# DC-DC converter in parallel



## geosynch (Oct 30, 2012)

Hello,

I apologize if this has been hashed out already -- but I didn't find anything after a search.

I have a '99 Miata that I converted and have been driving it for a few months now, and am getting used to its personality.

It appears I undersized my DC-DC converter and I'm going to have to add some beef.

I am using a 12 V battery with a 440 W DC-DC converter to keep said battery charged. Well, the 440 W DC-DC is at least _attempting _to keep the 12 V charged. I'm on my 2nd 12V battery now and it looks like I may have killed it, too.

I've looked for a higher output DC-DC converter and while I'm willing to pay a few hundred dollars for a good one, I'm wondering if I can just get another one exactly like I have now and run them in parallel...? Anyone have experience in this?

I am using this DC-DC converter now. The installation is very straightforward. The input section is connected directly (via fuse) to the traction pack (33 Sinopoly 180's in series). The output (13.8 V) is connected to my 12 V "deep-cycle" lead-acid battery.

Alternatively, anyone have a >800W DC-DC converter they would recommend?

And while I'm at it if anyone has any other advice for not killing my 12 V battery, I'm open. I've reduced the key-off parasitic drain to about 5 mA (had been almost 200 mA). Replacing bulbs with LEDs is on my to-do list, but so is adding in heated seats. My message is, that I think I've got the low-hanging fruit covered. But I also understand that this forum has a lot of knowledgeable people out there, so, I'm open for suggestions.

Thanks in advance,

geosynch


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## major (Apr 4, 2008)

geosynch said:


> ..... I'm wondering if I can just get another one exactly like I have now and run them in parallel...? Anyone have experience in this?
> 
> I am using this DC-DC converter now. ....


Hi geo,

I have paralleled Vicor DC converters and they have a method to use a gate signal to drive the slave in synch. I would be cautious with yours. They may not behave in parallel. I don't know. If you have a known heavy load like the seat warmer, perhaps you could use a dedicated converter for it on a separate circuit without the aux battery.

Have you metered the 12V system? Are you using in excess of the 30 some Amp output from the converter and draining the aux battery while driving? Or is it getting killed being drained while the car is off? Reading your post I'm not sure what's your problem.

major


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## geosynch (Oct 30, 2012)

major said:


> Hi geo,
> 
> I have paralleled Vicor DC converters and they have a method to use a gate signal to drive the slave in synch. I would be cautious with yours. They may not behave in parallel. I don't know. If you have a known heavy load like the seat warmer, perhaps you could use a dedicated converter for it on a separate circuit without the aux battery.
> 
> ...


Hi Major,

Thanks for the reply. Unfortunately, I do not have an ammeter wired in to the 12V aux system. I have a Fluke clamp-type ammeter coming from Amazon that will be here tomorrow or so, so I will be able to test the loads on my 12V system. I'll update this thread when I get actual data.

Right now I'm running on assumption and the knowledge that I've trashed two 12V lead-acid batteries in the space of four months.

I don't think my problem is the key-off "parasitic" load. That's only 5 mA, which is measured with an inline Fluke multimeter. The car does get driven daily, so, I don't think I am draining the battery while the car sits in the garage.

When the car is driven, the 12V system powers all the auxiliaries you'd expect on a Miata, including



lights


OEM fan for the heater (using a ceramic heating element driven off the traction pack)


power windows, antenna (very low duty cycle usage)


radio


etc.

I also run a 12V vacuum pump for the OEM power assit brakes, but that is also a very low-duty cycle. I think the biggest 12V load is a water pump, that is keeping the Curtis 1238 inverter/controller cool. The water pump is a 10 amp draw by itself and it is a 100% duty-cycle. Putting the pump on a thermostat is on my to-do list as well, but right now it is on when the key is on.

You've answered my question about DC-DC converters -- my unit is *not* specifically made to be used in parallel, so there is a high probability I'll simply upgrade to a more powerful DC-DC converter.

Any suggestions for one that is in the 800 to 1 kW range?

Once I get a reading on my key-on current draw, I will post the results. And then make a real assessment as to whether or not I am actually overloading the 12V system while I'm driving down the road.

Thanks again for your response!

geosynch


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## Frank (Dec 6, 2008)

Be sure to measure actual output voltage from the converter. It is possible that your pack voltage is too low under load to charge your aux battery. 30 amp output may be at max input voltage...


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## geosynch (Oct 30, 2012)

> Once I get a reading on my key-on current draw, I will post the results. And then make a real assessment as to whether or not I am actually overloading the 12V system while I'm driving down the road.


Well, that was illuminating. With the car sitting in the garage, but in the "on" state, there was 25 amps coming out of the DC-DC converter with all the auxiliaries on (lights, fan, radio, aforementioned pump and so on). Two amps was going into the battery. Voltage measured at 13.6 V.

This suggests to me that I should not be trashing my 12V battery. But it also tells me that I need a bigger DC-DC converter if I ever add anything else on to the 12V electrical bus.



> Be sure to measure actual output voltage from the converter. It is possible that your pack voltage is too low under load to charge your aux battery. 30 amp output may be at max input voltage...


That is an excellant suggestion. Thanks. I don't have a conveniant way to measure the V while I'm driving the car (a voltmeter on the 12V system is on the aforementioned to do list) but I can jerry-rig something. I'll do that this weekend. This could be the root of my problem.

Would appreciate any further insights.

Thanks to all.

geosynch


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## kennybobby (Aug 10, 2012)

*Better get a handle on those loads*

Were you running all of these at the same time Water pump motor? Cooling fan motor? heater fan motor? power-steering pump motor? vacuum pump motor? Probably will need a hefty converter for all that plus lighting and seat heaters, etc...


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## major (Apr 4, 2008)

Overcharging the battery? What kind of battery is it?


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## geosynch (Oct 30, 2012)

major said:


> Overcharging the battery? What kind of battery is it?


It's this "deep cycle" lead-acid. 

The first battery I ruined was a starter battery that was made specifically for the Miata. When it went, I realized I had significant parasitic loads. I have those fixed now (only 5 mA when the key is off).

Then I replaced that battery with the one linked above. It's all but gone, too. (Won't hold a charge, voltage sags from 12.6 V to 12.0 V overnight).

Sorry, to be stupid, but I thought I could keep 13.6 V on one of these batteries indefinitely, ergo, I ruled out overcharging. But I'm open to hear otherwise.

geosynch


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## geosynch (Oct 30, 2012)

*Re: Better get a handle on those loads*



kennybobby said:


> Were you running all of these at the same time Water pump motor? Cooling fan motor? heater fan motor? power-steering pump motor? vacuum pump motor? Probably will need a hefty converter for all that plus lighting and seat heaters, etc...


The car does not have a cooling fan or power-steering pump. Nor does it have heated seats, but I was planning on adding those before next winter.

The car does have a 12 V vacuum pump, although that wasn't running when I did the test above (Vacuum pump runs for only a few seconds... so I presume it can dip into the battery without really affecting energy consumption).

What was running was:

* lights 
* water pump
* radio
* lights
* fan for the heater

And the draw was 25 A, 3 of which was going into the battery. I will eventually put the water pump on a thermostat so it is only running when the inverter actually requires cooling; but anecdotal evidence is that in the summer months the pump will run a lot.

Yes, I agree. I need to get a beefier DC-DC converter! Any suggestions?

geosynch


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## major (Apr 4, 2008)

geosynch said:


> It's this "deep cycle" lead-acid.
> 
> The first battery I ruined was a starter battery that was made specifically for the Miata. When it went, I realized I had significant parasitic loads. I have those fixed now (only 5 mA when the key is off).
> 
> Then I replaced that battery with the one linked above. It's all but gone, too. (Won't hold a charge, voltage sags from 12.6 V to 12.0 V overnight).


Seems like it is not a case where you are damaging the battery by over discharge. So why has it failed in just a few months? Might be a bad battery to start with. Try to get a warranty replacement. Barring that, buy one from a reputable dealer who will honor a warranty.

I've used and tested a thousand or more lead acid batteries; most of them top quality for racing application. We always tested incoming new batteries, sometimes lots of a hundred. There were always stinkers to the tune of 2 to 10% of the lot. I don't recognize Scorpion Power, but I've been out of the Pb-Acid game for a while. Don't rule out a bad battery (baddery) from the getgo


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## Tesseract (Sep 27, 2008)

geosynch said:


> Sorry, to be stupid, but I thought I could keep 13.6 V on one of these batteries indefinitely, ergo, I ruled out overcharging. But I'm open to hear otherwise.


13.6V is closer to the proper float voltage at 25C and will be insufficient to desulfate the battery after even a moderate (<50%) discharge. After a few dozen cycles of this kind of abuse the battery is toast.

If you can kick the voltage up to at least 14.0V your /next/ battery should last longer; this one is likely ready for the recyclers.


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## major (Apr 4, 2008)

geosynch said:


> It's this "deep cycle" lead-acid.
> 
> ......It's all but gone, too. (Won't hold a charge, voltage sags from 12.6 V to 12.0 V overnight).


I looked around on the web about that Scorpion Battery and found some complaints. Not much in the way of solid information about the company or battery type/spec. And your link had this to say:


> Absorbed Glass Mat (often referenced as Gel Cell)


 So what is it  AGM or Gel Cell? http://en.wikipedia.org/wiki/Float_voltage Gives float voltage of 13.6V for AGM and 13.05 for Gel. 

Maybe you can tell; I'm blaming the baddery  

On the other hand, what do we know about your DC converter? Have you inquired of Thunderstruck about the problem? How many have they sold? What is the field experience? Clicking on the manual link doesn't instill confidence in the product. 


> *Specifications*
> Input Voltage: 70V~140V DC
> Output Voltage: 24.0V at 15A


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## geosynch (Oct 30, 2012)

Tesseract said:


> 13.6V is closer to the proper float voltage at 25C and will be insufficient to desulfate the battery after even a moderate (<50%) discharge. After a few dozen cycles of this kind of abuse the battery is toast.
> 
> If you can kick the voltage up to at least 14.0V your /next/ battery should last longer; this one is likely ready for the recyclers.


This is extremely helpful and answers oh, so many questions. I wish it otherwise, but this all makes sense now. See next post.

Thanks!

geosynch


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## geosynch (Oct 30, 2012)

major said:


> I looked around on the web about that Scorpion Battery and found some complaints. Not much in the way of solid information about the company or battery type/spec. And your link had this to say: So what is it  AGM or Gel Cell? http://en.wikipedia.org/wiki/Float_voltage Gives float voltage of 13.6V for AGM and 13.05 for Gel.
> 
> Maybe you can tell; I'm blaming the baddery
> 
> On the other hand, what do we know about your DC converter? Have you inquired of Thunderstruck about the problem? How many have they sold? What is the field experience? Clicking on the manual link doesn't instill confidence in the product.


Thanks for your insights and help, Major. I'll be getting a new battery and DC-DC converter soon.

I did contact Thunderstruck. They are usually really good about getting back to me, but haven't yet. Still, based on results I got from testing today, along with insights from yourself and Tesseract, I think I know the problem is a whimpy DC-DC converter.

geosynch


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## geosynch (Oct 30, 2012)

So today I performed a few more tests on my car's DC-DC converter. 

The voltage output of the DC-DC converter is very stable over the range of input for my car's traction pack. I did not see it vary by a even a millivolt.

However, the output does vary quite a lot depending on the load on the 12V electrical bus. With no load, the output is 13.8 V. With nominal load, it is 13.6 V. But I found today that fully loaded, it is closer to 13.2 V (that's with lights, heater fan, stereo blasting, water pump).

I'm talking with Belktronix about a new DC-DC converter with more punch and hopefully more stability in the output. I'll also be looking for a new 12V battery.

If anyone has a suggestion on a good quality *light* 12V battery, I'd love to hear suggestions.

thanks all for your help.

geosynch


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