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12v Rechargeable Batteries, Suggestions?
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12v Rechargeable Batteries, Suggestions?

by GAHorton on Tue Oct 14, 2008 11:28 am

Hello,

So my current project is putting a car horn on my bicycle. I need 12 volts of power to get a good honk and the only 12v rechargeable battery i have laying around is lead acid which is HEAVY.

Does anyone know a lightweight alternative for a lead acid? I would need something with a charger as well to power this project.

thanks
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Re: 12v Rechargeable Batteries, Suggestions?

by The_Don125 on Tue Oct 14, 2008 2:21 pm

What about 2 9V size rechargable batteries? The rechargable 9V batteries are really only 7.2, so if the horn can take 14.2V, those might be something to look into.
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Re: 12v Rechargeable Batteries, Suggestions?

by adafruit on Tue Oct 14, 2008 5:13 pm

try NiMH battery packs such as those used for cordless drills

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Re: 12v Rechargeable Batteries, Suggestions?

by eil on Tue Oct 14, 2008 7:22 pm

By "lead acid" I presume you mean like a car or motorcycle battery... They do make smaller lead acid batteries, like the ones they use in uninterruptable power supplies for computers. All the advantages of a car battery only smaller and far safer. One of these would probably fit perfectly under the seat or above the pedals. Just take care that it won't crack open if you bike falls over.
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Re: 12v Rechargeable Batteries, Suggestions?

by Amberwolf on Tue Oct 14, 2008 11:12 pm

I used this battery:
http://www.directbatterydeal.com/index. ... &Itemid=78
for a while to power both my lights and a small horn (though it wasn't automotive, it would still have worked if it was). It is an SLA, so it's a little heavier than some other rechargeables it's size, but it's also cheaper. I bought mine from BatteriesPlus local store, rather than that place I linked, but BP doesn't have it on their site to link to. I think it was only $11 when I bought it, about 3 years ago.

It's an AGM SLA battery, which is a lot better than just a car or motorcycle battery, for the purposes you want. AGM's can be operated in any position, including upside down for most of them, and they are very tough--the 12v12Ah ones I use for my motors have had road impacts a few times and have no exterior damage, and still operated the same after as before.

They make even smaller ones; this one has 1.4Ah capacity at 12V, so it ran my lights for a whole trip, within a few miles.

Nowadays I use a 7Ah, as I have many more lights, and it will last me several days' worth of trips before needing recharging.

If I could afford Li-Fe-Po I'd get those to replace the SLA, both for my lights and the old abused ones I have for my motors. I'm considering gutting some old laptop Li-Ion batteries for the purpose, which probably won't have any better life than what I have now, but will be very much lighter; the catch is building a charge and monitoring system to keep from damaging them to the point of fire. :roll:

That said, Ladyada's suggestion makes it easy to build a whole unit for this:
Buy a cheap rechargeable cordless screwdriver or drill (harbor freight has good sales often), then take the unit apart so that you have the battery contacts out of it (and any charging circuitry if it is integrated), and hook that to your horn, mounted perhaps in a water bottle.
If the charger is a separate device, then you can just take the battery out and recharge it as needed. I expect it would last you quite a long time between charges, even with NiMH's self-discharge rates.
If the charger is integrated and just has an ac adapter to plug into the wall, well, all you have to do is plug it in at night to recharge and it'll always be fully charged.

You can run some decent lights off the same battery, too, if you get one with enough capacity.
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Re: 12v Rechargeable Batteries, Suggestions?

by jsc on Wed Nov 05, 2008 2:06 pm

You can use a MAX761 (Digikey part# MAX761CPA+-ND) boost converter to get 12V from 2 AA cells (although using 4 will give you better efficiency). I did this recently to get 12V for an op-amp circuit because I was tired of using 9V batteries (expensive, low on power). Just follow the example schematic in the data sheet, and refer to Lady Ada's mintyboost design process for some useful information. The MAX761 is similar to the chip used in version 1.

Oh, I just realized you're going to use it for a bicycle horn. This probably won't be suitable due to current requirements, unless we're talking about some fantastically efficient horn.

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Re: 12v Rechargeable Batteries, Suggestions?

by jbrandt01 on Sat Nov 22, 2008 6:39 am

Amberwolf wrote:
If I could afford Li-Fe-Po I'd get those to replace the SLA, both for my lights and the old abused ones I have for my motors. I'm considering gutting some old laptop Li-Ion batteries for the purpose, which probably won't have any better life than what I have now, but will be very much lighter; the catch is building a charge and monitoring system to keep from damaging them to the point of fire. :roll:


You are not going get LiFePO4 cells to catch on fire by overcharging ever. After a small accident to the tune of 40 amps at 14 volts to a single cell all that happened was the end cap poped much in the same way a capacitor does and that was the end of it. You can damage the cells easily but they are not going to put you in a hazardous situation ever.

Li-Po's are bombs waiting to happen and I strongly advise not using them. Especially given the LiFePO4 are in every way superior for the same price.

Either way if you want a charge controller that actually does both: http://www.slkelectronics.com/lipodapter/index.htm I have one for R/C usage and it works great and isn't a bank breaker.

You are on your own for a voltage cut-off switch though.

Another viable option much along the same lines as the drill/tool battery suggestion are R/C battery packs. They are normally 7.2v (6-cell) wiring 2 in series is not going to damage any automotive part (as charging systems normally put out 14.4 anyways). In this way you are more likely going to get higher quality cells that are matched a lot better than tool ones.
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