Question for the experts

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greaser_monkey_87

New Member
I need a little advice with my electrical system. It works, and all the wiring is correct, but my battery does not charge as well as I would like. Before I get into details, let me add that this is not a kit bike, and there is no way to draw any power from the engine, which is why I built the system that I have. So here is my setup.

I started with a tire-driven generator. I wired this to a rectifier/regulator to convert the generator current to DC. I am using a 12v, 1.2Ah battery and I am using LED's for my headlight and brake light. The system peaks at 17-18v at speed.

The problem: the battery charges fine when the headlight is off, but when I run the headlight the battery drains, not super fast, but what I guess would be a normal amount for a battery not hooked up to a charging system.

What I theorize is the cause: both lights can run on anywhere from 8-30v, so the headlight is taking whatever is being thrown at it, i.e sucking up all the current in the system. This doesn't cause a problem with the brake light, because the brake light is only on when I'm stopping (tail light is separate from brake light).

On to my question: what is the best way to limit current to the headlight, so that the battery will charge while the headlight is on? A resistor? A zener diode? I considered a voltage regulator, but a linear regulator will just bleed excess voltage to ground, not allow it to go to the battery. I'm not sure if a switching mode regulator will do what I want it to or not.

To keep replies short and simple, I don't need a long-winded explanation of electrical theory, or why a certain part will work better. I just need to know which part will work best, and any pertinent info related to the installation/application of that part. I can get to radio shack faster than I can read a lot of extraneous info on theory, lol. Not trying to be rude, just that installing the right part doesn't require understanding the theory behind it. So just trying to help keep the answers as short and simple as possible. Thanks :D
 
As you asked, I will not share electrical theories, only charging/ lighting system facts.
Still, it most likely will not be what you want to hear.

Two questions:
1. When you performed your voltage test was it done with the generator unloaded or under the load from the battery needing a charge while the lights were in operation?

Tire driven generators produce 6 volts under a load. Unloaded they can easily exceed 20 volts if spun fast enough ( very, very fast).

2. Does the total amp draw from the lights equal less than what the tire driven generator can produce? I ask "less than" because any excess available power will be used to charge the battery and the system needs to be balanced to do this.

I suspect that you are drawing more power than what the generator can make.
Limiting the power to the head light will reduce it's brilliance, possibly even causing it to shut down if it has a driver circuit (which I highly suspect it does).

You cannot begin to tweak and adjust the system until you know exactly how much and where the power is going. It needs to be balanced first.

Using a resistor to limit the power to the headlight will only waste the energy as the resistor converts the energy into heat.
A Zener diode will limit the voltage only. You do not have a voltage problem. You have an amperage problem.

I cannot recommend any specific part(s) other than a higher powered generator such as a Shimano, Sturmey Archer or Sram Dynamo hub. The problem with these in your application is they only produce 6 volts under a load. You have a 12 volt system.

Radio Shack does not sell what you need.
 
Answer to your first question, on-board voltage gauge, in-depth explanation is this. All my positive wires are connected to a power distributor, so the voltage meter measures battery voltage when the generator is not spinning, and it reads the voltage from the generator when the generator is spinning. Remember that the generator is wired to a rectifier/regulator, so the voltage is being read coming off the rectifier. I was driving one day, and I flipped on the voltage meter (led readout, only displays voltage) and it peaked between 17-18v. The generator is 12v, not 6v. I am not sure how many watts/amps the generator produces at motor speeds, but the headlight is 5w, so at 18v, that's 0.27A. I don't think it's the amperage, but the no-limit load voltage of the bulb that's the problem (up to 30v anyway, and the generator peaks at 18v, so the headlight isn't anywhere near it's load limit). I don't think the headlight will shut down from limiting it's draw, since it can run on as low as 8v and I want to limit it to 12v. I guess what I'm asking then is what can I add to the circuit to put a lower load limit on the headlight. Seems to me that the battery has a lower load limit than the headlight, which is causing the system to be out of balance? Am I way off here?
 
It can't be an amperage problem. The generator came with a headlight and tail light, and the bulb for the headlight is 0.3A. And that's at 12v. The generator peaks at around 18v, and the headlight only draws 0.27A at that voltage. So something else is going on here.
 
Just looked at my purchase history on ebay, the generator is 12v 6w, and the headlight is 5w, so it can't be a wattage/amperage problem. The generator puts out half an amp, the headlight only draw a quarter of an amp.
 
Now I'm really not sure what's going on. According to my figures, the headlight draws 0.27A at 18v. Initial charge current for the battery is 0.36A. Even when I'm stopped, the headlight is drawing about 0.38A, so it can't be draining the battery faster than it charges. When the generator is producing voltage, the headlight is drawing 0.27A. These figures are based on peak voltage of the generator (18v), and the battery when fully charged (13v). Is the voltage drop from charging to discharging enough to cause the battery not to charge?
 
A 12 volt battery does not like more than 13.6 volts put into it for charging. Higher voltages will cause overheating.
A slow charge will occur at 0.1C rate which for your battery is 0.12 amps. Charge time will be 14 to 16 hours.
A fast charge time will occur at 0.3C rate which for your battery is 0.36 amps. Charge time will be 3 to 6 hours.
Your calculations show that you have a very good charge capacity of exactly 0.36 amps.

The volt drop between charging, 18V (too high of voltage actually) to discharging 12V I can only see as a problem because the excess voltage is boiling the electrolyte on the battery plate surface which will disrupt the charging ability of the battery.

Amp wise your system is well balanced. Volt wise it is way off balance.

I suggest doing a live system amperage draw test instead of a "on paper" amperage draw calculation test to find exactly, beyond a doubt, what is actually happening in your system. The real world facts instead of the on paper facts will be more help to you in figuring this problem out.

Also, I have strong reservations against the 18 volts going into your 12 volt battery.
6 volts excess is no good.
 
I put this system together last summer. I live in a city with stop and go traffic, so the battery is getting the 18v in relatively short bursts. Overcharging is not really possible, and the battery never gets more than a little warm to the touch. Worst case scenario, the battery will have a shorter life, and then I'll get another one for $11 shipped. But I'm not quite sure the battery is getting all 18v. The current is filtered through a 4-pin scooter regulator/rectifier. A scooter generator can put out up to 30vac, and scooter batteries don't blow up. I think the battery is getting the right amount of current. I am wondering though if I should be using a larger capacity battery. I don't know if that makes a difference, but I'm using a small 1.2Ah battery. I'm wondering if I should be using like a 6Ah battery.
 
Today after I get my rear wheel back on, I'll dig out my multimeter and see what the battery is getting. I will test it with the headlight off as well as on to see if there is a difference.
 
Testing with the multimeter reveals no power coming from the generator. Guess it's fried. Wall-charging the battery till I can buy a new generator. Disappointed now for sure.
 
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