# Is your 12V DC-DC ignition switched or always on?



## dimitri (May 16, 2008)

Hi all,

over the past years on this forum I have seen numerous discussions on interfacing 12V system with high voltage system. There seem to be 4 main options, which I outlined in the poll selection. I wanted to bring this topic back because I am considering to change my original setup for a couple of reasons. Please vote which option you have or will have when your EV is done.

My current option is the first one, ignition switched DC-DC and small Lead Acid 12V battery from the lawn mower. Its been working pretty good for me, but not perfect, hence the need to improve the situation. Here is what's bothering me:

1. I do not want to carry full size 12V starting battery due to its weight and size, so I choose small 12V battery with much less capacity. However, if my EV sits for 4-5 days unused, especially in the colder days, my 12V battery gets too low and fails to power the contactor to bring DC-DC into the loop. This only happened once and it only took a minute to connect 12V charger and start the car, but it was still unpleasant.

2. Once ignition is on DC-DC starts to charge 12V battery while also sharing power load for 12V systems while your drive. However, EV run time is usually short, around one hour or less in most cases, so there is not enough time to boost the 12V battery to a full charge.

3. Choosing correct size of DC-DC converter is a challenge since its difficult to predict typical 12V systems load. You can fire up all lights and fans and radio and all that and determine maximum load, but that will be much higher than typical load and so getting a huge DC-DC will be a waste of money.

So I am thinking of replacing my 12V Lead Acid battery with small LiFePo4 battery made of 4 Headway 10AH cells. This solves the size and weight issue and it has very low self discharge and long lifecycle, so hopefully I won't have to replace it for a long time. However, even 0.1A load while the car is sitting in the garage ( I have Paktrakr and BMS feeding from 12V at all times and clock in the radio takes a few milliamps ) will drain 10AH battery in 100 hours, around 4 days, so that is not acceptable either.

So now I am thinking of connecting DC-DC to 12V side permanently, which will keep the 12V battery charged at all times and the battery will essentially become a small buffer when 12V load exceeds DC-DC capacity and a way to get home in case DC-DC fails in the middle of the trip. 

So what is the downside of having DC-DC permanently connected to the main pack and 12V side? It will put a little drain on the main pack, but compared to the size of the main pack this will be negligible. If I have to store the car for more than a week or two, then I would unplug both batteries anyway.

Unless someone tells me why I shouldn't have DC-DC aways on, I will be trying it soon.

Thanks in advance for your feedback.


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## TheSGC (Nov 15, 2007)

I leave my DC/DC on all the time. I do have a main circuit breaker under my E-Brake that lets me fully disconnect the pack from the EV for emergency/maintenance. I usually keep the main breaker (DC/DC in result) always on because I sometimes forget that I disconnected the power, and go off to do errands and turn the key to nothing but my power brakes pump. 

As long as you don't have heavy loads on it, your charger shouldn't care too much. For a while I was having some rather large drains that was preventing my charger from completing (we are talking 200 watts here), but I fixed that and haven't had a problem since. (car radio died and just sucked power, replaced it with a much nicer Sony). 

I see nothing wrong with leaving the DC/DC active, unless you have gob loads of drain and your charger complains because the drain is preventing it from fully charging the batteries.


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## tomofreno (Mar 3, 2009)

It introduces more risk, in that if something shorts, or develops significant leakage to ground in the 12VDC system it now affects the main pack. Series fuse would protect against larger draws from the pack. I once had a brake pressure switch go bad on a car so that the brake lights remained dimly on all the time. Too dim to see in daylight. The 12V battery kept being drained when the car sat for several days. I noticed it when I went out in the garage one night.


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## gdirwin (Apr 7, 2009)

One additional option (not listed):
12v battery and DC-DC switched on by ignition key or during charging

In my setup, the DC/DC is directly connected to the + side of the pack, but is separated from the - side by a 2nd contactor. This contactor is turned on (by my BMS) whenever the ignition is on (and the emergency switch and inertia switch have not activated) or when the 120/240 VAC charger is connected.

The other contactor is on the + side and connects the motor controller to the pack (controlled by the motor controller - when it completes its safety/diagnostics it will turn on the contactor). The 2 contactors provide added safety when you want to turn off under high loads...


This lets the 12V battery charge when the AC charger is connected...


I am in the same boat as you - a 90Amp DC/DC and a decent sized 12V battery (Optima AGM) - my 12V loads include the power steering, 2 x seat heaters, fans/blowers, rear defrost, headlights, foglights etc... and add up to a substantial 12V load. I like the idea of using Lithium (small/light) but didn't want the extra complexity just now..



One other related discussion is that the DC/DC consumes 12V parasitic power when it is left connected to the pack, and can drain your 12V battery. A suitably sized relay can be connected to the output of the DC/DC to prevent this - it can be controlled by the same signal as the - contactor (ie on whenever the ignition is on or when the AC charger is connected).


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## Guest (Apr 2, 2010)

Mine is turned off from the circuit breaker and I have not drain on my pack what so ever. I also don't have a 12 volt battery. My controller is dependent upon the main pack for power and not the 12 volt system for the rest of the car. My DC DC only powers the blinkers, lights, wipers and radio if I had one. So you see my only heavy drain on the DC DC is the head lights. The DC DC is not strong enough for good strong head lights and that is what I don't like. I'd like to see a DC DC put out a solid 15 volts so I can power my headlights with good power and if I had a 12 volt battery to charge I could put in a voltage regulator so the battery can't be over charged. 

Mmmm. Wonder if an inverter could be used and then use a step down transformer to make that 15 volts for my head lights. I'd prefer not to have a 12 volt battery. 

Pete


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## etischer (Jun 16, 2008)

I have a 12v battery, and DC-DC always on (13.8v)

With the ignition on the DC-DC is set to output (14.3v) 

This gets the headlights a bit brighter, and compensates for IR losses in my long power cable since the DC-DC is in my trunk.

I have a 2nd DC-DC (only 90W) that is powered by the AC charging cord. I like DCIRWINs suggestion of turning off the DC-DC with the ignition and using an AC-DC while charging.


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## dimitri (May 16, 2008)

Wow, what a great feedback in less than one hour! Looks like we all spend too much time on the forum 

Gdirwin, I also thought of the same option as you describe, but I didn't want to add any more relays or contactors to my EV, mostly because I am too lazy to rewire stuff again, I am sick and tired of rewiring my car, I'm sure you all can relate to that 

I read of people reporting parasitic losses from DC-DC, but I can't find that in my IOTA, it doesn't seem to drain anything other than what I accounted for. Perhaps it depends on DC-DC make and model, I don't know.

I think the easiest option for me right now is to just flip one wire feeding DC-DC from one side of main contactor to the other side, to make it always on, and see how well it goes.

Obviously I have main battery disconnect which removes the battery from ALL circuits, including charger and in this case would include DC-DC as well, so there is no issue with maintenance work or long term storage. I will also add quick disconnect to 12V battery, so I can unplug it when needed for long term storage.

Thanks for feedback, it'll be interesting to see poll results once we get enough statistical data.


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## DC Braveheart (Oct 12, 2008)

Not done it yet, but I'll be leaving my Miata's 12V starting battery in place and using an ignition switched DC-DC to charge it when the car is on. 

There's another option for you ... if the lawn mower battery is too small why not go for a small LA car battery like the Miata's (I know you're familiar with them ) - I've left my Miata undriven for months as an ICE and the battery still started the car on the first crank ... so self-discharge is not much of an issue and it's a heck of a lot lighter than most car batteries.


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## EVfun (Mar 14, 2010)

I have 2 EVs under my belt. One has the DC>DC always on and a 20 amp hour 12v battery and the other has only the DC>DC converter. I don't really recommend battery-less operation because a failure of the converter would completely shut the EV down and leave it dark. That is fine for a beach buggy; they are not exactly a model of safety. It is not a great idea for an all weather and all season EV. 

I'm doing my 3rd conversion now and it will feature a DC>DC converter that will be always on and small 12v battery (another 20 amp hour Zappy scooter size battery.)

I have a good reason not to switch the DC>DC off. They generally have a good amount of input capacitance which will torment the switch or relay used to turn them on. Of course there are way around the problem, but I see little reason to switch it off so I don't find it worth the effort.


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## dtbaker (Jan 5, 2008)

Mine is 'always on' so there is power to the aux stuff like interior lights, radio, clock...


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## Dave Koller (Nov 15, 2008)

Was not sure how to check the way I have it..

12 volt Battery is always on as in ice... what I mean is any lights will come on (park,head, etc).

Ignition turns on PRIMARY only (pack) to DC/DC through 1st contactor. Pre-charge starts also.

Start switch latches a relay, through the clutch (that one latches a heaver relay, not contactor, for DC/DC output to battery) and makes pedal "hot" .
This was done because if it sits (off charger) the DC/DC output bleeds the battery down slowly... All Iotas do this by the way...


Pressure on throttle LATCHES second contactor and throttle relay - only unlatched on park brake, brake, clutch or ignition kills this latch - only the ignition kills all...


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## EVMAN (Jul 26, 2007)

I do it with switches.....(and relays)

To answer the Q's:
>>>>>>>>>
How do you manage your 12V system?
12v battery and DC-DC switched by ignition key?..........NO
12v battery and DC-DC always on?............................Yes/NO 12Vbattery goes to 12V stuff
DC-DC only and no 12V battery?................................NO
12v battery with separate AC charger and no DC-DC?....NO
>>>>>>>>>>>>>>>>>>>>>>>>>>


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## willitwork (Apr 9, 2008)

I have a relay installed that will I will turn on when I need to charge the 12v battery from main pack. Flick the switch and the high power will turn on dc/dc. I didnt want the dc/dc always on.


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## cycleguy (Oct 7, 2009)

I tapped 12V off the main pack to power the BMS and trigger a relay to turn on the DC-DC through the ignition switch. I don't think there is enough draw on the 12v tapped portion of the pack to cause any imbalance issues.


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## Voltswagen (Nov 13, 2008)

Etischer
What DC DC converter do you have which puts out 14.3v ?


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## willitwork (Apr 9, 2008)

I have the iota dls-30, it only puts out 13.4v.


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## gdirwin (Apr 7, 2009)

If you have an IOTA DLS, you can add the IQ4 smart charger to it (plugs into the telephone-looking jack):
http://www.iotaengineering.com/pplib/IQ4manual.pdf

The voltages in each mode are:
Bulk Mode: 14.8V
Absorption Mode: 14.2V
Balance Mode: 13.6V

Some IOTA DC/DC converters come with the IQ4 built in now...


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## Dave Koller (Nov 15, 2008)

gdirwin said:


> If you have an IOTA DLS, you can add the IQ4 smart charger to it (plugs into the telephone-looking jack):
> http://www.iotaengineering.com/pplib/IQ4manual.pdf
> 
> The voltages in each mode are:
> ...


Yep mine does..

http://www.diyelectriccar.com/forums/showpost.php?p=170432&postcount=73


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## etischer (Jun 16, 2008)

It's an Iota, 330vdc version. They provide a 4 pin jumper to boost the voltage from 13.8 to 14.3. I tied the 4 pins to a relay that is energized with the ignition. 



Voltswagen said:


> Etischer
> What DC DC converter do you have which puts out 14.3v ?


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## Olaf-Lampe (Feb 24, 2010)

My Renault Clio has a permanent_on DCDC. The car is already 14 years old and recently one cell from the aux-battery died. The DCDC tried to charge the battery to 13.8V. Thereby overcharged the remaining cells and the DCDC got pretty hot. If I hadn't used the car daily, it would have deep discharged my drive battery also. ( NiCD, no problem )

My other inverter ( Siemens Simovert ) has an inbuilt DCDC that switches on from ignition. I couldn't vote twice, just wanted to mention. I'll use an additional ACDC charger while charging. ( In case I ever start building this car )
-Olaf


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## ruckus (Apr 15, 2009)

This is a big dilemma for me right now. 

The car must be able to sit for months without drain.

I am considering a 12v charger that will go on when the main pack is charged.
or should the dc-dc be normally off, but on while charging so the 12v stays up?
will this cause charger to not shut off?

Headway seemed like a good place to start for a 12v batt?

I am thinking they might be good for mower too.


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## spdas (Nov 28, 2009)

What about leaving the dc/dc on (for clock, and small electric draw) and supplement the current draw with a small solar-dashboard charger? 
Francis


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## major (Apr 4, 2008)

ruckus said:


> Headway seemed like a good place to start for a 12v batt?


Hi ruckus,

You might consider NiCad for the aux battery. It is lighter than equivalent PbAcid and won't die if it is discharged completely. Using Lithium for the aux battery sounds like more trouble (BMS wise) and expensive than it is worth.

Also, some similar discussion for the rest of you guys on this thread by ruckus. http://www.diyelectriccar.com/forums/showthread.php/why-did-they-send-2-contactorsi-54396.html In posts #7&11 I explain how I use and wire a DC/DC.

Regards,

major


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## jackbauer (Jan 12, 2008)

I run 4 x 100ah thundersky cells for the 12v loads. no dc/dc and a 14.2v 20A chennic charger.


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## ruckus (Apr 15, 2009)

major said:


> You might consider NiCad for the aux battery. It is lighter than equivalent PbAcid and won't die if it is discharged completely. Using Lithium for the aux battery sounds like more trouble (BMS wise) and expensive than it is worth.


Ok, how many ah? I was thinking about 10-12 with the headways since they have good c ratings. Are nicad similarly productive?

does NiCad require a bms, over or under voltage protection?


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## Guest (Jan 24, 2011)

Nicads require proper chargers too like the lithiums. I'd use a bms on nicads before i'd use them on lithiums.

Pete


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## jeremyjs (Sep 22, 2010)

What kinds of loads does the 12v battery see? You could use this if the load is pretty light. Or this is the loads will be pretty large. They'll prevent over charge, over discharge, balance and shut off if there's a short.


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## EVfun (Mar 14, 2010)

NiCads biggest issue is that the cell voltage *falls* when they reach full charge. They can get into run away situations if the available charging current is high. If you are talking about flooded NiCads, like the surplus aircraft starting units sometimes used by EVers, and if the charge rate is limited then you can just overcharge them and not worry about it (just add water.) If you are counting amp hours you can just replace about 20% more than you remove (they are not real efficient.)

It takes 10 NiCad cells to make a 12 volt battery. They can be run to zero volts without harm, even slight reversal does little damage. These batteries are often stored in a shorted out condition and before putting back into service they are charged to about 130% of their capacity. Flooded NiCads are tough. They can last decades and survive abuse that kills other batteries. Still, I don't think I'd choose them.


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## major (Apr 4, 2008)

ruckus said:


> Ok, how many ah? I was thinking about 10-12 with the headways since they have good c ratings. Are nicad similarly productive?
> 
> does NiCad require a bms, over or under voltage protection?


Hi ruck,

I have used NiCad, 10 cell aux battery with a DC/DC and it worked well without any BMS. I just set the DC/DC output voltage to a reasonable level less than "full" charge on the NiCad. No low voltage protection required and that battery would routinely discharge completely during long periods of nonuse.

I don't know your aux requirements or Ahr you need. Maybe the NiCad is a bad idea for you because it will be a distraction. But I think Li aux battery is also a bad idea for you. I suggest you just use PbAcid and for your guy pull the fuse if it sits for a while.

Regards,

major


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## Guest (Jan 24, 2011)

Yes I guess setting the charge voltage below the threshold would work just fine. How about using a couple Prius Cells. They are 7.2v nominal each and connect two for a 14.4 volt and charge to 13 volts. No water and small footprint. 6.5 AH. Could prove to be a nice alternative. Yes? No? No watering and long life. 

Pete


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## ruckus (Apr 15, 2009)

Thanks for the inputs.

The link to the pcm bms is nice http://www.batteryspace.com/pcmprot...4cells128vlifepo4batterypackat16alimited.aspx

Nicad is an option but the owner is very wary of runnaway bat types. He is really sold on the safety of the lifepo4. (the Thundersky manual includes the AK47 test  ) Sounds like nicad is an option though...

This thing has power nothing. Not even 4-way flashers. 
Headlights, taillights/blinkers, gauges, wipers, crazy loud air horn, 2 contactors, and 2 super-efficient computer fans on the heatsink. That's it. Radio and 12v heater fan will likely be added.  Heater unit will run off 96v.

The dc-dc is 30A max (20A cont?), but I am thinking typical draw with everything running should be well less than 16A. Does this sound right? I figured some Headway 10-2Ah batts would run the lights at night for about an hour if the dc-dc failed. If I could only find some led headlights...

Does anybody know 12v amp usage on a similar setup?


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## F16bmathis (Jun 6, 2008)

I had my DC/DC switched, but had the same problem you had with discharging after sitting for a couple of days. I found my DC/DC (IOTA) had a trickle charge mode, which I was using, and also a fast charge mode engaged by installing a shorting plug. I had also been driving the truck at night with the headlights on, and now that winter is here, with the heater fan on, and radio, and... so with the trickle charge on my 17 mile daily drive, the trucks battery was getting lower and lower every day. I finally hooked it up so it trickle charges with the key off, and with the key on, it switched a relay that engages the faster charge mode. 

It been keeping up with my driving so far. Have not bought gas since mid December!, Driving the truck every day. Gas guzzling Prius stays in the driveway under a foot of snow!


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## ruckus (Apr 15, 2009)

Probably a good idea to run the Prius once in a while to keep the batts and fuel system happy? (and keep it full) I have seen gas turn into hard lacquer in fuel filters/fuel pumps when they sat a long time with low fuel.

So you are charging 12v when you charge the HV pack? That is what I was thinking of doing. Why did your dc-dc not take the load off the battery while driving? Were you pulling all the juice from the bat, but only pulling a trickle from the dc-dc? I thought the dc-dc was supposed to push the 12v load and all the 12v bat does is close the contactors, provide a cushion if the dc-dc is overwhelmed, and provide lights/power if the dc-dc fails.


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## F16bmathis (Jun 6, 2008)

Actually, I'm about to the point where I'll sell the Prius and be 100% electric.

I'm charging 24/7, just more with the ign key on. I am using the headlights, wipers, vacuum pump, 12V relays, heater fan, ect... the charger couldn't keep up in trickle charge or whatever Iota calls it, but trickle charging it 24/7 with full charge when driving seems to do the trick.


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## ruckus (Apr 15, 2009)

-so if the car was left unplugged for a year the dc-dc would drain the main pack?


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## F16bmathis (Jun 6, 2008)

ruckus said:


> -so if the car was left unplugged for a year the dc-dc would drain the main pack?


If it were unplugged for a year, I think the batteries would be dead even if completely disconnected from everything. What is it? 3% loss per month?

If it were unplugged for a year, I'd just pull the parts and call it a day like I did with the Ford Probe. That one I left sitting for about three weeks and the Iota on trickle and yes the Lead acids were close to dead.

If I were to leave the truck (lithium) sitting for more than a week, I'd simply unplug the charger.

The way I have it set up now works. The way I had it set up to only be on when the ignition was on, caused me to loose 12V power in the rain, (windshield wipers, lights...) so truck was dead. Really sucked!


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