12v Inverter Battery Query

[NewcastleSAR]

Hi all..

Rather than running electrics out to my shed for modelling use, I'd like to use a 12V Car battery and a 12V invertor to power small modelling tools. I also intent to use a 12V Solar panel to charge the battery but are inverters hungry monsters in relation to what they require from a 12V Battery.

If so, what calculations can I use to determine the ampage of the battery I would require to power the invertor satisfactory to give me sufficent time in the shed.

I've used Uninterruptible Power Supplies at work for computer use but they are more for supplying a specific amount of power for a short time to allow the computer software to shutdown the computer servers until normal line supply is established. In saying that, the 2 Batteries in the 700W UPS I have are only 7Ah which gives me up to 20 minutes (from what I remember).

Also, can the solar panel be kept connected to the battery whilst using the invertor or should it be disconnected?

Regards

Declan Barry

 

[Russlk]

How practical this is depends on how much power you plan to use. The inverter is 80% to 90% efficient. Assuming that you use 300 watts for an hour, the current from the battery is Ib = 300/12 + 300/12/.8 = 30 amps (I used 80% efficiency). Since the use is 30 A-H and you don't want a dead battery at the end of the hour, use a 60 A-H battery. The solar charger will have to supply 6 amps for 5 hours to recharge the battery, or 1 amp for 30 hours. Actually, since the charging process is not 100% efficient, it will take longer.

 

[HiTech]

If you are going to require any real amount of power, you're going to need true deep cycle batteries such as two 6v golf cart batteries in series or a 12v RV Group 27 or Group 31 deep cycle. The downside for lasting power from these batteries is their recharge times. You won't be recharging them effectively with a solar panel unless it's from NASA! Regardless, deep cycle batteries should not go below 50% charge to maximize their lifespan, and you would need a quality battery monitor of sorts. Trace Engineering and Xantrex make a combination inverter/charger, however the chargers often require AC power and these are high-end, expensive units often intented to power a home off of the power grid.

I have to ask what is the reason for not running an AC circuit to your shed? In the end it's alot cheaper, you can mount a small panel box with a circuit breaker that can also double as a master disconnect for when you leave the shed. You may be subject to local wiring codes such as burial or overhead drop details. Regardless if you use power from your home or inverter/battery, you should be cautious if there are flammable liquids/chemicals in the shed. Most sheds do not have the ventilation that homes have and fumes linger easily.

 

[philba]

I'd like to echo HiTech's point. I'd go for an AC circuit - the first time you run the batteries down before you are done you will wish you had. The amount of effort to run a wire and put in the breaker panel may not be that much more than getting all the solar panels, batteries, charger and inverter. It certainly will be cheaper.

Digging a trench sucks but you could probably hire a day laborer to do it if you dont want to - most cities have temp agencies that service the building trades. I don't know how far you'd have to go but around here they are around $15/hr for unskilled labor. You can run the wire (put it in conduit) and mount the panel as well as set up outlets. then hire an electrician the hook the panel up to your main box. probably not more than a couple of hours of his time. all that will probably cost less than a decent set of solar panels.

 

[captaincaveman]

How practical this is depends on how much power you plan to use. The inverter is 80% to 90% efficient. Assuming that you use 300 watts for an hour, the current from the battery is Ib = 300/12 + 300/12/.8 = 30 amps (I used 80% efficiency). Since the use is 30 A-H and you don't want a dead battery at the end of the hour, use a 60 A-H battery. The solar charger will have to supply 6 amps for 5 hours to recharge the battery, or 1 amp for 30 hours. Actually, since the charging process is not 100% efficient, it will take longer.

been looking into the solar thing myself in the uk a 15w panel cost around £150, the high power one go from £349 for a 50w to £599 for 100w ones and they are plug and play with charge controllers the 100watt one peak output is 6.7A@15 volts they also quote 400-500watts based on 5 hours of average daily peak sunlight hours

 

[NewcastleSAR]

Don't make me reach through this monitor to slap you a good one!

Hope you can't actually do the above but mainly my reasons were about using alternative energy. In saying that, next year when funds dictate, I'll be building a garage which will have electrics in it. I can run (and have) an extension lead out to the shed to do what I want but as electric costs are getting higher, I thought I'd look at alternative energies.

My modelling tools such as my soldering station is rated at 55W and my Combo drill is rated at 135W. I have a 12V charger for charging RC batteries which is mainly what I would be using the car battery with plus the solar panel (or Panels) to keep it topped up as I wouldn't be using the charger every day. Probably only once a week!

Declan

 

[Nigel Goodwin]

Your power requirements are quite modest, and would cost very little to be run off the mains - it's doubtful you would ever save the initial outlay of the battery and solar panels?.

 

[philba]

Hope you can't actually do the above but mainly my reasons were about using alternative energy. In saying that, next year when funds dictate, I'll be building a garage which will have electrics in it. I can run (and have) an extension lead out to the shed to do what I want but as electric costs are getting higher, I thought I'd look at alternative energies.

My modelling tools such as my soldering station is rated at 55W and my Combo drill is rated at 135W. I have a 12V charger for charging RC batteries which is mainly what I would be using the car battery with plus the solar panel (or Panels) to keep it topped up as I wouldn't be using the charger every day. Probably only once a week!

Declan

if you do a break-even analysis, you will discover a very long payback period.

 

[NewcastleSAR]

Cheers folks..

The responses have been enlightening. I suppose I was just trying to do something just for the sake of actually doing it without thinking if it was practical or justifyable in the the long run.

Many thanks to one and all.

Declan

 

[HiTech]

Before I would ever dump money into solar and inverters and deep cycle batteries, I'd get me a good inverter genset of 2kw or higher like the Honda or Yamaha. They are very quiet to the point that only a true, anal moron of a neighbor would notice it running. You'd also have the genset for use during a power outage or for camping. I have an industrial genset as well as two deep cycle batteries and an inverter. The genset is for daytime runs and the batteries keep the absolute bare essentials operating during night time. Mostly the stuff collects dust, since power doesn't remain off for that long.