Hello everyone, I'm working on my first origional project and the goal is to put under glow on my longboard ( it's like a long skateboard) with LED strips, by my calculations the length of 12v LED strip i would need (60" [ 2 - 30" lengths]) would require .6 amps.

But my question is how can i drop the 18v of two 9v batteries down to 12v? or is there a better way to power them ( excluding 8 AA's theres not really room for that under my board) I'm a bit fuzzy on Ohm's Law but by my research it is Volts = amps * ohms? so i would need a 10Ω resistor? what else do i need to know? and am i even remotely correct?

In summary, im looking to power 60" of 12v LED strip with 2 9v batteries, is this an ok idea? and what resistor do i want? Or is this whole concept FUBAR?

 

Your math is correct but 9 volt batteries are rather weak in the current department. Six tenths of an amp is askng a lot from those little batteries. If you can even get them to allow .6 amps, they will be dead in about 30 minutes. Ten AA NiMh batteries stand a much better chance of delivering the current and might last nearly 4 hours.

 

I was afraid of that, why 10 AA though? doesn't 8 x 1.5v = 12v?

 

Hello,

#12 is talking about rechargeable NiMH batteries, wich have a voltage of about 1.2 Volts each.
If you would use alkali batteries (one time use only), you would need 8 pieces.

Bertus

 

i see! thanks so much guys im so glad my math was right i was trying to use online circuit simulators and it was really throwing me off, thank you guys!

 

You might look at sealed lead acid batteries, so that you could get a single 12V battery to avoid the mechanical issues of keeping so many cells connected. It'll probably be more expensive than AAs, but maybe it would be easier overall.

Another approach would be a DC-DC boost converter that would allow you to use, say, 4 AAs and boost the output voltage up to drive your 12V strips.

 

You might look at sealed lead acid batteries, so that you could get a single 12V battery to avoid the mechanical issues of keeping so many cells connected. It'll probably be more expensive than AAs, but maybe it would be easier overall.

Another approach would be a DC-DC boost converter that would allow you to use, say, 4 AAs and boost the output voltage up to drive your 12V strips.

I was considering those, I'm looking at them and they would work great, but do i really need to pay for a charger? or are those easy to make or find not sold "as a pair" for cheaper? like these?

http://www.amazon.com/Tenergy-2000m...56725968&sr=1-2&keywords=10+aa+battery+holder

http://www.amazon.com/Tenergy-Smart-Universal-Charger-Battery/dp/B003C1HH32/ref=pd_bxgy_t_img_y

 

The voltage of a brand new name-brand (Energizer 9V alkaline battery drops to 7V in 10 minutes when its load is 400mA. With 600ma it might last 5 minutes. With two in series and are powering a 12V regulator IC then the IC output voltage will drop in 5 minutes.

I saw an Energizer 9V alkaline battery being sold for $4.95 today and you need two of them.
The cost of the battery is $9.90 for 5 minutes which is $118.80 per hour.

 

The voltage of a brand new name-brand (Energizer 9V alkaline battery drops to 7V in 10 minutes when its load is 400mA. With 600ma it might last 5 minutes. With two in series and are powering a 12V regulator IC then the IC output voltage will drop in 5 minutes.

I saw an Energizer 9V alkaline battery being sold for $4.95 today and you need two of them.
The cost of the battery is $9.90 for 5 minutes which is $118.80 per hour.

holy cow! thank you, haha so i suppose the idea of 9v's is out of the question, so what about 10 NiMh? where can i find this info so i don't have to bother you guys?

 

Go to www.energizer.com and click on Technical Info at the bottom. Select a battery size and chemistry.

Ten of their AA Ni-MH cells drop to 10V in about 3.8 hours when the current is 600mA.
Your LED strips might look dim when the voltage is only 10V.

Do not select their D size Ni-MH cells. They have a little AA cell inside.

Today I saw their new AA ADVANCED alkaline cells being sold at a very high price. Its datasheet shows that it is only slightly better than their inexpensive ordinary AA alkaline cells. You won't notice the difference.

 

... do i really need to pay for a charger?

Yes, if you don't have one already. Your charger needs to be designed for your battery chemistry, so that it applies the proper charging scheme and protects your battery pack from overcharging.

You can build your own, but you won't save much (any?) money by the time you incorporate the "smart" features. You can get away with an LM317-based, constant voltage charger for an SLA battery, but I'd use more elegant solutions for other chemistries.

 

A cheap and easy solution would be 10 "AA" NI-CD batteries. if size is an issue, you could use AAA. A typical cheap AA NI-CD is good for about 800mA-hr, I think the AAA is maybe 250? Thay are really cheap at places like Harbor Freight and easy to recharge with constant current.

 

The only Ni-Cad cells sold around here are extremely cheap (cheep, cluck cluck cluck) poor quality Chinese ones for solar garden lights.

I have 24 Energizer AA Ni-MH cells. Some are about 15 years old, are used every day and are still good. The Chinese Ni-Cad cells in my solar garden lights last maybe 1 year.
I replace them with Energizer Ni-MH cells.

On Boxing day a couple of days ago I bought 12 more Energizer Ni-MH cells at 35% off the normal price.