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[DPRG] Lithium Ion Batteries from BGMicro

Subject: [DPRG] Lithium Ion Batteries from BGMicro
From: David M Wilson davidmw at tx.rr.com
Date: Mon Mar 29 17:12:44 CDT 2010

Yeah I understand the motivation.   The built-in charger on the xport supports only NiMH but Drifter runs on a 4000 mAh Lipoly pack.  

In the case of the xport the charging circuit resides on the robot controller board and the voltage regulator relies on a large copper pad on the pcb as a heat sink.  Charging this way generates significant heat on the expensive robot controller and the pcb changed from green to gold/brown due to the heat.

It is quite common for LiPoly cells to swell.  That should be a signal to discontinue use.  The older the cell, or the longer the cell sits unused, the more likely some swelling will occur.

If you remember the story behind the external battery pack I had on the ipaq 2215 controller for Drifter.  I had not used the PDA for several months and on the first charge when preparing for the Lego First event, the 'newish' internal battery on the 2215 enlarged just enough to pop the battery door off.  This often breaks the door.  

We've had a few dozen cells for PDAs expand from a normal 4mm thick to over 10mm.  The PVC cell wrapper stretches like a vinyl pillow inflated by a gas.  These were OEM units internally mounted in HP PDAs with nice built-in chargers and on a constant use / charge  diet.  This kind of expansion could cause LCDs, mainboard and plastic housings to break.  The charger never detects a problem.

The standard battery box on the xport will only permit access after removing 4 long screws that hold it to the pcb.  A swollen battery would be hidden from view and the on-board charger would create a situation where a dangerous battery was charging in a hot environment.  If something goes wrong during charging there would be no way to extract the burning battery to save the robot.

Mobile robots, RC planes and cars can fall or crash, damaging batteries.   LiPoly cells are good solutions but the safest way to deal with them is to charge them outside the device.  This forces a visual inspection on each charge cycle and lets you position the battery away from expensive or flammable stuff.  I think this is more important on devices that we use sporadically than on devices we tend to use daily.  A 'new' robot battery that has been used only 10 times can easily be a few years old when you next need to charge it.

On Mar 29, 2010, at 1:56 PM, Ed Paradis wrote:
> So the design goal is the sort of 'fire and forget' charging of Li-Ion
> batteries such as we do with cellphones, mp3 players, and laptops.
> Charge it whenever you want, however long, and have it be smart enough
> to not let you over-discharge or over-charge.

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