# a question about reserve capacity for my batteryies



## greene (Mar 23, 2012)

I just hooked up my inverter to a single 100 watt cfl last night. I used a watt meter in line that tells me volts, hz, amps and watts. So I used an optima red battery that I got free, it had 12.27 volts at start, after 2 hr it had 12.15. ( I am throwing out all of the info, some useless) it drew .3 amps or about 15 watt and my inverter which is a cheap junker was putting out 120v. Hhere is my question. Say I put this exact setup in my cabin, I'm wiring it for 110 and going to plug in the inverter, and have a light switch flipped on powering this one bulb, how do I calculate run time until that would prob run out of juice. I know batteries are rated with reserve capacities but its in amp hours so is there a formula I can use? 
My plan is to wire the cabin completely 110 so I can upgrade inverters and charging as I go. I will for now run a generator as needed to top off the battery throughout the day and run tools as I'm completing the cabin. That way the battery will be used some at night. I am just curious to see how many hours total I could use the battry without charging. I have a 4ld battery to use later. Also at .3 amps at 120 volts should be a 3 amp draw at the battery + some for losses through the inverter? Thanks guys.


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## greene (Mar 23, 2012)

The batteries on my shelf at work are rated in minutes, say a group 31 is rated for 150 minutes reserve, does anyone know at what draw they rate them?


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## ZoomZoom (Dec 18, 2009)

greene said:


> The batteries on my shelf at work are rated in minutes, say a group 31 is rated for 150 minutes reserve, does anyone know at what draw they rate them?


It should be 20 amps.

Deep-cycle marine batteries would be your best bet. They're built to be run way down then re-charged. It can be run down and recharged over 1000 times. A regular car battery will only last for about 50 cycles like that. _Not sure if the Optima Red is deep-cycle or not._

You may want to consider switching to 12v lighting. Your inverter not only uses up (loses) around 20% of what it pushes through, it will also shut itself down once it gets down to around 10.5v. With 12v lighting, it'll keep going although the bulb will get dimmer.


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## The_Blob (Dec 24, 2008)

redacted due to already answered


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## Jimmy24 (Apr 20, 2011)

Though I have used marine batteries and they do work much better than regular car/truck batteries. There are 2 types of marine. One is deepcycle starting and deepcycle trolling. If you have to have 12 v batteries, then that would be the best.

However, at Sams you can get two 6v golfcart batteries for just a little more than one marine battery.

You will have a MUCH better battery reserve. You would need several 12v to match the two golfcart batteries. Plus they can be recharged many more times than the marine batteries. Sams recently had them for about $85 each. 

Jimmy


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## greene (Mar 23, 2012)

I will look at sams. I used to work in the marine field. A deep cycle will handle being pulled down and many redharges but at a slower charge rate. A starting battery can handle high loads for short time and handle a faster charge rate. A dc will die sooner if it is charged with most modern vehicle or boat charging systems. So my 4ld will handle a lot but not being drained way down. It will get me by for now. I just wanted to make sure I was getting my math mostly right. At 3 amps draw the little optima will last quite a while so my 4ld should last a night or so with a couple bulbs, and I won't even run it all night.


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## labotomi (Feb 14, 2010)

Be sure to find out the terminal voltage if whatever batteries(s) you use. Even deep cycle batteries don't take being completely discharged well


Also, battery capacity goes down as discharge rate goes up. You could have a 50ah (rated) battery last 50 hours at 1 amp (50ah) but only 3 hours at 10 amps (30ah). If your load doesn't vary its not an issue.


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## Tex (Oct 31, 2008)

Most of this has already been covered. W=VxI also Watt hours = V x Amp hours The given amp-hour rate on the battery is for a given discharge rate. Labotomi covered it, but basically, the slower a battery discharges, the more amp hours you will get. Avoid discharging a battery below 10VDC and don't let it rest in a discharged state very long.

Batteries in series have the same amp-hour rate. Since it is a higher voltage, the watt-hour rate will double. Don't mix different sized batteries in series. They won't charge or discharge evenly.

Batteries in parallel will add amp-hour rates.


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## boothfamily (Jul 3, 2012)

I run two optima batteries in my truck , red top for cranking and yellow top for accessories . I have a 1000w / 2000w peak inverter and I 200 w inverter and I can use both inverters for several hours running drills , grinders , saws , vacuum pump , air compressor , electric impact , etc . I have never had an issue with my batteries dying . I also run a 120 amp alternator which will bring the batteries up quickly. I have a mobile mechanic business and seem to have a good setup for electricity when I'm away from my shop. My inverter will also run my welder for short periods of time if I start the truck and let it idle as needed . Just throwing my experience out there .....


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