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Can I drain the 12V battery? (Or, How I plan to add my own Vampire loads!)

Discussion in 'Model S: Battery & Charging' started by SteveW25561, Sep 22, 2013.

  1. SteveW25561

    SteveW25561 Member

    Joined:
    Jun 10, 2013
    Messages:
    563
    Location:
    Vancouver, Canada
    I wanted to confirm that the 12V battery is continually being charged/maintained by the larger 85 kWh pack in my Model S (even when driving, or off and parked, not connected to any charging source). For most people, typical situations are listening to the radio/internet with the climate control on and the car not plugged in to charge. I know other threads say they did this for a long time (eg tailgating) without issue.

    But, I'm also planning to tap into a constant on 12V source and it will power the following:

    1. A dashcam -- Blackvue 400HD -- constant on 12V to allow parking mode recording enabled 24/7. I know this draws very little current, so not worried about it per se (uses around 3W of power if I read correctly).
    2. A cooler with an active peltier cooler for periodic road trips. This one made by Mobicool I got at Canadian Tire -- according to the Mobicool website these typically use a "4 A peltier cell with 12 V feed, cooling max. 15°C below ambient temperature"

    To do this, I have already used an add-a-circuit to tap into the frunk fusebox at the glovebox light fuse (5A) and already have the dashcam hooked to this for the last 3-4 weeks. Tonight, I have run an additional 12V accessory port (simply tapping into the same wire for the dashcam) to the trunk of the car for the cooler. The circuit for the dashcam and the cooler are protected by a 5A fuse. Having just installed it an hour ago, I obviously haven't used this yet, but will likely in the next couple of weeks.

    Note for the purposes of this discussion, assume I'm not terribly worried about losing driving range, but am worried about depleting the 12V battery excessively. Assume I can charge the car as needed for planned destinations.

    Currently when I need the Mobicool cooler, it's plugged into the standard 12V accessory port, but the disadvantage is when we are on a roadtrip with perishable items in the cooler, and we stop for dinner or go shopping, etc, the cooler shuts off when the car turns off/locks. I'd like it to keep running for an indefinite time (typical 1-2h, max 4h I think). While we haven't gone camping in a long time, I could forsee using the always on 12V port to keep groceries cool for a few DAYS at a time (24h/day) while camping.

    If I do this, will I come back to a non-startable Model S, or will the 12V battery be continually charged as needed by the larger drive pack, REGARDLESS of the car's current state (driving, charging, parked and not charging, etc)?

    Is there any way to check on the status of the 12V battery?
     
  2. SteveW25561

    SteveW25561 Member

    Joined:
    Jun 10, 2013
    Messages:
    563
    Location:
    Vancouver, Canada
    I found this device, meant for the Blackvue cameras, but I wonder if it could be used inline for my purposes: it shuts off the circuit if it detects the 12V battery dropping too low: Amazon.com: Blackvue Power Magic Pro Vehicle Battery Discharge Prevention for Parking Mode: Automotive

    I believe it can be set to trigger and shut down a circuit at a voltage of 11.1 and 11.4V and resume operation with voltages above 11.7-11.9V. Does anyone know if the lower voltage cutoff of 11.1 V still leaves enough juice for the normal operation of the MS?

    This might not be necessary at all assuming the main pack charges the 12V, but I'm still waiting for input...

    Edit: on second look, this device limits output rated 1A, max 2A, so it'll likely not be useful for me (cooler takes 4A)....back to the drawing board...
     
  3. captain_zap

    captain_zap Electron tamer

    Joined:
    Mar 27, 2012
    Messages:
    147
    Location:
    Redmond, WA
    Based on the 12V battery voltage logs I have seen elsewhere in this forum, the DC-DC converter allows the 12V to discharge to a predetermined threshold of voltage, then turns on for a charging cycle, then stops at a predetermined voltage. So the extra loads will cause the DC-DC to cycle on and off more frequently, creating a bit more wear and tear on the 12V battery. Remember the 12V battery is small, more like a motorcycle battery, so it can't support large loads. But as long as you keep the additional loads small relative to the vampire load (something like 200W pre 5.0 firmware), it should not increase the charge cycling that much, and you should be able to run them for days and days.
     

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