Headlight wire voltage drops from 52V to 6V

[emjayess]

Hi everyone!

When I connect my multimeter to the headlight wires without the light, they read 52.4V when lights are turned on. When I connect the light, it immediately drops to 6.x volts. When I disconnect the light from the wires, it slooowly increases back to 52.4V. If I turn the lights off and on at the control, it resets to 52.4V immediately.

It started with the stock light. Thought it was broken because it was so dim. Blew up a nice Lezyne light because I didn't check the output first and the initial 52V jolt fried it. Ordered a cheap light from Amazon for up to 60V input and it was great for a few seconds before it suddenly went dim. Thought "that's what I get for going cheap" and ordered a nice light from Juiced. Same thing, but this time I monitored the output and discovered it wasn't the lights at all. But what is it?

Everything else on the bike works great.

Thanks in advance!

 

[amberwolf]

If the light output comes from a display (most common), there is only a tiny transistor in there to pass the power to the light when the display turns them on (usually with the backlight control). It can't handle much current, so if a light requiring more power than it can handle (probably any more than the original) is attached, it will blow that transistor. It can also fail for other reasons, such as a problem with the original light (or wiring) that overloaded it.

If the display doesn't just use a transistor to pass battery voltage, but actually uses a converter to generate a lower voltage (about 6v is common) it may blow the converter's output stage when a light requiring more power than the original is attached, and actual behavior depends on how the stage fails.

In either case, replacement of the components responsible for the output is required, or replacement of the entire display with an identical one (and then using only a light that only requires less power than the max limits of that display).

If you need a higher powered light, then just leave that display alone (with it's failed output) and just wire a new connection from your battery to the new light, with whatever type of switch you prefer (that is rated for the voltage and current the light requires) to turn it on and off with, mounted wherever you want it to be.


Even if the light control is not in the display itself (is in the controller or some other module), the advice above still applies.

In any case you can open the system up to find out if the part is replaceable; locating the specific part will require tracing out the connections between the light and the parts on the board(s).

 

[emjayess]

So since the light is connected to the controller, you're saying it needs a new controller?

 

[emjayess]

Since the voltage is 52V and drops, either something in the controller is allowing the full power to come through before a load makes it step down to where it's supposed to be - or it's supposed to be full power and something is failing under load and drops power.

Either way, it's something in the controller, right?

 

[amberwolf]

So since the light is connected to the controller, you're saying it needs a new controller?

If you don't want to repair it, or create your own lighting wiring and switch, then yes, you'd have to replace it, if the controller has electronics that control the light's power and those have failed.

If the light you want to use requires more power then the electronics can handle, or there is a problem with the original light, then replacing the controller won't correct that problem, and they'll fail with the new light (or original) just like the one you have now.

So you should first determine the cause of the failure before replacing things, unless you have a big enough budget to just replace stuff until you get the result you want.

For instance, if your original light is designed to be powered by your battery voltage, you could just connect it directly to the battery to test if the light itself still works. (same for the new light). If there is a problem with it, then as long as the battery has proper protection, it will not damage the battery. (if the battery does not have proper protection, then the "right" type of problem with the light could damage the battery or the wiring between battery and light--if you like you can use a light bulb (like a regular incandescent 100w household light bulb) between the positive battery wire and the positive light wire, to act as a protection. If the light bulb lights up it can indicate there is a problem, because the light is probably drawing far more power than it should--but the light bulb will help limit the current to prevent this problem from overloading the battery).

If your light is not designed to be powered by battery voltage, then you would have to use a voltage source of the correct voltage, that can also provide as much current as the light requires, to test the light.

 

[amberwolf]

Since the voltage is 52V and drops, either something in the controller is allowing the full power to come through before a load makes it step down to where it's supposed to be - or it's supposed to be full power and something is failing under load and drops power.

Either way, it's something in the controller, right?

Unless the problem is caused by something in the light that is overloading the controlling electronics, which haven't failed but are instead turning off to protect themselves and/or the light.

 

[emjayess]

Thank you for your time and help.

I fixed up this e-bike and gave it to a disabled homeless guy (electrocuted while self-employed) and he rides it almost 30 miles a day even with his injuries. I want it to be reliable, so to avoid any other issues that may be undiscovered, I'll order a replacement controller from Bafang OEM.

 

[emjayess]

Update: I found the problem. On this e-bike there is an inline circuit between the controller and the lights. One of the outputs had failed and was not staying at 52V. I forgot I had an old kit I never used and it had a compatible controller and this circuit. I replaced the circuit first and viola! The lights come on and hold at 52V! Far easier than I thought considering I had to remove most of the wiring to get the controller out of the frame! LOL

Bonus: the kit also had the same display, so I can also swap that out and give him a working USB port on the display again!

 

[ZeroEm]

I'm out of the 48/52v group so always add a stepdown converter for how many amps that may be needed. Latest one is stepdown to 12v 20a. Have running lights, head lights, tail lights and a plug in 12v USB port.

If you need bright lights then don't strain the display, setup what you need.

 

[emjayess]

It's never been about bright lights. The controller puts full battery power to the light connectors so I had to find a light that can handle it - and most of those lights are 1000LM+.

Originally, I assumed it was like my e-bikes and threw a new Lezyne 500 on there. POOF! $65 gone! That's probably what killed that circuit, which I'm assuming is some sort of surge protector? I checked the voltage before and after the new one and it was the same. So since the headlight worked fine connected to the tail light wires after the issue started, it must have been what caused the issue.

 

[ZeroEm]

There are the light that can handle up to 60v. Running 72v so i'm stepping it down to 12v and can plug in a usb port and motor cycle lights.

 

[E-HP]

There are the light that can handle up to 60v. Running 72v so i'm stepping it down to 12v and can plug in a usb port and motor cycle lights.

I'm using one of these. Seems to handle 72V (from the cycle analyst battery level connector). I have a second one, but not sure the CA can output 40W, and one is plenty bright.
https://www.ebay.com/itm/354179613114?hash=item5276c001ba:g7AAAOSwST1i2hXi&amdata=enc%3AAQAHAAAAwNMtcfnjnN6m42aMZvvlPB1Qt7rSVHPfBWgrJYOa8x1owrZSzK2XVePV%2BBTXp3GuRQ%2FVa2WEMFqyesppcWN%2FXqULEa9dJrHql6R0Wvkuq7p%2FmYxn5RAnUmsnAV0zYPMaelateu%2FVX%2F5UGRj7facQwk%2FV1Y3SHtV0qkmZOoWO2rgC4Yd961duXqYgYAxuiVuYV6Qt%2B0CC7hDejrtxHjgd5OAYkG5e64MKMAX4rWVIHq1GawPtKBgAAqjHvDghW3CQtQ%3D%3D%7Ctkp%3ABk9SR7De4rOiYQ

 

[ZeroEm]

That's great had given up on 72v lights. Times change, what's not possible today maybe tomorrow.