Two down and one to go…
The 3rd preconception to go was the idea of calculating battery capacity based on the energy needs for a day (or defined time period) without a generator.
In my case the generator is going to have to run every day. I could theoretically go a day or two with minimum energy use, but as a practical matter, if I am actually living in the truck, I need to run the generator for at least a couple of hours a day. Web’s have to be surfed and TV’s have to be watched you know…
The limiting factor is charge and discharge current, or the 25% rule. The maximum practical battery current with the generator and the charger working, and not running the microwave at the same time as the air conditioner, is about 87A. That means I need at least 4 x 87 Amp Hours, or 348Ah. The batteries I am looking at come in increments of about 220Ah, so 2 batteries in parallel is 440Ah, which more than meets the need.
There are still lots of details underneath these high level figures that I haven’t shared with you. In true engineering fashion, that involves a spreadsheet, or multiple copies of the same spreadsheet to play out different scenarios like a maximum energy usage hot day, or a minimum energy usage comfortable day.
I made a list of all the electrical devices that I could think of. Everything from 12V electric blankets, to a Play Station 3 with a flat screen LCD TV. Don’t forget the basics like overhead lights and a refrigerator, etc. Then I researched the current, or power required for each of these. A good source for some of this information is:
http://www.oksolar.com/technical/consumption.html or
http://www.psnh.com/Residentia...l/Applianceusage.asp or
http://www.warehouseappliance.com/elecfridge.htm
All these devices break down into two general categories, AC and DC. The AC currents can then be translated into DC currents using the conversion factors I provided earlier. A time factor can be applied for each device, in other words, how many hours per day it will be on. That allows the calculation of Amp Hours, or Watt Hours if you prefer. With all that information in one place, plus battery charger current and a few others, it is possible to calculate a bunch of interesting metrics. At least these are the ones that I thought were interesting.
11.4 Battery Energy replacement time at full charge current (Hours)
7.48 Battery Energy replacement Time at Full Generator Capacity (Hours)
241 Maximum Battery Current if Generator and Charger are not working (A)
87.07 Maximum Practical Battery Current with Generator and Charger working and not running the microwave at the same time as the air conditioner (A)
348.2 Minimum battery Amp Hour rating based on 4X Maximum Practical Battery Current
66.1 Average Battery Current for 12 hours per day (A)
2397 Peak Inverter Wattage assuming everything possible is on simultaneously (W)
1597 Maximum Practical Inverter Wattage assuming the Microwave is not on at the same time as the Air Conditioner (W)
10.7 Generator Load Assuming only the Battery Charger (A)
81% Percentage of Generator steady state capacity at maximum Generator Load
0.6 Battery Hold Up Time (Hours) with everything on without the generator or the Battery Charger
2.2 Typical Expected Battery Hold Up Time (Hours) without the generator or the Battery Charger
4.2 Typical Expected Battery hold Up Time (Hours) without the generator or the Battery Charger or the Air Conditioner
83% Average Battery Current for 12 hours per day as a percentage of Battery Charger Capacity
The above figures are for a maximum energy usage hot day. By changing the number of hours that each individual appliance is on you can model any sort of day you like. As it turns out, if it’s a nice day, meaning I don’t need to run the air conditioner or the overhead fan much, and I don’t watch an excessive amount of TV or play on the computer all day, I actually can go the whole day just on batteries.
If anyone wants the actual spreadsheet, PM me with your E-mail address and I will send it to you.
To be continued…