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Old 11-08-2006, 01:27 PM   #166
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Spanky,

Your assertion would be correct for a constant-voltage system like a battey has on its output, or like most DC supplies - the lower the resistance, the higher the voltage, at least for any real-world current-limited application.


The problem is that we use chargers that function as constant-current sources to restore the energy in that battery. In reality they are variable-voltage sources that use current sensing to maintain a constant current, but the effect is the same.

For example:

Constant voltage (battery, power supply):

V = I * R

V= 5V, R = 1 ohm, I = 5A,
V=5V, R = 0.5 ohm, I = 10A
or, if current limited at 5A, then for 5A and a 0.5 ohm resistor, V = 2.5V



Constant Current (charger, laboratory current supply):

I = 5A, R = 1 ohm, V = 5V
I = 5A, R = 0.5ohm, V = 2.5V

Again, as you pointed out - Ohm's law. If current is constant, then the lower the resistance, then the lower the voltage. If voltage is constant, then resistance simply determines current.


If you charge a battery pack at 5A constant current and it peaks at a voltage that is lower than another pack that was charged on the same charger at 5A, then the general indication is that the first pack has lower internal resistance.

There are other factors that can effect this. If a pack were severely mismatched, then it is possible that only one cell is peaking. If that is the case, then the pack that is more mismatched than the other could show the lower peak voltage while still potentially having a higher internal resistance.

If they're within a few millivolts of eachother, then it's really not deterministic. As always, the best way to measure internal resistance is during discharge, not during charge. But it is a decent rule of thumb.

I think you are missing 2 key points:

1) the peak voltage of the pack is not its maximum output voltage under load - these are totally unrealted.

2) Current, not voltage determines the speed of the motor - it just happens that our motors are largely resistive, so a higher voltage gives a higher current and thus more speed

Mike


edited to add math and explanation
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Old 11-08-2006, 02:06 PM   #167
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HUH...
Now I remember why I didn't become an electrical eng.
can the damn thing make coffee that's what I want to know...

I'm trying to decide DPD or MM CTX.
i'm so confused.
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Old 11-08-2006, 02:45 PM   #168
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Just find one of your oldest battery pack and your newest one.
Charge them with the same settings/charger and you'll see which pack peak at higher voltage.
To me, my newest pack always peak at lower voltage but i live in New Zealand so things may work a bit different here


DPD vs CTX-D
I've got a DPD and i've borrowed my friend's CTX-D 2 and used it for a week or so.. and here are some comments:

I got very similar results when i discharge my batteries using both discharger. (e.g. if DPD tells me my battery pack 1 cell #2 got 15% lower capacity than other cells, cell#5 got highest IR...etc, then the CTX-D would tell me same result too)


DPD

Good:
-Doesn't need external power so can use it anytime anywhere.
-Cheaper
-Display is large and clear
-The clamp system is faster/easier to install/remove the battery
-Display capacity as mah/internal resistance as ohm
-Has Dynapulse function
-Can discharge stick packs

Bad:
-I got some error and it refuse to do anything if the battery pack voltage is too low when you start or the internal AA batteries power is low.
-Looks much cheaper/plastic compare to CTX-D
-I got some rattling noise from the unit/fan just after a few use...



CTX-D
Good:
-Discharge faster than the DPD
-Looks very nice. Usual muchmore quality feeling
-The "screws" hold the battery better. (but takes a longer time to secure/remove the batteries)
-Can deadshort battery (but then most latest cells don't like deadshort)

Bad:
-IT'S REALLY NOISEY!!!!! i thought my muchmore cellmasters are loud but it's louder than my 2 cellmasters running at the same time. I can't use it in the late evening.
-Need 12V external power.
-Depends on which discharge mode you choose, it only shows you the IR or Average Voltage .. but not both.
-The IR and capacity displayed are not in ohm and mah.




If the CTX-D and DPD are same price, even there are a few things i don't like about the CTX-D (noise, can't display IR and average voltage at same time, need external power), i would get the CTX-D just because i love Muchmore products and it "looks" much better on your pittable But since the CTX-D is quite a bit more expensive than the DPD, it is a much harder decision as i think both are great products

Last edited by yellow15; 11-08-2006 at 03:00 PM.
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Old 11-08-2006, 02:46 PM   #169
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can u actually set the DPD cut-off voltage or is it fixed at 0.9v?
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Old 11-08-2006, 02:51 PM   #170
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Quote:
Originally Posted by "Spanky
this is hilarious, do me a favor and charge your batteries and take a piece of wire with no resistance and place it across the battery terminals hahahaha. I need to delete this forum so I don't feel so stupid from reading these replys.
NO MATTER HOW MUCH RESISTANCE IS IN A CIRCUIT, VOLTAGE REMAINS THE SAME. Go to this website and put in the voltage then raise the resistance and see what happens, ohhhhhhhh wowwwww look at that!!! the more resistance the less current meaning less power for your motor http://micro.magnet.fsu.edu/electromag/java/ohmslaw/
Owned.
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Old 11-08-2006, 02:59 PM   #171
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Quote:
Originally Posted by wyl03
can u actually set the DPD cut-off voltage or is it fixed at 0.9v?

adjustable. From 0.2V to 1.0V per cell.
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Old 11-08-2006, 05:41 PM   #172
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anyway......I would get the teammuchmore CTX D2. I noticed better run times from using this. Instead of getting technical, I am going to throw my electrical engineering degree away and just say, the batteries have more punch for a longer time yo.
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Old 11-08-2006, 08:19 PM   #173
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Okay enough about the Muchmore vs DPD.

Lets talk about the "original" BLUE DPD vs. the "new" BLACK DPD

Is this a New & Improved?

Or is it the one that finally works like the original was suppose to?

Neverthless, its currently being sold as the replacement to the original one and at a higher price.
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Old 11-10-2006, 08:15 PM   #174
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i just got mine yesterday
it's the black one. i reckon it's the same thing as the earlier blue-colour DPD.

built quality is fantastic.

i've instantly identified the weaker cells in my battery packs
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Old 11-11-2006, 12:05 AM   #175
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Quote:
Originally Posted by wyl03
i just got mine yesterday
it's the black one. i reckon it's the same thing as the earlier blue-colour DPD.

built quality is fantastic.

i've instantly identified the weaker cells in my battery packs
so what do you do with the bad cell.... time to let me try your DPD.
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Old 11-11-2006, 04:03 AM   #176
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bad cell go towards a designated practice pack

sure .. u can have a go with it.. i'm unlikely to use it during race days
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Old 11-11-2006, 05:33 AM   #177
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Just got my DPD very please with it so far good information, It allows me to manage my batteries much better than I have been able to before. My routine for 4200 shv cells is as follows:

Linier discharge my pack then charge at 6amps run in the car once finished put back in my box at home I pulse discharge then charge back 400mah into the cells and store till next week then start over again I set the cut off to 1volt per cell as I have been told below this will damage the cells not sure if this is correct as you hear so many different ideas people have I guess if it works for you do it.

DPD better than CTX its not as loud and dosent need a 12v I can discharge my batteries watching tv in the living room the missus would not be happy to have a 12v on the floor and the tv would have to be turned up to be able to hear it over the CTX.
DPD all the way.
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Old 11-11-2006, 10:24 AM   #178
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Quote:
Originally Posted by yellow15
Bad:
...
-I got some rattling noise from the unit/fan just after a few use...
Had the same problem. I opened the box and found that the cooler was mounted in the wrong direction. All of this types have the fan (the rotating thing) open at one side. At the other side the fan is covered by parts of the plastic casing of the cooler itself. I hope you understand what I mean.

Make sure that the open side is facing towards the inside of the box. In my case it was mounted the other way around such that the rotating fan was touching the DPDs fan grill. To check without opening look through the fan grill or check that the air is being sucked out of the DPD.

MHA
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Old 11-11-2006, 07:54 PM   #179
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This thing is great. Found out the weakest cells in 2 packs and assemble a practice pack with a closer "match". I was suprised by how bad some of the cells were. Would have never known without this product.
What sucks is I learned I need new batts.
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Old 11-11-2006, 08:26 PM   #180
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Finally! i got my hands on to this discharger. now im just waiting for testing it in the track. for now, i say its good
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