I got a B&M Dynamo Toplight Flat Plus recently and am trying to hook it up. Everything's fine, except there's no indication on the unit, no instructions included in or on the packaging, for which electrical connector tab at the bottom of the unit is positive/current or negative/earth.

Does anyone know for sure which is which?

B&M part no. is 329ALK-02 and you can find the light on the B&M webiste here:
https://www.bumm.de/index-e.html

Follow the menu chain: Catalogue --> Rearlights --> TOPLIGHT Flat plus

There is a pdf available, which is the packaging that came with the light. Note the helpful instruction "Electrical connection via current cable to current contact, earth cable to earth contact." Which is great except there's no diagram and nothing printed or stamped on the unit indicating which is which...

No one else is running this unit? Huh--you don't even have to know what you're talking about electronically, just let me know which side the wire with the white stripe runs to.

OK, so I have queried:
1) Bikeforums
2) The bike shop of international renown where I purchased the light
3) B&M in Germany

Any bets on who answers my question first?

Hmm, bad instructions. If this is like any other LED I've worked with, it won't stand up to a reversed connection. Better to wait for the manufacturer's response...

If its one light running off the dynamo it shouldn't matter. If it doesn't work try the other way.
If it was critical there would be a warning.

Ask these guys. The sell B & M lights.
https://www.peterwhitecycles.com/index.html

I'm not sure about this. I don't have personal experience with the B&M system, but it appears that the taillights connect to a special connector on the headlight rather than the dynamo, and it may be that this connection is rectified/regulated. Sorry I can't be more helpful without more knowledge of the system, but I just thought I'd inject a note of caution.

Sorry, more info: Shimano dynamo hub, running a B&M LUMOTECH IQ Fly. So the taillight is run off contacts on the back of the headlight unit. The Toplight doesn't ground to the frame, so it's a double wire running from the headlight. Ground v. current is clearly marked on the headlight, but no marks anywhere on the tail.

Heard back from the mfg:

"Dear Mike,

if you connect the "TOPLIGHT Flat plus" to the dynamo or the headlight it is alike which contact is current and which earth.

Best regards"

Great...

What say ya'll--if I'm looking at the back of the unit on the headlight and the ground is on the left, current on the right, based on this response, I should assume the same on the tail light--ground, left; current, right, looking at the back of the unit? I mean, it's couldn't possibly be something silly like: on the headlight, ground is on the left side of the bike and so it should be on the left side of the bike at the tail light, which would actually be on the right as you're looking at the back of the tail light...?

As a distributor, too.

Answer from Peter White Cycles: "You can barely see the symbols if you look at the back of the unit in bright light, but it is ground on the left, current on the right."

Have to check again when I go home. I stopped just short of going over it with a loupe over the weekend...

Thanks for the suggestions...

Peter White, as mentioned before, would be a good source for info.

Here is a link to tailights at his site. Your light is discussed, but your question is not answered.

https://www.peterwhitecycles.com/taillights.asp

EDIT. I see you have already contacted Peter White. Sounds like you are good to go.

Thanks again for the Peter White info, but yes, I already have the answer from them.

BTW, from their website you:
"Most of the taillights we sell have screw posts for the wiring. The double strand wire we sell has connectors on one end only, for attaching to the bottom of the headlight. And you then use the bare wire ends for the taillight attachment. But the Toplight Flat Plus has only male connectors, meaning you'll need a wire that has female connectors at both ends. This doesn't exist all made up, so you'll need to make your own."

This is actually incorrect. The Toplight Flat Plus comes with a T-shaped plug unit. There's a post that locates into a hole in the back of the unit and then the rest of the plug swings down--thread the wires in from the bottom, bend them over toward the back, and then the plug unit snaps into the light connecting the wires securely to the connectors. So this unit can be used with any wire ending in bare or stripped ends. Also, although it did not come with wire, the Lumotec IQ Fly comes with wire to connect to a tail light. FYI.

Thank you very much for this information! I haven't found it anywhere else, but it is quite obvious, now I know, how the plug can cover the connectors.

What I can't get my head around, though, is how to thread the wires through the connectors and the plug. Should the wire go through the plug first and then through the connector or how? Has anybody got an illustration of this?

And yes, I know this is an old thread, but it is the only information about this subject, I have found.