Better formula to estimate energy output from a battery

[xibalban]

I find most people using the following formula to estimate energy outputs from a battery for a given load:

Assume a 12 V, 50 Ah battery (deep discharge type)
Assume a load of 100 Watts, so using power formula
P = VIcosΦ
Or I = P/V [assuming power factor = 1]
I = 100/12 = 8.33 A

Now, h = Ah/A
= 50/8.33 = 6 hours

So, the 12 V, 50 Ah battery would last 6 hours on a constant load of 100 Watts. Right?

However, we assumed that the voltage 'V' of the battery remains constant, which isn't true. The battery voltage keep diminishing, and hence the current drawn by the load keeps increasing. We need a better formula for estimating the "hours" above, don't we?

What, may I request, is the true solution?

Thanks in advance.

 

[FvM]

It's quite simple mathematic in any case. But before you jump into conclusions, you should specify the load behaviour. You are assuming a constant P load. That's rather unlikely, except for a view electronic devices.