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How to Size the 12V Battery for an Electric Car?

12V battery die hard brandI just recently had to replace the 12V auxiliary battery in the electric MR2 after almost 2 years of driving.

The 12V battery on converted electric cars, serve mostly the same purpose as the 12V battery in a gas car.

It powers:

  • cabin lights
  • headlights
  • instrumentation such as the “fuel” gauge
  • ignition circuit that starts the car
  • battery management system

Have you ever left the lights on after driving in the rain and come back to a dead battery?  Pretty much the same thing would happen in an electric car.  The main difference in an electric car is that this 12V battery need not be so big because it does not need to “crank” the starter to start the engine, it just needs to close electrical contactors in order to drive the car.  Sizing your 12V battery is important, as too big of a battery takes up precious space and adds weight to the car.

So how do you properly size the 12V battery on a converted electric car?

Here is what you need to do:

  1. Measure the amp draw from your 12V system with everything turned off using a multimeter.  My system draws 0.085 amps (see below for how I measure this).  
  2. Based on your normal driving pattern, estimate the max downtime you will have between driving the car.  For example, if you travel and are sometimes away for a week at a time, you would use a week.  I sometime go on travel for 5 days at a time.
  3. Multiply the amp draw result from step 1 by the number of hours representing the time you will not be driving the car on a regular basis from step 2.  This is the size of the battery that you need at a bare minimum.  Double it to give yourself some safety margin.

0.085 amps X 5days X 24hours/day  X 2 = 20.4 amp-hours

How long can I park the electric MR2?

The electric MR2 uses a 22 amp-hour (AH) deep cycle sealed lead acid battery, which should   give me ample energy to park the car for over 10 days.

22 amp-hours / 0.085 amps/24 hours/day = 10.78 days

12V deep cycle sealed lead acid battery

connected 12V battery

Secured and ready to go!

12V battery with 50amp fuse and SB-50 connector

How to measure amp draw from a 12V system

Here’s a video on how to measure amp draw for your 12V system:



An Alternative 

The 12V battery is charged when the car ignition is on.  A DC-DC converter converts the 144V drive train battery voltage to 12V to charge the auxiliary battery.  An alternative to this is to get rid of the 12V battery and to connect the DC-DC converter directly to the 12V system.  If the electric MR2 were setup to do this it would could be parked for theoretically over a thousand days.

( (144V/12V)  * 180 amp-hour )/0.085 amps  = 25,412 hours

25,412 hours / 24 hours/day = 1058 days.


I don’t, however, recommend doing this because if you ever leave your lights on, you could deplete you main pack battery much quicker.

For those of you driving electric cars, how many days can you park your electric car without the 12V battery draining?  Anyone with a Nissan Leaf?  Tesla model S/Roadster?  What’s the longest you’ve parked your car?

For those with your own conversions,  anyone not using a 12V battery?  For those with battery management systems, I suspect your parked amp draw will be higher, have you guys measured it?  Please share below, thanks!



Posted in 12V Battery, Instrumentation, Parking

10 Responses to “How to Size the 12V Battery for an Electric Car?”

  • Honn Chhim says:


    The amp draw of .085A or 85mA is very small. Does this include your EV Display, radio preset memory and clock memory?

    My amp draw of the 12V may be more. I have a EV Display, radio preset, clock and alarm. The alarm by itself probably draws more than 85mA already.

    I’m going to measure the amp draw of my 12V and get back.


  • Honn Chhim says:

    My thought of the amp draw on the alarm system was wrong. I just measured the amp draw on my 12V. It measured as low as .09 or 90mA. The amp draw remains the same whether the alarm is armed or disarmed. It’s good to know that with my alarm is armed, my 12V only drains additional 5mA compared to your 85mA draw.


    • Ken says:


      Thanks for the numbers, that makes me feel a bit more confident that my amp draw is inline with yours. What DC-DC converter do you have? Do you have a diode installed to prevent back flow of current from the battery to the DC-DC converter? I’ve heard that my AstroDyne SD-350 may draw from the battery when it is off. I need to check into that.


  • Honn says:

    I have a Kelly dc-dc converter. I don’t have a diode installed to block the current flow from the 12V battery. Anyway I measured the amp draw on my 12V again by disconnecting the dc-dc converter output from the 12V battery. It measured at 80mA, so my dc-dc drains 10mA from my 12V system. This tells me that my dc-dc has an internal resistor connecting between the output positive (+13.5V) and output ground. The purpose of this resistor is to slowly discharge the output capacitors inside the dc-dc converter when the dc-dc converter is turned off. Since the dc-dc output is always connected with the 12V battery, the resistor instead of slowly discharging the capacitors, it’s slowly discharging the 12V battery. According to Ohms law, the calculation of the resistor value is R = 13.5V/10mA = 1350 or 1.35K ohms.


    • Ken says:

      Do you have an ignition relay between the DC-DC converter and the battery? I still have to measure the amp draw with the DC-DC converter disconnected, but I think the relay should prevent the DC-DC converter from drawing amps from my 12V battery. Here’s a schematic of my DC-DC converter to the 12V battery.

  • Honn says:

    No, I don’t have an ignition relay between the dc-dc converter output and the 12V battery as your set up shown in the schematic diagram. That’s right the relay prevents the dc-dc converter from drawing amps from your 12V battery. I thought about installing a relay between the dc-dc converter output and the 12V battery too. However, I never had a chance to do it. Since I know that my dc-dc converter only draws 10mA, so it’s not much to worry about for now.


  • ken says:

    Thanks to all those that responded with your suggestions (Here’s a thread of discussion that I posted on diyelectric car and the many useful response I got on the post). A few things that I modified based on the feedback:

      1. Adjust the DC to DC converter output voltage to adequately float charge the 12v battery. For my battery, this voltage range is 13.5 to 13.8 volts (specsheet). I set it to 13.75volts.
      This only partially charges the battery. On occasion( I.e. once a month, I will fully charge the 12v battery using my hobby charger. This should extend the life of the 12 volt battery.
  • Honn says:

    I think 13.75V should be good enough for float charge. Once a while just top charge your 12V battery with a hobby charger. That’s what I do to maintain my 12V battery as my Kelly dc-dc converter puts out a fixed 13.5V, slightly low to charge a 12V sealed lead acid battery.


  • Joey says:

    Ken, I followed the link from the comment you left on my blog over to this site. Very nice job on your project and your blog. I will be spending some time reading up on your project. About your question above, I would always recommend having a battery even if it is small with only a couple of Ah. I would have one even if the DC-DC could handle peak loads on its own. The reason is that if there is a DC-DC failure, a battery will slowly drain down giving you warning and enough time that you can safely get off the road. Without the battery, you would suddenly be without headlights and motive power because the 12 volts is used to hold the main contactor closed.

    • Ken says:


      Excellent point about the potential DC-DC failure being a safety concern! I would say 12V battery is a must also. My Astro Dyne SD350 DC-DC has been pretty reliable though I have to say, almost 2 years and not an issue.

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