My first homemade spotlight

[Recounter]

Caution, noob ahead, I have lots to learn! I want to make a very bright spotlight. I got a 75W HID and ballast off of ebay but I am having trouble powering it. If I hook it up to a couple of lead-acid gel cells (feeding it 12V) it lights up very well. I want to run it off of rechargeable lithium's. I tried hooking it up to 8 18650's but I only get a single flash it will not run. What do I need to power this thing?



http://www.flickr.com/photos/8393404...in/photostream

[HotWire]

8 18650 batteries would be over 30v! You might have damaged the ballast. Try it again on 12v to be sure it's alive.

[Recounter]

8 18650 batteries would be over 30v! You might have damaged the ballast. Try it again on 12v to be sure it's alive.

I wired them up in series/parallel to provide 15V. I just tried it again at 12V and it is working. Are the 18650's not able to provide enough amperage to start it up?

[moderator007]

I wired them up in series/parallel to provide 15V. I just tried it again at 12V and it is working. Are the 18650's not able to provide enough amperage to start it up?

May depend on which 18650's you are using.

[Recounter]

May depend on which 18650's you are using.

These are what I got. They are supposed to be good quality.

http://shop.monsterflashlight.com/1-...?categoryId=-1

[kramer5150]

its possible you are tripping the protection circuit... or you are right on the border line.

On a cold start an HID ballast will draw almost double the current, IIRC they are rated at steady state after warming up. I think my 55W ballast draws 7-8 Amps (approx) on a cold start. Although admittedly I have never been brave enough to meter it... dont want to fry my meter or blow its fuse.

[kramer5150]

further thoughts...
4S-18650 would add up to 16.8V... your ballast should be OK with that. But that's not a given by any means.

Heres a short vid of a 35W ballast that draws over 60W (5.35A) on a cold start. Thats a difference of 25W, or a 71% increase over steady-state. AFAIK all automotive HID ballasts behave this way to some degree.
http://www.youtube.com/watch?v=1CSeQCVPCHk

75W / 12.5V = 6A at steady state for your build
It wouldn't surprise me if your ballast is drawing ~100-105W on a cold start though.

when you split that between the parallel cells thats over 50 Watts from each 4-Series cell arrangement. Factoring in voltage sag the 16.8V will drop to ~14.8V

50W / 14.8V = 3.37A

3.37A is a pretty hefty current draw for an inferior 18650. I would contact monster flashlight and find out what current their protection circuit trips at, I would also try and find out what raw cell they use... Sanyo, sony, panasonic (etc). With 8 cells thats a lot of opportunity for at least one to trip. I would think even if one cell trips, that would disconnect its 4S array so all 100W will be dumped on the second 4A array and the current inrush would trip that set as well... resulting in 0V.

[Recounter]

I checked with Monster Flashlight for specs on their 18650 batteries (see below). If I am reading this correctly, they should not cut out below 5A current. They said their battery mfgr. is "proprietary". I am going to attempt to measure how much amperage this ballast pulls when it starts up.

• Monster Flashlight Extended Capacity 18650 with PCB
• Li-ion Lithium Rechargeable Battery
• Capacity: TRUE 2600mAh
• Nominal Voltage: 3.7V
• Recommended for optimal performance in all Monster Flashlight products.
• Safe and environmental friendly.
• life time is about 10 years. Over charge and discharge protection.
• Maximum Charge Voltage: 4.2v
• Maximum Cut-off Voltage: 2.75v
• Maximum Charge Current: 1C5A
• Maximum Discharge Current: 1C5A
• Impedance (Max at 1000 Hz.): 70m ohm
• Charging Temp : 0 - 45C
• The voltage of full charge & empty condition is 4.2v & 2.75v respectively.

[BVH]

I checked with Monster Flashlight for specs on their 18650 batteries (see below). If I am reading this correctly, they should not cut out below 5A current. They said their battery mfgr. is "proprietary". I am going to attempt to measure how much amperage this ballast pulls when it starts up.

• Maximum Charge Current: 1C5A -so which is it? 1C OR 5A? 1C = 2.6 Amps with this cell
• Maximum Discharge Current: 1C5A Same as above?
  
  wiring in 2P will still give only a theoretical 5.2 Amps which is most likely not enough for this ballast
  

Your ballast is most likely tripping the over current feature of the protection circuit. Also, unless you have a megabucks meter that can measure DC inrush current exceptionally fast, (many thousands of counts per second) or you have a Oscilloscope you will not see the actual inrush current. The 2.5 - 3.0 times current happens for only nano seconds and most meters cannot react that fast.