TYPICAL BATTERY RUN TIMES

Or how long will my battery supply last before it goes dead? That depends on a lot of variables such as the state of your battery and the amount of power you use. The heavier the demand, the quicker it will drain your battery. This is basic information and is only used as a rule of thumb. You may get more or less run time from your battery. Also as the battery voltage drops the current (amps draw) is higher.

We are making a few assumptions about the batteries on the trailer and they are:
A. The battery is fully charged.
B. The outdoor air temperature is 80 F (26.6 C). Hotter or colder air temperatures will affect run time, heat kills batteries and cold reduces their available capacity.
C. The battery is NEW. As batteries age, this voltage reading will gradually get lower.

1.     With the battery fully charged, you then need to determine what the Percent of Charge is on your battery. You will need a good digital volt meter to read this.

100% = 12.70
90% = 12.62
80% = 12.50
70% = 12.37
60% = 12.24
50% = 12.10
40% = 11.96
30% = 11.81
20% = 11.66
10% = 11.51
0% =      10.50
TYPICAL STATE OF CHARGE. (may vary among manufactures)

2. Next you will need to find out the batteries 20 Amp-Hour Rating. Below is some typical ratings
Group 24 – 80 amp hrs.
Group 27 – 90 amp hrs.
Group 31 – 105 amp hrs.
Group 4D – 200 amp hrs.
GC2 (6 V) 2 ea. – 220 amp hrs.

The Amp-Hour rating tells you how much amperage is available when discharged evenly over a 20 hour period. The amp hour rating is cumulative, so in order to know how many constant amps the battery will output for 20 hours, you have to divide the amp hour rating by 20 hour.
   Example: If a battery has an amp hour rating of 105, divide by 20 hours = 5.25 amp. Such a battery can carry a 5.25 amp load for 20 hours before dropping to 10.5 volts. (10.5 volts is the fully discharged level)

3. Some typical current draws on my trailer
Furnace (Fan) 2.8 amps
Refrigerator on DC 11.00 amps
Fridge cooling fan .32 amps ea x 2
Refrigerator on LP .10 amps + fans
TV .80 amps
TV/DVD     1.20 amps
Water Pump 5.7 amps
Bathroom Exhaust Fan     
1= 1.00 amps
2= 1.30 amps
3= 1.75 amps
LED Light, Porch .15 amps
LED Light, single .28 amps
LED Light, double .55 amps
Propane Alarm .07 amps
Slide out 4-6 amps

Pick what lights and accessories you will be using and add up the amps. Some items are only on for a short period like the slide out motor.

4. Determining your batteries run time to a 50% State of Charge. Going below 50% will greatly impact the life of your battery. Some people recommend not going below 70-80%. I use 50% because the math is easy do in my head.

For example if you have 2 group 31 batteries with an amp hour rating of 105 each.

105 amp hrs X 2 battery = 210 amp hours (total).

To calculate run time to the 50% level, multiply the total amp hours by .50    210 AH X .50 = 105 amp hours.

5. Next take the 50% amp hour total and divide it by the total amps you are using and this will give you an approximant run time. If you want to run your bathroom fan at speed 1 all night you need to divide it by the amp hours you have.

105 amp hours / 1 amp = you can run your fan for 105 hours

If you want to watch a DVD
105 amp hours/ 1.2 amps = you can run your TV/DVD for 87.5 hours

If you want to run your refrigerator on the battery
105 amp hour / 11 amps = you can run your refrigerator for 9.5 hours

On my trailer I have a single group 31 with a 105 amp hour rating, so that gives me about 50 usable amps before I get to the 50% state of charge.
105 amp hours X .50 = 52.5 amp hours


That means that if I want to run my furnace
50 amp hours / 2.8 amps = I can run it for 17.8 hours
(Typically I will go to 75% state of charge so the math is 25 amp hours / 2.8 amps = 9 hours of running time)

So now I have to put that charge back into my battery, so how long will it take to fill them back up? (The following is from the Interstate battery Web Site. I changed it some to reference my battery)

Use your voltmeter to measure the remaining voltage in the battery. If the voltmeter shows a voltage reading of 12.1 volts then the battery is approximately 50% charged. Since the battery is 50% charged, then this means that there are approximately half of the 105 amp-hours in the battery. Therefore it is necessary to put about 52.5 amp hours plus 15% more to compensate for the internal resistance in the battery for a total of 52.5 amps + 52.5 x 0.15 = around 60 amp-hours back into the battery.

Charging the battery at a 10-amp rate, 60 amp-hours needed by the battery divided by 10 amp charge = it will take around 6 hours to recharge the battery to 100% state of charge. The WFCO charger on my trailer is a 3-stage automatic charger. The most I have seen is around 6 to 10 amps charging because I try not to get below a 70% State of Charge. (It is capable of putting out a lot more current to charge the batteries faster.) The charger doesn't output the maximum amps during the charge cycle because it automatically limits the voltage and the amperage during the charge cycle. Most of the time I charge my battery back up with Solar. I have two 100 watt panels. On a good sunny day I can get 8-8.5 amps with the panels flat, if I aim the panels at the sun I can get 9+ amps.

60 amp hours / 8 amps from the solar panel = about 7-1/2 hours charge time to 100% state of charge. Typically you will get to 80% state of charge pretty quickly

Your WFCO will charge at 55A until 80% SOC. After that it starts reducing output and charge rate. A dead battery will take only 60 - 90 minutes to reach a solid 80%, but another 4-6 hours to reach 100% SOC.  

Thanks for the most informative post regarding battery use! 

This weekend I was able to measure the current draw of my water pump and it was 5 amps. The 4.4 amps was the manufactures specification. The refrigerator only draws .10 amps to run the sensors not .13 amps. I could not get a current measure of the 2 cooling fans on my fridge because by the time I heard them running they would stop by the time I got my current clamp in place. The .32 amps for the fans is the manufactures specification.

JandL,

Being DC motors, larger wire would actually increase the current, opposite from AC motors. Clamp on ammeters can be inaccurate at the lower end of their range, especially if it is a 100A or 600A range, and make sure it's zeroed. I bought a 40A meter for just this reason.