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Battery Bank for Mobile Medical Business

Discussion in 'Energy, Environment, and Policy' started by Timothy Meredith, Apr 8, 2016.

  1. Timothy Meredith

    Joined:
    Sep 13, 2015
    Messages:
    40
    Location:
    Murfreesboro, TN
    The company I work at operates ~20 30' mobile dental units that treat patients at long term care facilities, we build our units from an empty shell into very nice trailers.

    We operate in ~10-12 states in various climates and all of our units are powered by noisy unreliable 13kw generators in a truckbed. We hate them, residents hate them, and everyone hates them.

    Having a tesla, I had the idea when the powerwall first came out to use that in our mobile units. After several emails / voicemails to Tesla went unreturned I finally got someone to call me back and they basically all but said they wouldn't even sell us one if they knew it was going in a mobile unit. (That killed that route, sadly)

    In the spirit of not giving up, I decided it may work out to DIY it and build our own. They don't like the idea of solar panels due to wear and tear (ever driven on AR roads?), holes in the roof, theft, etc ? Even with Solar panels we'd have to cover the whole roof to come close to making a dent, I fear.

    That being said, I had the idea of potentially upgrading to ~3 aftermarket high end alternators that can output 200amps at idle each. Throwing a dart at the wall I think we use ~4.5kwh depending on the time of year for ~8h, everything in the unit is 110v and nothing 220v. The other benefit here is it could be charging the batteries in transit to facilities which can be ~1-2 hours away and on the way back. I envision once I talk them into it down the road potentially having a mixture of solar panels + alternators.

    Long story short - I'm having trouble with the following:
    A. Calculating the size of the battery bank necessary. Based on my math we'd consume ~41a at 110v per hour, needing 102ah per hour at 48v or 204ah at 24v. I know with other battery types its advisable for the lifespan to not go below ~40-50%, how much should I oversize it with lithium? Our goal over time is to try and make them more energy efficient but there's not a whole lot of areas to cut as it is.
    B. Deciding between Lithium or other battery types. I like lithium due to size/weight savings - but we plenty of curb weight headroom to easily add 1ton or more if we had to to our units.
    C. Deciding where to purchase them and who might provide us with decent volume discount - we will eventually convert all 20 units plus the additional we build (~5-6 a year currently) to all battery banks.
    D. Deciding which inverter, charger, and batter meter/monitor we should get - we care more about reliability than price - again 4.5kw was an average but we could easily have spikes up to 8-10kw.

    If anyone has any insight I'd love all the advice we can get, I hope I posted in the right area.
     
  2. gimmi80

    gimmi80 Member

    Joined:
    Mar 30, 2015
    Messages:
    162
    Location:
    Pittsford, NY
    I think that would be a great idea!
    However isn't the Tesla wall 7kWh?
    Given you number I think you might need something five to ten time that size...
    Plus in the evening when all the trailers come back home you would have to charge them up (and pretty fast because you only have 8 hours) basically you need a small power plant...
    what do you think?
     
  3. Timothy Meredith

    Joined:
    Sep 13, 2015
    Messages:
    40
    Location:
    Murfreesboro, TN
    Ya the powerwall is 7kwh, would need at minimum 4-5 of them - however that route went out the window and I want to DIY it like I said above. (Tesla won't sell us the powerwalls because they said they're not certified for mobile/portable usage).

    The good thing is most of the trailers do not come back to our HQ, they go back to storage lots with 220v access.

    That's why I love the idea of having 3-4 aftermarket alternators to charge them in transit and throughout the day as needed. (Eventually paired with solar panels)

    In the long run I'd love to cut the power need down - I can cut corners in some spots squeezing a few hundred watts here and there - more efficient air compressor with bigger tank, etc. One big item would be to get rid of the 2 RV air conditioners and get an actual home/commercial air conditioner that would be a lot more efficient. However for now I'd plan around what we have for the first unit and work down with the next few units.

    I figured after seeing several highly detailed battery posts some fellow tesla folk would have some valuable input on a DIY battery bank.
     
  4. freds

    freds Member

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    Location:
    Bothell, WA
  5. SageBrush

    SageBrush Active Member

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    Location:
    Colorado
    I was just thinking about how much energy a unit uses in a day: the generator maxes out at 13 kW, so your upper bound is that power times however many hours they are run in a day.

    The Tesla hesitancy to sell you product is harder to parse. I don't know if it is a CYA thing or they are giving good technical advice.
     
  6. Timothy Meredith

    Joined:
    Sep 13, 2015
    Messages:
    40
    Location:
    Murfreesboro, TN
    When I spoke with them it was an act of congress to get them to return a call and they were nice about it, mentioned they had a lot of interest for mobile applications but basically said that they didn't engineer it to be mobile and didn't like the idea of it being on a truck or trailer.

    The units currently use 13kw generators but several of us here aren't sure how our unit production crew came up with that number other than for obscene peaks or to give a lot of headroom.

    While they use more power (a lot more) when first starting up, 2 of our RV air conditioners consume an avg ~1500 each, 3kw right there.

    I'd say between low-energy lights, air compressor running most of the day, tv, 2 computers, etc we could add an easy 1kw. We also have a autoclave (sanitizer) that runs 2 or 3 times a day for 20 minutes each time but uses a ton of energy ~1800w.

    Realistically I think the 4.5kw per hour is fairly accurate but it could be a lot less during fall or spring when air conditioning (or heat) isn't needed.
     

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