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NiCd/Nimh Battery Tech Info
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timmeh
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Ok i have two diff types of wire i can choose from for my battery pack leads.

1.The normal 8g wire from dicksmith or jaycar with copper strands.

2.Some new stuff i found at jaycar that looks like speaker wire and is see threw red and black with supra octopower 8 made in sweden written on it and im asuming its 8g cos of the 8 on it but anyway it has less strands then the normall 8g wire but they are thicker and more stiff and silver colered but i think it is copper centered but its very hard to tell?

Which would be better to use?
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Post Mon Nov 01, 2004 5:08 pm 
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Nexus
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WARNING Long Post ahead!

Have been making up a few battery packs and have been developing a concept for them. Was getting frustrated trying to find heat saver rings and it got me thinking. In essence all they really are is spacers for the batterys.

More I thought about more it seemed that rings arent the best thing to use when passing air through them as they add extra edges and lips that will reduce the air flow.
Probably talking about microscopic bees dick measurements but beleive there is some truth to that.

So with this in mind have been working on the idea of using tubes instead and I think its a promising idea.
The way they are put together is that the long horizontal pieces that run through the layers are set on the inside creating 2 horizontal channels for the air to move through the batterys.

With this method it actually increase the surface area of the battery to help lose heat as the only contact made on the batterys is at its tips not its circumference.
The vertical pieces act as spacers but are flush with the outside edges. All gluing is done on the outside areas where the vertical and horizontal spacers meet.

Reason for all this is that it creates a smooth glueless inside area for air to travel through as efficiently as possible whilst all the gluing is done on the outside of this area where the air cooling isnt as important.
A fan on the front works well but seems to blow it out the sides as well reducing the air travel getting through the other end so some tests have shown that placing something on the sides of the batterys stops the air escaping out the sides giving a better air flow through the other end of the pack.

http://robowars.org/forum/album_pic.php?pic_id=340

Pack will have the batterys mounted on the floor hoping that the Ali floor pan will help disipate some heat as well and will proberbly leave the pack with an open top for rising heat to escape.
Final thing to do is make the packs in a way that when secured in the robot they line up with a cooling fan that is part of the robot body. That way only need one fan to cool down any battery I put in there and have a switch to power it.
Would be cool to have the fan monitor the battery so it doesnt have to run all the time but thats not a priority.
Funky green spacers are just plastic tubes that are meant for holding up an aerial in model cars.
GIves it a sort of nuclear radio active quality so might start calling them matrix fuel cells.
By the way, dont take me too seriously.
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Post Tue Nov 02, 2004 2:43 pm 
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Glen
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looks very sweet!

on the subject of an automatic fan i believe you can buy a special transistor type thing that is temperature sensitive and will turn on when the termperature reaches a certain level. nicks power supply has one on it and i think pc power supplies do as well (the one i have does at least.)

stupid as it sounds i was also having a ponder about water cooling for the cells. (i hear its done in rc boating along with water cooling the motors) where one would have a tiny pump and tank and use a setup like yours to pump water past the cells. (not touching them directly mind you).

id like to try add a water cooling system to the motors and batteries on kang (aka computer case bot) as its just a no battle fun bot.
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Post Tue Nov 02, 2004 3:14 pm 
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kkeerroo
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I think water cooling is just adding un-needed weight to the robot and that trying to find the best way to keep a battery cool is a waist of time at the moment considering that no-ones battery has over-heated yet.
Just wondering about the strength of your batterys Nexus. Also the part about placing the batteries on a ali base, I hope the batteries are insulated from the metal!
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Post Tue Nov 02, 2004 4:52 pm 
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Nexus
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Sure its overkill but whats the harm. At RCM they rate the amps from a battery different when its placed in a certain pack. So its not just about heat but getting more amps out your batterys.
Its just experimentation.
IN regard to the strength they havent been finished yet. Will be binding them.
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Post Tue Nov 02, 2004 6:14 pm 
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andrew



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i swear George u have the most interesting imagination.

Not only do your robots look unreal but even your nicad spacers.

I would do it purely for the fluro green tube look alone really.
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Post Wed Nov 03, 2004 5:41 pm 
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Knightrous
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I didn't want to create a new thread, so I'm posting my NiCD based question here.

I recieved my 40 CP2400SCR's today (Thanks Glen) and I'm just thinking about battery configuration. I had an idea, but I wanted to get some confusion settled before trying it. My idea is to make 8 x 6v packs from 40 cells. My reason for this is that I can then make 12v/18v/24v/30v/36v pack combinations by just putting 6v packs in series to make the required voltage. This would save me remaking packs when I want to change voltage.

The thing I'm questioning come back to the discussion of resistance of batteries. As Jeff F said, if you put packs in parralel, you 1/2 the resistance, but if you put them in series, you double the resistance (Correct me anyone). My question is, if I was to make a 24v pack from 4 x 6v packs, ignoring wire and plug resistances, would I have the same resistance as if it was a single 24v pack? I don't think it will be different, but I have this annoying feeling that something isn't right.

Comments, corrections and suggestion welcome and wanted!
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Post Tue Feb 08, 2005 8:01 pm 
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Nexus
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Sounds about right, the only thing I can think of that would be dif would be the resistance from all the extra plugs and wire that would e needed.

Think of doing similar modular packs but considering using maybe Deans plugs as they are small and have less resistance than wire but we are probably talking bees dicks here, dont know in all honesty if it makes any noticable difference but to me the head ache would be the plugs so thinking of making some sort of panel where I could just plug in the packs directly rather than having mutliple Y plugs.
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Post Tue Feb 08, 2005 9:02 pm 
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Spockie-Tech
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You're both right.. Laughing

a 24v pack does have more resistance than a 12v pack, but it also has more volts to push current through that resistance.. Its resistance relative to voltage you have to think about..

[lecture mode] [
In a 24v system, to get say 480watts of power through, you need a current flow of 20 amps (watts = volts * amps).

If you put .09 ohms of resistance in the way of that 20 amp flow (20 cells * .0045 ohms per cell - not including braid/connectors etc), then that resistance will cause a 1.8v drop (Volts = I(current) * R(esistance)).

1.8v lost * 20 amps of power is 36 watts of heat being generated in your batteries.. coincidentally, since we have 20 cells (as well as 20 amps), then thats 1.8 watts of heat per cell..

Leaving an actual 444 watts being delivered to the load and 36 watts keeping our batteries toasty warm.

--

Now, at 12v, to get 480 watts of power, we need a current flow of 40 amps..

now our 10 cell pack has half the resistance (.045 ohms) of the 24 pack since we only have half the number of cells.. , but we are pushing twice the current through it to get the same power.

40 amps flowing through .045 ohms gives you, yes, a 1.8v drop again. (V=I*R)

but now the current flowing through those cells (and causing that voltage drop) is double the 24v pack, so the power loss is 1.8v * 40amps = 72 watts ! Double the power loss that we had before ! Leaving only 408 watts getting through to your load..

And to make matters worse, not only are our batteries radiating double the heat they were before, but that heat is now all coming from half the amount of cells (10 instead of 20), so now instead of each cell dissipating 1.8watts of heat, they are now dissipating 7.2 watts each, or 4x as much ! MMmm. marshmallow packs anyone ?

---

So to sum up.. yes, a 24v pack has double the resistance of a 12v pack, but if it is being called upon to deliver the same amount of power (watts not amps), then it will run much cooler and more efficienctly than a 12v pack will.

Of course if you hook up a 12v pack and a 24v pack to the *same motor*, then the 24v pack will be delivering twice as much current as the 12v pack would (and 4x the power), so you have to redo the numbers).

but if Power requirements are equal, then its better (more efficient, and cooler) to do it with a higher voltage and lower amps than a lower voltage and higher amps..

Thats why main power transmission lines run at 60Kv and more, then get converted to 240v only when it gets close to you..

[/lecture mode] [

Clear as mud now ? Razz
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Post Tue Feb 08, 2005 10:53 pm 
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Knightrous
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Bundles of 6v packs it is then Smile

That will be Saturday's venture Cool
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Post Wed Feb 09, 2005 7:39 pm 
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Glen
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when wayde was over a few weeks back charging up the new 2400mah packs, they all kept on charging up to something like 2600mah before we unplugged the triton.

they wherent hot at all so is it normal for batteries to take this much more? (is the 2400mah their absolute max capacity or is it just the normal rated capacity.)
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Post Sat Feb 12, 2005 5:28 pm 
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timmeh
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My 3600packs get abit over 4amps so id say its normal.

I asked brett a while back why my packs where going over there normal capacity and i think he sed the triton over charges the packs untill the voltage drops then the charger can tell the pack is charged.

Its called peak detection.
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Post Sat Feb 12, 2005 5:52 pm 
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ffej
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No, thats not really it, its true that the charger does overcharge them a little bit, but no where near that (batteries would be getting cooked all over the joint if it were). Most NiCd's and NiMh's (pretty much all batteries actually) require 140% of the total in to get 100% of power out, due to inefficencies during charging, Eg . . . a 2400mAh pack requires 2400 / 100 * 140 = 3360mAh in order to get 2400mAh from a pack that was close to dead flat.

Also, if the pack isnt even warm after charging, chances are its not fully charged, assuming your charging at 1C or greater.
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Post Sat Feb 12, 2005 6:58 pm 
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Valen
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at a high charge current cell temperature is a fairly good indicator of charge status.
when the packs hit 40-50C they are charged (50 is generally just over charged)
if they are "hot" to the touch but you can keep your finger on it for 10-20 secs then you want to pull them off.
off a triton your packs should probbly get to the "about as warm as you can hold your hand on continously" level.
if you are charging from cold at about C
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Post Sat Feb 12, 2005 10:00 pm 
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Philip
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quote:
Originally posted by Spockie-Tech:
Having said that, a lot of people (and pack manufacturers) do go and solder their packs anyway, and if done by a skilled soldering person with a high-heat-capacity (not temperature, heat storage) iron so it can be done quickly, the seal *should* survive a second or so of heat. then you get the advantages of the lower resistance, mechanically strong solder joint over the small-contact patch spot welds.


Has anyone actually tested the difference in performance between soldered cells and spot welded cells?
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Post Thu May 05, 2005 7:06 pm 
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