I worked out my math from my second generation prototype external battery that yo can get about 10hrs battery life for under 30$
Stuff you will need
1. At least 2 laptop batteries- 15$ (7.5$ each based on a quick ebay search on buy it now, lowest price and new)
2. Soldering gun, solder, electric tape and small tools like pliers, wire cutters and so on 0$(if you don't have tools, can't make stuff.)
3. A 4S balance charging cable 1$ or so on ebay(optional but highly recommended)
4. A low voltage buzzer to warn you when you drain battery too much.2$(it goes on your 4s lipo balance cable. optional, but safe)
5. An apple adaptor with extension cord, the extention cord will be consumed, unless you have color coded wires.
6. Male and female connectors for cctv power cable 1$ or so.
7. Box, on the cheap, I grabbed conduit boxes, 18cm and 22 cm 2$ at walmart, 1.2$ at home depot.
8. Heat shrink and electric tape, hot glue, zip ties 5$ or so... and double side foam mounting tape is good for securing the batteries from the edge of the case. They are also available at dollar stores.
Before you begin, please read all, and please always be safe.
Please always be safe! I cannot be held responsible if you do something unsafe, and my instructions are just how I did it, please prepare yourself to do solder and wire cutting correctly.
Step one, buy all your stuff. Unless you want t be like me and procrastinate for like a month before starting, buy everything first. If you don't have a lipo charger, that will cost you $ too, but that's really not part of the battery.
Step two: So after you get your batteries prepare your magsafe charger.
Cut your magsafe connector with some cord on it, bind the end that connects to laptop with a female(or male) CCTV power cable connector using sothing I called the uneven solder technique to make pretty wire. and take some extension cable to make a power cord for + and - connection(extension cable is power coded, use white and grey for + and -) if you want a long balance chargin cable, pull out the green wire.
-(negative wire on outside, cut 1 inch shorter)\solder--- (CCTV neg wire longer) ---plug
--(positive wire inside, longer)-------/solder----(CCTV pos wire shorter)--plug
(I tried to illustrate the uneven soldering procedure I used, but didn't come out well, I have an instructable on uneven soldering as well.)
if you don't have another magsafe, please buy additional cctv plug and make a connctor for power brick so you can still use it.
use the same solder technique for power brick and battery cord, they should be the same, for example, if the plug to computer is male, both power brick and battery should be female and vise versa.
Step three, carefully harvest your battery for 18650 cells, if they both have 6 cells, make 12 cell battery by parallel 3 cell each and serial connect them. This step is crucial, and if you don't know how to work with lithium cells, don't do it!!!
Strip the lable from batteries to reveal cells, the label is printed on a peal. carefully lift plastice case away from battery cells, you don't want to hurt your cells, then break plastic case. Harvest cells by cutting off one cable at a time, and tape with electric tape the ends of cables before cutting next cable. Some cells are connected together with tabs and leads, cut these only when they are serial links.
Note, always use cells from new batteries and with same capacity, having unbalanced battery is no joke.
First, make sure you know how to solder lithium cells and controlling temperature, if you don't know how, stop and get help. After harvesting your cells, solder them 3 or 4 or maybe 6 in paralell depending on the number of cells you have, use color coded wire you strip from apple extension cord to link cells or use metal tabs and leads to solder them.
After you have soldered 4 groups of cells each group in parallel within the group, solder color coded master leads from each group, and then wrap each group fully with tape. Mark 1,2,3,4 on each group with stickers, wire balance charger wire 1 to group 1 positive and power cord positive, make sure you can feed the wire into the conduit, and solder. Then solder balance wire 2 to group 1 negative and group 2 pos. Solder balance wire 3 to group 2 neg and group 3 pos. Solder balance wire 4 to group 3 neg and group 4 pos. Soder balance wire 5 to group 4 neg and power cord neg.
Take your multimeter and make sure the power from cctv connect is between 12v to 16.8v.
Step four, add safety mesure such as low voltage warning or fire proof lipo bag to conduit enclosure.
Charge your battery on a balance charger and then enjoy!
Please always be safe.
Step 1: Battery Strip
This is more than two bricks worth, but two is only a start. strip cells and tape them immediately after cut. Don't short anything and be safe. careful with positive end of the battery, because the outside is neg.
Form batteries into 4 groups, solder their terminals together with leads, and leave out one pos and one neg wire from each group.
Step 2: Wrap Up the Groups
wrap up the packaged groups of batteries and make sure they fit. You can wrap them flat or in squares, or in diamonds, just make sure they fit, plus there's space for wires. I am using a 22 and 18 conduits.
Step 3: Strip Extension Wire
Strip extension cable by cutting off the end, cut into the green wire and split the insulation. Solder cctv male to one end. Split the other end of the wire and feed it through. Some cells may be replaced in the future, having a flexible wire helps with that. Remember to balance the length between battery and power cord. Be safe, don't cut yourself.
Step 4: Connect Batteries
I lost my balance lead so I will simply say to connect 1 pos to main pos balance 1 wire goes here, 1 neg to 2 pos, 2 neg to 3 pos, 3 neg to 4 pos 4 neg to main neg.
Try to trim off unnecessary wire and after making sure everything fits, secure batteries to case with double sided foam mounting tape. make sure you protect batteries from each other too.
Step 5: Organize Box and Make Sure It Fits
Step 6: Uneven Soldering
solder one wire shorter and the other one longer, then bind each with small heatshrink and the whole with thicker one. Shorter and longer is always shorter for apple neg wire, which is wrap arround, and longer for inner so the neg does not reach to possibly short, this is for aesthetic and safety purposes, and test if the wire works before final wrap up of wire. Remember, always be safe!
Step 7: Plan Ahead
Because there will be a case and a finished product, you will lock it up with zip ties and put it in a safe container like an ammo box, you want to plan ahead for size, capacity. as per demonstrated 4~6 cells per half box is no sweat, you can fit an entire battery in one conduit combo if you pack tight for 12 or 14 cell, make sure you get 22.5 cm conduits. Additionally cig lighter with fuse, 4S lipo protection chip, digital voltage reader, and low voltage alarms are all good ideas.
For MacBook air 3S batteries are better, meaning you divide them into three groups instead of 4, you can also choose to buy a 12 cell battery for that. The downside is the battery will not work well for MacBook pro, also cig lighter socket is out. The upside been less danger of deep drain. 4S batteries are better for multiple applications.
Step 8: PS, Some Laptops Work Too
carefully choose the connection tip to your like. I use male connection for battery so I can also use it to power my Acer led monitor or my acer laptop, but the same conversion to power tip can be made to suit your other machines.
Step 9: Finished Product
Remember, lipo are stable so long as you charge them on a good genuine charger and monitor their voltage when discharging. Don't let any single cell fall below 3volt. 2.7v is the absolute danger zone, but don't go below 3v in general. A beeping balancing volt meter works great, all the function and reasonable cost(about a buck and a half).
Good work, this under 30 dollar project actually only costed about 25, so knock yourself out with some skin wrap like the auto detailing ones. some fire proof lipo case or charging bag is a good idea too.
Happy project building, and be safe.