I recently got a cygolite hotshot 2w rear light. Being a curious person, I disassembled it to see it I could find out the capacity of the rechargeable battery. Mission accomplished, it is 830mAh 3.7v Li-ion battery.

What I am wondering is hot does the light perform as the battery is running low/out of battery? Their is a secon small LED that flashes while charging, I am wondering if it flashes when the battery gets low?

I believe that is purely for indicating that it is being charged when plugged in.

I ran my hotshot all the way to the end and it took a while like a couple of months. Never bothered to get off the bike to look at it for any sign of low battery indicators.

This topic was discussed before, but no one had torn one down to know the answer.

Does the battery look like a little cell phone battery (flat) or is it cylindrical?

it is a flat battery. Looking at a side view of the light, there is a circuit board where the uber bright LED is mounted, then all battery behind it. They used the volume pretty efficiently.

Thanks.

A LiPo (flat) cell is usually harder to find than something like a 16340. I had wondered if the cell would be economically replaceable, since heavy users might wear one out in under a year.

Still wondering if anyone has discovered what happens when the battery gets low, see the OP.

PM mechBgon, ask him your question.

https://www.bikeforums.net/showthread...able-taillight

IIRC, this light will "just die" when the battery runs down. no warning like changing to a "special" flash pattern, and only a short period of dimming before it quits.

if you've got it apart, you can test it by disconnecting the battery and running it from a variable voltage source. or just put it back together and run it 'til it quits.

I have one of these also. Just wondering, did you take a photo of the tear-down? Would be interesting to see what it looked like disassembled.
Does the battery come out easily or is it glued in or something?
Being as small as they are these batteries probably won't last more than a couple years. It would be nice to know whether or not they are available for replacement.

I did not take photos. The battery has a connector and is easily removable. I would be very surprised if you could not get a replacement battery already, as I suspect the case was designed around something already available. Water exclusion is accomplished via a rubber seal between the two halves, and rubber buttons.

Tear down is 3 phillips head screws. Takes maybe 2 minutes, probably less. Not complicated, and i don't have a worthy camera so I did not take pictures. You just need a very small phillips screwdriver. If you have any interest in electronics it is worth the few minutes just to watch the light work on the circuit board without the lens over it. They really could have made the whole thing half as thick by thinning the red piece. Not really sure why they made it so big, but probably for marketing reasons.

Thanks for the feedback. Good idea on the test idea, perhaps I will try that some time. Really would need a little connector to feed power into the board, and I don't have the connector.

I really like that this thing is serviceable. I have an idea worth sharing: If the old battery dies you could take it out and just replace it ( if available ) or take the old connector off the dead battery and rig an external battery. You would have to drill a small hole in the body but that shouldn't be too hard to do. Yep, one of these running off of a single 18650 ( or two 14500 cells in parallel ) would triple to double the run time.

Ok, I took it apart again, hope people are able to use this info to their advantage.

battery is about 8x28x40 mm. I looked around a bit and the batteries I am seeing in that capacity range are thinner and longer/wider. Did not look too hard though, as it is not broke today, so I am not trying to fix it.

Here are a couple of pics:

I am a bit frustrated to not be able to know when to charge. Just my personality I guess. I looked at board some more, and see that the power into the board to charge the battery goes directly to the smaller diode D1 which flashes while charging. The marking on U1 (8 pin IC) is 12F683 MSN1217. The 12F683 is a microcontroller chip. Guess that is how they allow the programming of intensity, rate and such. Looks like U2 is the IC responsible for charging the battery, can not read any marking on it.

Measured the battery remaining, 4.03V. I have used this in various flashing modes for at least 5 hours of riding. Seems like no need to charge it again anytime soon. maybe once a month? we will see . . .

The battery has 3 wires coming out of it, and is a bit different shaped on the end with the wires. This suggest to me there is some sort of battery protect circuit board as part of the battery.

Finally for now, the small black rectangle on the cover appears to be how the circuit card is held securely in place. I noticed the first time I removed it, the circuit card "clicked" back into place when I put it back together. Not any more, but it looks like no big deal, as the manufacturer but an assurance that things would not move around. Best I can tell the battery is held in my just the circuit card and friction, so it is nice/required that the card is held securely in place. Nice simple design.

Fun investigation, not exactly how I planned on spending my day off work, but now it's time to ride a bit. :-)

Prior to my original post, I queried Cygolite to see what their response to my question would be. Finally got an answer back today affirming there is no low battery indication. I followed up with asking,

Will the battery be damaged by allowing it to discharge to the point where the light stops working?

The response I got back was:

It is possible the battery will be damaged if it is discharged to the point where it will not function so we do recommend having some kind of charge on the light at all times.
Putting it on the charger for an hour or so every month it is not being used should prevent that from happening.

??? Most all Li-ion powered devices have an automatic cutoff that prevents discharging the battery completely. Considering this device has a circuit board with ICs, it's almost inconceivable it doesn't. Perhaps they're talking about self discharge that would occur if left discharged for a long time after it has been run down to the cutoff?

I never let mine run out completely - end up charging my hotshots every 2-3 days

Same here. I usually charge on Wednesday and Friday, depending on my commute schedules and weekend riding. I also use the pulse setting which uses more battery than the pure flash setting. I could easily go all week, but I'm nervous about having my lights go out.

I plan to just check the voltage on the battery every week or so at first. As I listed above, after 5 hours ish the voltage was down very little. I still had roughly 75% capacity. I commute less than 10 hours a weeks, so I will likely go 2 or so weeks between charges

+1

sbslider, thanks for all the info, especially the pics.

you are welcome.

So far , here is how my hotshot is discharging, initially charged 12/25 100%
measured 4.03v on 1/2. ~76% left
measured 3.91v on 1/6 ~62% left. Did a lot of riding last week
Measured 3.83v on 1/11 ~43% left, rode 107 miles since last check

Seems like every 3 weeks ought to work, not too discharged but tons of time between charges. Your mileage may vary