I have a device (a vaporizer) which contains a rechargeable battery. The battery only takes a couple of hours to recharge, but sometimes I end up leaving it charging overnight. I have monitored the current drawn by the device when charging, and to my mild annoyance it still draws about 0.02A even when fully charged.

I don't know how to stop this small current draw. I also don't know whether I need to worry about this (i.e. whether it's decreasing the battery life). I have two choices when leaving the device charging overnight -- I can either use the mains, or a portable USB battery (i.e. use a second battery to charge the device). My portable USB battery turns off automatically when current draw is zero -- however the current draw of 0.02A pulled by the device even when fully charged stops my USB battery from switching off.

Is there something which I can buy, which has a USB input and output (or a micro USB input and output) and which stops conducting electricity if less than about 0.05A is flowing through it? Or is there a USB battery which turns off when less than about 0.05A is being drawn from it?

Some more details -- I know very little about the rechargeable battery in the vaporizer. There is no mention of what battery type it is in the manual; it's a portable vaporizer, so its job is to heat something up to 200 degrees celsius and keep it at this temperature. The specs on the device are that it wants a 5V input to charge and max power input is 10W; the charger that came with the device is 2A so this checks out. I bought one of these:


to monitor the current of the device whilst charging; it typically starts at 1.5A and then drops to around 0.02A when fully charged.

My main concern is that this constant drawing of a small amount of charge will degrade the battery in the device. A secondary concern is that leaving it to charge overnight represents a fire risk; most devices I have stop taking charge after some time and if I'm charging them with my USB battery then the battery switches off; this does not.

Am I fussing about nothing?

  • \$\begingroup\$ Yes you are. That behavior you describe is normal. Also, what are you measuring with which has better than 20 mA accuracy? \$\endgroup\$
    – winny
    Commented Jun 22, 2017 at 19:40
  • \$\begingroup\$ @winny 20mA isn't that little, most multimeters will do that with a reasonable accuracy \$\endgroup\$ Commented Jun 22, 2017 at 20:03
  • 1
    \$\begingroup\$ It could be the current draw of the battery charging/tending circuit. Or just whatever needs to be drawn to maintain a "full" charge. \$\endgroup\$
    – Chris M.
    Commented Jun 22, 2017 at 20:11
  • 1
    \$\begingroup\$ @MarcusMüller A multimeter is fine. I just imagined one of those mains power meter on the primary side and no-load + offset will give you 0.02 A. Still, even with multimeter on the secondary I forsee stuff connected in paralell with the battery, perhaps an 20 mA LED for indication. \$\endgroup\$
    – winny
    Commented Jun 22, 2017 at 20:19
  • \$\begingroup\$ yeah, true, 20 mA really happens to be the usual LED current \$\endgroup\$ Commented Jun 22, 2017 at 20:24

1 Answer 1


Addressing your concerns in turn:

  1. Battery damage.

All depends on how good the battery charger is. The best treatment for a lead acid battery is to maintain a trickle charge after recharge. The best treatment for a lithium ion battery is to cut the charge to zero. The charger could be doing either and still be drawing 20mA for its own purposes.

The manufacturer could care about battery life or they could not. Whether its causing damage is probably up to how much you trust the manufacturer!

  1. Fire risk.

Certainly not while operating correctly and only drawing 20mA, but anything left on is more risky than being physically switched off.

In general, I sympathise with the concern but think it's not worth worrying about:

  • 20mA @ 5V is 100mW and wont make a dent in the grand scheme of things. Charging from another battery is its own issue.
  • Whether the battery is being treated well or not is really a product quality consideration - is the product well made and likely to last a couple of years? Then the battery charger is probably well made too and nothing to worry about. Is the product crappy and likely to be chucked out in a year? Then who cares if the battery only lasts 2 years?
  • Of all the things that could catch fire, is this one likely? Probably not. Consumer products have safety requirements to meet, and there's bound to be someone that treats the product worse than you that will be the first to prompt a product safety recall.

If you're not satisfied, then join the campaign against Phantom Power - there are plenty of guides, tips, products and advice associated with reducing phantom power. You can use automatic detection, timers, rules and triggers to turn off devices, but it's a rabbit warren to navigate. The Lawrence Berkeley Laboratory's Standby Power site is a good place to start - there's even a chart with battery chargers on it.


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