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MountainBikeReview.com's Forum Archives - General Discussion
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Power question for lights, check my math (12 posts)
|  Power question for lights, check my math | pimpbot
Sep 18, 2003 3:14 PM | | I was wondering, If I wanna make 12v to power a homemade light with 1600ma AA batteries, and I chain 10 together, will it overhead from all the current to power a 15W light?
Just so you can bash my thinking (please do) I have a buttload of NiMH AA batteries already, so this would minimize my parts purchace list, which is good, cause I'm kinda on the broke side lately. I'm trying to figure out if I can build a cheap but effective bar light, maybe dual beam, if I can schwing it.
My math:
1.2v each * 10 cells = 12 Volts
15W/12V= 1.25A or 1250 mA
each cell is 1600 mA/hour so I can expect the light to run for just over an hour (in ideal conditions). IIRC, the current does not add up with multiple cells, just the voltage. Is this right?
So, I really need more like 12v @ 4500 mA to run for a few hours, right?
So either I have to series/parllell my batteries down to 6v to double the current and use a 6V lamp to double my run time. No, wait. The current would have to double to keep the same wattage. Or does the 1600ma add up with all those cells to 16 Amp hours? Nah, that doesn't make sense. Okay. I need bigger cells. |
| Your maths correct | heatstroke
Sep 18, 2003 7:59 PM | | Not recommended to run in parallel rechargeable batts - problem with strong batts running down weak ones, you'll shorten the life of the batts. Especially don't recharge in parallel.
if you're limited by cell capacity, then the other option is to increase the bulb voltage and keep the wattage the same . that way you drop the current and will need to run more cells in series (to make up the voltage). with the lower current your cells last longer.
unless you can find a DC-DC converter ? |
| re: Power question for lights, check my math | Anonymous Coward
Sep 18, 2003 8:08 PM | | In order to get the 12V, you'll need to connect them all in series. If you do this, you are correct in that you will still only have the 1600mA/hour, but you'll get that rating over 12V,instead of 1.2V. If you were to connect all the batteries in parallel, you'd only have 1.2V, but you'd have 16A/hour. |
| Thanks, but I fell into a new plan | pimpbot
Sep 18, 2003 10:13 PM | | I just picked up a used Ultra Death Ray from re-cyclerey in San Rafael for $35. The battery is weak, but I priced out a new one for $8 (not a typo). It's a 4A/h SLA, so it weighs a ton, but once I get more dough scraped together, I'll go NiMH in a water bottle. |
| Here's some fun math for you about over/under volting (w/pics) | DeeEight
Sep 19, 2003 12:37 AM | | Go here and read section 6.2 and what it says on overvolting and undervolting.
There are a few morons on this site who can't work their brain around the math but to the folks at BLT, Lupine Systems, Nightsun, Turbocat, Niterider and others who run 7.2V battery packs with 6V bulbs, and 13.2V battery packs with 12V bulbs its a quite well understood phenomenah. Its also why they'll advertise their model X light as having 40% more light than the other guy's light with the same rated bulb.
Now 10-20% overvolting is pretty safe for most MR11/MR16 type bulbs (which is the most commonly used type in rechargeable bicycle lights today) but if you're feeling daring a 100% overvolting CAN be done with some manufacturers bulbs (the safety margins for excess voltage they build into the bulb design is usually pretty broad). I've run 12V through a 6V-10W rated MR11 bulb and the light output you get is staggering and puts most metal-halide systems to shame. In fact, it is possible to comfortably read a newspaper with the light 50 feet AWAY from you. The catch is the battery sucks down pretty damn fast (12V-2.2Ah battery lasts about 30 minutes running with a 100% overvolting). If I ever manage a cheap source for high-capacity 3.6V Li-Ion rechargeable cells though, I'm going to wire three in series for a zippy 10.8V pack for these 6V bulbs (so about 7.8 times the normal light output, for 3.24 times the normal power consumption - sounds pretty high on the consumption still but a D-size li-ion cell is about 7000mAh capacity, so the bulb normally needs about 1600mAh so I'll still manage better than an hour of light from the pack, better still if I wire a digital voltage regulator into the system, I can step down my overvolting for different levels of light output/battery life).
Oh, as an example, here's a series of five night pics taken with my digital camera (without flash) of the same 6V-10W light. First two running a 6V battery pack and then the next three using a 12V pack, all pics take from 50 feet away and the fifth I managed to steady the camera a bit more (its windy here right now, we're expecting rain from the ol' hurricane in a few hours) and also lowered the camera a couple feet to show the light going thru the tinted car windows a bit better (oh and there's three halogen spotlights on the porch, about 75W each, aimed downwards and the 12V overvolt setup easily outpowers them for light output).
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| Can you afford $36 for a new battery? | Metroid
Sep 19, 2003 5:26 AM | | 3800mAh cells are available at the Batttery Station for $2.50 each. I ordered a dozen and it was $36 with shipping. These have over twice the capacity as you AA cells and weigh only 52g each. I ordered a dozen so I could either wire two 7.2V packs or one 13.2V pack. These offer the most power for the size/weight/price.
As far as the rules go, voltage in series is additive and current in parallel is additive. What you want to do here is wire the cells in series. The voltage adds up, 1.2V * n cells. The Amp-Hour capacity rating is what it is. Some companies like Marwi wire their 6V batteries in parallel pair wired in series to boost the capacity but this less than ideal as the other poster noted.
Either way you go, 6V or 12V, I would highly recommend overvolting by one cell. You do loose a bit of bulb life but most have at least a 2000 hour rating. Droping that to 200 hours isn't a big deal. I wouldn't recommend doing any more than one cell though because overvolting exponentially reduces bulb life.
http://www.batterystation.com/nicads.htm
HR-4/3AUXS overstock Sanyo Ni-MH 3800MAH 52GRAMS $ 2.50 |
| Dang, that's cheap! | pimpbot
Sep 19, 2003 10:28 AM | | They now have the 3/4 A size listed for $2.50 a cell. I still have to figure out how to charge it, but that would fit nicely in a water bottle right there, eh?
I was all set to go buy a 12v 3.5 AH SLA battery for $10, but did not look forward to adding 4 pounds of batery to my bike. Looks like I can buld the same power into one pound this way. |
| Question for Metroid | ismybrakepadrubbing
Sep 19, 2003 11:19 AM | | I have a 5 year old Niterider Classic . The battery does'nt hold much of a charge. If I order the new cells and open up my old battery will I be able to copy what Niterider did? The way it's wired. I'm asking this question because paralell and series are... I failed electricity class. It can't be that complicated to solder some wires. Can It? Thanks in advance for your input on this |
| I'm not Metroid, but.... | rb
Sep 19, 2003 12:02 PM | | ...I just cut open my Niterider battery pack to check it out. If you order from Battery Station (I just did) for an additional $5 they will solder the batteries together and give you either a lead or tabs to solder your existing lead to. I had them leave tabs, it will be quite simple to unsolder the leads from my old pack and solder them to the new tabs. |
| Question for Metroid | pimpbot
Sep 19, 2003 12:05 PM | | parallel means that if you have two (or more) batteries wired together the positives are wired together and the negatives are wired together. So in this case two 1.2V cells yields a 1.2v battery but can deliver twice the current (measured in amps or miliamps).
Series means that the positive of the first 1.2V cell is wired to the negative of the second 1.2V cell and the circuit is wired using the first cell's neg and the second cell's pos. This yeilds twice the voltage (electric pressure) but the same current (amps). So we get 2.4 volts
Since wattage (power) is voltage (pressure) times amps (current), both circuits can deliver the same power, but one will be more useful to you, say to drive a 12V lamp.
If you had a 6V 10 Watt lamp, you need to deliver twice the current to get the same amount of light as the 10 Watt 12V lamp, cause you have half the voltage. A 6V 10Watt lamp has half the resistance (ohms) of a 12V 10Watt lamp to allow twice the current to flow.
So to answer your Q, take your battery apart and count the number of cells, figure out what type they are (NiMH or NiCad) and you can just order a bunch of tab connector cells (rather than regular button top batteries) and build another one just like the one you have now by reverse engineering the current one. You can use some connector wire and solder the new batts togeter just by copying your dead one. Just be sure to black tape it up really well so the wires and terminals don't short together.
If you upgrade to NiMH batteries, you will also need a new charger. You can't typically use a NiCad charger on NiMH cells. |
| You can still use your old trickle charger but... | Metroid
Sep 19, 2003 1:01 PM | | You already see what that has done to your battery. I already have a fast smart charger from my Jet light but I want another to charge my new DIY battery. I'm looking at getting a Dewalt charger (e.g. one for a cordless drill). It can easily be modified to work with your battery. The site below shows the mods but I would recommend using a different plug. Most bike lights use a standard 5.5 outer diameter (o.d.) and 2.5 inner diamter (i.d.) plug. This is a "Type N" and available from Radio Shack. I would use something like this in the side of the case so there are no exposed wires. Also, make triple sure the positive (+) wire is wired to the center pin and not the outside case. You could really get hurt.
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F003%5F005%5F000&product%5Fid=274%2D1576
Also the Dewalt info is a bit dated in the article. The Dewalt 9116 seems to be the standard these days and will charge any battery up to 18V. It also has an automatic conditioning cycle that will increase the life of your battery.
Please read and heed the warning about working with AC/DC power!
http://www.myra-simon.com/bike/charger2.html |
| Not complicated at all... | Metroid
Sep 19, 2003 12:42 PM | | Series means you wire the cells positive to negative in a string. For a water bottle, it's easiest to wire them in pairs then string the pairs together. The tabs aren't all that sturdy so be careful. You can re-foam the bottle with Great Stuff foam insulation from your local home store.
This website will give you some ideas...
http://users.rcn.com/icebike/Equipment/batteryrepair.htm |
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