12v power calc confused

[ebg18t]

I am confused on something simple. I have an Engel MT-45, running dual battery setup. My 2nd battery is a new Duracell AGM group 34 and the fridge is wired direct using 10 gauge wiring. Battery is charged using a DirtyParts dual battery/isolation kit.

CCA - 775
Res Cap - 120
Ref CA - 955
20 AH rate - 55

If my MT-45 draws 0.5-2.5AH how long would my dedicated battery last.

I tried a online calculator and it indicated 2.9 hours. That just seems so short considering the low ah draw.. Or am I missing something?

 

[coax]

I suspect something is amiss in the calculation or data points entered. At that low current draw it should last for more than 3 hours. I read that as it should provide 55Ah if used over 20 hours at 2.75 Ah draw. (2.75A x 20 hrs=55Ah) I believe thats how the 20 amp hour rate is measured. So even if you draw 2.5A it should last for 20 hours. It will pull a little more amps as the battery voltage drops (I would guess) but still you should be far more than 3 hours. To preserve the battery you wouldn't want to draw it down all the way, so you'll have some factor of Ah reduction.

Plus, unless its really hot out the fridge won't run all the time.

Check out this thread as well.
http://www.expeditionportal.com/forum/threads/134-Power-System-Requirements-Determining-Capacity

 

[Wicked 2007]

55amp/hr - typically you only want to discharge 50% but 80% is ok, the battery will just have less cycles in it.

55a/hr @ 50% = 27.5a/hr
55a/hr @ 80% = 44a/hr

Speaking in terms of my ARB 50 it uses on avg. about 27amp/hr over a 24 hour period - divided evenly this would be 1.125 per hour. I would assume the engle to be the same or close (only way to do this is measure your actual draw from testing). But that battery with my ARB would be at just about 50% discharge after 24.5 hours or 39 hours if you go to 80%.

Many factors come into play - outside temp, how often you access things in your cooler, does it have a thermal blanket, etc.

 

[Ramjet]

I've ran my Engel MT45 for 16 hours straight with about 60% draw on my Yellowtop. I'm using an IBS system though. I don't think that makes much difference though. It's always good to start your car, which I really don think will be that hard of a problem when your out exploring.

 

[ebg18t]

Thanks for the help guys. Yes I do have the transit bag on it for insulation. Truck is silver with ceramic tint to keep temps as controlled as I can.

If I understand everything correct the AGM batteries will recover faster and handle more deep discharge cycles than regular batteries. So even if I ran it down to 75-80% I should be able to charge it fairly quick and safely.

I am on eBay right now trying to find an inline battery monitor I can just mount on the battery to monitor the voltage. one thread said they were a dime a dozen on there.. Anyone have a suggestion there?

 

[Wicked 2007]

Thanks for the help guys. Yes I do have the transit bag on it for insulation. Truck is silver with ceramic tint to keep temps as controlled as I can.

If I understand everything correct the AGM batteries will recover faster and handle more deep discharge cycles than regular batteries. So even if I ran it down to 75-80% I should be able to charge it fairly quick and safely.

I am on eBay right now trying to find an inline battery monitor I can just mount on the battery to monitor the voltage. one thread said they were a dime a dozen on there.. Anyone have a suggestion there?

AGM/Marine batteries do not recover faster, but they can recover more often from deep discharge than a standard battery. Most manufactures recommend 50% but max is 80% here is what a typical AGM battery looks like over the life pending on how deep you discharge it and the amount of cycles you will get out of it (number of cycles on the Y axis and depth of discharge as a % on the X):



From the chart you can see your life (in terms of number of cycles you can get) doubles at 50% when compared to 80% (50% = 1000 cycles vs 80% = 500 cycles)

while volts are good to know while under load it could be misrepresented. using an amp meter would be best, a lot of folks use this. With this you can measure actual amp/hr consumed (kind of like a gas gauge for batteries):

http://www.powerwerx.com/digital-meters/watts-up-meter-dc-inline.html

 

[ebg18t]

while volts are good to know while under load it could be misrepresented. using an amp meter would be best, a lot of folks use this. With this you can measure actual amp/hr consumed (kind of like a gas gauge for batteries):

http://www.powerwerx.com/digital-meters/watts-up-meter-dc-inline.html

So if I am reading that link correct I could use that in a variety of applications. I could just use the leads to spot check the battery by just touching the +\- on the battery as needed. Or I could run wiring to my cab and install this meter wired direct to the battery. This would allow easy in cab monitoring of the battery. Please pardon my basic electric questions.