Duration for light from Battery

theuns · January 2012

HI
I am a newbie to all this and is still trying to get my head around all this, so here is a couple of questions.
1. As i understand it deep cycle battery capacity is given in amp-hours over a twenty hour period.
eg. 105 ah battery provides 105/20= 5.25amps per hour for 20 hours, correct?

2. If you have a 20Watt, 230Volt, CFL light and a 105ah, 12V deep cycle battery fully charged connected to a 1500W inverter, how long will the light stay on?

a) The battery can deliver 105amp x 12V = 1026 watts, therefor the light can stay on 1026/20 = 63 hours, is this correct?
b) or, the light needs 20/230V = 0.08 amps , therefor the light can stay on 105/0.08 = 1312.5 hours, is this correct?
c) or .... the correct way to work out the amount of stay on time for the light

Which one is correct? option b) does not feel right.

I would appreciate any feedback, thank you in advance

rplarry · January 2012

Re: Duration for light from Battery

The only thing I (others will see something else) see that you forgot was to figure in what the inverter draws and add that to your amphour useage. Most inverters are 80 to 90% efficient. Also the way you are figuring the amphour useage on the battery will take your battery down to 100% DoD. Normally you wouldn't to go any lower than 50%.
Larry

Cariboocoot · January 2012

Re: Duration for light from Battery

You will never get a 100% accurate figure from calculations. You can only come close. This is because there are so many factors which will affect the power available from the battery (like temperature) and the power used by the load.

rplarry is on the right track: the first thing you have to do is reduce the available Amp hours by 50%, unless you don't want to recharge it. So 105 Amp hours becomes 52.5. Then there is the Voltage. A "12 Volt" battery starts out at, say, 12.75 Volts (depending on the battery) and will go down to 12 (inverters frequently have 10.5 Volt low Voltage shutdown, but you want to stay above 50% DOD).

Now you have to recalculate the load to include the inverter's consumption and its conversion efficiency. At 90% efficiency the 20 Watts becomes 22 Watts, but the inverter efficiency will not hold steady at 90% through the whole Voltage range of the battery. Add in the inverter's draw, which could be another 20 Watts. Your load is now 42 Watts. At a nominal 12 Volts that's 3.5 Amps. But of course the Voltage isn't consistent, and at 12.75 it will draw 3.29 Amps. Theoretically.

Now we go back to the battery and the Peukart Effect. In short form, a battery has a different Amp hour rating depending on the rate of current drawn. This translates into different Amp hour ratings at different "hour rates". Look at this chart for Sun-Xtenders: http://www.solar-electric.com/cosuagmba.html The 20 hour rate is based on 5 Amp draw, so at 3.5 Amps the "105 Amp hour" battery will have slightly more capacity due to the lower discharge rate.

As mentioned, the temperature will affect battery performance, and there inevitably will be losses in the wiring and connections. How much so depends on the wire length, size, and again temperature.

Now aren't you sorry you asked?

For practical purposes, you use the 1/2 Amp hour rating (52.5) and the nominal Voltage against the total Watts including inverter draw. So 52.5 * 12 = 630 Watt hours / 42 Watts = 15 hours of running the inverter and light. You could get more, you could get less.

Pretty horrible isn't it?

theuns · January 2012

Re: Duration for light from Battery

Cariboocoot wrote: »

Now aren't you sorry you asked?

Not at all, thanks allot for the detail explanation. Its much appreciated. Most people give short answers, where a "magnitude" of information is not in the answer, and it is presumed you know all the missing bits.
About the horrible part, yes, lots of stuff to consider, but i think it comes with experience and understanding, of all the technical issues.

Thanks again.

theuns · January 2012

Re: Duration for light from Battery

Few more questions
You mentioned that you should not discharge the battery more than 50% DOD, and that most inverters cut out at low voltage 10.5 volts.

1. Now, at 10.5 volts, is the battery then at 100% DOD or at 50% DOD ?
2. If the battery will be recharged daily with solar panels, can you go more than 50% DOD, and what percentage of DOD is advisable when its recharge daily?

Looking forward to the reply, thanks

icarus · January 2012

Re: Duration for light from Battery

Let's make this easy.

For starters,, read and. Understnd the following:
http://www.batteryfaq.org/

http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries

This will give you a pretty good. Understanding of how batteries work.

Tony

LandKurt · January 2012

Re: Duration for light from Battery

Yes, do read the battery FAQ. But the simple answer is that a battery at 10.5 V resting is at or close to 100% DOD. So don't count on the inverter low voltage cutout to protect your battery lifetime. It should keep you from completely trashing your battery as a battery at 10.5 volts shouldn't be unrecoverable if recharged fairly quickly. It was just used hard and probably has its lifetime shortened a bit. As I understand it anyway. Likely it would shut down before 100% DOD if there was much load on the inverter. The battery voltage would recover a bit once the inverter shutdown.

Am I correct that it is pretty difficult to set up a system that automatically enforces a 50% discharge rule? I think you can get voltage controlled cutoff switches but judging SOC from battery voltage under load is notoriously unreliable. According to a SOC chart 50% DOD would be about 12.06V while resting. But you couldn't rely on that since it would vary a good deal under load. I imagine the only decent method would be a computer system reading a battery monitor like a TriMetric and shutting down the load when it indicated the battery had reached 50% DOD. Do some of the more expensive systems with network communication between units have that capability?

BB. · January 2012

Re: Duration for light from Battery

Ideally, you probably would set a voltage controlled switch to ~11.5 volts as the minimum battery voltage cutoff. Unfortunately, the battery output voltage is current and temperature sensitive, so to pick "a voltage" is not really practical.

Also, even if you pick 11.5 volts as your battery cutoff voltage, you still have voltage drop of the wiring, fuse/breaker/, and DC disconnect to worry about. That 1 volt maximum drop is actually a bit on the difficult side to achieve--You need short runs of heavy wire to meet that requirement.

For example, a 1,200 watt 12 VDC inverter supplying 2,400 watts of surge power (starting a refrigerator motor as an example) would take around:

2,400 watts * 1/0.85 inverter eff * 1/10.5 volt cutoff = 267 amps
V=I*R; R=V/I
R= 1 volt wiring drop / 267 amps = 0.0037 ohms maximum circuit resistance

-Bill

niel · January 2012

Re: Duration for light from Battery

landkurt,
please be aware that it is more than just a case of being there to quickly charge up a battery that went to 100% dod for cells can reverse polarity too. we cite the 50% dod point for better cycle life, but 80% dod to help insure against a reversal of a cell.

Cariboocoot · January 2012

Re: Duration for light from Battery

theuns wrote: »

Few more questions
You mentioned that you should not discharge the battery more than 50% DOD, and that most inverters cut out at low voltage 10.5 volts.

1. Now, at 10.5 volts, is the battery then at 100% DOD or at 50% DOD ?
2. If the battery will be recharged daily with solar panels, can you go more than 50% DOD, and what percentage of DOD is advisable when its recharge daily?

Looking forward to the reply, thanks

The 10.5 Volt disconnect on inverters is pretty much the point of no return. A battery may recover from it, but don't do it too often. In terms of 50% DOD vs. Voltage, the battery nominal is pretty much it (in this case 12 Volts). The discharge limit is with daily recharging, as when you go below it the matter of "will it recharge?" arises. Forklift batteries, incidentally, are designed to go to 80% discharge and recover. But that's another scenario.

If you replace that 1500 Watt inverter that probably uses 20 Watts for itself with a little Morningstar 300 you will gain time for running the light. Not only is it capable of supply the 20 Watts & 230 Volts (get the right model, not the standard 115 Volt unit) but it only draws 6 Watts running. That means your total load goes down to about 28 Watts from 42. Your light would run for around 22 hours, not 15.

theuns · January 2012

Re: Duration for light from Battery

Thank you, everyone for the replies.
I have read the suggested articles, and understand a little more about batteries, I hope

Try not to go more than 50% DOD, on occasion its ok.
Try to never go 100% DOD (10.5volts).
So design the system for 50% DOD from the start.

Thanks Again

Cariboocoot · January 2012

Re: Duration for light from Battery

Depending on how critical the application is, you might want to design for 25% DOD: the sun does not always shine brightly every day.

techntrek · January 2012

Re: Duration for light from Battery

And if you design to 50% DOD, after just one year you'll be going over 50% because of loss of battery capacity. The next year you'll be even more over and eventually your battery will die faster than you expect.

icarus · January 2012

Re: Duration for light from Battery

Personally I design for 25% max daily discharge,, and closer to 20% if I can. Read the battery FAQs and look at the relationship between Dod and cycle life.

Tony

Duration for light from Battery

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