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12 V 4 Amp hour
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Bernie66



Joined: 14 Jan 2005
Posts: 13967
Location: Eastoft
PostPosted: Sat Apr 30, 11 9:10 pm    Post subject: 12 V 4 Amp hour Reply with quote
    

Car battery, in good condition, fully charged.

How can I calculate a ratio of watts per hour times length of time before failure.


For example if I run it through an inverter and then run a 60watt load how long before the battery dies?

I am sure I will lose capacity by putting it through an inverter but don't know how much and what the calculation would be.

Any ideas?

arvo



Joined: 04 Dec 2006
Posts: 3321
Location: Somerset
PostPosted: Sat Apr 30, 11 9:14 pm    Post subject: Reply with quote
    

You will lose something, but I'm pretty sure the manufacturer wouldn't be that forthcoming about losses.

What voltage is your inverter inverting up to? Could be anything from 220 to 240 and that will make a little bit of difference to the sums.

And whats the Amp/Hours rating of your battery?

Bernie66



Joined: 14 Jan 2005
Posts: 13967
Location: Eastoft
PostPosted: Sat Apr 30, 11 9:27 pm    Post subject: Reply with quote
    

4 AH, and the inverter is up to 80w 240v
I'm hypothesising at the moment about heat trace at 10W per metre to prevent a water tank freezing.
I may be way off but can't work out the figures right now.

Treacodactyl
Downsizer Moderator


Joined: 28 Oct 2004
Posts: 25705
Location: Jumping on the bandwagon of opportunism
PostPosted: Sat Apr 30, 11 9:42 pm    Post subject: Reply with quote
    

4AH, is that correct? My small spare car battery is about 40 - 50 Ah.

Bernie66



Joined: 14 Jan 2005
Posts: 13967
Location: Eastoft
PostPosted: Sat Apr 30, 11 9:42 pm    Post subject: Reply with quote
    

probably not then hang on..........

Bernie66



Joined: 14 Jan 2005
Posts: 13967
Location: Eastoft
PostPosted: Sat Apr 30, 11 9:46 pm    Post subject: Reply with quote
    

Startup Power: 740 Amps
Type: Lead Acid
AH Value: 80
Bench Charge: 8.0 Amps

Reserve capacity: 140

RichardW



Joined: 24 Aug 2006
Posts: 8442
Location: Llyn Peninsular North Wales
PostPosted: Sat Apr 30, 11 9:46 pm    Post subject: Reply with quote
    

Its quite easy as long as the load is reasonably sized in relation to the battery size. If the loads are big then you need to allow for the Peukerts effect.

Nearly all manufactures of inverters give a power efficiency curve. However as most are in the 90+% range you can make it very simple on a 12v system with a bit of wiggle room by using 10 instead of 12 as the voltage.

so a 110ah 12v battery discharged to 50% (so you dont kill it) will give you 550wh. Or 0.5kWh so about 6p worth.


(thats 110ah x 10v div 2) If using a car bat you would be better off only discharging to 80% SOC or 20% DOD
(SOC state of charge, DOD depth of discharge)

The battery actually contains more like 1354wh as its average working voltage would be 12.4v but again it gives you more space for errors.

Just spotted your other post.

So,

I doubt that you can use a 4ah battery to run 10w per meter cable unless you are looking at a very short run.

4ah x 10 = 40wh, Discharging to 50% will give you a run time of 15 mins for 1 meter BUT that is a large load so allowing for peukert you would be down to about 10 mins or less.


The total energy stored in the battery at its real voltage is less than 50wh

RichardW



Joined: 24 Aug 2006
Posts: 8442
Location: Llyn Peninsular North Wales
PostPosted: Sat Apr 30, 11 9:49 pm    Post subject: Reply with quote
    

Just spotted your next post so here we go again,

80ah x 10 / 2 = 400wh

So you can run 1m for 40 hours or 40m for 1 hour

(actually less again due to the load size compared to the battery size)

Bernie66



Joined: 14 Jan 2005
Posts: 13967
Location: Eastoft
PostPosted: Sat Apr 30, 11 9:55 pm    Post subject: Reply with quote
    

Thanks, shame though

Bernie66



Joined: 14 Jan 2005
Posts: 13967
Location: Eastoft
PostPosted: Sat Apr 30, 11 10:01 pm    Post subject: Reply with quote
    

RichardW wrote:
Just spotted your next post so here we go again,

80ah x 10 / 2 = 400wh

So you can run 1m for 40 hours or 40m for 1 hour

(actually less again due to the load size compared to the battery size)


or 3 metres for 10 hours.

So would 3 metres have a chance of heating the tank enough to stop it freezing?
depends on size i guess...........

Is it a non starter though?

RichardW



Joined: 24 Aug 2006
Posts: 8442
Location: Llyn Peninsular North Wales
PostPosted: Sat Apr 30, 11 10:03 pm    Post subject: Reply with quote
    

We are just about to buy & install some more heat trace cable. It will bring our total up to 19m or 190watts. If that was left on for a full 24 hour day it would be 4.56kWh. That's more than we use per day for running every thing. In winter we average about 3.5kWh per day but less in summer so the yearly average is about 3kWh per day.

Now obviously we cant use that much extra so we will be trialling just running it for a few mins at a time at say 10% duty cycle or poss even just running it for a few hours before needing the borehole pump running to top up the storage tank. We would still need to have the section from the tank to the building on more often. We will have to see how things pan out. As its just needed for 2-4 weeks per winter we might just have to bite the bullet for the convenience of running water.

RichardW



Joined: 24 Aug 2006
Posts: 8442
Location: Llyn Peninsular North Wales
PostPosted: Sat Apr 30, 11 10:06 pm    Post subject: Reply with quote
    

Bernie66 wrote:

or 3 metres for 10 hours.

So would 3 metres have a chance of heating the tank enough to stop it freezing?
depends on size i guess...........

Is it a non starter though?


Is it a non starter with your battery? I think so yes.

In fact even with my 1550ah one its iffy.

Volume of tank?

Level of insulation?

Exposure (delta T IE difference between temp of air & temp of water) level?


If its a tank a fish tank heater might be more suited & cheaper, but use more power but be more effective due to the built in thermostat.

Bernie66



Joined: 14 Jan 2005
Posts: 13967
Location: Eastoft
PostPosted: Sat Apr 30, 11 10:09 pm    Post subject: Reply with quote
    

but off battery not a chance?

RichardW



Joined: 24 Aug 2006
Posts: 8442
Location: Llyn Peninsular North Wales
PostPosted: Sat Apr 30, 11 10:21 pm    Post subject: Reply with quote
    

Bernie66 wrote:
but off battery not a chance?


We run off battery but we also have a PV array & generator back up.

Off your 80ah battery not for long.

A typical hot water tank can loose 1 or 2 kwh per 24 hour period. You would loose less as your Delta T would be lots lower. Your battery could not replace that loss without being on charge in which case you might as well just plug the heater in the mains.

Would you really want to have 3 batteries (one in use, one ready to use & one on charge) & have to swap them over every day?

Again without more details about exactly what you are doing I cant say for sure but I would 99% say you dont have a hope of doing it with that 80ah battery even if it was brand new & I am guessing its one thats lying around & would most likely be stuffed any way.

Bernie66



Joined: 14 Jan 2005
Posts: 13967
Location: Eastoft
PostPosted: Sat Apr 30, 11 10:25 pm    Post subject: Reply with quote
    

Trying to stop a water trough freezing solid in the middle of a field.
just thinking aloud

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