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Old Oct 23, 2006, 11:22 AM
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A123 overdischarge protection

My two DeWalt packs haven't arrived yet.

Do the packs, or the DeWalt tools they're meant for, include an overdischarge protection device? If so, maybe we could use it in our airplanes.

If not, I forsee alot of very unhappy new DeWalt tool owners.

- RD
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Old Oct 23, 2006, 07:52 PM
Crash Master
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I magine it does.

The electronics pack is cast in thermal epoxy. There is a +, -, ground, and 12 tap leads as well as a thermistor. The terminals to the charger/tool is 3 heavy quick connects, and a port of about 20 contacts.
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Old Oct 24, 2006, 03:48 AM
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What are you trying to say RD? Don't run these cells right down?
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Old Oct 24, 2006, 06:00 AM
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Thanks, Gene - I'll have a look when my packs get here.

LOU2

Post #1 asks a question. Can you answer it?

- RD
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Old Oct 24, 2006, 06:25 AM
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I didn't try to figure out what the module was doing... Enough of that crap at work I'm sure someone who cares can figure out what all is going on. If I had their charger I might have tried, but for the use I intend, there was no reason.

Found a cheap tap connector, BTW... An old IDE ribbon cable. Chopped off 6 pins wide worth (5s packs), clipped every other wire, and shoved toothpicks in the unused row of sockets... Now I can check and balance the cells from outside the shrink.
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Old Oct 24, 2006, 06:36 AM
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Quote:
LOU2

Post #1 asks a question. Can you answer it?
Not for certain but it sure looks like it is meant to balance them.
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Old Oct 24, 2006, 08:07 AM
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I found myself sufficiently curious about the function of the electronics module that I unpotted it and partially traced the circuit.

Schematic: http://web.mit.edu/ruddman/Public/Ro...lt%20Board.GIF
Photo: http://web.mit.edu/ruddman/Public/Ro...t/1C2T7291.JPG

There are two different ways to draw power from the battery through the module. The lone spade connector is the positive terminal, and is directly connected to the battery positive terminal. The outer of the paired spade connectors connects to the battery negative terminal through a surface-mount 15 amp fuse. The inner spade connector sends current through a large MOSFET and a current-sense resistor, then to the battery negative terminal.

The module contains two custom ICs: one appears to be an integrated 10-cell balancer, while the other appears to be a microcontroller. Thus, it looks like the module should be able to perform almost any battery-management task, so long as the inner spade terminal is used. Certainly, it is equipped to provide over-discharge protection and current limiting. Heck, DeWalt could even use the FET in the battery module as an ESC for the power tool, though they do not appear to do so. However, the circuitry is not powered unless some of the signal terminals are connected, so don't expect the electronics to do anything unless you use the proper charger and proper power tools. (Or unless someone reverse-engineers that interface...) With the electronics inactive, the only way to draw power through the module is through a hard-wired fuse - draw more than 15 A and the fuse blows, rendering the module useless.

-Bryan Ruddy
Ministry of Bad Ideas Combat Robotics
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Old Oct 24, 2006, 08:36 AM
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Thanks, Bryan!

Doesn't look like the microcontroller IC can be adapted to our needs.

- RD
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Old Oct 24, 2006, 08:42 AM
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Great work Bryan!
But now my thinking goes into economical mode. We take the deWalt packs for around 90$, we see all this electronics, the box...
May the cost of each cell be...? 5$? 6$?
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Old Oct 24, 2006, 12:44 PM
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Can You check the voltage, at which will original dewalt pack cut itself off during Discharge? I guess it would be good start point for us..
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Old Oct 24, 2006, 01:48 PM
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Specs. here

http://site.buya123systems.com/ANR26650M1specs.pdf


DOD 2.0V per cell.
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Old Oct 24, 2006, 07:44 PM
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Bryan,

Nice job exposing that circuit board. Are you going to tell us how you got that potting compound off so nicely?

I have noticed that the top and bottom row of the 14 muti-pin connector seem to be the same top and bottom, so really only 7 connections besides the 3 main terminals. I can get the FET to turn on by pulling the center multi-pin to the positive side of the battery, but only with no load across the positive and FET output terminal. With a 470 ohm resistor across the positive to FET output, I get a short series of pulses across the load and then nothing untill I reset the process. I'm wondering if the input requires some type of pulse width control? Or it could just need a combination of control pins connected. I suppose the wise move would be to get a charger and drill, and then scope the logic on the multi-pin while in all modes of operation.

Kevin
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Old Oct 25, 2006, 02:35 AM
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To explain clearly why I am asking for voltage in post #10
- Optocoupler like PC817 and a set of resistors (cca 2k2 or 5kOhm, ), ONE for each cell, is all I need to asure approximately 1.1V (voltage to light a led in optocoupler) minimal voltage over every cell. The weight as well as a price of this voltage guard approaches to zero. Series connected optocoupler outputs are expected to connect between ESC and RX into signal wire, causing short power sag in flight if any cell falls below 1.1V, with immediate power recovery after cell voltages are back within limit. Being optocoupled = completely galvanic separate, it could be used to stop bench discharger at any possible signal point eventualy.
I know that it is too simple, but if it will be sufficient, it is sometimes as simpler as better. Fixed voltage is biggest question for me - is it too low, or too high?
Spec sheet says 2.0V recommended cutoff at 25 degree celsia, but 0.5V in freezing - that is the reason why I am unsure.
http://www.rcuniverse.com/forum/fb.asp?m=4737609
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Old Oct 25, 2006, 04:50 PM
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It turns out that reverse engineering the battery interface is pretty easy. We got in the battery connector and wiring harness from the 36V impact wrench at work yesterday, and I was quick to trace the circuit. (What better way to build a prototype cordless tool in an academic research lab than to use the case and battery system from an existing tool, especially at ~$70 for a case and wiring harness?) The impact wrench just uses an on/off switch, so the circuit is awfully simple.

Circuit diagram: http://web.mit.edu/ruddman/Public/Ro...t%20Wrench.GIF
Connector photo: http://web.mit.edu/ruddman/Public/Ro...t/1C2T7862.JPG
Board photo: http://web.mit.edu/ruddman/Public/Ro...t/1C2T7864.JPG

The circuit board in the impact wrench shorts two pins together, supplying power to the battery module. It also includes a resistor divider to apply voltage to a different pin, presumably indicating the presence of a tool (and perhaps even identifying the tool as well). If the signal connector pins are labeled with pin 1 next to the positive spade connector, then pins 3 and 4 are shorted and 2/3 of that voltage is applied to pin 5.

The board otherwise contains a freewheeling diode to protect the battery from polarity reversal (during motor direction changes) and a pair of transient voltage suppressors on the motor leads. The common terminal for the TVS's goes to pin 1 on the connector. Data logging, maybe? There is a 40 amp automotive-style blade fuse on the power input as well.

So, if you wanted to use unmodified DeWalt/A123 batteries with their original charger, you could purchase part #636747-00 (wiring harness, $39.90) and part #630141-00 (tool casing with battery latch, $28.76) to build something to properly interface with the battery. Other cases or harnesses might be cheaper; just poke around www.dewaltservicenet.com.

-Bryan Ruddy
Ministry of Bad Ideas

PS: A nice, long soak in methylene chloride takes the epoxy potting off easily. The soaking also destroys the connectors for the thermistor and balance leads and softens the circuit board itself, but leaves the main connector curiously unharmed. I would not recommend doing this without a fume hood.
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Old Oct 25, 2006, 05:04 PM
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Bryan,

Thats awsome! I'll look over your schematic later. I'd like to use to module during charging to keep the pack in balance. Thanks for all your info.

Kevin
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