An Old Technique for getting 12 Volts from a 36 Volt Battery String

[LowGear]

I found this diagram while looking at 36 to 12 VDC converters.  Would this protect the center battery (my string is three twelve volters rather than six sixers) from getting pushed seriously out of balance.

I want the 12 VDC to power some lights - ATV level - and a starter solenoid that is used to turn off the 36 volts when the accelerator is released.

Casey

[mobile_bob]

that is still going to whack out your string, not from the discharge cycle, but from the charge cycle

here is another option, one i would strongly suggest,

http://www.ebay.com/itm/DC-DC-Converter-36Volt-down-to-12V-15A-180W-GOLF-CART-voltage-reducer-converter-/170876681863?pt=LH_DefaultDomain_2&hash=item27c90b3e87

sufficient power to cover lights i would think, and because it takes its power off the complete string the batteries remain balanced, both on discharge and on charging.

bob g

[TimSR2]

 On a Golf cart or larger battery set,  I'd do it, and expect no ill effects to the pack. You will be able to monitor the results  by your water consumption and SG of your cells in your logbook. You keep a battery logbook, right?

You may find that your charge voltage is so high for so long that the small 12 volt gets overcharged.  A single large diode will give you a .7 volt drop, and will correct this. You can use dropping diodes as a current limiters. You will need a heat sink.     One side of a RV voltage splitter will also do the trick.  

The 12 volt solenoid to control 36 volt traction motor loads is not a good idea. I think a quick trip to the golf cart repair shop will find you a nice 36 volt 500 amp continuous rated solenoid that will be a much safer bet. Call your golf course pro shop and get a referral to a Club Car or EZGO dealer, or independent golf cart shop. They have 36v and 48v solenoids made for this duty.

[Thob]

If I was going to add a separate 12V battery to carry the 12V loads, I would just charge it separately with a 12v charger.  But I like the converter that Bob pointed out better, assuming it really does what it says efficiency wise and is reliable.  Since it comes from China, I'd get two.

[Ronmar]

I agree with bob, Bad idea... It will disrupt your series string charge voltages and cause charging issues.  Been dealing with this onboard a USCG 110' patrol boat i work on for the past few years.  The boat went into the yards for a refit, and they did a major overhaul of the power system including installing a pair of trace inverters off of 24V AGM battery banks for emergency 120VAC power.  

Some bright light at the shipyard thought it would be a great idea and simple way to get uninterruptable12V for the pilothouse electronics by splitting into the 24V banks to derive 2 different 12V circuits. Besides leaving a bunch of circuits thats negative side is still 12V above hull potential, it horribly disrupted the 24V bank charging. Besides leaving one particular radio succeptible to destruction when a ground piece of test equipment was connected to it, they have been eating a very large and expensive AGM battery about every 6-8 months for the past 4 years.  After mine and others initial gripes about this horribly engineered design change, they changed the plan for all remaining boats and committed to retrofitting the 4 boats that had already went thru the refit process.  They finally got back to fixing this early boat just this past week.  Instead of 24V-12V converter they oppted for 120VAC to 12V power supplies powered off of the inverters.
 
DOnt mess with series strings.  It is asking for trouble and preamature battery replacement...

[mobile_bob]

here is another that might be a better unit?  higher output anyway
and is useful over a broader range of battery supply voltages

http://www.ebay.com/itm/Golf-Cart-Converter-DC-DC-36-48-12-Volt-voltage-reducer-/320991292653?pt=LH_DefaultDomain_0&hash=item4abc926ced

i am betting the golf cart shops have similar converters and probably could give you an idea of what works and what has a higher failure rate?  they might even have a better value unit on the shelf with a local warrantee?

bob g

[BruceM]

+1 for not tapping 12V out of a 36V string.  A buck (down) converter is the way to go.

[bentcrafter]

I have a dozen or more 12v 4W LEDs, and a 24v bank. I simply connect 2 LEDS in series. If I need more light, I simply add'em in pairs. No losses.

[LowGear]

Thank you for the good response and especially those of you that took the time to look up the converters on Ebay.

I've posted a photo of the factory wiring diagram.  Yes, I know Cushman did not write any journals on DC distribution networks or battery care.  But these barbaric machines lasted for years.  In fact with fresh batteries (do you suppose there's some correlation?) the unit I gutted for parts would have been ready to play golf if I could have found a golf course willing to risk rust contamination of their fairways.  

WARNING:  Parts of this photo may be disturbing to trained or knowledgeable DC circuitry personnel!  Remember; Electrical blasphemy may be stupid but it is not a Sin.

This drawing has the solenoid connected to 12 volts but of one the other diagram it's connected 18 volts.  Logic = If the batteries get low and can't produce enough of 12 volts to keep the solenoid closed then 18 volts will probably work as long as there is enough poop in the batteries to make the motor go roundie-roundie.  Neat thinking huh!  When my 12 volt solenoid fails - working off 12 volts I should have secured a 36 volt one by then.  Or I will have broken down and bought one of the 15 Amp converters. 

This hunger for 12 volts?  Every easy solution to lights, music and solenoids is prefaced by 12 Volts.  OK!  OK, I confess.  I have a 12 volt inverter (120 Volts /1500 watts which pencils out to 125 Amps at 12 volts.  And!  And a winch which pulls mucho Amps as well.  And who knows, I might need one of these every six months or so. 

I'm off to shop Ebay and Amazon.  The new rule here is to always price check Ebay with Amazon.  12 Volt converter it is. 

Casey

[LowGear]

Been shopping.

This unit:
http://www.ebay.com/itm/DC-DC-Converter-36Volt-down-to-12V-15A-180W-GOLF-CART-voltage-reducer-converter-/170876681863?pt=LH_DefaultDomain_2&hash=item27c90b3e87
Many of the negative feed backs are on this very unit.

I was wondering if I paralleled three of these:
http://www.ebay.com/itm/370639711002?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649
Would I get 12 Amps or would they self destruct?  They don't ship from China either although they're probably made there.  {Oops, Just reread the listing - Made in USA}  Or maybe I could use one for the headlight.  One for the other lights.  One for the horn and audio system.

Casey

[BruceM]

Casey, the former unit looks like a better match for your needs.  Buck converters are pretty simple, if you don't push the current to the max spec you'll probably be fine. The packaging and housing look first rate.  Seller made good on one defective unit, who knows if the customer actually fried it. (Very likely, given the typical human.)  If you buy it via paypal, you're ass is pretty well covered.

[Horsepoor]

Wow - what a timely thread. Recently purchased a 20 year old club car golf cart for a few hundred dollars and noticed the previous owner had wired 12VDC from two of the six 6VDC batteries thus unbalancing the string. Started shopping eBay and found a DC to DC step down 120 Watt unit for only $27 which includes shipping.

http://www.ebay.com/itm/370499721369;jsessionid=5987E2162D01FA08458D07AE3D319A96?ru=http%3A%2F%2Fwww.ebay.com%2Fsch%2Fi.html%3F_sacat%3D0%26_from%3DR40%26_nkw%3D370499721369%26_rdc%3D1

This was the best price I could find, ordered two units.

Bruce

[BruceM]

What a bargain. Great find, Bruce.

[LowGear]

OK Gentlemen,

I'll go with either the 10 AMP or the 15 Amp unit.  I called the island golf car recycler and he didn't have anything in stock.  The decision will be determined by the current requirements of one of this devices.  (I checked the feedback a bit closer on the 15 Amp unit and noticed they had sold a bunch of these so the return rate isn't so discomforting.)

http://www.ebay.com/itm/120994330629?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649
or
http://www.ebay.com/itm/120994628968?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2648

For the first week I'll just fake it with the factory settings and upgrade as the parts come in.  I must stop stalling on completion of this project in order to achieve perfection.  Going up the driveway is the most important step.

Again, thanks for the guidance.

Casey

[LowGear]

LOOK OUT!

Brain wrinkle forming.  These selonoids use 36 volts to activate as well as passing 36 volts at 50+ Amps.  WOWWWWW.

Didn't hurt at all.  Really.

So I can buy the purdy shinny one.

Casey