# DC to DC converter wiring



## mcmichaelev (Dec 3, 2008)

Several of the example wiring diagrams I see have the DCtoDC converter permanently wired into the system with no way to control it. Would this just drain the traction pack when the vehicle is not in use and overcharge the aux battery. Is a relay or switch a good idea? If so does it matter where on the DCtoDC it is placed?


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## EVfun (Mar 14, 2010)

Many of us leave the DC to DC converter on all the time. The drain on the pack when there is no output load is usually less than 0.1 amp. If the DC>DC is built to provide an output voltage between 13.2 volts and 13.8 volts then it can be left attached indefinitely to a 12 volt lead acid battery without harm.

Some DC to DC converters have substantial input capacitance that makes just switching them on somewhat difficult (hard on the switch, may blow a fuse.) Some DC>DC converters have the nasty habit of slowly draining the 12 volt battery if the power is turned off but they are left connected to the 12 volt battery. A few people don't have a 12 volt battery (not really recommended.)


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## mcmichaelev (Dec 3, 2008)

My DCtoDC is a Curtis, not sure of the model number right now. I'll check the output voltage... if its 13.2-13.8 I'll leave mine perm. attached.


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## dtbaker (Jan 5, 2008)

mcmichaelev said:


> My DCtoDC is a Curtis, not sure of the model number right now. I'll check the output voltage... if its 13.2-13.8 I'll leave mine perm. attached.



curtis dc-dc tend to be very solid and dependable. If you suffer from any 'blink' in headlights or radio with the vaccuum pump kicks in, the easy fix is to put a small 3a motorcycle/atv battery in parallel w/ the dc-dc... help take up the slack if the dc-dc can't keep up with a sudden load change.


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## lowcrawler (Jun 27, 2011)

EVfun said:


> A few people don't have a 12 volt battery (not really recommended.)


I've heard this before, but am not actually sure why... can you elaborate?

The only benefit I can think of is for 'surge' loads the battery would act as a buffer...


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## EVfun (Mar 14, 2010)

lowcrawler said:


> I've heard this before, but am not actually sure why... can you elaborate?
> 
> The only benefit I can think of is for 'surge' loads the battery would act as a buffer...


What all would happen if the DC to DC converter failed while you are driving on a cold and rainy night?

The main contactor would open, cutting vehicle power instantly. All the lights would go out and the wipers stop so you couldn't see where you where going and no one could easily see you. You would coast to the side of the road, but not have hazard lights to flash. Steering and braking would be hard if either of those are usually assisted. 

My buggy doesn't have a 12v battery. It also doesn't have power brakes, hazard light or wipers to not work. It isn't a normal car and doesn't have the same expectations. I have also converted a VW Pickup and Datsun. Both of those vehicles had a Zappy size (18 to 20 amp hour, about 3" x 7" x 7" tall) 12 volt AGM battery and an old Rudman reg modified to trip the alternator light if the 12v system fell below 12 volts.


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## lowcrawler (Jun 27, 2011)

Makes sense. Thanks.


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## dtbaker (Jan 5, 2008)

EVfun said:


> ... an old Rudman reg modified to trip the alternator light if the 12v system fell below 12 volts.


do you have specifics of this documented anywhere I could follow? I have a tiny 3ah aux battery now to cover the surge load when my vac pump fires up and headlights used to blink... 

I would not have a *lot* of time if the dc-dc failed and the tiny battery had to pick up the entire 12v load... but if I had a warning light it would give time to park safely.

so, a sample circuit and specific parts needed to create a dc-dc failure idiot light?


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## EVfun (Mar 14, 2010)

I don't have the specifics anymore. I used a couple of old Manzanita Micro Mark II lead acid regs. Both of the units where built at the same time, must have been around 2001. Rich told me to change "that" resistor by about "this" much to change the adjustment range to be around 12 volts. I did that, removed the built in shunt resistor, and replaced it with a small relay with about a 1000 ohm 12 volt coil. I added a diode across the relay coil so its kick couldn't take out the MOSFET that powered the shunt. I used the normally closed contacts of the relay to drive the alternator idiot light.

If I was doing this right now I would likely use an LM431, a couple of resistors, a small SIP or DIP relay with N.C. contacts, and a diode to protect the 431 from the relay kick when switching off. I like relays for this because incandescent dash lamp bulbs, even though small, still have quite a bit of startup current (a little 1.5 watt dash bulb has about 15 ohms cold resistance.) Oh, and relays conveniently have normally closed contacts, immunity to spikes that are all to common in auto 12v systems, and a nice crisp digital output.


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