - i'll change it to gas springs & a jam latch when it dies. the actuator does draw a lot of current, so any diodes would have to be pretty large.

You will need seperate wires runnning the batteries in series. Using seperate wires should keep it isolated from the rest of your system. But an Electronic Guru might want to chime in.

right, but i don't know a way to do this without sending 24 to the rest of the system. i want a simultaneous 12V to the existing system (dash, stereo, motor...) and 24 to only the lift (which already has it's own dedicated circuit).

You might want to post the question on a website where the hydrualics guys discuss their systems, seems like they would have encountered this challenge more then us boat guys. I don't know how you could do it without having another battery dedicated to the 24v system to raise it the other 12v.

Say you attach two batteries in series (positive post of battery 1 to negative post of battery 2). You'll get 24 volts if you attach something to the negative post of battery 1, and positive post of battery 2.

However, if you attach something to the negative and positive both of battery 1, you'll still get 12v. Same thing if you attach something to the negative and postive posts of battery 2.

I'll draw a pic if that's confusing.

I dont think so Boomer. I think when you hook up batteries in series, no matter where you connect another wire, you will end up with 24volts. It still makes up 24 volts no matter where you would connect the points on the batteries.

Luke you're wrong,

If you wire 2 batteries in series the total voltage across both batteries is 24 volts. If you move one of the end connections to the middle, you'll end up with 12 volts (the other battery is effectively removed from the circuit)

Current always flows in a loop from the positive post to the negative post (or negative to positive if you want to think in terms of electron movement).

For the OP, it really depends here how you have your boat batteries set up. I highly doubt you can do this easily and safely. 2 battery applications in a boat will either be tied in parallel, or have a "perko" type switch that switches batteries (A - BOTH -B - NONE), which basically connects battery A, battery B, both in parallel, or neither to the boat electrical/electronics.

Wiring both batteries in series is just asking for trouble if you don't sort out your electrical return paths. You don't want to apply +24V to your ECM or stereo

Trash

Trash,

If you reread the first post you will see that he already has a dual battery system for all boat systems. Your system will work but the second battery will only be useful on the 24v system.

So you guys are kind of both right and kind of both wrong. Trace has an idea to speed up his lift motor but I don't believe he will find a practical system that provides a safe and effective 24v. There are other ways to speed up the motor for the lift. If there any R/C car guys lurking here they can probably shed some light about different motor windings.

Time for a drawing...

batteries in 24v cfg.pdf (89.5 k)

As Trash said it can be done, BUT IT SHOULD NOT BE DONE because one of the batteries will be running in an inverted mode relative to ground causing a high likelyhood that something will get fried or go boom. It's OK to do this in AC systems but it's a bad idea to do it in a boat running on DC. Kind of reminds me of Dirty Harry when he says "DO YOU FEEL LUCKY"?


(Message edited by mikeski on April 13, 2005)

I don't have a dual battery setup in my boat, but there is no way that the batteries would ever be wired in series. For the OP to get what he wants to do, (assuming he has a battery bank switch...he doesn't say but I imagine he does), he would have to leave the battery switch in A or B. If he every (accidentally or not) moved the switch to BOTH with the batteries wired in series, you are in essence shorting one battery out. Kaboom!

I'm too lazy and inept to draw a nice picture explaining this better, but the end line is : don't do it. You're just asking for something to go wrong.

Trash

yes, i also have a Perko switch, but i don't want to have to remember to switch stuff around to operate the lift without frying my alternator. also, people not used to Rube Goldberg stuff need to just be able to hit the button.

thanks for the drawing Mike. by hydraulics guys, you mean bouncy cars? good thought.

my dual Optimas are onviously in parallel right now, but i also want what i'll call an isolated series circuit out of them. this could easily be accomplished with a third battery, but i also still want my alternator to charge everything.

i'd be interested in some sort of DC transformer, but that (RC speed control?) sounds expensive compared to what i'm imagining could be a couple wires & a big diode.

As trash said:
JUST DON'T DO IT

Kind of reminds me of when I was installing car stereo's right out of high school. This VW bug came in with a blown radio. There were a few other things like the gas guage, windshield wipers, and the heater (never works in a bug anyway) that also had recently quit working. I tested the voltage going to the radio with my meter it showed -12V, after double checking the meter leads and a little head scratching, I put my meter directly on the battery. The battery had reversed polarity! We nicknamed it the "Reverse Polarity Bug"... Would not have believed if I had not seen it myself. Anyway after spending a few days at an auto electrical shop they straightened it out and we sold him a new radio. I don't know if they ever fixed everything in that car, and I would never want something like that to happen in my boat.

Trace,
What you're looking for is not a DC transformer (there really is no such thing), but a DC-DC boost converter. But that'll be expensive, especially for the power you probably want out of it.
Actually, something stupid comes to mind. You could use one of those DC-AC inverters to get you 110VAC, then find a power supply meaty enough to drive the lift. Dunno what kind of power the lift takes, but if it's under a couple of hundred watts that may be your answer. Not electrically efficient mind you....


Mikeski,

Although not common here in North America, there are a few European (British) cars that have had a positive ground system. Everything electrical was at a lower potential to the chassis! Thankfully I don't think they make that system anymore.

Gives me the willies just thinking about it....

Trash

Put a gear on the motor between the ball actuator and the motor

You could make a noisy chopper circuit to convert DC to AC then run it to a simple doubler circuit, but it would be a PITA.

Or you could use a 12vdc to 115vac inverter and power a 24vdc wall wart. That is what Trash suggests, and while it is not efficient, it is simple. Plus, you could use the 115 vac for other stuff, like a playstation or christmas tree lights, other stuff essential for wakeboarding 300 watts is a lot of power, more than enough to drive a small motor. It would be noisy, too, you'd probably hear it in your stereo.

OR, you could buy the cheapest car amp that you can find, open it up, disable the output section and use the PWM power supply rail to the collectors of the output transistors which will be supply DC between 20 and 50 vdc. Cheap as dirt, and already designed to supply high current and be relatively quiet. You may already have one that the output section has already done been blow'd up. Just find the big output transistors, There's probably four of them. Measure the points on the circuit board, there will be +30 and -30 on each rail, give or take. I would not even remove the transistors, just short them with a thick piece of wire so that there is -vdc and +vdc right on the speaker jacks. If you are a snob, screw up a JL or Zapco for best performance)

Be sure to use an inline fuse on the former speaker ouputs, in case bad stuff happens. Even a cheap amp can supply a lot of power. Trace, I know that you could fab your own boat given time and resources, you are as badass as Grant IMHO. Otherwise, I would not recommend this. Put an appropriate load on it (inductive load like the exact same motor you are going to drive) for a while and make sure it doesn't combust before you put it in a boat.

(Message edited by cyclonecj on April 13, 2005)