Just thought I’d share a few little notes on some battery-related experimentation that I did with my GTM. Having rushed into the disassembly process, I ripped the sheating on the original Li-ion battery off, and not wanting to cause a battery fire, I chopped the leads right off almost immediately after. For the better part of my 1 year ownership, I’ve run battery-less with USB power always on. I was going to run this as a POE-powered relay, so I didn’t think it was going to matter to much.
It turns out it does. My GTM (v1.0 board dated 2017), does not like not having a battery connected at all. If operated without a battery, it will power on, however the red power LED with flash dimly, intermittently. It will pair, but messages will not go through. The white LED normally indicating a transmission just flashes over and over again until the unit is powered off and back on. I’m thinking this probably has to do with the radio pulling directly from the battery, with a diode setup that prevents the charge circuit from feed it directly. All other functions seems to work fine, I even did the firmware upgrade without a battery (reckless, I know).
First takeaway: if you have a dimly flashing red LED, your internal battery is likely toast.
I had some spare 18650s, very popular lithium cells that have the same nominal voltage albeit much high capacity, so I hooked one up to the battery terminals for testing.
The one you see here is a cheap chinese cell out of an aftermarket laptop battery that had failed. Simple voltage testing told me it was good, but capacity is both not on the cell, and untested since I don’t have the hardware to test that right now. It’s likely a 2000 mAh cell, which puts it at nearly 4x the capacity of the original Li-Po battery, rated at 589 mAh.
The charge circuit liked this one, which was expected since Li-ion and LiPo share charge profiles, from what I have read. Charging took a while, I measured charging at 15-16 mA at around 50% charge, but I don’t really have any data that would give away the charge curve. Charge voltage reached normal Li-Ion charge voltage, 4.3 if I’m not mistaken.
The charge level indicator is all sorts of messed up with the 18650 connected, it’ll show 0 and 100 and nothing in between. There is probably some sort of calibration mechanism that needs to relearn depth of charge, I have not tested that far.
I’ve mesured standby draw at around 20mA in bluetooth-connected mode, which would amount to a 4 day battery life on a single cell like this. I got to 4 days out of it in real-life testing, mostly with the unit staying in standby, spending maybe a few hour a day flashing (bluetooth disconnect state), voltage was still showing north of 3.6v, at which point I had to leave for a while, so I disconnected the setup. Maybe the cell I had is more than 2000mAh after all!
Quality 18650s can be harvested from all sorts of electronics, high quality cells can do 3Ah and more. That’s 150 hours on bluetooth standby, 6+ days, if we’re to trust the reports of power draw halfing in relay mode, that’s potentially 12+ days!! With a 3 cell parallel configuration, we’re looking at over a month of endurance potentially.
Other advantages include:
- no DC-DC buck converter loss from ready-made battery packs (potentially 4S 18650 array for a 12v config -> 5v usb -> battery level 3.7v nominal
- no loss from other off the shelf power bank peripherals (status LEDs mostly)
- More flexibility for battery array configuration if going the stripped-board-in-enclosure way
2nd takeaway: 18650s are a better choice than a USB battery bank and will work.
Since I still wanted to go battery-less for POE operation, I tried replacing the battery with a small cap rated for well about nominal voltage, 220uf 6.3v. The idea being that I’d want my unit to be in the attic of my house which gets VERY warm, and where heat-related battery failure could potentially mean important property damage a very unpleasant discussion with my insurance company.
The charge circuit did its thing and I got LiPo peak charge voltage quick enough, but still got the flashing red light. Transmissions did not work.
I’ve crunched some numbers, although these calculations are a bit arcane for me. The cap stores 0.00184900 joules at 4.3v. I’ve calculated around 60mAh draw while attempting to transmit, but this is likely really off since I calculated with a very low resolution multimeter. At full charge voltage, this is about 0.25 watts. Assuming half a second of this transmission power draw (here again I’m spitballing, no scope to test this out), that’s 0.129 joules. Even at 0.25s transmission time, we’re a full decimal place away from even being remotely close. Long story short you’d need a much bigger cap to make things work. It doesn’t seem worth it to get this working unless you have VERY serious environmental constraints which make battery operation a no-go.
Third takeaway: always-on, batteryless operation is not possible.
I hope that some of this is helpful to somebody.
PS: GoTenna, please start selling bare GTM boards for us modders!