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[Paul R]

Has anybody used these high capacity AA cells in a boat yet.. Looking at some web sites these latest High capacity AA size NiMh cells , (some are rated at 2,700 mah each) perhaps could be used instead of sub c size cells of similar capacity. In other words would these AA cells have the same output size for size as the sub c type cells. If so then the weight saving could be considerable. There also a lot smaller. Just a thought?

Paul R

[Mankster]

AA's cant supply a lot of amps, say 3-4 dependimg on the batteries you have. If you have very low drain motors then they will work fine.

[dougal99]

Mankster

Are you saying that AA cells rated at 2+ Amps do not provide that capacity?

I haven't used any in a boat yet, well not for drive, but my experience of RX and Camera use is that they last an extremely long time.

I would have thought that the usual equations applied, eg motor draws 3amps, 2100mah battery pack gives approx 35-40 mins run time.

Comments please

Cheers

Doug

[Guy Bagley]

check out multicell  in leicester, they have some great high capacity cells- pricing is very good too, they will also shrink wrap and assemble cell packs for you....

 sorry dont have a web address for multicell but i am sure it will be there on google !

[Hal]

Look at Miami Crashboat in June issue of Model boats and U Boat in April issue

[barriew]

Doug,

There is a difference between capacity and the ability to deliver current.

Barrie

[Mankster]

Doug,

There is a difference between capacity and the ability to deliver current.

Barrie

Yes thats what I ment. An AA cell rated at say 2500mAH should theoretically be able to supply 1Amp current for 2.5hour or 2.5Amp for 1hour or 5 Amps for 30min or 10 Amps for 15min, before being exhausted (in practice its less than this as you lose energy as heat).  But there is now way you will be able to reliably draw 10amps out of an AA cell (if every you did it, the batteries would be so hot that you would be losing a lot of power in heat) A C cell of the same 2500mah capacity will have no proplems supply a constant 10 Amps. Typically you use AA cell in low drain items like radio gear and small low power models.

[Hal]

All cells "lose" energy when discharged due to their internal resistance.
Small cells can deliver large currents, anyone who flys electric RC aircraft knows this.

[Paul R]

So... are we more or less in agreement that an AA size cell (Ni Mh) with a rated capacity of  2,700mah can provide the same current output as a sub C size cell rated at 2,700mah? Is the heat output an issue due to an AA cell's smaller size?. The fact is that battery technology is changing fast and some of the latest Ni Mh cells to come out of Japan, again i'm talking about AA cells have very high current ratings indeed..

Paul R   

[Mankster]

So... are we more or less in agreement that an AA size cell (Ni Mh) with a rated capacity of  2,700mah can provide the same current output as a sub C size cell rated at 2,700mah? Is the heat output an issue due to an AA cell's smaller size?. The fact is that battery technology is changing fast and some of the latest Ni Mh cells to come out of Japan, again i'm talking about AA cells have very high current ratings indeed..

Paul R   

No, C cell can supply far more current (Amps) than AA cells. Ever wondered why RC cars run on packs made up of sub C cells instead off AA cells of the same capacity inspite of the obvious weight advantage? The samller AA cells won't supply enough current and car won't go fast enough, despite being lighter. Like I said, for smaller, low powered model AA will be fine.

[cbr900]

I used to race Radio Controlled cars and the current draw if you are serious was in the 35 amp range
if the batteries were double AA with the same capacity they would last approximately one quarter of
the time that the C sized cells would last, even then the C sized packs were very hot when removed
from the car AA sized cells would more than likely melt or explode...

Roy

[dougal99]

Doug,

There is a difference between capacity and the ability to deliver current.

Barrie

Yes thats what I ment. An AA cell rated at say 2500mAH should theoretically be able to supply 1Amp current for 2.5hour or 2.5Amp for 1hour or 5 Amps for 30min or 10 Amps for 15min, before being exhausted (in practice its less than this as you lose energy as heat).  But there is now way you will be able to reliably draw 10amps out of an AA cell (if every you did it, the batteries would be so hot that you would be losing a lot of power in heat) A C cell of the same 2500mah capacity will have no proplems supply a constant 10 Amps. Typically you use AA cell in low drain items like radio gear and small low power models.

Thanks for the explanation.

Cheers

Doug

[Doc]

Are you ready for this?
The electrical energy produced by a battery is a result of a chemical reaction.  That chemical reaction also has a characteristic of producing heat.  The speed of production of the electricity is determined by what chemicals are used.  The amount of electricity is determined by how much of the chemical 'goo' is present.  So, larger quantities of 'goo' can produce/provide a larger current flow than smaller quantities of 'goo' (volume).  As long as the 'goo' stays the same stuff, don't switch types of batteries, nimh's, nicad's, lipo's, etc, if the volume is 4 times as large, the current flow can be 4 times as fast.  Not really, since the larger volume of 'goo' also means that the current has to flow a longer distance to get to the terminals of the battery.  But it does work out to something related to the 'size' of the battery (in comparison to the size of the battery it's being compared to).
That is a semi-scientific approximation of an explanation by someone who hasn't had enough coffee yet, and doesn't really know what he's talking about.  I warned you , didn't I?  And like they always say, "No matter where you're at... there you are.".
 - 'Doc


Oh yeah, the heat production.  As long as the electricity produced is greater than the heat produced.  And as long as the heat isn't produced much faster than the electricity, the thingy is called a battery.  When the speed of heat production is much greater, and faster than the electricity's production, it's called an explosive device.  Except if the device is produced by 'Radio Shack', and then it's called junk.

[sweeper]

Hi Doc,
For someone lacking their coffee fix,not a bad explanation.
Try thinking of the problem from the point of view of the cross sectional area of the plates in the cell rather than the volume of goo (love that one!) aka electrolyte.
The larger the surface area of the plates,the larger the amount of active material in them, the greater the cell capacity - doesn't matter if the cell is circular or rectangular.The larger plates also reduce the current density and thus less heat is produced.It also helps if the plates are mounted very closely together as the current will travel more readily through the plates than through the electrolyte.

Now if you're feeling generous, can you pass a nice steaming mug of your coffee across? I could really go for that!

[Doc]

sweeper,
I sort of quit worrying about what's really in a battery when they quit making wet-cells with clear glass containers.  Plates, 'goo', some other mysterious stuff, whatever. (no, I'm not really that old, I'm just lazy!)
 - 'Doc

PS - Get your own coffee!  I don't mind giving away money, shirts, even beer, but I draw the line at coffee.

[meechingman]

My RTR Graupner Southampton uses a pack of 6xAA NiMh cells. I've got one set of 2300maH and one of 2700maH, both from Modelpower.

Can't remember what pair of motors she has - Ghost in the Shell will probably know - but she's been run for 2 1/2 hours pretty much non-stop on the 2700 set. Didn't get over hot either.

Andy G

[Paul R]

Hi Andy...

You must be the only chap running a model boat on AA power. Since I started this topic know one else seems to reccomend using AA cells for motive power. The ampre hour capacity of AA cells is increasing all the time and for small models, they must represent the ideal solution.

Paul R

[warspite]

so if you have a small model where weight is an issue, believe me i have this problem, how long would a MFA 3-7.2v 457-re380 last, running on a team magic Ni-MH 6v 1200mAh?

[Mankster]

My RTR Graupner Southampton uses a pack of 6xAA NiMh cells. I've got one set of 2300maH and one of 2700maH, both from Modelpower.

Can't remember what pair of motors she has - Ghost in the Shell will probably know - but she's been run for 2 1/2 hours pretty much non-stop on the 2700 set. Didn't get over hot either.

Andy G

If you can run her for 2 1/2 non stop, you cant be drawing anymore than 1 Amp current from those 2300mAH AA cells. AA's are fine for 1 Amp. Its when you are drawing a higher current that they give up the ghost.

[Paul R]

so if you have a small model where weight is an issue, believe me i have this problem, how long would a MFA 3-7.2v 457-re380 last, running on a team magic Ni-MH 6v 1200mAh?

Probably not very long. I would use the high capacity Ni Mh AA's rated at 2.7 Amp/Hr. In theory you can draw 2.7amps for 1 hour, or to put it another way, draw 1 amp over a period of 2.7hrs). One of the major drawbacks with Ni Mh cells is that they have a high self discharge rate, typically 50% higher than that of their Ni Cad counterparts.

Paul R

[Colin Bishop]

One of the major drawbacks with Ni Mh cells is that they have a high self discharge rate, typically 50% higher than that of their Ni Cad counterparts.

See my separate thread on the new Uniross Hybrio cells which claim a very low self discharge rate.

[Paul R]

Hi Colin... Have seen your thread on the new Uniross hybrio cells. Looks very interesting... I understand their based on NI Mh technology but without some of the drawbacks. It'll be interesting to see how these new cells perform

Paul R

[zetec]

I've just been having a nose about on this subject and found AA cells 2700mA rated with a discharge rate of 3C, correct me if I'm wrong but that is about 7 amp!

I'm building a small speedboat with my son and fitted with a 480 motor, which I suspect drinks about 4-5 amps, these would be ideal as 6 AA are about the same weight as 2 C cells! Even if I fitted 2 packs of 6 to give 5700mA capacity it would be 2/3 of the C cell weight.

Might email the company to check if this is true...

http://www.vapextech.co.uk/acatalog/High_Power_Consumer_Batteries.html


I think with a bit of thinking I could mount these smaller cells to get max cooling. Think of the advantages of these for something like a 1/24 MTB running 3 X 480 size motors....You could get the balance just right, and they are cheap(ish) 60 for £58.

[Fast Electricals]

If you have a look at the Overlander site ( http://www.overlander.co.uk ) , you can see the maximum recommended current for all the the batteries they sell.

A few years ago I purchased an electric flight pack which was made up of 7 GP1300 NiMh AA cells. These worked well with a Speed 400 motor drawing about 6 amps in a small fast electric. The performance was similar to a Sanyo 600AE pack but with double the run time.

Although these cells worked well, I cannot vouch for any other models or makes.

Neil

[roycv]

Hi all, I seem to vaguely remember the maximum power theorem from which you can calculate the maximum power that can be drawn from a power source.
This is based on the internal resistance of the cell, which is frequently quoted in battery specification.  High current requirement also means a low resistance (ohms) and if you shunt the internal resistance of the cell then the voltage drops.
Maximum power is obtained when the internal and external resistances are the same.  But I believe that the internal connections of cells varies a lot.
As an experiment with Nicads If you connect a meter switched to current across the terminals of a charged cell then a good AA  Nicad (650 mah.) will show a reading of 7 amps.
 I bought some very cheap (4 for a pound) 800mah Chinese cells and this test gave only 1.2 amps.  So I use these in Tx's on test they lasted 5 and a bit hours at a discharge rate of 120 milli amps.  This is quite close to the quoted 8oo mah capacity.

I have just done this test on recently charged 2500mah Sanyo NiMh AA cells and got a reading of 5 amps which equates to 2C discharge rate as against 10C for the Nicads.  The test connection was for just long enough to take a reading.
How long it can maintain this current I do not know, but it will not supply a higher current as might be suggested from doing any arithmetic beyond the 2C discharge rate.  So cells with a 3C discharge rate are a big improvement.
regards to all Roy