# EEVblog Electronics Community Forum

## Electronics => Beginners => Topic started by: danshtr on April 27, 2017, 08:36:38 pm

Title: Battery loglivity calculations for a small radio project.
Post by: danshtr on April 27, 2017, 08:36:38 pm
Hi All.

I have a radio project (LORA) which consumes in average 17.5ma and with 200ms bursts of 150ma every 5 seconds (accounted for in the 16ma average) . All at 3.3v.

I want to keep it compact but lasting 24 hours.

What battery/regulator combo do you suggest?

I would love to use a single  AAA battery with step up, but I don't know how to calculate the waste. Do I just look at the graph at https://www.pololu.com/product/2563 (https://www.pololu.com/product/2563) and see that with 1.5v battery the efficiency is between 70% to 80%. So 800mah (of AAA battery) * 0.7 (efficiency of the stepup)  / 17.5 = 36 hours?

Am I missing something?  :-//

Thanks !

Title: Re: Battery loglivity calculations for a small radio project.
Post by: james_s on April 27, 2017, 08:41:24 pm
That should get you pretty close. Given the low draw, the battery should deliver very close to if not a bit over the rated capacity. I'd say build a prototype and run it from a battery until it stops working. I don't think you'll have much trouble getting over 24 hours.
Title: Re: Battery loglivity calculations for a small radio project.
Post by: ebastler on April 28, 2017, 12:58:54 am
I think you overlooked the impact of the actual step-up conversion. Let's assume you had a perfect, 100% efficient step-up converter. In order to provide 1 mA @ 3.3V, that converter would draw 1 mA * 3.3/1.5 = 2.2 mA @ 1.5V. I don't see that represented in your calculations. The losses due to real-world efficiency below 100% then come on top of this factor.
Title: Re: Battery loglivity calculations for a small radio project.
Post by: james_s on April 28, 2017, 03:10:26 am
Oops, that explains that feeling I had in the back of my mind. You'll need to measure (or calculate) the current drawn by the converter, it's looking like 800mAh AAA won't get you there but a 2,000mAh AA ought to. There are small lithium batteries that might be worth looking into as well, the higher voltage would eliminate the need for a boost converter.
Title: Re: Battery loglivity calculations for a small radio project.
Post by: digsys on April 28, 2017, 05:15:49 am
I agree with the lithium cell path. Even if you used a LDO linear regulator, you're still running at 90% efficiency, with a simple circuit. They have much higher energy capacity as well.
For the low energy required, you could use a switched capacitor buck cct, usually run at 98%+, to reclaim most the LDO loss. You'd need a reasonable 3.3V storage cap though. Perfect.
Title: Re: Battery loglivity calculations for a small radio project.
Post by: sleemanj on April 28, 2017, 06:17:03 am
Work in Watts.