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Old 06-14-2010, 09:03 AM
rickeolis rickeolis is offline
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Default Car battery still necessary when using DC-DC converter?

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-


Rick
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Old 06-14-2010, 09:08 AM
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Default Re: Car battery still necessary when using DC-DC converter?

Quote:
Originally Posted by rickeolis View Post
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-


Rick
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.....
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Old 06-14-2010, 09:19 AM
rickeolis rickeolis is offline
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Default Re: Car battery still necessary when using DC-DC converter?

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.
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Old 06-14-2010, 09:45 AM
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Default Re: Car battery still necessary when using DC-DC converter?

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.
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Old 06-14-2010, 09:51 AM
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Default Re: Car battery still necessary when using DC-DC converter?

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.
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Old 06-14-2010, 10:10 AM
rickeolis rickeolis is offline
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Default Re: Car battery still necessary when using DC-DC converter?

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!
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Old 06-14-2010, 10:58 AM
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Default Re: Car battery still necessary when using DC-DC converter?

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)

Last edited by Stunt Driver; 06-14-2010 at 12:15 PM.
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Old 06-14-2010, 11:55 AM
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Default Re: Car battery still necessary when using DC-DC converter?

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Originally Posted by Stunt Driver View Post
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.
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Old 06-14-2010, 12:16 PM
rickeolis rickeolis is offline
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Default Re: Car battery still necessary when using DC-DC converter?

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
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Old 06-14-2010, 12:59 PM
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Default Re: Car battery still necessary when using DC-DC converter?

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.
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