discussion / Acoustic Monitoring  / 13 October 2021

Using rechargeable lithium-ion batteries for AudioMoths and/or Swift's

Hi all,

I'm wondering if anyone has experience using rechargeable lithium-ion batteries for passive acoustic devices, specifically Audiomoths (AA batteries) and Cornell's Swift (D batteries) units.

1. Will the device even accept that kind of battery?

2. How long do they last compared to NiMH rechargeable or regular lithium ion batteries.

3. What brands would you recommend? It doesn't seem like any of the "common" brands (Duracell, Energizer, etc.) are producing these for consumer purposes. 

Thanks!




Hi Carly,

While NiMH rechargeables (1.2V nominal per cell) are broadly compatible with alkalines (1.5V), the lithium ion chemistry isn't (3.7V).  But it is possible to replace 3 NiMH or alkalines with a single Li-ion rechargeable if you change the battery holder.  For the Audio Moth, a single 18650 (these look like AAs but bigger) will contain roughly the same energy as the usual 3 alkaline AAs (about 2800 mAh).  

But before you do this, realise that lithium cells require babying, which is probably why Duracell et al do not sell them to consumers.  In practice, lithium cells are integrated into packs with circuitry to prevent fire and other infelicities like an e-cigarette exploding.  You can buy single 18650s with this circuitry already built into them; they are about 70mm long instead of the usual 65mm and are generally considered consumer proof.

In terms of energy per unit volume, lithiums are roughly on par with alkalines.  But in terms of energy per unit weight, lithiums come out ahead, so these might be considered if your deployments are difficult to access.

Hi Carly.

To answer your questions:

>> 1. Will the device even accept that kind of battery?

The Audiomoth comes with a built in AAx3 alkaline battery holder. You would need to solder some leads to the (+) and (-) side of that battery holder to connect to a lithium-ion cell. In terms of voltage compatibility, according to the Audiomoth schematics v1.1.0 which is published, it should be no problem, although it's a bit tight. The system runs at 3.3V and as long as you have a voltage supply at least 0.3V above that, things should run fine. The nominal voltage of a lithium ion battery is 3.7V so it's just within that threshold. You may not "officially" be able to use the full capacity of a lithium-ion battery though as it would drop down to 3.0V which is too low for the regulator. However it's likely that things will probably function through most of the battery's life. 

>> 2. How long do they last compared to NiMH rechargeable or regular lithium ion batteries.

@htarold gave a good explanation above, but one caveat is that you may not be able to use the full life of the battery depending how the system behaves once the battery voltage droops below 3.6V. you can double or triple up lithium-ion batteries though which would increase the battery life. 

>> 3. What brands would you recommend? It doesn't seem like any of the "common" brands (Duracell, Energizer, etc.) are producing these for consumer purposes. 

Again as @htarold mentioned, Lithium-ion batteries are harder to come by than akalines. They require proper handling and protection, otherwise, they can chain-react and quickly release their stored energy. Those were the hoverboard fires and Samsung mobile phone fires that made the news a few years back. That said, you can purchase lithium-ion batteries with protection circuitry or battery holders that incorporate protection circuitry to prevent any errant behavior. Jacinta and I are working on battery modules like this for our field designs because we need to be able to charge them via solar panels. 

I'd recommend playing with them to start getting familiar with lithium-ion batteries. I haven't had one explode into flames on me and I use them alot. Usually I have the opposite scenario. The batteries get drained too much where they get damaged and can no longer be charged. There are ways to revive them, but this is also one of the reasons you need protection on them. They need to be protected from undervoltage (<3.0V), overvoltage (>4.2V), and overcurrent (enough current where they start heating up uncontrollably).

Hope that helps.

Akiba 

 

I think I maybe didn't explain correctly, let me clarify. I meant rechargeable lithium ion AA batteries like these: 

https://smile.amazon.com/gp/product/B08RZ5NDMM/ref=ppx_yo_dt_b_asin_title_o00_s01?ie=UTF8&psc=1

Or D batteries like these: 

https://smile.amazon.com/gp/product/B08CN9YZJR/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

I asked about this in the AudioMoth support forum and Alex Rogers said "We’ve never tried them but they should work fine. I can’t find a decent discharge curve plot for them but they claim to maintain close to 1.5V over their capacity. They have some additional voltage regulation inside them which might create some additional noise but worth trying."

Carly--

We looked into it but decided against it, mostly because of worries about TSA / customs issues with any attempts to travel to deploy than because of any particular performance concerns. We do use NiMH batteries for some local sites that we drive to though.