# DC/DC converter, no battery



## eric-ev (Apr 28, 2009)

Hi there! I can't imagine any reason you wouldn't hook the +positive output from the converter to the + cable where the battery used to be - where it goes almost directly into the fuse bank - and connect the negative to ground. Thats on the 12v side of course. Then the high voltage side would connect ON THE CONTROLLER side of the HV system between the contactor and the controller - that way it would turn on when you power the car up. 

If it was me my big concern would be with the contactor. I have a contactor which requires 12v on its coil to close the contact and activate the HV system. Since you wouldn't have the DC/DC running all the time, there would be no 12v to close my contactor.

If the contactor is rated ON THE COIL SIDE at whatever your battery pack voltage is, you should be fine!

Also it comes to mind that sometimes you'd pull more than 300watt (or whatever your DC/DC happens to be rated at), such as lights on, wipers, and horn. All of a sudden you're trying to pull more than the DC/DC can handle and it burns out (unlikely unless its a continued overload), it blows a fuse(more likely), or your lights/wipers/horn all dim and don't work right(most likely). If it was me I'd use even a small motorcycle/garden tractor type battery to absorb the extra load. It would charge off the dc/dc and you'd never have to think about it! You could also use contactors rated at 12v coil voltage.

And, you can always add that motorcycle/tractor/whatever battery down the line if it becomes a problem. Heck, a large capacitor might even work, think car audio sized. The battery would be cheaper though, and ultimately more reliable.

Also, it may be tempting to use one of the 12v traction batterys from your main pack and tap it for 12v - don't do it! It'll lead to premature failure of that battery or the whole string, as they'll unbalance themselves. This MIGHT be able to be overcome if you have seperate chargers or some battery management system, but I'd avoid it.

Hope this helps!
~Eric


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## electric85 (Apr 10, 2008)

thanks a bunch! that really helps. i guess i can't turn on my contactor if the converter isn't on, i guess i'd need a battery...but...is it possible to keep the converter powered by the traction pack all the time? doesn't seem like a great idea but it would be interesting to know how that would work


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## eric-ev (Apr 28, 2009)

Could you keep the converter powered up at all times to eliminate the need for a battery? Sure, I suppose. The converter uses electricity even when there's nothing 'running', it will power your clock in the dash, its own cooling fan, and it wastes some electricity from ineffeciency (this turns into heat).

Honestly, this place is called the DIYelectriccar forum - with something like this if I were you I'd be tempted to wire it together (the converter on the HV side of the contactor) so it runs all the time, and try it out. Would it drain the HV pack in a week, a month, I don't know. If it seems like a good idea, try it and tell us how you like it! Most likely, worst case is you end up putting a small battery in later.


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## electric85 (Apr 10, 2008)

i like that answer...i'd like to save space and weight, i know a small motorcycle battery isn't that big or weigh that much but still i want to make this car simple and effective. so less a 12v battery sounds great.


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## kittydog42 (Sep 18, 2007)

I don't recommend leaving the battery out. It doesn't have to weigh that much. I built a car like that once and got stuck twice when the DC/DC converter cut out. I eventually added a battery as I have done on every other conversion, and changed out the DC/DC converter for another brand.


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## Qer (May 7, 2008)

kittydog42 said:


> I don't recommend leaving the battery out. It doesn't have to weigh that much. I built a car like that once and got stuck twice when the DC/DC converter cut out. I eventually added a battery as I have done on every other conversion, and changed out the DC/DC converter for another brand.


I concur. Also, if you drive in the night and the DC/DC shuts down it could be very dangerous when all the lights go dark and motor cuts off. On a small road without street lights you'll have to stop the car without seeing where the road is or if there's anything in the way and on a highway you can't use turn signals or hazard to indicate where you're going or that you're having problems.

A small MC battery on 5-10 Ah weights maybe 5 pounds and can literally save your life. It will also make it possible for you to pick a smaller (and lighter) DC/DC since you only have to dimension it for average current rather than top current. Top current can easily be 2-3 times higher than the average (probably more).

Just add that battery, it's just better that way...


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## JimDanielson (Oct 19, 2008)

I thought about not using a battery, but if you were using a contactor/relay sorta thing to turn on the car when you turn on the key, you will need 12v to go through the key switch and to leave the dc-dc converter on at all times would be a waste of energy.


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## Zemmo (Sep 13, 2007)

This has been discussed before, different people feel differently about weather or not to have a battery. I had a DC/DC converter and battery setup. The battery went dead so I just pulled it out and only ran the DC/DC converter. I had little bugs here and there with the car by only running a DC/DC converter. When I lost a connection to the pack, the DC/DC converter also went dead since no battery. So I lost all power, since no power, I could not put on my hazard flasher or head lights or anything. Now I run just a small AGM deep cycle battery (33 Ah motorcycle size battery) for the 12 volt system with a smart charger. That small charger comes on the same time my main charger comes on. So all batteries are always charged when I plug in. I like this method the best, doesn't drain my main pack at all and if there was ever a problem again with the main pack, I would still have 12 volt power for safety. Thats just my 2 cents.


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## electric85 (Apr 10, 2008)

you guys raise a good point with a failure of the traction pack, or i have to hit the breaker switch in emergency...i'd be without lights and hazards. thanks for all the input everyone! i should post a pic of the dc/dc converter to show you guys what i'm working with. i don't want one that will crap out like kittydog42


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## kittydog42 (Sep 18, 2007)

It was an Astrodyne/Meanwell. They have a self-protection shutdown that requires a power cycle to reset. I don't know what was causing it, but adding a battery mostly eliminated the problem. I changed over to an Iota later and never had another issue since.


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## mamodbasher (Apr 22, 2009)

Qer said:


> I concur. Also, if you drive in the night and the DC/DC shuts down it could be very dangerous when all the lights go dark and motor cuts off. On a small road without street lights you'll have to stop the car without seeing where the road is or if there's anything in the way and on a highway you can't use turn signals or hazard to indicate where you're going or that you're having problems.


This is sound reasoning to me - here is another thing - has anyone investigated the possibility of using LED bulbs in the EV to cut way back on power consumption, especially at night? Seems to me that collectively, this might be a benefit in the long run.  Ken.


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## Qer (May 7, 2008)

mamodbasher said:


> Seems to me that collectively, this might be a benefit in the long run.  Ken.


In theory, yes, in reality it won't make that much of a whooping difference. The nice log data I've got from Dimitri shows that his EV will typically draw several kW, at highway speeds we're talking around, or over, 30 kW and this is for a pretty ordinary car, no truck or so. Compare those numbers with the typical 110/130 Watt for the head lights and even if you count in all the lamps all around the car you'll probably end up at 200-300 Watts.

Of course, if you put on tons of extra lights, turning the car into something that will remind people of when the alien ship lands in E.T., you might reach, and even surpass 1 kW but it will still only give you a few percent better range tops if you replace them all with LED. Compared to that even regen will be a bigger improvment...

Save those LED-money for a LiFePO4-pack instead.


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## mamodbasher (Apr 22, 2009)

Qer said:


> In theory, yes, in reality it won't make that much of a whooping difference. The nice log data I've got from Dimitri shows that his EV will typically draw several kW, at highway speeds we're talking around, or over, 30 kW and this is for a pretty ordinary car, no truck or so. Compare those numbers with the typical 110/130 Watt for the head lights and even if you count in all the lamps all around the car you'll probably end up at 200-300 Watts.
> 
> Save those LED-money for a LiFePO4-pack instead.


Again, sound reasoning. I guess if you look at that way, then why take out the accessory battery. Seems to me that if your EV weighs in at 3000 pounds, then a 50 pound battery won't make much difference in the grand scheme either - especially when compared to the added safety factor.


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## Zemmo (Sep 13, 2007)

I replaced all of my Light Bulbs with LED bulbs except for the head lights. It did cut the power usage a lot. It is still an expensive option to go with all LED. I think I spent around 125 dollars to replace everything on my car.


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## electric85 (Apr 10, 2008)

i plan on changing all my lights to LED ya it will be 100-200 bucks but i just want to make this thing as effecient as possible. and it will make me feel better sitting in traffic knowing i'm using as little juice as possible.

There is also a black pontiac fierbird on evalbum that has had all its lights converted to LED alot of them custom made


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## rillip3 (Jun 19, 2009)

mamodbasher said:


> This is sound reasoning to me - here is another thing - has anyone investigated the possibility of using LED bulbs in the EV to cut way back on power consumption, especially at night? Seems to me that collectively, this might be a benefit in the long run.  Ken.


This bloke did!

http://www.driveev.com/jeepev/home.php


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## mamodbasher (Apr 22, 2009)

electric85 said:


> you guys raise a good point with a failure of the traction pack, or i have to hit the breaker switch in emergency...i'd be without lights and hazards. thanks for all the input everyone! i should post a pic of the dc/dc converter to show you guys what i'm working with. i don't want one that will crap out like kittydog42


Speaking of DC to DC converters, I just bought one from EV Parts (their model # DC2416). The catalog advertized it as "48 to 72 volts DC input - 12 volts DC output". When I got it in the mail, the paperwork that comes with it seems to suggest 48 volts only, and if you look at the schematic, it refers to 36 to 48 volts on the ignition switch input. No big deal to run a contactor with a 12 volt coil to provide this, but the voltage suggestions are confusing to say the least. I have contacted their tech-line and still waiting for an answer - has anyone used one of these? Is it good to 72 volts input, or am I stuck at 48 (that would be a real bummer ). Ken.


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