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For a portable application, I would be using a single 3.7V 2600mAh li-ion cell. The circuit requires 2.5V to 5V to function with the current requirement being 100mA.

So obviously a voltage regulator is required.

I would want to know for how long, using an appropriate regulator, could the cell power the circuitry continuously. Is it almost 20 hours? I guess the health of the cell also needs to be considered, so it should not be discharged below 3.3V, right? If we consider this, then what will be the run time?

Thanks.

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    \$\begingroup\$ This is off topic because you're asking us to read the datasheet to you. \$\endgroup\$ – Olin Lathrop Nov 14 '14 at 19:32
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    \$\begingroup\$ It's not obvious to me that a voltage regulator is required. If the circuit will work from 5V down to 2.5V, why do you need a regulator? \$\endgroup\$ – Spehro Pefhany Nov 14 '14 at 19:32
  • \$\begingroup\$ Maybe he needs to have his circuit function at full potential. Sometimes this is the case with audio amplifiers. Even though it has a reasonable voltage range, like he/she has mentioned, to power up, at the upper limit it gives a big output. \$\endgroup\$ – arjun Nov 15 '14 at 5:18
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If you believe the battery manufacturer, and you're sure your circuit consumes 100mA, then the answer is:

2600mAh / 100mA = 26h

But you obviously need to verify your circuit's actual consumption. The above calculation should be considered an estimate.

My experience flying radio controlled planes have shown me that the estimate is actually quite good. Depending on the manufacturer it could even be conservative.

Modern RC batteries tend to understate the mAh numbers because hobbyists have started to depend on the simple calculations above to plan really long range flights. Also because reviewers started to compare battery performance on youtube. Generating more than the stated mAh when the battery is drained down to 0% (around 3.2V or 3.4V depending on opinion) makes the brand of battery very popular among RC pilots.

Non-branded or non-RC batteries however don't have this competitive pressure. So they may even over-rate the mAh numbers.

As for regulation, your circuit can take up to 5V and operate down to 2.5V. This is well within the battery operating voltage range. You don't need any regulator. You only need one if you plan on using 2-cell batteries (7.4V).

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    \$\begingroup\$ I should note that the mAh number is usually based from fully charged (4.2V) instead of nominal (3.7V) \$\endgroup\$ – slebetman Apr 18 '15 at 0:53
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If you're really going to use a regulator (see Spehro Pefhany's comment that this may not be necessary), you're going to need:

  • The efficiency of the regulator when supplying 100mA, so that you can calculate the current of the whole circuit
  • The discharge graph of your battery at that specific current

With the discharge graph you can see how long it will take before the battery reaches the cut-off voltage of the regulator, or the 3.3V for battery health reasons.

Just to be sure, always add some margin, the circuit may draw different currents under different environmental circumstances. (I once made a GPS logging device that should run for about six hours according to the calculations and the tests - then I put it in my pocket and went hiking. It became quite hot inside the pocket, the current increased and I could log for only two hours.)

Also note that the quality of batteries degrades over time, and that its capacity may decrease.

If you would be drawing a higher current, you should also take the possible voltage drop into account.

In any case, not much can be said about the exact calculations without part numbers and datasheets.

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