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Battery Questions?

PostPosted: Thu Oct 15, 2009 7:22 am
by mobile
Hi Guys

How do I calculate how many batteries I need to run for example a geyser?

When inverting 12-24VDC at 10 Amps to 230VAC is their an amps loss?

How do I calculate what voltage, and amps do I need to charge one battery?

Can you run a geyser off batteries with a standard inverter or do you require a true sinewave inverter?

If a geyser says 2.5kw is this how much it requires per hour or is this how much it pulls immediately on turn on

The reason I want to know is that I know that a geyser thermostat only switches on when the water drops by 2 to 3 degrees. I would like to take my geyser off the grid.

Re: Battery Questions?

PostPosted: Thu Oct 15, 2009 5:36 pm
by windgat
Wow thats a lot of questions. Let me answer some of them:

Inveters have an efficiency, which varies depending on the power being drawn. So a 80% efficient inverter will lose 1/5th of the power. Ask for specifications before buying.

Geysers are heating elements, so they don't care if its square or sine wave. Motors and some other equipment do not like square waves.

2.5kW means instantaneous power. So if its on for 5mins it will use 0.2kWh. Bear in mind 2.5kW will draw over 100 amps from a 24V system. That will mean a LOT of batteries (or fewer BIG batteries), and fat cable.

To take your geyser off the grid, by far the best route is to go for a solar heated geyser.

Re: Battery Questions?

PostPosted: Thu Oct 15, 2009 6:23 pm
by mobile
Hi Windgat

I have so many questions but do not know where to start. Thanks for the reply. Let me tell you what I am attempting to do.

I want to run my geyser from batteries via an inverter and charge the batteries using a turbine. My geyser is 3KW geyser. My turbine will run a DC motor high voltage low RPM/s. The idea is to charge the batteries constantly with wind turbine and if the batteries go under a certain voltage due to lack of wind it will go over to eskom power.

No water really gets used during the day. I have a blanket on my geyser and it is set to 55 ?c. I am not sure how many times my geyser comes on.

I have calculated that 6 x 12VDC should do the job. I want to create 2 battery banks of 3 batteries in parallel then connect the 2 battery banks in series. From what I understand this should create a 24VDC 15 AH which gives me 3600 Watts.

Do you think this is a worthwhile project or should I scrap it and get a solar geyser?

Re: Battery Questions?

PostPosted: Thu Oct 15, 2009 9:31 pm
by windgat
I think scrap it and go solar.

Sun makes heat makes wind makes turbine turn makes electricty makes charged battery makes DC to inverter makes AC makes heat for hot water.
Look at that carefully... and then look for a shorter way to make hot water!

Make a wind turbine for sure, and use it for lights, fridge etc.

Re: Battery Questions?

PostPosted: Fri Oct 16, 2009 4:11 pm
by mobile
Hi windgat

I dont see a worthwile purpose of running it on lights and fridge as my lights in my house are all 7-11watt energy savers. As to the fridge this leads to another problem of running cables to it which would look quite messy. Connecting it via the db board by disconnecting the lights from the grid and feeding it from the batteries also does not help as the builder of my house wired lights with plugs. The next issue is that of my fish tank which runs 5 pumps, which will not work well if the sine wave is not of a good quality the pumps will not work

Is it possible then to use an AC motor which produces a true sine wave and enough power to connect it directly to the geyser element skipping the whole battery problem and inverter problem. What I can do is then build a switching circuit so if therre is not enough power from the turbine it will go over to ESKOM.

Re: Battery Questions?

PostPosted: Fri Oct 16, 2009 5:25 pm
by windgat
Perhaps... but small turbines tend to be variable speed (turn faster in higher winds), so you need to think how your loads will respond to variable Hz AC. Of course a heater element doesn't care about Hz, so that would work. But you would need quite a big rotor, or high wind (or both!) to get anywhere near 3kW.

A point of running lights might be if Eskom power is unavailable. Either because their infrastructure starts to crumble, or if they cut you off because you cant pay your R3000 a month bill!

Did you get what I meant with the "Sun makes heat..." sentence?

Re: Battery Questions?

PostPosted: Sat Oct 17, 2009 9:42 am
by mobile

Yes I understand what you meant about. I was thinking about a steam generator. So my costs would only be burning wood as fuel or something else. Heating water and generating electricity from a steam turbine. I know this would be labour intensive to build and to keep the fire going ata constant temperature to keep the turbine going.

What are your thoughts on this matter?

Re: Battery Questions?

PostPosted: Sat Oct 17, 2009 10:12 am
by windgat
I think that could work. Depending on the size, you could get a large amount of power from a steam engine, which means you may only have to run it for a short time each day to fill the batteries.

An alternative is a very small steam engine, with a fire you keep going constantly. But that is a lot of work - it might cut into your sleeping time!

We have got so used to power at the flick of a switch, that I think one needs to plan something that is relatively automated, otherwise you will just stop using it once the novelty wears off.

Re: Battery Questions?

PostPosted: Sat Oct 17, 2009 10:31 am
by mobile
Their is one good thing that will come out of this whole power problem is that it makes people become very innovative. Like all the inventions that were created in world war 2. The microwave for example.