1100 mAh battery vs. 300-400 mA circuit current draw

Hello Arduino peepz!

I have the circuit for our wireless fire alarm project already mounted on a PCB and it kind of bothers me that it draws around 300-400 mA (random spikes from 260 to 300 but since its on normal operation and not on alarm condition (buzzer on, LED on, XBee transmitting and receiving), I made the end estimate at 400 for allowance).

According to my noob knowledge and estimates (I’m still not into electronics that much), it should draw just around 100-200 mA with the major components just by reading their specs. Now I’m just stuck with that and I only have a few options with the battery as well, considering both the battery dimensions and capacity as well. This one is kind of okay since it has a small size for its capacity (usually ones at this size are around 800-900+ mAh) and is cheap. I need at least 55 mm x 45 mm x 6 mm for the dimensions considering the fixed enclosure size.

Lastly, I need the wireless fire alarm node (the whole circuit) to last at least 2 hours with 1 hour charging from the TP4056 module. Considering all of this, should the battery suffice or do you have any other suggestions? I just can’t find any good ones readily available here (btw I’m from the Phillipines).

To be honest, without a battery specification, it's impossible to know how long the battery will last on your circuit. Even if you had a specification sheet, and understood how to read one, it would only help in a parametric search. The best way to know if it will last as long as needed is to put the battery in service and see how it performs. A lot of LiPo batteries are rated in mAh @ 0.2c meaning a 1100 mAh battery can supply 220 mA for 5 hours. There is usually a derating curve in the spec sheet if you discharge 400 mA.

According to the battery charger's datasheet, an hour of charging could yield at most 1000mA. Assuming that it gets maybe 900mA, 2 hours of current draw at 400mA would still leave 100mA left in the battery as long as you have a large enough battery. You should be able to get away with a thin lithium battery with a capacity of 1000mAh or higher depending on how long you would like the maximum run time to be without being charged. I haven't looked at your design but I know there are ways to get microcontrollers in a low power mode or you could try with a different microcontroller but I agree, ideally you need a smaller power draw. Best of Luck!

btw, most batteries charge anywhere from 0.5C to 1C depending on it's size. I personally use 2C as a general max discharge rate for quick estimates but some can go much higher. You would only be discharging at under 0.5C. (C stands for capacity in mAh)

1200mAh Li-Po Battery (51mm x 34mm x 6mm)

Why are you using a tiny battery for such a current-hogging circuit?

If the circuit is taking twice the current you expect, its worth measuring each part's current draw
separately to find out what hogging the current, it might be a fault.

Perhaps the transceiver is permanently transmitting and you've not realized?

Perhaps some resistors are completely the wrong value (can't read any of them in those images,
please post version with legible text)...


I see but it's already on a PCB. I don't think I could measure current with that idea except with the battery to load (which is the circuit itself) connection. Perhaps a breadboard setup will do but yeah it takes time lol. Will see what I can do.

For the transceiver, I don't think so. The XBee Explorer's Tx and Rx pins are not lighting up in any way. The resistors on the other hand, I really do know if they do affect the circuit that much.


Yeah I do understand your concern, it's just that any bigger dimension than the 1100 mAh battery won't fit inside the enclosure. Unless I make huge modifications and compromises to the design (which would make it uglier and bulkier somehow), then perhaps I wouldn't have any much of a problem with this.


Employing low power methods for the Pro Mini only makes a small difference. I'll see what I can do. Anyway, thank you for the additional battery info.


That's what I've been thinking for quite some time but due to a limited budget, yeah. Hahahaha. But thanks. Think I've got no choice for this. I'll see what I can do.

Employing low power methods for the Pro Mini only makes a small difference.

It would be an excellent idea to figure out what is consuming all that current.

The resistors on the other hand, I really do know if they do affect the circuit that much.

I looked at your schematic, but the resolution is too low to see what values are on the resistors. I'm especially concerned about the voltage dividers. Too low a value will just flush current down the drain, so to speak.

Some bit of news:

I measured the current of the circuit now and it’s a bit weird the readings were different compared yesterday. I think it’s more consistent now.

Here are the current readings:

  1. Whole circuit during normal condition (260-270 mA)
  2. w/o smoke detector circuit (225-230 mA)
  3. The lower PCB only (95-100 mA)
  4. Lower PCB with buzzer on @ alarm condition (170 mA)
  5. Lower PCB w/ buzzer on, red LED on, and XBee on during alarm condition (no smoke detector) (290-300 mA)
  6. Whole circuit during alarm condition (320-330 mA)

So I think it boils down to this:
1.) The transceiver consumes the most current in discrete terms (with the radio itself + the onboard regulator and 4 SMD red LEDs) @ around 50-60 mA.

2.) The resistors do consume current a bit more current than I expected despite their values (I’ll post a better image, sorry)

3.) The smoke detector circuit eats more current than I expected too (10 mA from the IC specs vs. 35-40 mA in actual, well perhaps of the resistors and the sensor (but the sensor is at the magnitude of pA according to the HIS-07 ionization sensor datasheet!).

4.) I couldn’t think of anything else much more significant?

With those, I think it’s kind of better than the 300-400 mA estimate, don’t you guys think?

The transceiver consumes the most current in discrete terms (with the radio itself + the onboard regulator and 4 SMD red LEDs) @ around 50-60 mA.

This is when not transmitting. Depending on which transceiver it is, it can draw 150 mA or more when transmitting.
You can add a capacitor that charges between broadcasts that will reduce your peak current.
You can also get the Xbee into the microampere range by using it's sleep features.

@ Perehama

What do you mean by adding a capacitor? Do you mean from VCC to GND? I observed that the current is not very stable when I try to connect it. Also, I don't think that I would be able to utilize sleep modes for the XBee (since it's designed as a router device).