The other day a reader contacted me with a problem — their power bank was discharging all by itself with no device attached. They assumed that the problem might be that the power bank was broken or worn out, but it seemed to charge up properly, and it appeared to charge up devices properly.
Also, the power bank was quite new.
I resisted my initial temptation to tell the reader to send it back to the manufacturer for a replacement, and instead asked a question that put me on the path to solving this mystery.
“What do you charge with the power bank?”
I got a predictable response. “iPhone.”
Something in my head clicked.
“Do you keep a Lightning cable connected to it?”
“How did you know?”
Here’s how I knew.
Lightning cables, specifically USB-C to Lightning cables, draw a small amount of current when they are plugged into a USB-C port, and this can be enough to trick some power banks to stay switched on, slowly discharging it. This can be more of a problem with smaller capacity power banks than the big ones where the discharge is much less noticeable.
True story: I had a power bank that did just this that had an LCD display on it. One day I was going though baggage checks at a train station and the mysterious flashing black box with what looked like a timer on it and wires coming out raised a few eyebrows!
Many modern power banks have a low-power trickle-charge mode that has to be activated to charge devices that draw low current to prevent this kind of discharge. Problem with those is that you have to remember how to activate the mode when you need it!
So, there you are. A cable with nothing connected to the other end can discharge your power banks.
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