I got a couple of these USB batteries from Costco, expecting nothing and hoping for the best.

This morning we had a dead battery issue on the Hyundai. Now we had a scheduled power outage a few weeks ago and I pulled the USB battery out and plugged it in to charge and never bothered to unplug it, so this morning I had a fully charged USB battery.

I popped the hood, hooked up the cables to the terminals, powered the USB battery on and then plugged the cables into the port. The dome lights came on as well as the door chimes. And she started right up, as if the car battery were fully charged.

Ok, so these little things work. They have only one obvious flaw that I can see; they have to be charged before you need to use it. Now I could leave this plugged into the wall at home (micro USB) and I’ll always have a charged battery. The problem is I want to be able to do jumps away from home. So I could plug this into any of the car’s USB ports and leave this in a continuous charge mode. I’m just concerned something will get jostled and then when it’s time to step up and use it, it’s dead.

Any suggestions on how to have this thing ready at the moment’s notice?

2 Answers 2


I wouldn't be too much worried about discharge. Put a reminder to charge it once per year and you'll be fine. If after that year they still have nearly full charge, you can put the next reminder after 2 years.

All of the new USB batteries are lithium ion. There are two ways a lithium ion battery could discharge:

  1. Self-discharge. However, that's very minimal. Unlike non-LSD NiMH and lead acid, lithium ion batteries self-discharge very slowly.
  2. Soft off power usage. This is for the case where the battery device doesn't have a mechanical on/off switch but rather a circuit that's always powered on and then a push button that merely sends a signal to the circuit to turn the device on.

Out of these two, (2) is the most likely cause of lithium ion battery losing charge, since self-discharge isn't really an issue with lithium ion.

I recently purchased an Ecoflow power station (210 Wh, 300 W true sine wave AC). I set its charge limit to 95% so it never charges above this SoC to prevent rapid full charge degradation. This charging was a week ago. Today, I turned the device on and it still has 95%. So I can conclude it has lost less than 1% of charge, and therefore the soft off power usage is so small that the device will have charge for at least 1.8 years if not more. (This device doesn't have a mechanical power switch but rather a soft off circuit.)

However, it isn't entirely impossible for a poorly designed soft off circuit to rapidly discharge a lithium ion battery. I bought a beard trimmer that I use every 2-3 months. Annoyingly, if I charge it to full, use it, and wait 2-3 months, then the battery is completely empty and I need to charge it again.

There's no reason for a soft off circuit to consume so much power that recharge every 2-3 months is required. However, this beard trimmer is very poorly designed. I have several flashlights with soft off circuit and they consume so little idle power they retain the charge for at least years if not decades.

If your USB battery has a state of charge indicator, you can do an experiment: charge it, wait for a while, then see what charge it has left. If the indicator is percentage (0-100%) then you will very quickly get an estimate for how many years the charge is retained. If it's only 4 or 5 bar indicator, then obviously getting an estimate may require months if not years.

None of my USB power stations have high soft off power usage. I have several USB power stations (USB only, not AC like the Ecoflow) that also retain the charge for very very long time.

According to Wikipedia, lithium ion has 2-3% self discharge per month. If that's accurate, then it would take a year and half before the self discharge would be a serious issue.

  • Lithium ion is not that common in jumpstarters. More often LifePO4 is used, as it has a way higher power density (but a lower energy density), so it has a far easier time to crank the starter. USB packs for jumpstarters are not that commonly designed for general purpose powerbank, because their low energy density
    – Ferrybig
    Dec 6, 2022 at 15:03
  • 1
    For the last few years I've had a Costco-purchased jump starter pack tucked away behind a seat in my truck. The truck is kept outside in a high-desert climate. About every 6 months my memory nags be to look at it and recharge it. I bring it inside, check the charge level (turn it on) which is always above 95%, recharge it anyway, and throw it back in the truck. Using it to start a Honda Civic drained all of 2% off the charge level.
    – Jon Custer
    Dec 6, 2022 at 17:49
  • Many jump starters have lithium cobalt oxide batteries for some reason (I'm not sure why), so definitely not LiFePO4. Lithium cobalt oxide is the oldest lithium ion, I think.
    – juhist
    Dec 7, 2022 at 15:55

Rechargeable batteries do self-discharge and will lose their charge in the matter of a couple of months. Depending on the battery type, the self-discharge rate will be different, but will happen.

With that said, your best bet is to do exactly what you stated: leave it plugged in in your car. By doing so, it should always be ready to go unless you don't drive your vehicle for long periods of time. There are a couple of things here:

  • To ensure it is always charged, make it a part of your regular maintenance. You should be checking your tires and filling your car up on a regular basis. When you do this, check the charge of your battery pack. By making it a part of your regular maintenance, you'll also know if the battery pack is no longer taking a charge, at which point you'd want to replace it.
  • These battery packs are designed to only be charging as needed, so you shouldn't have a problem with overcharging or ruining it from being constantly charged.
  • Before you put it in your car for long term storage, ensure the pack is fully charged. You never know when you're going to need it.
  • When you plug your battery pack in, it needs to be on a switched power source. If you don't, it will continually be pulling off your vehicle battery. If the car were to sit for a long period of time, it could deplete your car battery, and then the battery back up would self-deplete, leaving both batteries in not so good of shape.

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