The question is: is it okay to use a battery to power a headlight (or the like) while charging the same battery?

The whole story is: I'm trying something a little different on a bike I want to use for long rides, possible camping trips, etc. I have plugged the wire from the dynamo into a device that converts the AC current to 5v DC in the form of a standard USB socket. I can use that to charge a battery, cell phone, GPS, etc; or I can use it directly to power a headlight.

But if I do both at the same time, am I going to damage the battery or something?

Im , thinking A/B 2 rather than 1 .. charge 1 drain the other . lights if also dynamo powered , Off.

your bike dynamo traditionally only puts out 3w @ 6v.

drain may negate the charge so you dont actually get any charging done.

I don't see why you would be damaging the battery. It probably is just stealing current from the charger.

My USB lights will not turn on when plugged in. Check that first, but otherwise no problem.
I use a dyno for my most of my riding and would only consider a battery for roadside repairs.

Thanks for advice!
[MENTION=391744]catgita[/MENTION], to clarify, I know there are rechargeable lights that charge with a USB charger; but this is not that. My light has no internal battery. It's pretty neat but when powered directly from the dynamo and converter, it cuts out when my speed drops below something like 8 mph. That's annoying and means I have to mount it where I can reach it, so I can turn it on again when my speed goes up again. So it would be better to have a battery (or a capacitor) in there.

I did know, however, that the rechargeable ones won't turn on when charging. That's actually the reason for my concern. Is this a safety feature? If so... maybe this isn't wise?

[MENTION=197614]fietsbob[/MENTION], my longer range plan is to use two batteries as you suggest, and I have ordered two that are being shipped from China. But for now I have one biggish one. I thought I'd try this scheme out tomorrow and want to be sure I'm not going to ruin the battery.

my guess is that it's just a design decision. Or maybe they are limiting the current that the charger has to supply.

Hmmm... do you unplug your car battery while driving/charging? Note, that a 12V car battery can discharge down to 11V or so, but charge at 14 or 15V. Lithium batteries may not have such large voltage swings.

With Lead Acid batteries it is common to use the battery as a power sink, and smooths out any current ripples. I don't see why that wouldn't work with other battery types.

One issue you may run into is that most Lithium battery packs come with a built in power controller. At 75% charge, it should work reasonably well being connected to the battery. At 100% charge, it may tend to cut out rather than letting you discharge.

Since the charge controllers are small and cheap, I'd probably add 2 charge controllers to the battery pack. One for charging, and an independent one for discharging. That way both functions are independently regulated. For your lights, also add a battery bypass switch.

Look for one with "pass through"; will power a device while recharging.

If it works it's fine to run the light and have it connected to the charger.

Problem is finding one where it works, a lot of lights turn off if you plug a charger into them while they're on, and won't turn on without removing the charger.

There are a lot of variables in the design of a light, a charger and a battery module. Nowadays they are not simple electrical devices but can employ a fair amount of electronics and even firmware. Without knowing the design details of a particular setup, it's impossible to say with certainty.

My B&M Ixon Core headlight can run while charging, but a lot of lights won't do this. I suspect it's hard to design such a thing, or maybe it costs more to make it that way.

I suspect you won't damage anything if you can get your project to work. You may want to test the voltage while it's charging. I don't know what Li-Ion batteries do when the input charging voltage is too low, but I suspect they'll just ignore it, so to speak.

Okay, another update to this hairbrained scheme of mine!

Turns out, the short answer to my question is: no.

Several of the replies above already anticipated this, but here's the whole story. I soldered a USB plug to the wire to my taillight so it would function the same as my headlight (but the taillight has no on/off switch). I plugged both into my large USB battery, which has two outlets. The battery has an on/off switch. Powered the battery on, taillight came on; switched the headlight on, headlight came on. Plugged 5v input from the dynamo into the battery; both lights were on, good and bright, so far so. Rolled the bike out to the driveway, and noticed the lights were off. WTF? On closer inspection, the battery was off. Turned it back on, got on, turned the light back on, and as soon as the wheel turned it was off again. Well, it turns out the battery automatically turns off its power outlet(s) when it is charging.

So I left the taillight plugged into the battery, and powered the headlight direct from the transformer thing. The only problem with that scheme is that I had to press the on button on the light every time I started from a stop, which happened about five times.


By the way, the power converter, or transformer, or whatever we should call it, is the silver box included in this set:
Generator Bicycle Bike Dynamo w USB Charge for Cellphone Mobile 12V | eBay

AC power comes from a Shimano Alfine dynamo hub; I'm not using the dynamo that was included in the photo.

The headlight is this one:
New CREE 1200 LM XM L2 T6 USB LED Headlamp Headlight Bicycle Bike Light 3 Modes | eBay


I don't remember what the taillight is; something similar to a Spanninga Pixeo.

Many of us think this ought to be easy, but so far, it's proven to be hard to accomplish. Maybe it will be solved at an acceptable price level. It does seem ideal to be able to run off a dynamo when possible and off a battery when the dynamo isn't putting out current.

Actually, what I've got going is pretty good.



This is one of those cheap mini-racks that Nashbar used to sell complete with a miniature pannier set. The transformer and battery are neatly tucked into the pocket on the left. The headlight is made for handlebar mounting, but the improvised mount shown (consisting of a small piece of handlebar and a zip tie) is remarkably robust. I realize it's not going to win any style points, but this is all about function over form.

Mechanically, it looks great. I hope you can work it out so it works the way you want it. When you do, I hope you post the results.

Flashlight Bulb just replaced by an LED .. lazy optical design ,

to my experience, after using B&M's IQ 2 Shaped beam headlights for several winters .

But I guess it was Cheap to try, as Is.. for your experiment.

Yes, look for a battery pack that supports pass-through charging. However, even if it does work, you might shorten the lifespan of the battery pack.

I had one of these https://www.amazon.com/External-UPGRA.../dp/B00ZDQFT6Q and I did something like what you mention. I used my dynamo to charge the battery while using the battery to keep my phone topped up. After doing that on a regular basis for a few months the battery pack stopped working. The manufacturer was nice enough to send me a new one, but I will minimize how much I use it for pass-through charging from now on.

Should not damage the battery at all. Some packs may not support this mode though.