# Car battery still necessary when using DC-DC converter?



## vpoppv (Jul 27, 2009)

rickeolis said:


> So I was thinking about either using a small car battery as my 12 volt supply or buying a medium duty (20 or 30 amp) DC to DC converter. I was wondering if the converter acts as the 12 volt supply at all times, or does it just charge the 12 volt battery?
> My battery pack wiill be 165 volts.
> Thanks-
> 
> ...


 I can't comment on the DC to DC converter, but that's what most people on here use, so I'm sure you'll get comments about that; but as far as using a SMALL battery, forget it. You're not just running turn signals and lights, but the occasional windshield wipers and headlights- the Exide Orbital I have is great for keeping the voltage up, but anything smaller and I'm sure the wiper blades would barely move.....


----------



## rickeolis (Aug 13, 2009)

Thanks-
The battery I would consider using is a lawn and garden lead\acid style which is about one half the size of a normal car battery.


----------



## TX_Dj (Jul 25, 2008)

You can run with or without a DC converter, and with or without a 12v system battery, but you can't run without one or the other.

Thing is, with a 12v battery, you have a stand-by power source permitting you to switch your mains from 12v rather than mains voltage. It's kind of the chicken or the egg debate. How do you activate from the 12v system if the 12v system is only powered by the DC converter, and the DC converter doesn't activate until the mains are switched on?

It is possible to construct the system such that it doesn't need the 12v battery to operate correctly and safely. 

But, having the 12v battery permits you the redundancy to keep your headlights going in case the DC converter dies on you. The smaller the 12v battery, the less time you have to run those high-current 12v systems until you're stuck. Similarly, if your system requires 12v to energize the main contactor, even indirectly, if you have no 12v battery, then if your DC converter fails, you're stuck without the ability to move.

My truck will have a sizeable deep-cycle 12v battery, with the DC converter keeping it charged and handling the bulk of the energy requirement. This setup keeps the battery in "float service" rather than "cycle service", and it will last considerably longer than if I used the same without a DC converter at all, but charged it every time the mains were being charged.

Hope this answers some of your questions.


----------



## Woodsmith (Jun 5, 2008)

Also if you are using both DC-DC and a battery then get the 13.8v DC-DC so that it can add charge back to the battery when needed.

I would be tempted to run both so the intermittent large loads can be handled by the battery while the DC-DC can be less then the maximum load and keep the battery topped up.


----------



## rickeolis (Aug 13, 2009)

Thanks guys!
So, now I'm thinking that I can get a small DC-DC (at 13.8 volts), say 2-5 amps just to keep my battery topped off and so I would not have to have an additional battery charger.
That way, even if the main power (165 v) is drained, I still have 12 volts for a while...
I love these forums!


----------



## Stunt Driver (May 14, 2009)

I used standard car battery only, without DC-DC unit. It gave me enough time if no headlight used, and my run time was limited at night, but still was enough.

But battery dies after 12 months - standard batteries, seems like, are not designed to run to flat out.

So I have added one IBM laptop PSU - it takes 80-90V DC in, and produces 16v 8-9A out. I am bringing that voltage down to 14v, and still keeping charger (that replenishes aux battery when car is charging). It is plenty during day, but at night aux battery has to help.
After adding brake vacuum pump - I may add second power supply (for 17A total)


----------



## TX_Dj (Jul 25, 2008)

Stunt Driver said:


> But battery dies after 12 months - standard batteries, seems like, are not designed to run to flat out.


This is why I mentioned a deep cycle for the 12v system, a regular car battery isn't designed to last in a cycle application like that. That's why we have alternators/generators in ICE vehicles.

Having the DC converter lets this be more of a float application, because the DC converter will shoulder a lot of load (much like the alternator) and let the battery take the spikes.

rickeolis, I don't think a 2-5 amp unit will do what you're looking for effectively. You will slowly drain the battery unless you're charging it as part of your charge cycle, in which case you're still stuck in a cycle application on that battery.

For daytime running 2-5 amps may be close to perfect, until you turn on the radio, or the heater blower, or headlights. Even incandescent brake lights can draw up to that much power. Your contactors can use a few amps too, combined, depending on the coil rating.

I'd feel much more secure with something in the 20-30 amp range, as this is closer to the low end of what an alternator would put out. The alternator in most vehicles is still well over that, however. The DC converter will only put out as much power as is required to hold the system up at 13.8v.


----------



## rickeolis (Aug 13, 2009)

Well, from the looks of some of them, they are pretty small, so going with a higher amperage would be just a few more $$ than the cheaper ones, so I will probably just do that. Then when I get my car running, try running on just the DC-DC for a few days and decide whether or not to install a battery too.
 Thanks guys-



Rick


----------



## Sunking (Aug 10, 2009)

I am not an EV guy, just a NEV or golf cart kind of guy. We use a 12 volt deep cycle auxiliary battery to power our radios and head lights.


----------



## Duxuk (Jul 11, 2009)

I'm just using a DC-DC which gives 13.8V. It works well but I accept that if it were to fail I would be stuffed! Do they fail often? Mine is a no name Chinese. I could add a motorcycle battery, perhaps, to improve it since I only have lights, horn and indicators.

Andrew.


----------



## rickeolis (Aug 13, 2009)

Right, seeing as how these batteries will not be powering anything high amperage like a starter or anything, the deep-cycle style will be the way to go here!


Rick


----------



## Woodsmith (Jun 5, 2008)

My plan is to use around 30A DC-DC and a small deep cycle battery. What sort will depend on what I can afford or find.

This is what I am looking at on Ebay at the moment. I will get a 48v-13.8v for my tractor and maybe 144v-13.8v for the MR2.


----------



## CrunchTime (Feb 13, 2009)

Remember that if you have an electric Power Steering pump it can draw high amps under load - doesn't the MR2 pump uses 50-70A depending on model?


----------



## TX_Dj (Jul 25, 2008)

CrunchTime said:


> Remember that if you have an electric Power Steering pump it can draw high amps under load - doesn't the MR2 pump uses 50-70A depending on model?


Exactly the point of having the battery with the system, the DC converter will give all it can, the rest comes from the battery. Then when the intermittent load cuts off, the converter tops off the battery.


----------



## Qer (May 7, 2008)

Duxuk said:


> I'm just using a DC-DC which gives 13.8V. It works well but I accept that if it were to fail I would be stuffed! Do they fail often? Mine is a no name Chinese. I could add a motorcycle battery, perhaps, to improve it since I only have lights, horn and indicators.


The main problem is that when it fails your car will go instantly out of power. This could be a real traffic hazard since your head light would go out (not so fun if you're driving in the dark), you can't use the turn lights to inform other drivers that you want to get off the road before your car stops, you can't enable the hazard lights to warn others that your car is now an obstacle and so on.

Depending on the laws your car might also be illegal. At least in Sweden (and probably EU) your car battery must be dimensioned for being able to run the hazard lights for at least 24 hours if it breaks down. And even if the law doesn't require it, it's still a pretty good idea to have a small battery that makes it possible for you to safely get off the road before you lose all power.

And, of course, if you manage to pull too much power from the DC/DC it will brown out to protect itself, which might be a bit impractical...


----------



## Duxuk (Jul 11, 2009)

Actually Qer, I have a 2Ah burglar alarm battery to run the hazards. I will charge it separately on occasion. My indicators only use 10W bulbs so it will be good for an hour. The drive side of things is totally separate to the 72v to DC/DC so I will never lose drive, only the lights. I'm sure in time I will fit a small battery to keep things safer.

This thread has got me thinking, so thanks for the information.

Andrew.


----------



## EVfun (Mar 14, 2010)

The headlights look nearly like a short circuit when they are first switched on. My buggy has only a Todd PC20-LV DC>DC converter and when I hit the headlights the neon lights briefly go off. It is not long enough for the Albright contactor to open but I suspect a Zilla would toss an error code and possibly shut down due to low 12v supply. A car stereo stiffening capacitor should take care of that problem. A small 12 volt AGM battery also takes care of that problem and provides a little run time in case of DC>DC failure. 

I've built EVs both ways, but my preference is to use a small AGM battery. My usual choice is a Zappy scooter size battery, about 20 amp hours, about 3 by 7 by 7 inches and around $35. It is a common UPS battery.


----------



## Woodsmith (Jun 5, 2008)

CrunchTime said:


> Remember that if you have an electric Power Steering pump it can draw high amps under load - doesn't the MR2 pump uses 50-70A depending on model?





TX_Dj said:


> Exactly the point of having the battery with the system, the DC converter will give all it can, the rest comes from the battery. Then when the intermittent load cuts off, the converter tops off the battery.


Yes, that's what I am expencting to happen. Unfortunately I have the earlier high current pump, I think, so the battery is going to be essential to get around a parking lot!


----------

