Protected Litium Battery Pack - Anyone Made One?

[Curly]

I would like to build a battery pack with the equivalent voltage of one 18650, or maybe two but one would be fine. So that's 3.7 - 4.2v if it was the single version.

Basically I want a portable, rechargeable pack that I could wire to an overhead light under my Easy-up canopy when camping. I plan to use 3 or 4 Cree LEDs or cannibalize 3 flashlights for their reflectors to create an adequate 'ceiling lamp' to hang over the cooking area. So to power it for several hours at night I think a smaller sized battery pack would be a good solution. (we bring a marine battery now, but oh man they are SO heavy)

Has anyone made a pack of maybe 10 cells in parallel and somehow managed to run it through a protection circuit for charging etc.? I have done this with old NiCad batteries but I realize lithiums are a bit different.

Then there is the issue of charging... Would a typical Ultrafire charger work, or would I have to use some other method to charge the whole pack each morning?

Thanks!!

 

[steve6690]

If you put them in parallel I don't think you'd need a protection circuit board if you use protected cells. You could probably use a charger like this :

http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=1231

 

[SilverFox]

Hello Curly,

There is no problem hooking up 10 Li-Ion cells in parallel. There is no problem charging 10 Li-Ion cells in parallel, but you have to charge at a rate that is above "trickle" charging.

An ideal charge rate is 0.7C. If you had 10 cells that were 2400 mAh, that would be a battery pack of 24000 mAh. 0.7C charging would be at 16.8 amps. There aren't too many chargers capable of doing that.

Let's look at the other side of charging. The charge algorithm should end the charge when the current drops to about 0.5C. For a 10 cell battery, this would be 1.2 amps. This means that you could charge at 2 amps, and then terminate the charge when the current drops to 1.2 amps. There are several chargers capable of charging at 2 amps.

The maximum charge rate would be 1C, or 24 amps.

If you charge at 1C, your battery should be charged in around 1.5 hours. If you charge at 0.7C, the charge time would extend out to a little over 2 hours. Charging at 2 amps would extend the charge time out to over 14 hours. Charging at lower rates is getting into the trickle charging rate and is not recommended.

Decisions, decisions, decisions...

Tom

 

[Curly]

Is there something special about the way Lions are charged that would preclude me from using a high current RC car or other 'pack' charger? I can see your point Silverfox about the low charge rate and time. It makes perfect sense. I take it that for the most part its the voltage peak that would trigger the charger to stop?

The next step then would be to find a decent charger that will turn off once the pack is full, or reaches that 4.2 volt point, that delivers a high current charge rate. Any suggestions?

Also, there are several varieties of tab top batteries that I think would be better suited for this pack formation. If I used them instead of protected cells, is there a protection circuit available that I can use to control or 'protect' the pack as a whole?

 

[SilverFox]

Hello Curly,

Li-Ion cells are charged differently than NiMh cells.

With NiMh cells, the charger charges at a constant current and the charge is terminated when there is a slight drop in voltage.

With Li-Ion cells, the charger charges at a constant current until the cell gets to 4.2 volts. Then the charger holds the voltage constant and the current tapers off. When the current drops to somewhere in the 0.1 - 0.05C range, the current is shut off and the charge is complete.

The RC chargers that charge Li-Ion or Li-PO chemistries will work very well. I have a Schulze charger and it works very well. I have also heard good things about the Bantam BC-6, as well as a lot of other hobby chargers designed for Lithium chemistry.

I believe BatterySpace has some protection circuits that may work in your application.

Tom