Battery pack for very slow charging of USB device

Question

I have a GPS device that has too small of a battery pack (300 mAh) and only lasts a month or so - I want something that can last 6 mo to a year. It can recharge from a USB connection, but only draws power until it's charged then disconnects.

So my clever idea initially was to just hook it up to an external USB battery pack of much larger size (like 10000 mAh) and never have to worry about it.

And that's when I learned that most battery packs have auto shut-off, and even if they don't there's issues with over discharging batteries.

So in lieu of trying to hunt down an "always on" battery pack that I can feel safe has over discharge protection (since starting a fire on something in deep storage sounds like a really bad idea), I thought I'd build something.

So I got some cheap boost converters, specifically:

https://www.jacobsparts.com/items/USBMOD-B

And I'm planning on putting together a simple battery back of a few cells. Originally I was thinking 4 18650 Lipo cells, but started worrying about getting some with proper protection and also the consequences of putting them in parallel (since the input is only 1-5 V). That got me looking at NiMH which evidently handles discharge much better (especially in parallel?) but still can damage the batteries if discharged too far.

I've thought about using a simple protection PCB with a number of unprotected cells, since the current is so low (I'm trying to get longevity out of the parallel cells, not higher current output after all) - but at this point I'm realizing that I might just not be considering some other issue and I might be putting my storage at risk.

So what's a reasonably safe and cheap (I have a few of these I'd like to make) way to continuously supply USB power over many months and not damage the battery pack and/or start a fire or other such damage?

Answer 1

if this helps- I'm solving this problem (wanting much longer battery life in a well-hidden LWT3) with one of these- a programmable low-drain solid state timer.

hook it to your power supply (in my case- a 12v motorcycle battery), use it to cycle power to a 5v USB charger, and set it to come on every week for ~20 minutes of charge time. (or every 2-3 days for 5-10 mins, or whatever you wish.)

~Reuben