Using a laptop battery?

[leew]

I have found a store selling li-ion laptop batteries cheaply, possibly as they are for now obsolete laptop models.
I was wondering if anyone here has used a Li-ion laptop battery to power a light? I notice that alot of laptop batteries have about 8 terminals where as my regular Li-ion battery packs just have 2, a positive and negative for both charging and discharge. Any idea what the extra terms are for? Is there protection built into the battery pack or is the protection in the laptop itself thus would not be able to use the battery by itself without buying a protection PCB?

 

[Gadget_Guru]

I have found a store selling li-ion laptop batteries cheaply, possibly as they are for now obsolete laptop models.

I was wondering if anyone here has used a Li-ion laptop battery to power a light? I notice that alot of laptop batteries have about 8 terminals where as my regular Li-ion battery packs just have 2, a positive and negative for both charging and discharge. Any idea what the extra terms are for? Is there protection built into the battery pack or is the protection in the laptop itself thus would not be able to use the battery by itself without buying a protection PCB?

This is a good idea, but there are some pitfalls to avoid.

Firstly, even if a lithium-ion notebook pack is advertised as "new", be sure that it is either not more than maybe a year old, or has been charged to the proper 3.8V per cell storage voltage on a regular basis since it was manufactured. Bare Li-Ion cells seem to do very well in extended storage, when charged to 3.8V per cell. But some packs will self-drain, due to the parasitic load of the protection circuit.

I recently bought a "new" genuine IBM notebook pack on eBay, intending on using it with my POB HID light. The pack was a 11.1V, 7200mAh model. Fully charged voltage would be 12.6V, and the pack's built-in low voltage cutoff would probably kick in around 7.5V - 8.25V, which would allow the pack's full capacity to be used with the HID light.

When my pack arrived, it was obviously a new in sealed package pack, but the manufacture date was three years ago. The pack voltage tested at the terminal was 0V. To see if I could revive the pack, I charged it manually using a variable DC power supply. First I charged it at low current until pack voltage eventually rose to 9V, then I boosted charge current to 4 amps or so, fully charging to 12.6V. I let charge current taper off to a few hundred mA at 12.6V, then connected the pack to my Triton to measure discharge capacity to 9V. It was under 5Ah, WAY under the rated 7.2Ah, making the pack effectively "dead". I think I've read that a Li-Ion cell that tests at anything less than 80% of rated capacity is considered dead, and may even be a bit unsafe.

So for "new" packs, measure the pack's voltage at the pack terminals with a good DMM. If you see less than the following voltages, reconsider paying good money for what may be a bad pack:


Nominal pack voltage/ Minimum open-circuit pack voltage:

(7.2V/7.4V) / 7.2V
(10.8V/11.1V) / 10.8V
(14.4V/14.8V) / 14.4V

Any Lithium-Ion battery pack made for consumer electronics will have protection circuitry built in. The extra terminals may be for communicating pack health to the PC. You can probably charge the pack by simply connecting your charger to the pack + and - terminals (often located at opposite ends of the row of contacts). The pack's built-in protection circuit should keep the cells balanced during charge.

Keep in mind that the maximum discharge rate for the cobalt Lithium-Ion cells used in notebook packs is usually around 2C. So in the case of my 7.2Ah IBM pack, maximum discharge rate would be 14.4A. The pack's over-current protection may kick in below that figure. And for hotwire use, remember the huge inrush current of a hotwire filament. Cold starting a tungsten filament requires way more current than would be implied by the bulb's current draw at operating voltage.

 

[Marduke]

I think the extra terminals are for individual addressing to balance cells. At one time it was common to open up old laptop batteries and salvage the unprotected 18650 cells inside. But now when you can buy protected ones for cheap, the benefits of salvaging old unprotected cells is diminished.

 

[marschw]

A lot of laptop batteries have a rom chip inside to identify the battery to the computer; such identification takes place over those extra pins, usually by a serial protocol of some sort. Some have more advanced microcontrollers in them, on the same board as the protection circuit. I suspect that laptops don't normally bother with balance charging, since it doesn't have all that much of a benefeit "out of the gate" with a fresh (that is, recently-purchased) battery; it just prevents it from wearing out too soon due to a single weaker cell. Since laptop manufacturers make a good profit on replacement batteries, especially for older models, I doubt they'd go too far out of their way to make sure their batteries last for a lot of charge/discharge cycles. But maybe I'm just too cynical...

As for charging the pack, I'd just as soon crack it open and use the cells inside; discounted/old laptop batteries often have a few cells that are in much worse shape than the others, so the performance of the whole pack is greatly diminished, even though 90% of the cells in it are just fine. Also, many laptop batteries' contacts are recessed in the plastic, and are almost impossible to get at.

IMO just take the cells out of the laptop battery's case, since it's a terrible form factor for most flashlights, and either keep using the protection pcb that was inside the laptop battery's case, or use a separate protection circuit.

A place near me sells "as-is" laptop batteries for dirt cheap; consequently, I have so many 18650s that I could use them as primaries.