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I want to make a battery pack that can make 7.5V at 3.0A.
- Thread starter daffy1234
- Start date
Hi daffy
welcome to the forums
yes you can get that from a Li-Po battery pack, be prepared to pay a reasonable amount for one
cheers
Dave
Hi Dave,
Is there a way I can pull this off with only standard, store-bought batteries? Like AA, C, or D?
EDIT: I can use button/coin cell batteries too, but I doubt they have enough power.
A typical D cell has a capacity of about 10000-12000 mAh but not really recommended for large current drain. You could in theory get about 3A for 2-3 hours but it could also in theory overheat and blow up in your face 
A typical D cell has a capacity of about 10000-12000 mAh but not really recommended for large current drain. You could in theory get about 3A for 2-3 hours but it could also in theory overheat and blow up in your face
That doesn't sound too good.
If I were to use D cells, I would use 5 of them (1.5V * 5 = 7.5V). If I do this, is that in danger of blowing up?
On an unrelated note, what you said reminded me of my "hand warmers" I used to make. They were just two 9Vs hooked up to each other. I was so ignorant
Would 6 nimh D cells suffice?
I think 5 would. 6 would probably be too much voltage. I think I have a charger somewhere, but it's probably only for AA and AAA batteries. Good idea.
EDIT: I just did the math and 6 is too much voltage. My project calls for 7.5V and that gives off 9V.
davenn
Moderator
your maths is out
6 x 1.2 = 7.2V which is less than you need .... OK fully charged you may get 7.5 - 7.7 V but its gonna start dropping quickly especially with a load up to 3 amps
Its not really going to be feasible with AAA or AA batteries, doing a series/parallel pack to get the voltage and current requirements would be ok but those small cells are
not going to have much "staying power" ie. wont last overly long.
Li-Po's are the best choice for compactness and operating time.
A SLA, as KJ6EAD suggested, would be the other way but would be really be getting bulky
Dave
I would likely look into the 7.4 volt RC (radio controlled helicopter) lipo packs they are designed for high drain applications and the tenth of a volt less probably won't cause an issue... Just remember you need the proper lipo charger as well...
You can get 7.4 lipo packs over the counter with mAh ratings above 15000, so you can get several hours out of them, you just have to pay for it...
You can get 7.4 lipo packs over the counter with mAh ratings above 15000, so you can get several hours out of them, you just have to pay for it...
your maths is out
OK fully charged you may get 7.5 - 7.7 V but its gonna start dropping quickly especially with a load up to 3 amps
Its not really going to be feasible with AAA or AA batteries, doing a series/parallel pack to get the voltage and current requirements would be ok but those small cells are
not going to have much "staying power" ie. wont last overly long.
Li-Po's are the best choice for compactness and operating time.
A SLA, as KJ6EAD suggested, would be the other way but would be really be getting bulky
Dave
I settled on using D's. Are those powerful enough?
Good idea, but I'm trying a more low-cost method first. If it fails, not a lot of money is lost.
Update: I just went to radioshack and got some D holders, wired them all together, then found out that I bought the wrong connecter
I'll have to wait till tomarrow to get what I need because radioshack is closed now.
I was impressed by the power that 5 D batteries put out. I accedentally shorted it for a second and in that time the wire turned red and broke. Guess that's just a part of learning.
Thanks for everyone's suggestions.
I'll have to wait till tomarrow to get what I need because radioshack is closed now.
I was impressed by the power that 5 D batteries put out. I accedentally shorted it for a second and in that time the wire turned red and broke.
Guess that's just a part of learning. Thanks for everyone's suggestions.
The discharge curve for alkaline D cells shows a 3A rate with a service life of 1 hour.
http://www.google.com/url?sa=t&sour...ivygBA&usg=AFQjCNGN4NuU1LwEbZvYO_LHDjbH8dMxGg
http://www.google.com/url?sa=t&sour...ivygBA&usg=AFQjCNGN4NuU1LwEbZvYO_LHDjbH8dMxGg
I'm using 3A over 5 batteries. Will that raise the battery life?
if you need to extend the battery life a little you can opt for 10 batteries. just make 2 lots of 5 in series then put them in parrallel. this will give you a rough voltage of 7.5volts (give or take) and will double the amps of having a single row.
lipo packs are an alternative as are nicd but the drain on them is big. and the cost is big especially for lipo.
the 2 alternatives that I would recomend are a plug pack wall charger if you can find one (try a laptop charger too) or if it has to be transportable try the same from a cigarette lighter in the car (you can get a porabtle 7ah battery then that'll work but you'll have to worry bout keeping it charged)
hope that helps a bit
lipo packs are an alternative as are nicd but the drain on them is big. and the cost is big especially for lipo.
the 2 alternatives that I would recomend are a plug pack wall charger if you can find one (try a laptop charger too) or if it has to be transportable try the same from a cigarette lighter in the car (you can get a porabtle 7ah battery then that'll work but you'll have to worry bout keeping it charged)
hope that helps a bit
At that point you might as well go to a lead acid (or gel) cell, or even lantern batteries in weight and size
You run into a cost vs size issue, Lipo packs are small for the juice they push out and they are designed for high drain, but they are also initially more costly... But then again they are rechargeable while most people won't bother recharging D cells so over the run the Lipo pays for itself pretty quickly over name brand D cell purchases...
You can get 5000-6000mAh lipo packs for about $30 a piece, get two so you can charge one while using the other... As KJ6EAD the run time of D cells is about one hour at that drain, the Lipo at 5000mAh (even though less rated capacity) should perform as well at that drain rate due to design... A 5000mAh pack drained at 3000mA is only 0.6C for that pack, most lithium packs are rated for 1C or more many reliable up to 3C... Of course that isn't solid data, run time will depend on several factors, and battery companies love to overestimated capacity... But, regardless lipo packs perform better in high drain situations...
When you factor in that 5 'name brand' alkaline D cells is going to cost you $2-3 each cell or about $10 - $15 a load up, it won't take time to recoup the $30 on the Lipo investment, heck it won't take long to recoup a $100 lipo investment...
Actually, that helps quite a lot. I think if I run into problems with battery life, I'll try those ideas.
At that point you might as well go to a lead acid (or gel) cell, or even lantern batteries in weight and size
You run into a cost vs size issue, Lipo packs are small for the juice they push out and they are designed for high drain, but they are also initially more costly... But then again they are rechargeable while most people won't bother recharging D cells so over the run the Lipo pays for itself pretty quickly over name brand D cell purchases...
You can get 5000-6000mAh lipo packs for about $30 a piece, get two so you can charge one while using the other... As KJ6EAD the run time of D cells is about one hour at that drain, the Lipo at 5000mAh (even though less rated capacity) should perform as well at that drain rate due to design... A 5000mAh pack drained at 3000mA is only 0.6C for that pack, most lithium packs are rated for 1C or more many reliable up to 3C... Of course that isn't solid data, run time will depend on several factors, and battery companies love to overestimated capacity... But, regardless lipo packs perform better in high drain situations...
When you factor in that 5 'name brand' alkaline D cells is going to cost you $2-3 each cell or about $10 - $15 a load up, it won't take time to recoup the $30 on the Lipo investment, heck it won't take long to recoup a $100 lipo investment...
Yeah. The battery life of the D batteries at 3A probably wont be enough.
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