HOMER Energy Support

Creating a Battery

Last Updated: Jul 25, 2016 03:12PM MDT

I want to add my own battery


Let's start with the capacity curve.  HOMER uses the capacity curve to calculate the kinetic battery model constants, the most important of which is the "maximum capacity", which is essentially the y-intercept of the curve.  In our experience, the c and k values, which relate to the shape of the curve, have surprisingly little effect on the simulation results.  And they typically do not vary much from one battery to the next.  So any reasonable-looking curve shape will do.  I would take the capacity curve data from the Trojan L-16 and scale it to the size of your battery.  For example, if your battery has a maximum capacity of 800 Ah, then multiply each capacity value by 800/392 (since the L-16 has a maximum capacity of 392 Ah).

Now the lifetime curve.  HOMER uses that to calculate the suggested value of the lifetime throughput, so you can do without the curve as long as you enter a reasonable value for the lifetime throughput.  Fortunately it's easy enough to come up with such a value.  A typical good-quality deep-cycle battery (like the Trojan L16 or the USB US-250) will have a lifetime of about 500 full cycles.  A really excellent (i.e. expensive) deep-cycle battery like the Surrette 4KS25P might have a lifetime of over 1000 full cycles, but 500 is more common.  So guess a value between 500 and 1000, and then you can calculate the lifetime throughput [Wh] by multiplying the full cycles times the maximum capacity [Ah] times the nominal voltage [V]. Divide by 1000 to get kWh, and enter that as the lifetime throughput.

You can estimate the rest of the battery inputs too.  For round trip efficiency, a good guess is 80%.  The minimum state of charge is almost always 30-50% for a deep-cycle battery.  The float life is usually 5-10 years, but it rarely comes into effect anyway, since the batteries usually die of overuse rather than old age.  (See "battery bank life" in the help file for more info.)  Guess 10 years if you don't know any better.  Use 1 A/Ah for the maximum charge rate, and for the maximum charge current enter a value equal to the maximum capacity divided by 10 hours.  So if the maximum capacity of your battery is 800 Ah, enter 80 A for the maximum charge

That should give you a reasonably accurate model of your battery. 
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