Is the runtime not long enough to get you through the day?
Do you carry one of those huge external battery packs?
This instructable is intended to show how one may replace the dead li-ion/li-poly cells of a laptop battery and how one may increase the capacity of the battery by adding extra cells.
The reason why adding extra cells to the internal battery is suggested compared to carrying around an external battery pack is that for the same amount of cells in an external pack, the laptop can run significantly longer if those cells were used internally.
Step 1: External battery pack? Rip it apart and never think of using an external pack again!
What's the rating on your power brick?
Lets use my tablet computer as an example.
The battery pack is rated for 3.7v * 3 cells or about 11volts. However, the power brick outputs 20v.
I've bought external battery packs before and have calculated how long it should last using watt hours instead of amp hours. However, that was wrong. The battery pack would need to output 20v in order to power my tablet, thus the battery cells used to increase the voltage does not attribute to the amp hours, which is indicative of how long the battery pack should last. Thus, a difference of 9 volts is significant. That's about 2 li-ion cells wasted just to match the voltage.
Another problem with using external packs is that the laptop would think that it's connected to an outlet, thus is not so weary of watching how much power it drains. The power brick for my tablet outputs 2.5 amps, thus it is possible at times my tablet is drawing 2.5 amps from the external pack. However, the internal batt pack only requires an average of 1 amp per hour.
So what would one do in order to increase the runtime of one's battery pack? Forget the external pack, just add more cells to the internal one.
Step 2: How laptop batteries work
The image below is a typical smart circuit. It has four wires running out of it: ground, power, and two 'intermediary power' wires (actually the ground wire is just the tab on the right).
Lithium cells output about 3.7 volts. Like all batteries, in order to increase voltage, they are connected in series. However, charging a "pack" by adding power through the positive node and negative node of the whole battery pack is dangerous. They are not guaranteed to charge evenly (refer to resistance in series in a physics text). This means one cell may overcharge and explode, which is very bad especially since it's lithium. The intermediatary power wires are sandwhiched between every series connection of the battery pack so that it monitors each individual cell.
Now on to the nitty gritty. Most people would say not to mess around with the smart circuit, and they are correct. But if handled correctly, it won't be a problem. The core of the smart circuit does not control the charging cutoff and output cutoff when the gauge goes to 100% or 0% (older model laptops do, but not anymore). The smart circuit merely lets the enduser (laptop user) know how long they have before the battery runs out and let them turn on special options such as hibernation in order to save their work. Charging and discharging cutoff is done by a secondary monitoring circuit that monitors a one of two states called "end voltage" or "end amperage". So for those who believe that they must charge and discharge their batteries once a month or so to "recalibrate" the battery are wrong; it only recalibrates the gauge, not the actual capacity of the battery. That is, if one is missing about 20% of their battery capacity due to the gauge being offsetted, the only reason why one would need to recalibrate is because they want to utilize the hibernation/shut off option when the capacity reaches too low. If one were to turn that option off, one can use the battery pack until it drains fully, completely ignoring the fact that the battery meter is flashing 0% (Because the meter does not control the battery's cutoff point, just the computer's). However, if the li-ion cell is dead/dying, no number of charge and discharge cycles can bring the battery back to life; the cell is physically dead (so forget about the term 'digital memory loss').
Step 3: Parts list
Solder (but of course)
SAND (A MUST)
BIG METAL CAN WITH LID (A MUST)
fire extinguisher (somewhat a must depending on how careful you are)
undead battery (zombie batteries) I mean, new lithium ion or lithium polymer cells (make sure you know which your battery uses)
duct tape (geek's best friend)
Other things as you see fit (second hand soldering helper, wire cutters, wire strippers, etc)
Step 4: Preparation/setup
-put sand in can (picture below)
-place fire extinguisher someplace close
-if you're just replacing your dead cells with new ones, obtain the same number of cells. As for choosing the capacity, bigger is better.
-Note how the cells are connected in series and parallel, and solder your new battery pack the same way.
-NOTE: do not remove dead battery cells from battery pack (explained later on)
-if you're increasing the capacity of battery pack, obtain nth times the number of cells in your battery pack (an original pack of 3 cells can have 6,9,12 cells, etc)
-for every series connection in the original pack, you can add cells in parallel. (a pack with 3 cells in series can accomodate 6 cells (pairs in parallel) in series. That is, two in parallel, and attach those pairs in series, etc) any number of cells in parallel is ok.
-NOTE: once again, do not remove the original cells from the battery pack.
My battery pack below has 3 sets of 4 cells in parallel, which are connected in series. (note: parallel groups are separated as left, middle and right. ) Wires are also connected so that I can solder the new pack to the smart circuit easily.
Step 5: Safety first: test the darn thing
-So, attach alligator clip w/ wires to the new pack and bury it in the sand (don't forget which clip belongs to which wire)
-Here's the tricky part (yet another thing other how-to's messed up.) The reason why I said not to disconnect the original (dead) cells from the smart circuit (which I inadvertently did. Don't worry, it was my test battery) is because the circuit requires a constant power supply or the smart circuit guage messes up. You might wonder why worry about the gauge if it doesn't contribute to charge and discharge cutoff. This is because the laptop requires a signal from the circuit before the laptop will turn on (in case the cells are thought to be drained and draining more, even for a second can kill the li-ion cells. Or simply, something's wrong with the battery). So, connect the new pack to the circuit before disconnecting the original battery cells.
-However, what if you're using alligator clips, which is a temporary connection? How can you disconnect and solder on permanent connections? Either, solder on the new connections while leaving the clips connected, or you can even use a power brick with about the same voltage as the whole battery pack (a 11.1 v pack actually ranges from 12.68 v to 7 v so a power brick at 12 v is ok). But remember to add a resistor between either the anode or cathode of the power brick and the circuit, so you won't kill the circuit board. (Not connecting intermediate pins should be ok, I haven't tried. If you worry about this, you can reuse your dead cell as a temporary power source while soldering on the new pack.)
-Plug in the battery circuit into the laptop and place it FAR AWAY from the battery pack. Test the battery pack. Charge is first, then completely discharge it, then charge it again. This is when you should watch over the battery pack intently, because it might explode (sand should stifle the fire, but immediately unplug the battery from the laptop.) The fear here is the type of secondary circuit used to monitor end states. End Voltage type circuit is ok, but end amperage is no good. If you can tell what type you have just by looking at the circuitry, then you're in good shape, but if you can't there's always the fear of an explosion. From my experience, the circuit is typically end voltage.
Step 6: Completion
-Solder it permanently to to the circuit, and fit it back into the plastic housing if its the same number of cells, if not be creative and line the cells up so it fits nicely below/behind/etc your laptop. Use duct tape or if you have, shrink wrap it with rubber shrink wrap.
-And remember, be careful around rechargeable lithium batteries.
-Update: I forgot to mention. Depending on the type of "smart" fuel gauge, adding more cells won't change the 'estimated hours left' displayed by the laptop, this is because the number of hours might be a fixed range. One might think that even if it's a fixed range, the number of hours left or % capacity left might be proportional to the actual number, however, depending on the type of circuit used to count the "electrons" (some use ic's called electron counters), it might assume the capacity to be fixed as well, thus the estimated capacity won't be proportional, just truncated. However, from my experience, the capacity gauge stops at about 7%, until the physical battery drains until 7%, so it still effectively alerts the user when the battery is drained after below 7%.
-Update 2: At first I thought my smart board fuel gauge circuit was of fixed capacity, but after a few complete discharges, it recalibrated. Now it knows the capacity of my new pack and estimates accordingly (ranges from 9 - 8 hours total runtime depending if I'm constantly using my secondary hard drive accessed via USB and/or lcd backlight levels)