1. Oct 18, 2009

UGVteam

Hey there, we are creating a hybrid system on a prebuilt UGV (unmanned ground vehicle) for our senior design project and are looking for some technical advice. We are all mechanical engineers so our electrical engineering knowledge is lacking so we are looking for some help.

Here is the setup:
-Two 36V batteries running off 3 12V Lifeline batteries connected in series.

Here is the goal:
-Recharge the three batteries using a small gasoline engine (3-6 HP basic engine). Have the system be fully automated and have the engine turn off and on via an electric start whenever the batteries run low on juice.

-The Issue:
What is the best way to go about getting a 36V charge from the motor?

The original idea was to use a basic 12V alternator and alter the internal regulator to output 36V. However, we are not sure how easy this is or if it is even possible.

Then we started to brainstorm about possibly charging in parallel and keep the alternator at 12V. However since the batteries have to output 36V, we don't think it will work to charge in parallel and output in series.

So now we are in a bind on how we should go about charging these 3 12V batteries. Any suggestions?

Also the electric start for the engine runs on 12V so somehow we are going to have to get 12V from the battery bank. Which seems like it won't work if they are all connected in series.

Any help would be very appreciated.

Thanks, UGVteam

2. Oct 18, 2009

Bob S

If the alternator rotates fast enough, you can get 36 V (actually you need 3 x 13.8 V) out of it. This would require removing any internal regulator, and bringing both the diode array output and the slip ring wire (terminal) input out. You would have to build a new regulator for the higher voltage, or maybe use a 3:1 voltage divider and use the old one. The question then will be whether the diode array rating and the wiring insulation can handle the extra volts. It gets a little messy if you have to replace the diode array, because I think it might be 3 phase.
Bob S

3. Oct 18, 2009

vk6kro

To charge a 36 volt battery, you need about 42 volts.
I doubt if you could get that by rewiring a 12 volt alternator.
I don't think you would want to reconnect the batteries in parallel every time you charge them.

So, maybe you need a 12 volt to 42 volt DC to DC converter? This would be something to research. Have a look for boost voltage regulator circuits. Maybe someone has already designed one for another application.
I can't think of an off-the-shelf solution, though and that is probably what you need.

The electric start issue is a bit easier. Can you just use the bottom battery of the string of 3 ?

4. Oct 18, 2009

Bob S

A good automotive alternator can produce 12 volts at an engine speed of 1000 RPM. So it should be able (with a different external regulator) to produce 36 V at 3000 RPM (engine speed) without changing the coils. If you decide to charge the batteries individually at 12 volts, remember that one side of the alternator is grounded, so you will have to disconnect the batteries from the circuit while charging.
Bob S

5. Oct 19, 2009

arithmetix

Could you please tell us the speed of the shaft which will drive the motor?

6. Oct 19, 2009

Bob S

.
Do you mean drive the alternator?
This is a guess. First determine what minumum engine RPM produces 14 volts on the alternator. Look at or measure alternator belt pulley diameter ratios to determine the alternator RPM. Multiply alternator RPM by 3 to get 42 volts (with a different regulator). (This is because the alternator works in the Faraday induction law principle. Note the d/dt in the equation.) Are the diodes in the alternator rated for the higher voltage? What about coil insulation?
Bob S

7. Oct 19, 2009

UGVteam

Thanks you for all the help so far!

The engine can run up to 3500 rpm.

I think modifying the regulator in the alternator will be a little above our electrical engineering abilities.

1st Option
Attach a normal 12V alternator to the engine and connect it in parallel with the batteries. We will then run the parallel batteries to a convertor that will take the voltage from 12V to 36V.

Here is the convertor: http://www.powerstream.com/dc36.htm

We plan on running the engine at around 2500 RPM with a goal of creating around 50 AMPS. That means each battery would see around 15 Amps. Do you guys think this will fry the batteries?

2nd Option
Find an alternator that will output 120V AC and run it into a normal golf cart battery charger that outputs 36V. Now many amps do you think we would need coming from the alternator for the charger to work correctly?

Here is a possible charger:http://www.batterymart.com/p-dual-pro-i3625-36v-charger.html

8. Oct 19, 2009

vk6kro

I think modifying the regulator in the alternator will be a little above our electrical engineering abilities.
Yes. I have one open in front of me and it would be a major job to modify it. Also, it uses a 12 volt field coil so you would have to bring this back from the bottom 12 volt battery in your 3 battery series string.

Find an alternator that will output 120V AC and run it into a normal golf cart battery charger that outputs 36V. Now many amps do you think we would need coming from the alternator for the charger to work correctly?
I like option 2 but it seems like an expensive exercise. At least the alternator will be a standard item. And you could use the 36 volt battery charger if you charge the batteries at home before you go out in the field.

The maximum output of the battery charger would be about 1000 watts because its output voltage would be higher than 36 volts. So, maybe 12 amps from the alternator allowing for some losses?

9. Oct 20, 2009

dr dodge

alternator conversion kits to output 110 V have been around for years
I had one on my service truck
flip the switch and plug in the drill

http://islandcastaway.com/stuff/windpower/Alternator%20Secrets.htm [Broken]

dr

Last edited by a moderator: May 4, 2017
10. Oct 21, 2009

UGVteam

Dr. Dodge: So do you just use a switch to bypass the rectifier and regulator?

So if we do figure out a way to get 120V/110V out of the alternator can anyone think of a cheap way to get converted to 36V/42V? The cheapest golf cart battery charger we could find was like $250, and we are trying to keep everything under$400 so that would take a huge chunk of our money.

Thanks for all the help,
UGVteam

11. Oct 21, 2009

Bob S

This URL, shown in post #9,
http://islandcastaway.com/stuff/windpower/Alternator%20Secrets.htm [Broken]
states that it is straight forward, and probably safe (diode PRV, etc.), to make an alternator put out 42 volts dc, just by modifying the regulator. I suspect that the alternator will have to run at >3000 RPM to do so. Remember that the neg side is grounded to the alternator case.
Bob S

Last edited by a moderator: May 4, 2017
12. Oct 23, 2009

dr dodge

The chrysler voltage regulators are a silver box, filled with "breadboard style" electronics. The potting melts out with heat. I would bet that the circuits could be modified to output 42 volts regulated. If you were down the street, Id give you a couple bad ones...lol
look for a regulator for `72-88 year dodge, chrysler, plympouth the last one I bought was about \$30
The converter box on my old service truck was 0on it when I got it, and it seemed that it broke the single output wire to the battery, and switched it to the plug in, and switched in a 2nd "black box" to do the regulating. It's output was 85-90 VDC which was plenty to use a drill or electric impact. When the alt broke, I took it to an alternator shop who disected it and then "lost" the original conversion parts, and handed me back a stock rebuilt alternator...I was bummed!...should have fixed it myself...as usual

dr

13. Oct 24, 2009

famousken

You shouldn't have any problem using a 12v alternator, I have gotten over 100VDC out of a Gm style alternator with the stock rectifier, so 42V should be safe, And Yes, they are 3 Phase. modifying the regulator isnt all that hard either, if you open up the case, it is sitting on the opposite side of the diodes and it looks like a little black box. Its got three wires comming out of it, B+, field +, and field -, ground is taken from the alternator housing where the regulator mounts. What you need to do is take the B+ lead and either add a rheostat so you can adjust the output voltage, or install a voltage divider using a few resistors. It is important to use 5W or greater components though, because they must pass the current for the field coils to get power. As for the starter, can you have a seperate, 12v battery for starting the motor? As far as I know, most small engines use the starter as a generator as well to recharge their batteries, so thats one thing you wont have to worry about.