milliamps and testing battery load

[David Scott]

Hi all,

Well, I've been busy fixing up my 1974 Compact II (pics will be coming soon) and my wife, 3-month-old and I are leaving for our first camping trip tomorrow... YEAH!! My question is regarding my 12v setup. I installed a 115 amp hour Everstart Marine battery and basically want to just use it for 2 things: lights and the occasional 150W inverter. Forgetting the inverter for a second, I installed 4 8-led dome lights (http://autolumination.com/fixtures.htm) rated at 80 milliamps each. I have a switch to turn 2 blue ones on at night (night light for baby) and 2 white ones in the evening ( may switch to the 24-led lights for this as the 8-leds are a bit weak). So I just want to see if I have this:

1 amp = 1000 milliamps
1 hour with 2 lights = 160 milliamps
10 hours of lights (approximate for a day) = 1.6 amps

Since I can use up to 50% of a 115 amp hour battery (so 57 amp hours), that would mean for lights only, I could run them for 57/1.6 or 35 days at 10 hours per day... right?

Another question: I have a voltmeter but I think that readings are different for open circuit vs. under load. If I leave my battery hooked up, but nothing turned on, will my readings be considered open circuit or under load? I ask because according to the following website, the battery at 50% should read 12.06 under load and 12.24 open circuit. (Website: http://www.ccis.com/home/mnemeth/12volt/12volt.htm)

THANKS!

[Brian B-P]

Very close... just missing parts of the units of measure:

1 amp = 1000 milliamps
1 hour with 2 lights = 160 milliamp-hours
10 hours of lights (approximate for a day) = 1.6 amp-hours

If I leave my battery hooked up, but nothing turned on, will my readings be considered open circuit or under load?

The "load" is the current being delivered, so "nothing turned on" means no current, or "open circuit". Since the amount which the voltage drops under load depends on the size of the load (amount of current), it's hard to say what the "under load" condition should be. For a 115 amp-hour battery, a fraction of one amp is certainly not under much load; I don't know what load is intended in that specification, but it is likely at least the "C/10" rate (11.5 amps for the 115 amp-hour battery).

[Morgan]

Hi all,

...

1 amp = 1000 milliamps
1 hour with 2 lights = 160 milliamps
10 hours of lights (approximate for a day) = 1.6 amps

...
THANKS!

David,

The first line above is correct, but the others are incorrect. When you specify a period of [b]time and current, you're talking about power consumption:

1 hour with 2 lights = 160 mAH (milliampere hours)
10 hours of lights (approximately for a day) = 1.6 AH (ampere hours)

[David Scott]

David,

The first line above is correct, but the others are incorrect. When you specify a period of [b]time and current, you're talking about power consumption:

1 hour with 2 lights = 160 mAH (milliampere hours)
10 hours of lights (approximately for a day) = 1.6 AH (ampere hours)

Okay, but the calculation of days is correct, right? I just didn't express it in the right terms? The battery should still go over 30 days since half of 115 amp hours should be more than enough.

[David Scott]

Okay, so I don't have to disconnect to have an "open circuit", I just need to turn everything off? So basically I am going to be watching for the 12.24 volts as my cut-off point?

Very close... just missing parts of the units of measure:
The "load" is the current being delivered, so "nothing turned on" means no current, or "open circuit". Since the amount which the voltage drops under load depends on the size of the load (amount of current), it's hard to say what the "under load" condition should be. For a 115 amp-hour battery, a fraction of one amp is certainly not under much load; I don't know what load is intended in that specification, but it is likely at least the "C/10" rate (11.5 amps for the 115 amp-hour battery).

[Morgan]

Okay, but the calculation of days is correct, right? I just didn't express it in the right terms? The battery should still go over 30 days since half of 115 amp hours should be more than enough.

All your calculations are correct, David.

[Brian B-P]

Okay, so I don't have to disconnect to have an "open circuit", I just need to turn everything off? So basically I am going to be watching for the 12.24 volts as my cut-off point?

Right, everything off is essentially disconnected. You normally need to wait a little while after removing the load (turning off) for the battery voltage to stabilize, but your load is so small that my guess is that it will change very little anyway, so when you hit 12.24 volts with the LEDs on you'll be close to the desired cut-off point.

[David Scott]

Thanks everyone. We're off now. Let's see how my little Toyota Corolla does pulling the Compact II now that it's packed. I've been working hard for this moment and am so excited about arriving to our camping spot tonight (San Simeon, CA) and setting up.

Happy trails to all...