If the batteries run out while you are riding, it can be risky if you are riding in the dark without functioning tail light. So is there a way (beside using fairly new batteries) to know the tail light is no longer lit while you are on the saddle? Actually not just the batteries, but whatever reason making the tail light stop functioning (maybe it fell on the road). I tried looking at the mirror but it doesn't show the light.

I wish I knew as that very thing happened to me last week. I kinda gave me a sick feeling in the stomach when I noticed it was non functioning. I think I will get another and use two. I may get one to go with the Magic Shine.

Depending on the light it may have a failsafe mode. When my batteries get too low to continue running in whatever flashing mode I have set it to the light turns on solid red. Granted it is at a much reduced power setting but at least there is something. Other than that I will periodically at a stoplight check it. The light I am using is a Dinotte 140R-AA-R

You could get a piece of fiber-optic "cable" and monitor your taillight that way. In darkness, my own taillights (DiNotte or Nova BULL) are strong enough that a glance in my helmet mirror usually shows they're running. It never hurts to have secondaries, though. If you can clip a light to the back of your helmet, that also helps you show over the top of passenger cars in traffic, so that's one way you could go.

Good point! I do this too when I am out in pitch black. I actually use my Dinotte on EVERY ride, day or night.

I use a helmet mirror, and now the MS tail light, and before with a PBSF.
I can tell it's working by the red reflections coming from behind me, or even the red illumination of the ground behind me if it's dark.

My primary commuter bike has two PBSF tailights and a Dinotte 140L. Losing one isn't critical.

I can usually tell if my tail blinky is working in pitch black conditions just by glancing back, chainstays even reflect enough to check there. In twilight or daylight conditions, I check on my seatpost mounted light by sticking my hand behind it.

I just use two tail lights -- if one dies, the odds are very good that the other one is still working.

#1 - use rechargables and never let them get anywhere near empty. The light is running much dimmer for probably weeks before it gets to the point where it doesn't work at all anymore, so it's not smart to let the batteries get anywhere near low enough to quit during a ride. I recommend low self discharge cells like Eneloops.

#2 - do not ever ride with only one taillight, for precisely this reason. Then at least you've got one light if you lose one. I ride with at least two, sometimes as many as 4, plus a reflective vest and nearly a square foot of reflective crud on my bike. My primary taillight is a Magicshine, secondary is a Superflash clone on the helmet, #3 and 4 are two superflashes on the seat post pointing to the sides.

+1 on always having 2 blinkies. They are so cheap, and the batteries last a long time so there is no reason not to run 2 if you ride much in the dark

Another reason is that you don't have to worry as much about aiming them. If you have 2, it's more likely that one is pointed at the guy behind you

I have a Blackburn Mars rear light, it is about 8 months old and I have yet to need to replace the battery...

Cateye LD1100 mounted at base of seat tube. Just look between my legs and there it is.

I use a Dinote 600L and 140R, both powered via a Y-cable from a 4-cell battery in back.
If the battery is low, the headlight will let me know.

I use a blinky taillight, but I consider my primary back-facing light to be reflectors. Vest, reflective tape on the helmet, cranks, rims between the spokes, etc. I use rechargables and try to keep a good idea of their run time. It's not always perfect, but its pretty good.

This is what I usually do for evening running and walking; one of the PBSF knockoffs clipped to the back of a ball cap or headband. It goes well with the class III vest.

My night use bikes both have dynamo hubs and both dynamo powered headlights and taillights installed. The primary bike also has a Dinotte 400R battery powered taillight mounted which is bright enough so you can definitely tell if it stops blinking. The second dynamo hub bike runs a PBSF or Radbot 1000 taillight.

I also just installed a dynamo hub on my Big Dummy and have a dynamo light set on order for it.

I like dynamo lights as unless the wiring is damaged or there is a electrical failure you are independent of batteries and their discharging on you unexpectedly. As most battery taillights that run AAA batteries do not have voltage regulation the light starts dimming shortly after new batteries are installed, particularly if running alkaline batteries. If using NIMH batteries the light is never at full brightness due to the lower voltage of NIMH batteries compared to fresh alkaline ones.

I bet that if you put new batteries in it you would be surprised how much brighter it is.

As already suggested it's a good idea to use two taillights. I mount one under my seat and one on my backpack. One flashes the other solid.

I always check my lights before I go on a ride. One light I can sense is the light I attach to my helmet. It lies low to my neck causing me to feel it's warmth.

Sounds cozy--good for winter . Just remember it will cool down only a while after it stopped functioning.

Just in case this may happen to someone: this evening I clipped the tail light on my backpack, new batteries. All the way I rode with confidence knowing the light is flashing behind me. Then once when passing by a window I turned to look at the window to see if the light was OK. I didn't see any light. Turned out the hood of my jacket was resting on it and covered it. I was only lucky that no accident happened as a result.

I can see my tail lights with my helmet mirror.

I can't unless there's a sign behind me. The MagicShine will light up a street sign up to about 1/2 mile away. But if there's no sign, there's not really anything for the light to bounce off of, and the pattern of the light only goes to about 180 degrees so I can't see the light directly either. Besides, it's on the back of my saddle and I couldn't see that anyway.

Like others have said, redundancy is a good thing. Both front and back.