Help!!Bicycle headlamp wiring with Philips Rally H4 130/100W

Thread Starter

Rahulk70

Joined Dec 16, 2016
451
I'm wiring my brother's bicycle with a Philips Rally H4 Bulb 12V 130/100W. He needs it for like 15-25 minutes everyday.Since I have only a 12V 7Ah battery and its insufficient for this bulb as it will drain the battery really bad and damage it. Also I'm not sure if it would be legal to use 130/100W(since at 12V it will be at the rated value ) on a bicycle. So I have connected the bulb to a 6V 10Ah battery. So the bulb wattage will be like 32.5W(current draw of 5.41A) & 25W(4.2A) at 6V and its quite bright in my opinion.I have a few questions:
1. Can I run both the filaments at once?(Total watts will be like 57.5W)
2. Right now I have connected it directly through a Universal Ignition Key Switch(3 wire-3 position), but it does heat up after running for 20 minutes.Do I need a headlamp relay?
3. Also what wire gauge would be the best? Right now I'm using 18 AWG I guess.
4. Will a 12V 10A fuse be enough?
 
Last edited:

wayneh

Joined Sep 9, 2010
16,229
I'm wiring my brother's bicycle with a Philips Rally H4 Bulb 12V 130/100W. He needs it for like 15-25 minutes everyday.Since I have only a 12V 7Ah battery and its insufficient for this bulb as it will drain the battery really bad and damage it,I have connected the bulb to a 6V 10Ah battery. So the bulb wattage will be like 32.5W(current draw of 5.41A) & 25W(4.2A) at 6V and its quite bright in my opinion.So my questions as follows:
1. Can I run both the filaments at once?(Total watts will be like 57.5W)
Yes, but it will consume power quickly from the battery. A bulb filament has a negative temperature coefficient, meaning the resistance drops as it gets hot (bright). I wonder if running it below target temperature means that the efficiency is worse? You might get more light per watt using a proper 6V bulb or a 12V battery.
2. Right now I have connected it directly through a Universal Ignition Key Switch, but it does heat up after running for 20 minutes.Do I need a headlamp relay?
I think you know the answer. If the switch is getting more than just warm, then a relay is indicated. Also, this will spare the contacts of the switch. It might wear out prematurely otherwise.
3. Also what wire gauge would be the best? Right now I'm using 18 AWG I guess.
That should be fine as long as it's not too long, and it really is 18. I wouldn't go smaller.
4. Will a 12V 10A fuse be enough?
As long as it's not a "quick blow", it should be. Any bulb will have a higher in-rush current while the filament is cold and low resistance. This surge is over very quickly, but you don't want to fuse to blow before it's over.
 

GopherT

Joined Nov 23, 2012
8,012
I'm wiring my brother's bicycle with a Philips Rally H4 Bulb 12V 130/100W. He needs it for like 15-25 minutes everyday.Since I have only a 12V 7Ah battery and its insufficient for this bulb as it will drain the battery really bad and damage it. Also I'm not sure if it would be legal to use 130/100W(since at 12V it will be at the rated value ) on a bicycle. So I have connected the bulb to a 6V 10Ah battery. So the bulb wattage will be like 32.5W(current draw of 5.41A) & 25W(4.2A) at 6V and its quite bright in my opinion.I have a few questions:
1. Can I run both the filaments at once?(Total watts will be like 57.5W)
2. Right now I have connected it directly through a Universal Ignition Key Switch(3 wire-3 position), but it does heat up after running for 20 minutes.Do I need a headlamp relay?
3. Also what wire gauge would be the best? Right now I'm using 18 AWG I guess.
4. Will a 12V 10A fuse be enough?

Use an LED replacement for H4 bulb.
 

RichardO

Joined May 4, 2013
2,271
Not a direct answer to your question, however...
It is a lot more complex but you may want to consider using a high power LED. The battery will last 3 to 4 times longer.
I have a bicycle head lamp that only consumes about 15 watts and is good enough for me to ride at nearly full speed (15 MPH).

edit: Typed to slowly. @GopherT beat me to it.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
451
I think you know the answer. If the switch is getting more than just warm, then a relay is indicated. Also, this will spare the contacts of the switch. It might wear out prematurely otherwise.
The ignition key switch contacts weren't burning hot but it was slightly hot. You are right, it will be better if I use a relay.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
451
Use an LED replacement for H4 bulb.
I did think about an LED replacement. But since I had all the above parts lying in my garage I just thought I'll make one with those parts. If this project doesn't go well I might switch to an LED. Also I have a Chinese HID 35W Kit also with me. It draws a lot less current once it starts operating too. I thought about using that but then I wasn't sure if it would be legal.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
451
Not a direct answer to your question, however...
It is a lot more complex but you may want to consider using a high power LED. The battery will last 3 to 4 times longer.
I have a bicycle head lamp that only consumes about 15 watts and is good enough for me to ride at nearly full speed (15 MPH).

edit: Typed to slowly. @GopherT beat me to it.
What do you think about these two?

https://www.amazon.com/gp/slredirect/picassoRedirect.html/ref=dp_sp_detail_2?ie=UTF8&adId=A01010683S7BS8CVIVIIS&url=/dp/B01K6PCPN8?psc=1&qualifier=1483184035&id=721085468835687&widgetName=sp_detail

https://www.amazon.com/Xtreme-Bright-Waterproof-Light-Taillight/dp/B00CULFR4Q

I even saw those cheap ones for like $6–9 but I assume the LED in those would be those simple low powered LEDs.
 

Alec_t

Joined Sep 17, 2013
10,896
A bulb filament has a negative temperature coefficient, meaning the resistance drops as it gets hot (bright).
Not so for the usual tungsten filament lamp. What filament material has a negative temperature coefficient of resistance, and what would prevent thermal runaway?
 

wayneh

Joined Sep 9, 2010
16,229
Not so for the usual tungsten filament lamp. What filament material has a negative temperature coefficient of resistance, and what would prevent thermal runaway?
I just KNEW I would get it backwards if I didn't check it, and of course I did. Yes, a filament has a positive temperature coefficient, meaning the resistance goes UP with temperature. Maybe if I make this mistake a few more dozen times, I'll start getting it right.
 

Dr.killjoy

Joined Apr 28, 2013
1,196
Personal I would pick up a couple 18650 flashlight off ebay with a couple mounts and be done with..
Is you going for brightness like the rally racing lights they use in the desert at night. The second gen of rally lights have turned to Hid for there long range and pulling less current then LED or Halogen over a period of time..
 
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