If a battery has "180 amps / hr" (and the right voltage) can I assume it can deliver 1 amp for 180 hours at the same voltage? Or 1/2 amps for about 360 hours at the same voltage?
Thank you.
Sam
SamBrownADK:
If a battery has "180 amps / hr" (and the right voltage) can I assume it can deliver 1 amp for 180 hours at the same voltage? Or 1/2 amps for about 360 hours at the same voltage?Thank you.
Sam
You have your units all wrong.
Battery capacity is measured in Ah, (ampere-hours, i.e. amps x hours), not amps/hour (completely, utterly different - that's a rate of change of current).
Capacity is simply electric charge. 1 amp for 1 hour is 3600 coulumbs of charge. coulumbs (C) are rather
unwieldy units so Ah and mAh are typically used for batteries.
But yes your basic point is valid - lower currents are available for longer times. However batteries are
never ideal, and at higher currents the capacity of a battery will drop due to chemical inefficiencies.
Often capacities are quoted at the 10 hour rate. For lower currents the capacity may be somewhat higher.
But real batteries are not ideal as I said, very not ideal. Capacity of a lead-acid battery drops with
age, temperature, number of discharge cycles, and depth of discharge.
For a long life with good capacity try to never discharge a lead acid battery fully. And over-discharge wrecks the
battery and drops the available capacity by a large factor.
Or put another way if you want 10Ah capacity, buy a 20Ah battery and treat it well.
quote from the book "Programming Arduino, Next Steps, going further with sketches" by Simon Monk
page 88
"as a rule of thumb, you can calculate the number of hours that a battery will last before it is discharged by dividing the capacity in milliamp hour by the number of milliamps being drawn;
Battery life in hours = batt capacity in mAh / current in mA " end quote
this is a good book with lots of good stuff in it and pretty cheap on Amazon
rule of thumb: "a broadly accurate guide or principle, based on practice rather than theory"
A calculation like that is purely from theory. A rule of thumb might be something like "the capacity at
the 10 minute rate is about half the nominal capacity" (I made that up, as an example).
The other "complication" is, what does "discharged" mean? The voltage drops (non-linearly) as the battery is discharged, so near the end of the battery's life the voltage is low, but at the end of the it's useful life, the voltage is not zero. I Googled to find the datasheet for a [u]AA battery[/u]. It has a chart showing the different mAh at different discharge rates, and it says it's discharge is 0.8V. (That's 53% of the rated 1.5V.)
The datasheet also shows discharge curves, showing how the battery starts loosing voltage immediately, then it levels-off a bit (while still dropping) and then drops rapidly as it get's more discharged.
Depending on your application, you may consider the battery "dead" (discharged) before that, or if you have a 12V battery with a 5V regulator, you may be able to get a little extra time out of the battery. (Although you do have to consider the regulator's drop-out voltage, and a linear regulator will waste energy.)
that's just what it says in the book...
I imagine that "discharge" means its past its useful point.
MarkT:
However batteries are never ideal, and at higher currents the capacity of a battery will drop due to chemical inefficiencies.
Exactly. For example at 180A draw the battery capacity will drop to 120Ah or so. Additionally 180Ah battery sounds like a Lead Acid battery, most Lead Acid batteries can only be discharged to about half their capacity, beyond which will cause damage to the battery.
Thank you everyone.
I see batteries with the following specs:
Battery Capacity
106AH at 20hrs (1.80V/cell, 25°C)
100AH at 10hrs (1.80V/cell, 25°C)
68AH at 5hrs (1.80V/cell, 25°C)
58AH at 1hr (1.80V/cell, 25°C)
Which should I used to get a rough rule of thumb idea? My project will draw (with sensors and all) at most say 1 amp. So is it (roughly) 106 x 20 = 2120 amp Hours? or 58 x 1 = 58 amp Hours?
Thank you and I am sorry for the simple questions: I am still a learner.
Sam
The 106AH is the AH capacity. The 20Hrs figure is the time period over which the battery can theoretically deliver the 106AH capacity. In other words the battery will deliver 5.3amps for 20 hours
Battery capacity is usually specified at a 10Hr or a 20Hr discharge period (C10 or C20)
For example, your 106AH/20 battery will be derated to maybe only 90AH if discharged at the 10 hour rate so if you try discharging at 10.6 amps you'll only get around 8.4 hours of discharge. This degradation is termed the Peukert effect and relates to all types of lead-acid batteries.
Conversely if you take the 106AH/20 battery and discharge it at only 1 amp you may well get say 150AH out of the battery ie 150 hours of use.
jackrae and everyone else: thank you for the help!
jackrae: So then when i see :
Battery Capacity
106AH at 20hrs (1.80V/cell, 25°C)
100AH at 10hrs (1.80V/cell, 25°C)
68AH at 5hrs (1.80V/cell, 25°C)
58AH at 1hr (1.80V/cell, 25°C)
It is really saying "if you want all the power in one hour, I can give 58 amps for an hour. but if your willing to take less power over 20 hours, I can deliver a total of 5.3 amps per hour for a total of 106 amps over that 20 hours" and so forth. Taking that logic one step forward. "If your willing to accept 1 amp per hour, I can deliver the one amp for aprox. 150 hours for a total of 150 amps."
Am I understanding this correctly?
Thanks for bearing with me. I am trying to figure out how to set up my batteries.
Sam
Yes if all 4 figures relate to the same battery. BUT, if you discharge a lead acid battery below say 10.8 volts (1.8 x 6cells) it will never recover to full capacity. The 1-hour rating of 58AH may do damage to the battery even if you don't exceed the 10.8 volt discharge due to internal heating and plate material breakage.
SamBrownADK:
It is really saying "if you want all the power in one hour, I can give 58 amps for an hour. but if your willing to take less power over 20 hours, I can deliver a total of 5.3 amps per hour for a total of 106 amps over that 20 hours" and so forth. Taking that logic one step forward. "If your willing to accept 1 amp per hour, I can deliver the one amp for aprox. 150 hours for a total of 150 amps."
It would help if you get used to using the terms correctly. Amps (A) are instantaneous current, Amp hours (Ah) are capacity.
So it can deliver 5.3A and keep it going for 20 hours for a total of 106 Ah.
But I can see nothing in those figures that suggest 150Ah under any circumstances. At 1A you may or may not get more than 106Ah. There's no easy way to tell.
Steve
"may" was the word I used. But you will get more than 106AH if the battery meets the 106C20 specification.
This will be for a little project i am whipping up for the farm. 10 Unos (R3) each with PIRs at a remote coop. The idea is to have them run off of one big battery rather than a bunch of smaller ones. I don't think each of the 10 Unos with 4 PIR sensors will draw too much current. So 10 of them with a total of 40 PIRs off of a big deep cycle 6 or 12V (with a buck converter) battery should last a good long time. I hope 
If you need 6v then go for a 6v battery. A buck would waste around 10 to 15% of the battery capacity just running itself.
If buying large Lead Acid batteries to power your project then make sure to buy Deep-cycle batteries (VRLA type would be good) and NOT the regular car starter batteries which are used for cranking and can only discharge only a small part of their capacity.
Noobian: Yup. I will be buying the best deep cycle battery I can afford.
How far are the chicken coops apart.
Can't you wire the PIR sensors to one central Arduino (Cat-5 is cheap).
If not, then why the power-hungry Uno.
Leo..
Leo: I have a lot of Unos from a prior project lieing around. I am very comfortable and familiar with it and I like the board. The coop is far. I am not trying to re-do the project, just solve the power consumption question. This will help with future projects I am sure, where other boards, CAT5 solutions, etc may not work.