How long will a battery last if connected to 10 LEDS in parallel?

Question by Bob T: How long will a battery last if connected to 10 LEDS in parallel?
Is there a way to calculate how long a 1.5v hearing aid battery will last if connected to 10, 35 mA /3.6V LEDS in parallel and left the circuit constantly closed?

Best answer:

Answer by John Lowe
That is really a trick question. The reason is that the 1.5v battery would not be able to cause the LED’s to conduct. Most LED’s require a forward voltage of 2V or so, therefore the battery has to have a higher voltage. Hence, the answer is it would last as long as its shelf life.

Ok, maybe we have three hearing aid batteries in series, giving us 4.5v. The largest hearing aid battery might be the 675 with a capacity of 600mAh. If we neglect internal impedance of the batteries which limit the current that it can supply, and we assume that the LED’s will still only consume 35mA at the higher voltage, we can divide 600 with 10*35 giving us the service hours of 1.7hrs. For LED’s in parallel – we multiply the current by the number of LED’s.

If we consider real life… the internal impedance of the batteries would limit the current that it could supply. The capacity of the battery would also reduce (current derating), because they are not expected to supply high currents. I would think that a number of minutes is quite likely as being the real service life of this scenario. Hearing aid batteries are normally operating at the 50-75 Hr rate, i.e. maximum current is capacity divided by 50 or 75 hours. Good luck and hope this helps.

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