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Old 07-16-2012, 09:59 AM
Rob_of_Waterloo Rob_of_Waterloo is offline
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Default dc-dc connection method

Hello folks

I am nearing the completion on my Miata conversion and have a question about the dc-dc converter. I have a 750 W (62A) dc-dc converter and plan to leave the original car (aux) battery installed. Input power to the dc-dc converter is disconnected when the ing is off. The dc-dc converter is connected directly to the battery.

I am concerned that the output stage of the dc-dc converter will slightly drain the aux battery when the car is off. Even though this will take a long time, I am interested in what other people have done. I can think of few different options

1) Always power the dc-dc converter, even when the car is off. (not preferred)
2) Have a power diode between the dc-dc and the aux battery
3) have a power relay between the dc-dc and the aux.
4) directly connect the dc-dc to the aux battery.

I am leaning towards option 4.

Thanks for any input.
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Old 07-16-2012, 10:06 AM
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Default Re: dc-dc connection method

What DC-DC is it?
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Old 07-16-2012, 11:07 AM
Rob_of_Waterloo Rob_of_Waterloo is offline
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Default Re: dc-dc connection method

p/n 500-dcdc-636 from EV Source
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Old 07-16-2012, 12:11 PM
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Default Re: dc-dc connection method

Personally, my DC-DC is always powered and connect to the auxiliary battery.
I need 12v power to run my BMS, door lock, etc.

When I don't drive (recharged) my car for long time, I disconnect the DC-DC.
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Old 07-16-2012, 01:15 PM
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Default Re: dc-dc connection method

Quote:
Originally Posted by Rob_of_Waterloo View Post
I am concerned that the output stage of the dc-dc converter will slightly drain the aux battery when the car is off. Even though this will take a long time, I am interested in what other people have done. I can think of few different options

1) Always power the dc-dc converter, even when the car is off. (not preferred)
2) Have a power diode between the dc-dc and the aux battery
3) have a power relay between the dc-dc and the aux.
4) directly connect the dc-dc to the aux battery.
Most likely there wont be any significant back current when the DC-DC is shut off. Measure it. More than likely it will be pretty low. A bleeder resister across the output caps is the most likely source of a load. If there is backflow then you can add a diode. But you will want to raise the output voltage of the DC-DC to compensate for the voltage drop across the diode or your DC-DC won't fully charge the battery. The voltage drop will be between about 0.6V and 1.5V depending on the type of diode and the load.
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Old 07-16-2012, 02:02 PM
Yukon_Shane Yukon_Shane is offline
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Default Re: dc-dc connection method

I can't speak to the dc/dc standby load but if you're planning to have your dc/dc disconneted from the battery pack while parked you might want to take a close look at your other "parisitic" loads in your vehicle.

If you're like me you may only drive the car for 30-40 minutes a day which would mean your aux battery would only have that length of time to be recharde by the pack while the remaining 23.5 hours a day it would experience a slow drain from the various standyby loads (bms, battery instrumentation, car stereo, etc.).

I think this is why most folks tend to end up just leaving the dc/dc connected to the pack all the time.
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Old 07-16-2012, 02:16 PM
Rob_of_Waterloo Rob_of_Waterloo is offline
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Default Re: dc-dc connection method

Interesting and a good point.
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Old 07-16-2012, 05:44 PM
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Lightbulb Re: dc-dc connection method

The best thing is probably to start by checking current drain. Then figure out how long it can be left connected without depleting the battery. If it is significant, you can get battery disconnect switches for about $5 at Harbor Freight.

You can also make a very good solid state switch with two MOSFETs. You tie the sources together and the gates together, and add a 10k resistor from the gates to the sources. The drains will be non-conductive and constitute an open switch with typically less than 10uA leakage. The drains, which are the tabs, can be used as the switch contacts to the battery. If you apply a voltage, like 9V or 12V from a small battery, to the gates, they will both turn on. Here is a 100 amp 40 volt MOSFET with about 2.8mOhm RdsOn for about $1.72 each:
http://www.digikey.com/product-detai...3-5-ND/2606362

At 60 amps they will each drop 0.17 volts and 10W, but that's at maximum charge/discharge and at more normal levels it's much less. There are of course many devices available so you can choose voltages and currents as needed and parallel devices to get lower resistance.

You could also use an optoisolator to turn the switch on or off using some other signal such as the ignition switch.
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