Battery selection for a small mobile robot

Thread Starter

salvus

Joined Apr 2, 2020
49
Hi,

I am making a small robot and want to select the battery that gives me the most prolonged use at the lowest price and fits into the smallest space allowable. It only needs to power the drive mechanism and no micro controllers etc.

I have already made the mechanical parts of the robot (e.g., gearing).

When I run the robot with a lab power supply, the wheels turn as I would like from 3V, showing as 130mA. When I resist the rotation with my physical strength, it goes up to around 230 mA. If I increase the voltage, the wheels spin faster, but I don't think I need this (although it would be allowable if it offers some advantages related to the above points). From observing it, I can see that anything above 5V would be too fast for what I need.

The motor was taken out of an sg-90 servo. I think it is a 5V motor, but I could be wrong. This motor is used because I have a lot of them on hand, so it is convenient for me to use them rather than buy another. For this practical reason, the choice of the battery should be selected to fit this specific motor.

I got some cheap CR2032 3V lithium cells from a local discount store (pack of 20 cost me €5). I can't see a mAh rating on the packaging. From Google, my understanding is that it is typically around 225 mAh. These are very cheap batteries, though, so I don't know if that would also apply to these batteries or more premium versions of the CR2032. The robot motors do not turn with one coin cell, but it does with two coin cells placed on top of each other.

The motors spin for about 30 seconds, then the robot motors sound like they are struggling, and it stops shortly after. I thought that the batteries had run out, but if left for a few minutes, the process can be repeated. I can see that the voltage goes back up. Do you know what the issue could be? If they are 225 mAh each and my robot only draws 130mA, should it not run for several hours?

If I used a 3V regulator, would this increase the runtime, or would it just add inefficiency and unnecessary cost?

Do you have any other suggestions about how I might be able to increase the runtime?

I might also use AAA batteries instead, but the rechargeable versions are pretty expensive for what I need. The CR2032 has a rechargeable version called the LIR2032, and I am thinking of using those if I can increase the runtime to at least 10 mins of use. I can see that these are also very cheap.

Many thanks for your help
 

k1ng 1337

Joined Sep 11, 2020
584
I think it's because you are discharging the batteries at around 0.5C which is probably not what they are rated for. I have a key fob that uses a coin cell that lasts forever because the current draw is very low and intermittent.
 

Dodgydave

Joined Jun 22, 2012
10,361
I would use a 18650 lithium battery, these are 3.6V rechargeable and have a high Ah capacity .

The button cells don't have a good current output for long periods due to their internal resistance.
 

Thread Starter

salvus

Joined Apr 2, 2020
49
I think these might be a bit too big for the robot but I had a look and saw the single cell lipos. The only concern I have is that kids will be playing with the robot. It seems like the consequences could be very high if something went wrong, and there are many ways that it could go wrong. Given the constraints that I mentioned, do you see any better approach than 4 AAA batteries?
 

Thread Starter

salvus

Joined Apr 2, 2020
49
Do you think two AA would be better than 4 AAA. Is there a cheap way to recharge these batteries though? Are the chargers not quite expensive, is there a simple micro usb approach?
 

BobTPH

Joined Jun 5, 2013
5,233
They are expensive, but these are 1.5V AA batteries that have a lithium battery, change controller and buck converter internally. The charge via an included USB cable via a phone charger. The have a little less capacity than an AA primary cell, but that is offset by the fact that they stay at 1.5V until they die.

Bob
 

Thread Starter

salvus

Joined Apr 2, 2020
49
I have seen that you can get them from aliexpress for around 2 euros each. Do you think it would be possible to use one of these AA batteries and do some kind of boosting with low cost circuitry?
 

BobTPH

Joined Jun 5, 2013
5,233
I have seen that you can get them from aliexpress for around 2 euros each. Do you think it would be possible to use one of these AA batteries and do some kind of boosting with low cost circuitry?
If you are going to boost it, why not use a lithium battery at 3.7V?

Bob
 

Audioguru again

Joined Oct 21, 2019
4,794
A Name Brand CR2032 coin battery cell is rated for 240mAh when it has a 15k load which begins drawing 2.9V/15k= 0.19mA and ends drawing 2V/15k= 0.13mA.
It is overloaded with a current of a few mA.
 

Thread Starter

salvus

Joined Apr 2, 2020
49
Which 3.7V lithium battery do you mean? Is this much different to the CR2032 that I tried?

I just tried using two rechargeable batteries that I have on hand (not the fancy usb ones shown in the link above) and those did at least run the motors for a few mins, I cut off the experiment after a few mins. It would have no doubt gone for longer.

I also tried using 1 off these AA batteries with a 5v dc dc converter that I have on hand (hw-105). The measured output voltage was 5V but this then didnt run the motors at all, I think the current was no longer high enough.

Any suggestions about how I might be able to use 1 of the AA batteries shown in the link above rather than 2?

Thanks again for your help, much appreciated
 

BobTPH

Joined Jun 5, 2013
5,233
If you were going to use 1 AA you might as well use two AAA, it will have more capacity than a single AA boosted to 3V.

But, again, a lithium ion battery will give you more capacity for the same size and weight. They come in all kinds of sizes.

Here is one I have used, compared to other batteries we have discussed. It would happily give you 120 mA for 3 hours.

2BEF548D-B1D5-444F-A482-B0BEBF13154B.jpegBob
 

Thread Starter

salvus

Joined Apr 2, 2020
49
I think I might not be understanding something. Appart from the physical size, are these differnent from the 18650 battery that we discussed? Does this battery have the same level of safety concerns?
 

k1ng 1337

Joined Sep 11, 2020
584
I was surprised to learn how many battery types and shapes exist. I pickup up some 1400 mA/h 18500 Li-ion batteries (15mm shorter) because I couldn't find 18650 for my price range and liking. When searching the datasheet should tell you capacity and charge / discharge rates as well as cutoff voltages.
 

BobTPH

Joined Jun 5, 2013
5,233
I think I might not be understanding something. Appart from the physical size, are these differnent from the 18650 battery that we discussed? Does this battery have the same level of safety concerns?
Do you consider your cell phone a risk? Practically everything has lithium ion batteries now. They are only a risk when DIY users charge them without knowing how. The battery in my photo has a built in protection circuit and I use a commercial charger which actually came with the batteries. You might notice that I have added a component in series with the connector. It is a fuse. This tiny battery can actually supply 10A, so I suggest you do the same if you used one.

Bob
 

bassbindevil

Joined Jan 23, 2014
457
Nearly all the lithium cells found in phones and portable devices include a tiny protection board. 18650 cells sold at retail for flashlights and vape pens ought to include protection, but you'll pay a premium price for those. And if you buy them from some unknown Amazon/ebay/Ali vendor, they may well be lying about the protection (as well as the capacity). You should be able to feel a small board under the heatshrink at the negative terminal of the cell, and the negative terminal shouldn't look like a bare cell.
So, anyway, your best bet could be a small rectangular lithium cell (protected); charging could be done using a TP4056 charging module. Some modules also include protection chips, but if your cell is protected, you don't need that feature. And for a small lithium cell, you may need to reduce the charging current.
https://www.best-microcontroller-projects.com/tp4056.html
 
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