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:


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?

  • \$\begingroup\$ USB is a bad choice for continuous battery power in the first place. It requires two voltage converters running constantly and drawing their idle power consumption at all times. And I am sure the second converter (inside the device, 5V => battery voltage) is not switching mode and not really that much efficient. How about hooking your battery pack directly into the battery terminals of the GPS device? \$\endgroup\$
    – fraxinus
    Sep 18, 2021 at 7:40
  • \$\begingroup\$ @fraxinus if only I had a choice. The GPS device is enclosed and only has a USB port. I suppose I could try to break it open, but I'm not a fan of that solution. \$\endgroup\$ Sep 19, 2021 at 4:47
  • \$\begingroup\$ Although... I suppose if I did I could just figure out what the battery pack was and make a monster version of it, and then not even worry about trying to charge the pack inside the GPS. \$\endgroup\$ Sep 19, 2021 at 4:47
  • \$\begingroup\$ And I've managed to find a teardown, courtesy the FCC: fccid.io/ZVS-LWT3/Internal-Photos/Internal-Photos-4603062 If I went that route, I assume I'd have to disconnect the internal battery and replace it with my 3.7V pack (to avoid putting my pack in parallel with that one?) \$\endgroup\$ Sep 19, 2021 at 5:22
  • \$\begingroup\$ Way better than your initial idea. \$\endgroup\$
    – fraxinus
    Sep 19, 2021 at 7:31

1 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.)


  • \$\begingroup\$ That's actually the same GPS device I am using - have you run this long enough to get an idea of what the best settings are for that device? (and how much longer you are seeing life in the LWT3 - or if it's essentially infinite depending on the size of the vehicle battery?) \$\endgroup\$ Sep 19, 2022 at 18:21

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