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Old Jan 29, 2013, 08:51 PM
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Question
battery power conversion

I have a friend that has a tank that is powered using 8 double A's, which is 12 volts I believe. so the question I have is it possable to converted it to a 7.2 volts. if so what is needed if any thing?
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Old Jan 30, 2013, 02:32 AM
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Portland Oregon
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that sounds like that large abrams about a foot wide and about 32 or more inche long every thing is big and have set of rockets mounted on the side.If it is what I am thinking it may that big 12th scale . I had one given to me a wile back one BIG Tank . I am going buy what your saying in the battery department for the one I have uses a 8 cell blue batter pack. I am not sure if you can even order those packs?
DON just wondering
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Old Jan 30, 2013, 02:41 AM
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big tank

Is your buddys tank look like this one ? FOR IT takes that 8 cell blue battery. Just wondering from what I understand there are not that many still around ? FOR it is an older tank from many years ago. Been told about 12 th scale I realy dont know for sure myself > O ya thanks for the name it work for what I needed NOW I can keep track of ya when ever you send. Don
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Old Jan 30, 2013, 02:10 PM
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Chief, nominal voltage is 9.6 on an 8 cell AA pack. Fully charged can be 12 volts. But for the most part, nominal is the rating used for info. Dropping to a 7 cell pack (nominal 8.4) volts might work, if the stock esc doesn't have a low voltage cutoff. Also everything else is going to run slower, kinda like running with a low 9.6 pack. And if it has a smoker it may not operate at all.
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Old Jan 30, 2013, 02:20 PM
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Thanks for all your replys,as of yet I have not seen the tank ,but was asked last night over the phone if a 7.2 battery can replace the 8 double a's. I then posted this question in hope from some one with this experiance. thank you.
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Old Jan 31, 2013, 12:37 AM
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I will agree on what Glacier says the 7.2 or 6 pack or lower will work but may be low on power . My self I would run the pack that comes with the tank. That is what I plan on doing with the one I just posted it take the 8 cell /12 volt pack . IF you do lower to another pack things may work slower or none at all. There are reasons for the different packs and voltage NOT a good idea I feel my opinon only . But on the other hand I can see why you would want to as well so it can go both ways GOOD LUCK.
DON THE Rustbucket
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Old Jan 31, 2013, 12:37 PM
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Something else to think about, maybe look at replacing the stock pack with a lipo.
You could get a similar voltage/capacity lipo that would be smaller the the stock pack.
You would need to add some type of voltage cut off to protect the pack.

Used to do it all the time on electric airplanes that came with round cells.
Nice thing with a lipo is it tends to hold voltage longer under load, doesn't suffer from voltage loss at a rate anywhere near what round cells do. And yes they also have some downsides, if you do not respect them.
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Old Jan 31, 2013, 01:33 PM
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I am trying to learn more with electronics and want to thank you for your reply but being electronics is not my high point,can you explain your thought compearing it with numbers. smaller voltage & for the low voltage cut out could I use an alarm that I have for Lipo batteries?
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Old Feb 01, 2013, 10:02 AM
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Why are you replacing the existing pack? Is it because you don't want to keep buying alkaline batteries?
How does the tank behave when the alkaline batteries are dying? If you can drive the tank slower and slower until it finally dies you could get away with a lower voltage pack and always run at that lower speed. Not much fun, in my opinion.
To replace eight alkaline AA's you will need a 10 cell nicad/nimh pack to get the 12 volts you need. Lots of options here (follow the link for 10 cell pack options), and some modifications to the tank may be required to make the pack fit. Then just get a charger that can handle 10 cells and you are all set.
3S lipo would be another option, but like mentioned already, a low voltage cutoff would be required to protect the pack. Plus a charger made for lipos.

Dave

http://www.onlybatterypacks.com/items.asp?db=37
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Old Feb 01, 2013, 10:18 AM
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Well there's charged voltage, and nominal. Charged is fresh off the charger, nominal is the voltage under load. So your 8 double A's dependent on construction could be we'll say 12 volts fully charged, but under load will drop to a working(nominal) voltage of 9.6.

Jump to a 3 cell lipo, or as it's called a 3S pack. It's constructed of 3 cells wired in series, where the "S" comes from. A 4 cell pack would be a 4S, and so on.

Each cell has a charged value of 4.2 volts, and nominal of 3.7. So a 3S pack would be 12.6 volts charged, and 11.1 nominal.

Okay, just a start, but look at the voltages under load (nominal) between the two.
Your 8 cell drops to 9.6 volts, the lipo to 11.1. More Power!

Also a lipo has just like your round cells capacity. Or in layman's terms a fuel tank.
How much fuel it can hold. Lipos tend to have better fuel tanks for size. You can pack more fuel often into a smaller lipo then it's round cell brother.

A lipo tends to have a better fuel delivery system, or as it's known as a C rating, the higher the C rating, the better the pack can deliver it's fuel rate over a period of time.

Just for S&G's, a C rating of 10, on a lipo cell of 2100 capacity (2.1 amps) would deliver 21 amps of power till it reached it's low voltage cut off. Same pack, with a 20C rating, could deliver 42 amps over the same time, and so on.

Round cells just don't have the capability of a lipo, C wise. There are lipos out there now rated way over 80C, something a round could only dream of.

What's all this mean to you? You may find that a lipo can deliver more performance, over a longer period of time then an equal physical sized round pack. A depleted lipo at cut off voltage, we'll use the 3S pack for reference, will be right around 9.9 volts or so. See what I mean? The lipo even when it hits it's cut off, is still putting out voltage equal/better then your round cell pack started out with.

Also a lipo does not suffer like a round cell, to voltage loss just sitting around.
You know how you have to fully charge your pack right before you go out to play?
Not so with a lipo, it can sit a week and still have nearly fully charged value, your round cell wouldn't even be close.

Those are some of the good points. And like said there are some not so good, but nearly all of them is due to ignorance and lack of respect of the pack. Failure to follow the rules of charging/discharging/storage a lipo can get ugly. But respect them for what they are and you'll never go back to round cells.

One of the biggies of lipos, is NEVER running them past their cut off voltage. That's where a low voltage cut off/ warning comes into play. Lucky for you a tank will just stop where it's at when LVC occurs. In my planes, when the same occurs, it can get real ugly as the stop is usually after a fall from the heavens.

A low voltage cut off, LVC, is a simple add on to a system not equipped with one. For example something that was built to run on round cells will not have the proper cut off lever for use with a lipo. LVC is built into higher grade systems, via the ESC or Electronic Speed Control(s) and can be set for the number of lipo cells you are using to protect your pack.

Like the Sabertooth esc I run in my 1/6th scale tanks. Great system, can operate two motors by itself, and has the built in LVC that can be set to control lipo or regular cells.

Long winded I know, but to learn more about lipos and such, take a gander over on the electric flight forums here for a lot of stickies about care and feeding of them.


As for a unit for your tank(s) take a look at this, cheap, simple, effective.
http://www.hobbyking.com/hobbyking/s...arch=low%20vol


And lastly, after all that, before you jump into a lipo conversion, you need the facts.
The facts being, what exactly is the charged and nominal voltage of your round cell packs.
And what is the voltage capability of your boards inside the tank.

If you need any help, give me a holler or PM me.
Brad
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Old Feb 01, 2013, 11:12 AM
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Brad, since your avatar has the P-38 Glacier Girl I must ask, are you/were you involved in that program? I met a guy at a boat show up here in Wisconsin who had somethig to do with the restoration of that plane. Can't recall his name, though. P-38's are great!

Dave
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Old Feb 01, 2013, 08:00 PM
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Thanke Brad for all the info. I have some understanding about lipos,charging ,storage and not to allow them to drop below 3volts per cell. I do have the On board low voltage alarm to help prevent the lipos reaching the danger level. as I go forward and if I need some directions I will take you up on a pm. thanks Wayne
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Old Feb 01, 2013, 08:03 PM
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And dont let me forget Dave and Don for your information.
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Last edited by CHIEFSONN; Feb 01, 2013 at 08:09 PM.
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Old Feb 02, 2013, 01:43 PM
Kamikaze Ace
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Dave no not with Glacier Girl. I wish!
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