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LoonWhisperer
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# Posted: 8 Oct 2014 11:59am
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Wondering if you seasoned 12 volt vets can help me make sense of the voltmeter readings I was seeing over the weekend.
Long story short, I constructed a power box (more on that later) with a 12v 130AH deep cycle battery. Mounted on the side of the box is a 1600 watt MSW inverter from Whistler. On the inverter is a display that flashes 2 different readings and by the time the weekend was over, one of them was showing 11.9 - 12.0. Later confirmed with my multimeter. The other display just showed zeros the whole time and I have no idea what that means (not referenced in the manual).
So, if the battery is down to 11.9, how discharged is it? I guess my problem is translating these readings to the actual discharge of my battery.
The good news is for the first time, we had cold food and drinks the entire time as it ran our Igloo 12V cooler flawlessly. Also ran a shop vac like a champ.
Thanks!
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razmichael
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# Posted: 8 Oct 2014 12:39pm
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Couple of references:
12 volt side
Faq
11.9 volts is about a 40% state of charge (ignoring temperature differences). General "guidance" is to try not to exceed 50% but this is guidance. Impact? depends a lot on the type of battery - is is a hybrid marine deep cycle - a 'true' deep cycle?
I'm going to assume that the cooler is a thermoelectric (peltier) type - these are extremely inefficient and will really drain a battery (as will a shop vac. The inverter is likely around 80-85% efficient and may also have a fairly high idle draw (you would need to check the specs to confirm). All of this is likely to really draw down your battery.
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razmichael
Member
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# Posted: 8 Oct 2014 12:45pm
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From
Igloo
"Igloo Thermoelectric coolers are designed to operate between 48 to 60 watts (depending on model) and draw an average of 4.2 - 5.0 amps on a continuous basis"
Also the inverter specs state <1.0 amps on idle, so just assume 1 amp, you are drawing up to 6 amps on a continuous basis plus any additional use like the shop vac.
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LoonWhisperer
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# Posted: 8 Oct 2014 12:51pm
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Thanks raz! The battery is a true deep cycle:
http://usbattery.ca/marine-batteries.php (model US 31DC XC)
So, is 6 amps considered a heavy load? (sorry, noob here) What length of time could I reasonably expect to run this off my battery?
And would it help if I plugged directly into a 12v connection instead of running it through the inverter with a 12V - AC adapter?
Cheers!
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razmichael
Member
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# Posted: 8 Oct 2014 01:57pm - Edited by: razmichael
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I would say that 6 amps on a continuous draw is a big load on a small system. Keep in mind that I do not know for sure what your pieces really draw.
Lots of factors such a temperature and rate of draw will influence this. I suggest you do a quick review of the references to get a better idea. How quickly you draw out the amps will impact your total available. Look at the specs on your battery and it is 130 amps at a 20 hr rate or 99 at a 5 hr rate.
Without worrying about some of the details (not really needed to give you a ballpark), assume you don't want to go below 50% discharge rate this gives you 65 amps to play with so 10-11 hours. Actually, as you are running this cooler through the inverter this will eat up another 10-15% (or more) due to the inverter loss. Keeping the inverter off unless needed will save the idle draw and the inefficiency of converting 12 to 120 and then back again to 12.
Run a vac and this will drop your time, possibly a lot depending on the size of vac and run time.
Does this all kinda match what you saw?
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LoonWhisperer
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# Posted: 8 Oct 2014 03:06pm
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Seems spot on actually. I appreciate you breaking it down for me and those 2 links are an excellent reference.
I am going to install a 12v plug on the box and run it from there going forward. The Igloo cooler is well insulated fortunately so it does not need to run continuously to keep things frosty.
Thanks!
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