# q about dc-dc power consumption



## madderscience (Jun 28, 2008)

If it is always on, it is always on and it will drain your batteries. I put mine on its own contactor that closes whenever the ignition is on, so that the DC-DC does not cut out when I am off the throttle (which opens my main contactor) but it does turn off when the car is parked.

The only downside to my setup is that the DC-DC does not charge the auxilary battery while the main battery is charging. This could be fixed with a couple more relays and switches If I felt the need.

As a hypothetical example, if a DC/DC draws 1A when idle off of your 200AH pack, sitting for only one day will take your pack down by about 5%.


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## rfengineers (Jun 2, 2008)

madderscience said:


> As a hypothetical example, if a DC/DC draws 1A when idle off of your 200AH pack, sitting for only one day will take your pack down by about 5%.


WOW! That's a big load for an idle state! What draw's that much power with your car off? Perhaps your accessory battery was depleted and the DC-DC was in charge mode when you measured the current.

My DC-DC is on all the time, although I do have a switch in the trunk that allows me to turn it off. The draw from my traction pack (with all the lights off) is less than 100mA.


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## dtbaker (Jan 5, 2008)

rfengineers said:


> My DC-DC is on all the time, although I do have a switch in the trunk that allows me to turn it off. The draw from my traction pack (with all the lights off) is less than 100mA.



that answers my q... probably nothing inside the dc-dc drawing power, just a tiny bit running the clock or whatever. sounds safe to wire to 'always on' side of the contactor so that the main auto 12v side 'sees' it like the battery used to be.


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## rfengineers (Jun 2, 2008)

dtbaker said:


> that answers my q... probably nothing inside the dc-dc drawing power, just a tiny bit running the clock or whatever. sounds safe to wire to 'always on' side of the contactor so that the main auto 12v side 'sees' it like the battery used to be.


As I said, I have a switch that allows me to turn off the DC-DC if I am parking the car for an extended period or working on the system. I left the power plug on the Iota and installed an outlet and switch in the trunk (picture attached).
I have a 32 AmpHour accessory battery that is recharged by the DC-DC, so if the battery is low the current draw by the converter is higher.
It is important to fuse the high-Voltage feeding the converter. Also, if you are mounting the battery far from the original fusebox (I put my battery in the trunk) you should fuse the 12V line close to the battery. I have a 20-Amp fuse on the 120V input and a 60-Amp fuse on the 12V output.


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## MBEV (Sep 28, 2008)

Along similar lines, I was just going to post this but noticed this thread going already….

Any Sevcon knowledge out there?

Am using a Sevcon 622/11086 DC to DC converter (72 v Input/13.4v output @300 w) to charge a small PowerSonic deep cycle accessories battery (as required by law here for EV’s). But I have found out the hard way that due to the large surge current and voltage at switch on, that switching the converter on via a solid state relay is *NOT* the thing to do - the SSR can’t handle the high in rush currents on start-up of these converters and after the first switch on the SSR is a piece of wire! They do not have a “soft-start” feature in their design.

A few simple questions?
1- Is it actually necessary to disconnect the DC to DC unit from the main 72 v battery pack or can one simply hard wire the input and leave it on continuously? I see some conversions do this….
2- When fully charged does this Sevcon disconnect or go to a “float” condition? I’ve assumed it’s a float condition but not mentioned anywhere in the info on this converter. If it is float, then maybe it makes sense to leave the converter connected as charging could take longer than the driving time usage?
3- Once the output has charged the battery, does anyone know how much the input “standby” current would be from the main pack?​So I guess 100ma might be reasonable to assume for question 3? 

The idea of a separate disconnect switch makes sense for longer non use periods and easier then pulling the Anderson connector apart. 

So any one with Sevcon experience??

Regards

Bruce


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## madderscience (Jun 28, 2008)

I used that exact Sevcon model while I was still running a 102V nominal pack. the sevcon is designed for an input voltage up to 100V (72V nominal). It was a bit cranky when my battery pack was just off the charger but would be fine once the voltage came down a bit under load. 

Replies to your questions:

1) Not necessary, but I think it is a good idea. (Clearly there are other opinions) whether it is 1A or 100ma, it is a load that will drain your battery pack over time. Regular cars don't need to charge their starting batteries while parked, neither does an EV.

2) The sevcon just keeps putting out 13.4V. If the battery comes up to the same voltage and there are no other loads, then zero current should be flowing on the output side. There will still be input side current as the converter is not 100% efficeint.

3) I don't know. 100mA is probably a better guess than my off-the-cuff 1A example in a previous post. I sold my sevcon unit once I went to a higher voltage pack.


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## MBEV (Sep 28, 2008)

Thanks

I’m running tests now and will post the results later…. 

Currently the input current _(with 72v pack)_ is 470ma with the battery currently sitting @ 13.22 volts. So the 100ma figure @ 13.4 could be close as the efficiency on spec sheet states “84% at 100% load” and “no less than 75% at 10% load” _(whatever this actual means)_


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## MBEV (Sep 28, 2008)

Testing completed and the standby current for the 300 watt Sevcon is 30ma.

These tests are done charging a PowerSonic PSH-1218NB-FR SLA (12 Volt/21 Amp Hour) aux battery with a 72 volt battery pack @ 30 degree C.

It is worth noting that this standby current drops to 10ma if the battery is disconnected once charged.

I also found that the charger actually delivered around 240-250 watts under charging conditions not quite the 300 watts quoted. 

Nice design thou – stays nice and cool with the addition of a small heat sink (see www.mightyboyev.com for details)


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## mechman600 (Oct 16, 2010)

All good info...thanks! I am building a 48V motorcycle and I want to eliminate the accessory battery altogether and just run a small 120W or so DC-DC converter straight to the keyswitch. If it is hooked up to the pack all the time, maybe through a disconnect for long storage periods, I can use a 12V contactor powered by the converter through the keyswitch/kill switch. I was hoping that this setup wouldn't drain my pack when not plugged in, and at 30mA, it will not.


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## Roy Von Rogers (Mar 21, 2009)

I'm of the opinion that NOT running an accessory battery is NOT a good idea, even though many are doing it. If there was ever an emergency and you had to hit the BRB, you wouldnt have any lights or warning flashers.

It is also law in some states to have a battery for such occasions.

Roy


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## PTCruisin (Nov 19, 2009)

I've only had my EV on road for about 3 weeks but I decided to wire the dc-to-dc converter as always on. I'm using the IOTA 55 amp model which was wired by Pioneer Conversions to switch to 14.2V output when operating and switch back to 13.4V when parked. I've measured the current draw at 100ma to 200ma when parked. I also kept the 3-prong power plug on the converter and will plug it into an AC outlet if I am parking the car for the weekend to keep a trickle charge on the AUX battery (a lawn tractor battery). So far so good.


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