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  #1  
Old 09-19-2007, 03:54 PM
SuperJason SuperJason is offline
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Custom DIY battery packs

I'm trying to get more runtime out of my car. I've been toying with the idea of building some custom battery packs out of AA Nimh batteries. Obviously I would have to do some wiring and soldering.

Has anyone here done that?

I can use 6 cells, and get it at 7.2 volts. Also, if I use 2300mah batteries, my math tells me the pack will be 7.2v and be 2300mah. Wouldn't that be better and cheaper than most packs you can buy? Obviously I'll have to get a good charger for the packs.

There are a lot of links online for building your own packs, but all the RC forums I see rarely mention it.

I was even toying with the idea of using 12 batteries, 2 sets of 6 cells in series, but each site in parallel. That would give me 7.2v and 4600mah, which could give me a potential run time of 1 hour! I'm not sure if the weight would be a concern.

Thanks!
Jason
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  #2  
Old 09-19-2007, 08:05 PM
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The quick answer: You may have a longer runtime but your vehicle might run slower. Reason being that typically the higher the mAh is, the slower AA NiHm's discharge.

Long answer: It's not just all about Voltage and mAh. It's also about how fast can your batteries discharge (Amp drain / C rating). For example, your motor may be asking for 4 Amps continuous discharge and the batteries may only be able to discharge at 2 Amps continuous. Wont matter how much voltage or how many batteries you have the motor just won't get the power fast enough. What you need to know is the 'C' rating of the cells.

Here's a link to a page that may give you insight (or just totally confuse the issue). At the very bottom of the page is an icon named 'Battery Info'. It gives some insight on mAh and 'C' rating. But you may find the rest of the site useful in understanding the capacities of different types and sizes of batteries. As well as what configurations work for what applications.

http://www.robotcombat.com/marketpla...ack_build.html

Nonetheless, if it was possible to get extra long runtimes and adequate power drain from AA NiHm's then it would be something a lot of people would have been doing for quite some time already. I think you will be adding a lot of extra weight for not very much gain.

cheers
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  #3  
Old 09-19-2007, 08:06 PM
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You will get longer runtimes doing that but i would not recommend that at all its just not feasible.......when it comes to a motor its all about amperage and AA's just dont cut it.......get yourself a set of 2/3a's and you will be happy......if you have a charger with adjustable amp and mv just set it too 1.5 amps for longer runtimes and 2amps for more punch.
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Old 09-19-2007, 08:09 PM
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Well, seems I have done it again. I have virtually duplicated Alfred's response at the very same time..

cheers
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  #5  
Old 09-19-2007, 08:11 PM
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You should go ahead and post it anyway.......you might just get something that i missed....lol....not joking at all.
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Old 09-19-2007, 08:14 PM
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Alfred, look above your post..
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Old 09-19-2007, 08:22 PM
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Lol, oh okay i see that we posted a minute apart......dont know by how many seconds you beat me though.....its all good.
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  #8  
Old 09-19-2007, 08:39 PM
SuperJason SuperJason is offline
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Figured it out

Ok, I did the math from the page you recommended, based off the battery data sheet from the engergizer website.

If I build a 6 cell pack, it would have a discharge rate of 3 amps. If I build a 12 cell pack (but still at 7.2v), I would have a discharge rate of 6 amps.

What is the standard propulse battery pack rated at?

I understand that you're probably right (I would bet on it), but I would just like to verify the numbers myself (that's how I am!).
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  #9  
Old 09-19-2007, 08:43 PM
SuperJason SuperJason is offline
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Oops

It looks like I didn't find the actual "c" rating. I just used the number they gave for a typical drain, which would last a couple of hours.

At least I know 6 cells would be 3 amps minimum.
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  #10  
Old 09-19-2007, 08:49 PM
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When you compare a GP1100 rated at 25-30amps versus 3amps thats a big jump in amperage and motor performance. And heres a graph for an Elite 1500mah cell......

http://cheapbatterypacks.com/cellinf...vid=ELITE1500A
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Last edited by alfred; 09-19-2007 at 08:54 PM.
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  #11  
Old 09-19-2007, 08:58 PM
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Quote:
Originally Posted by SuperJason View Post
Ok, I did the math from the page you recommended, based off the battery data sheet from the engergizer website.

If I build a 6 cell pack, it would have a discharge rate of 3 amps. If I build a 12 cell pack (but still at 7.2v), I would have a discharge rate of 6 amps.

What is the standard propulse battery pack rated at?

I understand that you're probably right (I would bet on it), but I would just like to verify the numbers myself (that's how I am!).
You might have it a little confused, which I was afraid of. No matter how you slice it, amp drain rate will not increase no matter how many cells you have. More cells will only allow you to draw that same amperage over a longer time because you have more cells to draw from.
Keep in mind that for the 'C' Rating examples on that page, each equation states a 'given' rating.
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  #12  
Old 09-19-2007, 08:59 PM
SuperJason SuperJason is offline
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Ok, Thank You!

Ok, we're on the same page now. Do you have a good recommendation for an inexpensive LIPO charger and battery pack that will work well in the stock Pro-Pulse? I wouldn't even mind if I had to modify how it sits in the car.

Next summer I would like to upgrade to a Mamba 25 and a brushless motor, but at this point, I'm just trying to get better run times, and hopefully buy something that I can keep using as I move up.
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  #13  
Old 09-19-2007, 09:23 PM
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Quote:
Originally Posted by SuperJason View Post
..Do you have a good recommendation for an inexpensive LIPO charger and battery pack that will work well in the stock Pro-Pulse?
"inexpensive" is a tall order. I have a Hobbico 'Accu-Cycle' Elite (Most Hobby Shops should carry them). It charges/discharges anything and everything with multiple rate options. Totally programmable for NiCD, NiHM, Li-Ion and Li-Poly. Rather than spending a little on an inexpensive charger that will be limited in it's capabilities, only to have to get another or better one later on, I would seriously consider the investment into a decent charger. It by far will be the smartest investment you can make in RC.
Quote:
Originally Posted by SuperJason View Post
Next summer I would like to upgrade to a Mamba 25 and a brushless motor, but at this point, I'm just trying to get better run times, and hopefully buy something that I can keep using as I move up.
Consider the amount of additional 'breakage' you will incur by installing super powerful set-ups in your vehicle. It could be an expensive venture that will trump your increased 'runtimes'. It's all kinds of fun no doubt watching the tires nearly fly off, but honestly, full throttle mamba is virtually out of control. Just my experienced opinion.
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  #14  
Old 09-19-2007, 09:31 PM
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I bought a trinitron charger 5 years ago $130 at the time.still use it for everything.I say get a good charger once you wont be disappionted.the october iss of rc driver has a review of 20 lipo chargers
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  #15  
Old 09-19-2007, 09:39 PM
SuperJason SuperJason is offline
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Thanks

I'm just getting started in RC, and I see this is going to be an expensive journey.

That charger looks awesome. Can you leave an Nimh battery on it and automatically have it top it off as necessary? I know that was bad on nicd's, but the biggest pain the butt right now is the 10hr charger that comes with the propulse. I either want it earlier than 10 hours, or I want to put it on the charger and forget about it. I guess if I had a good charger, it would probably solve my biggest complaints.

I was thinking that going brushless would be crazy. What about one of the motors that can just be dropped into this car? I drive it on my street, and I would like to increase the top speed about 50% if possible. I'm not quite as fast as the cars that drive by! I don't know of a hobby shop in my area, and I don't really know how to choose any parts for the truck. I only know what I can gather from google searches and this forum.

I really appreciate you all walking me through this stuff. It's very helpful to have such a great community.
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