Aux battery project

[yurizilla]

Good day,
I'm working on a project for my Moped. I'm looking to add an additional headlight to the bike, it will be used at a weekend house for fun.
My problem is that I'm trying to find the cheapest, yet effective way to make this work. My research so Far has led me to the following equation: For 12v systems
Headlamp Bulb rating 55W $40
Motorcycle Battery autozone 14 ah 12n14-3a battery $46

35w/12v= 2.9 amps (draw)
14AH/20hrs = .7 amps @20, 1.4amps @10, 2.8amps @5

12v Battery charger $40

So If I buy a motorcycle battery, hook it up to a car fog light that is rated 35W I can use it for about 5 hours and then recharge it overnight
and during the next day (battery will be removable). If I want to put back what I use during the night, then I reverse the math, say, I need a charger
that can deliver 1.4 amps over 10 hours charging time, and the battery must be able to accept this much.

What specs wil list this if it is a correct assumption for charging? I've also read about deepcycle batteries, sounds to me like they are used from full charge to discharge
rather than cranking. Is this a necessity or will the motorcycle battery suffice.

So I'm looking at adding a headlight for about $120. Could I make this project cheaper with some compromise, I'm thinking:

Moped headlight 25w $15
6V charger $20
But how many AH would I need in a 6v battery? is it 25w/6v= 4.1 amps draw 21ah/5= 4.2 So I need to find a 6V battery with 21ah, so for the same wattage, as the 35w fog light I'd need a 30ah 6v. I see 4.5 ah 6v's for 20 bucks, so I would need 5 6v , which then costs 100bucks, so I'm break even at 135, infact I loose 10w of lighting.

Does anyone know where to get cheap 6v batteries, they seem close to 12v prices in the bottom range, or they seem small, but I cant find the rating

I hope the post isnt too long or with too many questions all at once.

I'd like to make sure I'm thinking about this problem correctly.

thanks

 

[birdman0_o]

Just to point something out, make sure they are deep cycle lead acid, not the normal car battery type as they are not meant for continuous draw.

 

[MikeMl]

Right, most lead-acid "motorcycle" batteries are just a small starting battery, and are not suitable for deep-discharge.

 

[marcbarker]

Hey many many years back I used to have a bicycle with a really bright headlamp, had a 6 V (40W?) motorcycle headlight bulb, I used C cell NiCad batteries to power it, and I used the tiny little generator to charge the batteries. There was a light that went out when the batteries were full charged so I could click the generator away from the tyre. I only used it for shortcuts through dark alleyways and unlit sidestreets.

 

[Ghosty_Ghoul]

It's now law here for cyclists to have lights in the dark.

 

[audioguru]

A little 14Ah lead-acid battery will supply 0.7A for 20 hours or 1.4A for 10 hours.
It might power your 2.9A light bulb for a couple of hours.

If you turn the light on and off a lot then the battery will last for a shorter time because a 2.9A bulb draws 29A for a moment each time it is turned on.

 

[marcbarker]

A little 14Ah lead-acid battery will supply 0.7A for 20 hours or 1.4A for 10 hours.

If you turn the light on and off a lot then the battery will last for a shorter time because a 2.9A bulb draws 29A for a moment each time it is turned on.

Does the "20 hr A/hr rate" still hold true at 10 hr rate? There's probably not much difference between the two...

How many times does the lamp need to be switched on to use up an A/hr of capacity?

 

[audioguru]

If the battery is rated at a 20 hour rate then it will last less than half the time for double the current.
0.7A for 20 hours, or 1.4A for 8 hours, or 2.9A for maybe 2 hours. The light will dim slowly the entire time.

Don't turn it on and off a lot. There is a huge strain on an incandescent light bulb when it turns on.

 

[marcbarker]

Don't turn it on and off a lot. There is a huge strain on an incandescent light bulb when it turns on.

I'd better not use my car's turn signal lamps then, they flash on and off 1.5 Hz the whole time I'm using them!

If you turn the light on and off a lot then the battery will last for a shorter time

So anyway, how many switch-on surges do you think would amount to 1 A/hr's worth of extra drain?

 

[audioguru]

A light bulb lasts 1000 hours if it is not turned on and off too much. It might last "only" 100 hours if it is flashing all the time. 100 hours is a long time in a car.

Instead of wasting battery power heating incandescent light bulbs, why not use cool LEDs?

 

[marcbarker]

Yes I use red LEDs in my rear lights, Much nicer looking! I connected a load of them in series-parallel, with a few ohms in series with the ground return of both lamps.

:If you turn the light on and off a lot then the battery will last for a shorter time

So anyway, how many switch-on surges do you think would amount to 1 A/hr's worth of extra drain?

 

[audioguru]

So anyway, how many switch-on surges do you think would amount to 1 A/hr's worth of extra drain?

You won't know unless you find out how quickly the hot filament cools and how fast is the flashing.

 

[marcbarker]

Ooops cross purposes, sorry...

If you turn the light on and off a lot then the battery will last for a shorter time because a 2.9A bulb draws 29A for a moment each time it is turned on.

I was asking about how much shorter a time this is in the OP's battery.

That's what I mean when I ask how many of these 'switch-on surges' (of the OP's battery) do you think would amount to 1 A/hr's worth of extra drain?

I'm interested, since you'd specifically mentioned "29 A" in posting #6

 

[marcbarker]

It's now law here for cyclists to have lights in the dark.

I thought that was a law in UK here for the last 50 years?

Not that it makes much difference, nearly every night cyclist where I am has no lights!

 

[audioguru]

I think I saw cars without headlights at night in the city in England?
Here cars must drive at night toward traffic with low beams. High beams can be used if there is no oncoming traffic.

Cars since 1989 have "daytime running lights" that turn on automatically. Most cars have them as dimmed low beams or "front turn signal lights". But Chrysler cars (and Jeeps) use full brightness high beams to blind oncoming traffic on cloudy days.

Most bicycles here use very dim and almost useless LED lights at night.