Why do I need a DC to DC converter?

Why do I need to get a DC to DC converter?

You need it so that a small 12 volt battery for the lights, accessories, etc. can be kept charged up by the main battery pack.

If you drive during the day, chances are you can just run a separate battery for the radio, turn lights, etc. Most people just use the original battery for the car.

A lot of EV don’t have DC to DC converters.

If you start running a lot of 12 volt accessories, then you will need one.

Or you can just get the biggest 12v deepcycle battery you can find and make all your 12v loads as efficient as possible (leds, hid’s, low watt radio) You would just need a seperate 12v charger.

I have a kick ass cd - radio in my car I love music and when there is no noise from the engine it sounds just like home, my dc to dc converter is just 300 watts and well worth the bucks. Sometimes at night 300 watts isn’t enough to run the vacum pump lights turn signals radio and brake lights and it discharges the 12 volt system, even with the converter I can only imagine what would be happening without it, the headlights dim and I have a Die Hard for my 12 volt system, brand new 6 months ago.:wink:

It’s my understanding that if you have a DC to DC converter, as long as you have stored energy in your main battery pack, you will have power for your 12V components (lights, horn, radio, etc.) and this likely to be more than a single auxiliary battery can supply . However, I have been advised to ALSO have a single 12V battery tied together with the DC to DC converter in case something happens to your main electrical system while you’re driving at night. If you have a major failure, you loose your headlights, brake lights and emergency flasher - a scary thing in the dark because you can’t see and other cars can’t see you. In this application, your auxiliary battery doesn’t need to be all that big because it’s only for emergency backup. Whether or not, the DC to DC converter keeps the auxiliary battery charged or whether you will need to charge the auxiliary battery separately, I couldn’t tell you. That would be an interesting thing to know if anyone would like to post an answer to that question.

It’s my understanding that if you have a DC to DC converter, as long as you have stored energy in your main battery pack, you will have power for your 12V components (lights, horn, radio, etc.) and this is likely to be more than a single auxiliary battery can supply . However, I have been advised to ALSO have a single 12V battery tied together with the DC to DC converter in case something happens to your main electrical system while you’re driving at night. If you have a major failure, you loose your headlights, brake lights and emergency flasher - a scary thing in the dark because you can’t see and other cars can’t see you. In this application, your auxiliary battery doesn’t need to be all that big because it’s only for emergency backup. Whether or not, the DC to DC converter keeps the auxiliary battery charged or whether you will need to charge the auxiliary battery separately, I couldn’t tell you. That would be an interesting thing to know if anyone would like to post an answer to that question.

I wonder how the efficiency of a DC-DC converter compares with an alternator driven off a main motor stub shaft? The alternator of course, would provide no power to the 12V storage battery when stopped, but could be switched in and out for a poor man’s regenerative braking system…
-enganear

[QUOTE=enganear;3640]I wonder how the efficiency of a DC-DC converter compares with an alternator driven off a main motor stub shaft? The alternator of course, would provide no power to the 12V storage battery when stopped, but could be switched in and out for a poor man’s regenerative braking system…
-enganear[/QUOTE]

would certainly be cheaper, and with a relay hooked to the brake switch, you could have it kick in only under braking. maybe a micro on the accelerator pedal for off throttle 12 v regen also.

I thought about using an alternator instead of a DC-DC coverter. It could be set up on a toggle switch to turn it on only when needed ie. at night when its raining. The alternator would be capable of producing all the power needed to run every accessory on the vehicle. Yes it would put an extra load on the motor and reduce range but the DC-DC takes power from the pack as well. As far as regen it could be connected to the brake light switch to energize the alternator whenever the brake pedal is touched.

[QUOTE=Turbotom;3649]I thought about using an alternator instead of a DC-DC coverter. It could be set up on a toggle switch to turn it on only when needed ie. at night when its raining. The alternator would be capable of producing all the power needed to run every accessory on the vehicle. Yes it would put an extra load on the motor and reduce range but the DC-DC takes power from the pack as well. As far as regen it could be connected to the brake light switch to energize the alternator whenever the brake pedal is touched.[/QUOTE]

You would not want to wire it just to the brakelight switch. When stopped, you would be powering the alternator armature at the maximum current without any return. There would need to be some logic that only switched in the alternator when the speed was high enough to actually charge the 12V battery (or just do it manually with a pushbutton). A small PLC could handle this easily.
-enganear

ok, so lets compare:

system 1: Alternator driven off the motor.

The armature of the alternator will always drag a small percentage off the motor its connected to. You’ll always get that small drag, even when the alternator is off. Plus they can be heavy. You need 10A or more for lights on a motorcycle, and 2-3 times that in a car. Not a small alternator.

In order to enable the alternator, a current must flow through the armature in order to generate. Then you have the inefficiencies of the alternator. Plus you STILL have to have a small aux battery because alternators are fine for constant load, but they don’t like surges of current (like when you turn on lights).

Then you have the inefficiency of the rectifier regulator that makes the 12V.

Add it all up, MAYBE 70% for the alternator, 85-90% for the rectifier and 1-2% for the drag.

System 2: Aux battery.

You have the weight of the battery (50-100lbs) that is just sitting there to power lights. Its not adding to the traction pack at all, and is meant to only power lights/accessories. This will always produce drag due to the dead weight.

System 3: DC-DC converter only

you have the top of the pack coming right to the converter, always taking some current off the pack, but only a couple amps. The converter is 85-90% efficient. It will go into current limit when the lights go on, maybe causing other lights to him until the inrush goes away. You may get brownouts.

system 4: DC-DC converter and a small battery

This system has the efficiency of a DC-DC converter, and has the inrush capability for the lights. a small 20Ah battery, with a 20-30A DC-DC converter will handle most inrush currents from headlights/brake lights, and supply the constant current needed so the aux battery never dies. Adds minimal weight, but the most efficiency. You only take a little off the pack, but do so before the conversion to mechanical (through the controller/motor/wheel). No more brownouts.

Go to the Ev calculator at http://www.evconvert.com/tools/evcalc/

Put in your combo, then add 75 pounds for an extra battery under vehicle weight.

Your range will only drop about 2/10ths of a mile.

Now, a Dc to Dc converter has weight also, and plus it pulls power from the main pack. i got to believe it will reduce your range also. BTW, the most you could possibly gain in range is about 2/10ths.

DC to DC converters are nice, they are better. But a guy on a budget doesn’t need one right away if he has good battery laying around…

75lbs of dead weight, plus having the charger to charge just that battery…

I know my DC-DC is less than 2lbs, and 90+% efficient… True you don’t need it, but what is the cost of a 60+Ah battery to run accessories? And the charger for it?

2/10 of a mile less isn’t bad, I know, but lets say you have a 144V to 12V converter (12:1) and it puts out 30A, thats about 2.5A.

If you use an aux battery, lets say you’re using 300Wh/mile, and you say 2/10 of a mile, thats 60W it drains the main pack, plus lets say the same load as the DC-DC, thats 30A @12V thats 360W, plus 60 is 420Wh total drain off the Aux and the Main pack.

A DC-DC uses 2.75A at 144V is 396W. Pretty close to the 420W of using an Aux battery, but is a battery and Charger cheaper than a DC-DC converter? I’m not so sure it is/isn’t. There would have to be a back to back comparison.

Plus, some Vicor DC-DC’s use tons less room, and Lazzer has used a 500W power supply by feeding the rectifier with straight DC, and it works great. So for only 50 bucks or so, you get a DC-DC, less weight.

Sure, its basically the same efficiency, but you also have to fit another battery in there somewhere, and a charger. Its the simpler cheaper way to go for alot of people.

I’m just saying a small battery to stabilize and a decent DC-DC is the route I’ve gone.

I didn’t realize the DC to DC converters were so cheap.
Most i see are $180 and up.

its not a DC-DC converter Lazzer was working on, it was a normal computer switching power supply…

first you rectify it, charge a cap, and feed the switcher. He just put the 144VDC across the cap, skipped the rectifier. Said it works great.

Thats all a DC-DC is, a switcher without the AC frontend.

The DC to DC converter is really the best way to operate the 12Volt accessories/lights on our GEM cars. The converter spreads the load evenly across all six batteries. This makes it practical to use our headlights at night and prolongs the radio and accessory running times.

A DC-to-DC converter is an electronic circuit which converts a source of direct current (DC) from one voltage level to another. It is a class of power converter.

You need it so that a small 12 volt battery for the lights, accessories

It is just crazey to run without a DC-DC converter. I don’t know where the 2/10mile thing came from, but when I logged my commute and calculated energy lost to the accelerating after every stop, I was able to estimate the range increase for every 100 lbs less my car weighes. It came out to 3% range increase for every 100lbs lost with stop and go driving. I have a small motorcyle battery for my 12 volts. It is trickle charged from my DC-DC converter. The battery really only powers the initial contactor when I turn the key. After that my “12 volts” is supplied by my main battery pack. I can play my radio and my lights, etc for awhile when the key is turned to accesories, but not on. I also trickle charge my 12 volt battery when I charge the main pack from another AC-12volt power supply. It works great. I haven’t thought about my battery for over a year.
You can see my car at [http://www.electricmgmidget.com]
I have a wiring diagram posted on the site that shows this setup.