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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 11:57:10 CDT 2010

Ed,

It is unlikely that a cheap single cell battery pack with a protection circuit will control the charging process like you are hoping.

The cell itself will pull fewer amps as it reaches a charged state.  The circuits are generally designed to disable the battery if the cell voltage drops below 2.6v, they keep a little bit of data showing charge cycle count, a small table showing expected mAh capacity at given temperatures, often have a temp sensor, and log current flow in / out.  It isn't designed to replace an intelligent charger.

Devices typically update the data in this chip when in battery calibration mode (complete discharge and recharge).  We've long been installing larger batteries in our fleet of PDAs that had been designed for only 1400 mAh batteries.   It is easy to find 'hi capacity' batteries in the 1700 - 2200 mAh range but oddly the battery manufacturers seem to clone the data from the chips in the original 1400 mAh OEM units.  Consumers buy the 2200 mAh battery expecting longer run times but the default calibration makes the battery appear empty when plenty of juice is left.  We could not get reliable battery recalibration by running the PDAs beyond empty so we overwrite the data directly on the battery and gain over an hour of run time.

On multiple cell LiPolys the protection circuit is also supposed to prevent uneven discharging / charging of the cells in the pack.

The safest thing you could do would be to use a real LiPoly charger on these batteries.  If it can be external, use something like the Duratrax ICE battery charger used by the RC guys.  These units will terminate the charging cycle based on current, optional temp probe, and are time limited.  The graphing display is pretty neat and occasionally useful when the discharge curve isn't smooth and the battery needs some exercise.

I'd recommend peeling the labels off one of your batteries and finding the product sheet for the chips on the PCB. That is probably the only way you'll discover what is going on there.  A 600 mAh battery will still burn a good hole in your workbench if you over charge or charge a damaged battery.

Speaking of batteries, I've got another batch of 20 salvage units available to good homes:   3.7VDC, 2200 mAh Li-Polys.  They have maybe 1 yr of use.  They have an odd and pretty rare connector on the pcb but it can be bypassed or the pcb can be removed if you want the naked cells.

David


On Mar 29, 2010, at 10:10 AM, Ed Paradis wrote:

> Thanks guys.
> 
> I did a little experiment:
> 
> I connected the li-Ion cell to a 5V bench supply through a 10 ohm resistor.
> 
> The supply is current limited by its own crappy nature to around 300mA.
> 
> I monitored the voltage across the resistor to determine how much
> current was going into the cell.  My hope was that the "protection
> circuit" of the cell would somehow cut off current into the cell when
> the cell was 'done'.
> 
> At first, the cell drew about 150mA, then settled down to about 80mA.
> 
> I didn't want to leave it sitting there forever, so I kept
> disconnecting it when I left the house.
> 
> The "about 80mA" would slowly decrease over time, but I have no numbers.
> 
> I'll try to build some sort of monitoring circuit / data logger to get
> more info.  These are pretty small little cells.. I might end up
> building a little load balancer or series-battery-charging-monitoring
> circuit to charge and use more than one cell at a time.
> 
> Many li-ion packs from laptops have all the charge monitoring and
> limiting circuitry built in, and you really can use them in the same
> way you use Lead Acid cells:  1) don't drain them past some magic
> voltage   2) hook them up to a charger and just forget about them,
> they will trickle charge themselves.
> 
> I was hoping these cells were something like that.  It sounds like
> from John's experience that I can probably do that if I limit the
> current.
> 
> That being said, this battery would be great for little handheld
> devices.  I imagine that is where it came from.
> 
> Ed
> 
> On Mon, Mar 29, 2010 at 6:45 AM, Quinones, Jose <jquinones at ti.com> wrote:
>> Hi Ed,
>> 
>> Not certain about this battery pack in particular, but what I have often seen on lithium-ion battery packs is what the industry refers to as a gas gauge. Although, not filled with gasoline, the energy on the battery module is monitored in the same fashion your car gas tank is monitored.
>> 
>> Except it is pretty easy to know how much gas there is on your tank. With a battery, however, how much charge is left is not that a trivial effort. So these devices are made aware of how much charge the battery can take, and then are at all times monitoring whatever comes in and whatever comes out.
>> 
>> This allows the chip to know when the battery is reaching its full potential (either up or down), which is when you should not charge/use the battery anymore. Also, if the battery is above certain charge level (90% or 95% I think), the chip determines there is no need for charging, which is kind of dangerous.
>> 
>> Hence, the protection chip is much more than just controlling the current flowing into (or out of) the battery.
>> 
>> I think you could just allow a small current into the battery and monitor the temperature. If the pack starts to warm up, disconnect the charging current. I am not certain how super duper safe this open loop method would be, but I don't see why it would be unsafe. Run the experiment by your local firehouse, just in case ;-)
>> 
>> Best regards,
>> 
>> JIQ
>> 
> 
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