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Summary: what are the risks / is it advisable to run rechargeable bike lights at the same time as they're charging from an external power source?

There are plenty of bright rechargeable bike lights these days. They're great, but in my experience they tend to come with modest li-po or 18650 batteries, which, after a year or so, no longer run for more than an hour before requiring recharging. That means they run out during a long ride. Also, if you've been out riding with family, it's quite a drag to have to dismount front & back lights from several bikes and find micro-USB chargers for them all indoors, then remember to remount them all on the bikes before the next ride.

I am wondering about connecting a bike's front and back rechargeable lights via their micro-USB ports to a 10,000 mAH 'power bank' sitting in a little frame-mounted bag, and charging them WHILE they're lit. I am hoping this could increase run times and that it would be easier to take only the power bank inside occasionally for topping up.

However I am wondering if anyone is familiar enough with the commonly used bike light circuit designs to be able to comment on what might happen over the long (or short term) with a setup like this.

Most importantly: will the bike lights' batteries likely be damaged? Is there a high chance of fire? Anything else to watch out for?

Maybe most bike light batteries don't charge while the light are on, and will end up just switching off when their onboard battery runs out? If the bike light battery is full, will the load from the light itself be insufficient to prevent the power bank from going to 'sleep'? Either of these situations would make the whole setup pointless, albeit not a danger.

Daniel Tork
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Robyn Hines
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  • This is not a question about electrical engineering. That is to say, it is not a question about designing any new electronic thing. You only are asking about how to use some existing product. [P.S., I have done what you are asking about. Some bike lights will turn off when connected to external power. Others will operate from the external source. It's not a feature that they usually advertise, so try before you buy.] – Solomon Slow Aug 21 '22 at 13:35

1 Answers1

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Try it out. You are very unlikely to damage anything.

From a design point of view the external supply will typically supply up to 500 mA without negotiation from the lights. (Phones, USB PSUs, etc., use the data lines to signal the max power that the PSU can supply.)

The LEDs will draw what current then need (probably < 100 mA) and the rest is available to the battery charging circuit. You can't charge and discharge a battery at the same time so the battery will either charge or discharge and your tests will confirm that. You could connect a mA meter in series with the battery to see what's going on but I suspect you may not wish to go to that trouble. (The mA reading would change sign between discharge (no external supply) and charge.)

Transistor
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