# DC-DC Convertor control



## vmrod (Jul 2, 2010)

I'll have a 12V-30aH (or 45aH) aux. battery on my EV.

When do you guys have the 12V DC-DC Convertor switched on?

1. During 'runnning' only (like a car's alternator).?

2. During traction pack charging?

3. Always on?

4. A mixture of #1 and #2?


I rather having it cycle on and off completly, so that the cells in the aux. battery can fully balance.

I may just go with my rotator switch again, so I can change how it operates with a knob (rather than rewire it). However, it is a good bit of wiring to get it done.

opinions?


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## mora (Nov 11, 2009)

My DC/DC comes on almost like original alternator. Motor controller operates a relay after main contactor is closed. This relay connects DC/DC to high voltage. It takes few seconds for DC/DC to begin operating (soft start feature) but this way it won't interfere with controllers precharge procedure. Whole system is behind main contactor so DC/DC also gets disabled completely when I turn the car off.


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## dtbaker (Jan 5, 2008)

vmrod said:


> I'll have a 12V-30aH (or 45aH) aux. battery on my EV.
> 
> When do you guys have the 12V DC-DC Convertor switched on?



I have mine 'always on', powered by traction pack, with small (4ah) aux battery in parallel. I want 12v always on so I don't lose little things like clock and radio settings, have interior lights when keyed-off, etc.

The little 12v (an ATV starting battery) I added later because I was getting a 'voltage sag-blink' in 12v power when running at night with headlights on, heater on, and the vacuum pump switched on. I tried a couple different things.... a TINY 1.2 ah battery, a big stereo capacitor, then settled on the 4ah ATV battery as it levels out the voltage spikes, and the always on dc-dc keeps it at float voltage most of the time without doing a big draw to re-charge like it would if I turn the dc-dc off and let the battery hold the always on 12v load...

I usually drive every day....on occasions I'm gone for week, I disconnect both main batteries and 12v aux to prevent standby drain.


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## vmrod (Jul 2, 2010)

DTBaker,
I guess yours is always on, since the 12V battery has such a small capacity.

Mora,
what aH rating is your 12V battery?


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## TEV (Nov 25, 2011)

I kept the original 12V battery and the dc/dc it's switched on with ignition. After I would finish all the wiring on my conversion I think I would ad the "dc/dc on when charging" switch.


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## dladd (Jun 1, 2011)

Mine used to be switched on/off with the ignition switch, but I had problems with the 12v battery dying if it sat for a day or two. It's a cheapo garden tractor starting battery. Instead of solving the problem (finding any excess drains) or buying a larger battery, I wired the DCDC to be always on. This was about a year ago, no problems anymore. 

If I'm going to be gone for more than a few days I'll disconnect the main circuit breaker (which of course will shut off the DCDC also) and put a battery tender on the 12v battery.


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## wessss77 (Jan 4, 2013)

Hey VMRod....I am planning to have mine only on when the key is on off the contactor, similar to an alternator basically tap into the original alternator + side wiring and replace the original 12v with a deep cycle. This way there is no real change to the existing harness is my thought.


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## dtbaker (Jan 5, 2008)

wessss77 said:


> Hey VMRod....I am planning to have mine only on when the key is on off the contactor, similar to an alternator basically tap into the original alternator + side wiring and replace the original 12v with a deep cycle. This way there is no real change to the existing harness is my thought.



I would advise against switching dc-dc on/off with key-on.... just leave it on all the time, unless you are mothballing the vehicle for week or more. much better to leave the 12v side energized like the original battery; keeps the clock, radio, lights, aux.12v battery, and various capacitors and whatnot charged up.


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## mora (Nov 11, 2009)

vmrod said:


> Mora,
> what aH rating is your 12V battery?


It is 4x 90Ah old Thundersky cells. Top balanced. Too much capacity for me, I'd do fine with 40Ah cells, or even smaller ones. Doesn't matter if I forget to turn off parking lights for a night (or two) though. Oh and bigger battery doesn't care it it gets 30A charge. Charging at 1C might warm these cells a bit but not too much. I wouldn't push 30A to a small motorcycle battery though.


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## corbin (Apr 6, 2010)

vmrod said:


> I'll have a 12V-30aH (or 45aH) aux. battery on my EV.
> 
> When do you guys have the 12V DC-DC Convertor switched on?
> 
> ...


Only when the car is running; don’t drain your battery pack when you don’t need to!

I have an AC-DC converter that keeps my BMS alive when my car is charging.

corbin


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## vmrod (Jul 2, 2010)

Thanks for the replies.

1. I plan to have it on only during ignition and set it up so my 30aH LiFePO4 aux. pack will get 14V. (each cell gets 3.5V) I'll monitor them, and see if they either need increased voltage or longer charge time.

2. After the car is on the road, I'll add the selector switch to choose between "always off", "Ign. only", "Ign & Charge", & "always on". 

3. It should be noted that the DC-DC convertor only outputs current when it is needed. If I decide to leave it "always on", then I would adjust the voltage down to 13.6, so that the LiFePO4 aux. pack cells each get a float charge of 3.4V.

4. Final note: I installed a selector switch for my Zilla controller, so that it gets power either "always" or "Ign/run only". If I were to let the car situp, I could flip the switch to "Ign/run only" & eliminate a parasitic drain.


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## dtbaker (Jan 5, 2008)

vmrod said:


> Thanks for the replies.
> 
> 1. I plan to have it on only during ignition and set it up so my 30aH LiFePO4 aux. pack will get 14V. (each cell gets 3.5V) I'll monitor them, and see if they either need increased voltage or longer charge time.



the problem with dc-dc key-on is that when you first key-on the dc-dc gets hit with a big load until the aux battery is fully recharged. big loads are harder on contactor/relays when you key-on, and the dc-dc itself.... so I prefer the dc-dc always-on approach with the output voltage being a good float voltage for the battery you are using....


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## vmrod (Jul 2, 2010)

I believe it 'soft starts'. 
Also, if it is not needing to produce lots of current, I wouldn't think it would draw much initially.
Lastly, I could use a High Voltage contactor (EV-200), the same for my main contactor, so that should handle it if there were any, however there is a remote on/off which will be used to control the unit.

I plan on the Meanwell HPRG-600-15


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