Poor Man's Solar Battery: Power a TV from a Car Battery Charged during The Day

[Deactivated] on 03/10/2017 - 11:49
Last edited 03/10/2017 - 12:33 by 1 other user

Hey guys, any tinkerers wish to assess my logic and calculations here?

Right now I pay roughly 30c/unit for power and gain 7c/unit as feed in tariff for unused generation.

A simple logic gate and relay (IF solar power > utilisation THEN enable) could turn on a car battery charger, which could then power a TV (or whatever) using a cheap inverter at night. None of this is particularly complex or expensive, the sanity revolves around round trip efficiency - considering cheap low efficency equipment and extra losses from being inverted twice.

If I assume 80% efficiency at each step it seems like I'm getting 64% efficency

Solar inverter (excluded as same in all scenarios)
Car battery charger 80%
Car battery inverter 80%

If my assumptions and calculations are correct I reduce my bill by 19c per unit generated compared to only getting 7c feed in tariff.

If this is accurate we can move onto the next steps - whether the outlay is worthwhile and possible ways to build a logic relay.

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  pernunz on 03/10/2017 - 12:10

  That's 19c per kilowatt hour. How many kilowatt hours would you save this method over a day/week/month? Is it worth your time to set this all up?

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    [Deactivated] on 03/10/2017 - 12:19

    None of this is particularly complex or expensive

    If this is accurate we can move onto the next steps - whether the outlay is worthwhile

    It would take about 5 minutes for basic setup and it would be highly scaleable, this is a proof of concept so I'm okay to go the bigger calcs later. The time is in building the logic gizmo. To answer the question a 300w TV 3 hours per day would save $62 per year and provide power redundancy.

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      Seraphin7 on 03/10/2017 - 15:11

      While I cannot comment on likely total lifespan of your solution, what I can say is that I've just had to spring $188 for a new car battery. In an actual car situation, a battery has a lifespan of 3 to 4 years. As I say, don't know how that would translate to your case, but you are talking a cost of $47 - $62 per year in battery wear based on this analysis.

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  fantombloo on 03/10/2017 - 12:49

  +4

  Plus the cost of battery wear.

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  Ocker on 03/10/2017 - 15:37

  Reverse sex discrimination

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  Cluster on 03/10/2017 - 16:24

  +1

  Car batteries are worn out quite quickly if you deep discharge them. They're designed for providing a large amount of current in a short span of time, not for UPS/backup work. You need a deep cycle battery for your purposes.

  Origin's Solar Boost plan offers 18c/kWh FIT in SA. Other providers in other areas may have similar offers. There's no need to take the default 6.8c offer anymore.

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  pjetson on 03/10/2017 - 16:29

  Car batteries are designed to supply a huge amount of current for a short time (ie to start your engine), then have that charge immediately returned via the car's alternator. That's completely different to the way that you want to use the battery, where you will charge it as and when you can and then probably drain the battery significantly while watching TV.

  What you want is a deep cycle battery, not a car battery.

  Drats - Cluster beat me to it!

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  jmon on 03/10/2017 - 16:55

  If you're looking for a way to use excess power, use the logic to turn on your water heater. Cheap and efficient(ish) if you already have an electric system

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  tonyjzx on 03/10/2017 - 17:33

  you probably need to look into 12v inverter systems from the camper caravan set

  you then end up buying several hundred in deep cycle batteries, the solar panels and the charger and 240v circuitry

  this makes sense if you camp like a grey nomad etc but for general use??? hmmmm…

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  Bigboyburrito on 03/10/2017 - 17:49

  I know nothing about anything, but if we're talking batteries, I hear Laptop batteries are a great option for DIY power blocks that, when executed well, can rival the storage capacity of Tesla's power wall.

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  mskeggs on 03/10/2017 - 18:32

  +1

  How about start with a better power company?
  Powershop or Energylocals will charge around 25c per kWh with 12c/kWh feed in tariff.

  FWIW, the Tesla power walls pay off in approx 10years, so that is the benchmark to beat.
  Assuming you just want to power the TV, you need a minimum 80Ah battery, looks like about $200 is a good price. You will really want to get quite a bit bigger though, as if you are discharging completely every day you will only get 100 cycles or so. If you got a battery twice as large, and only discharged to 50%, you could expect 1500 cycles or more, say 5 years of use.
  I wouldn't fuss around with building logic relays, just plug the charger into the wall during the day, or use a cheap timer to charge when the sun is up.
  So, assuming your time is worth nothing, and you have a battery charger, inverter and timer already, you need to save the cost of the battery within 5 years.
  Ebay has a 160Ah battery for $298, so it would be right on the line whether this is worth doing (based on your $62/year calculation).
  If you change to a more competitive power company, you will lose money doing this.

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  tonyjzx on 03/10/2017 - 22:31

  protip: you can buy native 12v TVs that people use in RV/campers

  granted they tend to top out at 24" but you can run that straight off a 12v

  if you're that much of a cheapskate, you can slum it on a 24" w/ headphones

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  Elppa8 on 04/10/2017 - 11:19

  Took me a while but got the same figures in the end. I think that you would want to have the logic to come on when the solar was well above the charger load to avoid on/off oscillation at threshold levels.

  You may be better off with separate small solar panel and controller to charge the battery. That way you save the 30 cents consumption and still keep the 11cents solar rebate that wasn’t used to power the battery charger

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    [Deactivated] on 04/10/2017 - 12:08

    Is the 80% efficiency reasonable? I'm now thinking it might be a little optimistic for junk off ebay.