Electronics, Lighting, & Gadgets - Frugal DIY 7.4V Battery pack

Guide to a quick and easy DIY Li Ion Pack

http://img42.imageshack.us/img42/3363/186501.jpg

A)Take a 18650 cell holder, trim the little ramps from the ends with an exacto or dremel. Add 2 single terminals on one side, a dual on the other. These just clip on. Dremel or cut notches in the cover to allow it to close properly.

B) Remove the clips to solder lead wires, replace them on the box and BAM: a 7.2V 18650 pack of whatever capacity cell you choose to slap in.

http://img42.imageshack.us/img42/6853/186502.jpg

C) Connect 2 in parallel (postives all together, neg leads together) for a 2S2P pack (7.2V/~4400mAh). Hook up 4 for 2S4P or 7.2V/~8800mAh.

http://img69.imageshack.us/img69/1046/186503a.jpg

D) Connect to light plug. For a deluxe model, use quick connects (http://dealextreme.com/details.dx/sku.10788) for the connection to the light plug so it isnt "married" to anything and the packlets can be easily removed and reconfigured or replaced as needed.

E) Electrical tape or Plasti-Dip over the contacts to prevent shorting; Velcro strap to secure covers. For a deluxe model, hot glue the bases of 2 cases or packlets together to reduce stress on the wires when you drop a half of a 2S2P pack. Waterproof (optional) with used MTB tube, place in bag.


Costs and parts:
2.83" Cell Holder from DX (http://dealextreme.com/details.dx/sku.26339) $1.88 (also avail in purple, cyan and green)
2 Pos Clips from Mouser (http://mouser.com/ProductDetail/Eagle-Plastic-Devices/12BH228-GR/?qs=SeNEhayBclRa3EbSIZmhWw%3d%3d) .32
1 Dual Clip from Mouser (http://mouser.com/ProductDetail/Eagle-Plastic-Devices/12BH210-GR/?qs=ms0dagSGI4oj6mbYmvxsOg%3d%3d) .27
Wire, solder from toolbox .30
Connector from old pack 0 (optionally cut from a Y connector)
(optional) Bag (http://dealextreme.com/details.dx/sku.5061) ($3.97)
--------------------------------------
Total $ 2.77
x2 for 2S2P $ 5.44
x4 for 2s4P $11.88
Time: 15 mins per packlet
Never buying another Li Ion pack again, Priceless

Add cells of your choice from $3 to $13 ea. Total ~$18 2S2P, ~$36 2S4P (compare to $70+s/h)

The downside to a 2S4P configuration of 4 'packlets' is that you need 8 charged cells to ride and have 8 cells to recharge after. Also, in summer if you use those 8 cells less deeply but continue to top off the charge regularly you are reducing service life frivolously. It seems easier overall to me to carry 2S2P packs and change out as needed. This avoids congestion at the charger and extends cell life for a rather minor inconvenience.

Notes
1) The case will accept the cells either way, so mark + and - to be clear. Wire them the same way so if you have to fiddle with it or change cells in the dark, it is easy to tell or know where Positive is. Putting a cell in wrong will result in a short possibly to the whole batch. Putting them all in wrong will reverse polarity to the light.

2) It is best to mark and match cells in pairs (A1, A2, B1, B2...) and keep them together. Marking the date on them wouldnt hurt.

3) Use cells of the same capacity. Given Chinese cells vary widely and wildly, getting the same brand may be best. 2200mAh cells will give a max of 8.8Ah in 2S4P; 2600mAh might give a bit over 8.8Ah and 2800mAh will prolly give the same ;).

4) Dont use a pack charger. Use a cell charger. This is a PLUS!:
- All cells will fully charge each time
- It allows you to tell when a cell is getting weak when it takes longer to recharge (no more shortened runtimes because 1 or 2 cells are weak; confidence in cell health)

5) Avoid reverse charging by recharging all cells when any are recharged. Always replace cells in pairs. If B2 is obviously weak, replace B1 and B2. Dont be cheap (cheap is different than frugal): you can already 'replace' your pack 2 cells at a time instead of 4 or 8 at once!



No Bold Fonts were harmed in the production of this post.

More added bonuses:
Pack isn't waterproof
Extra fun of having the non-soldered connections momentarily break when you hit a hard bump, causing the light to cut off just as you're approaching a bad spot in the road.

Youre right it isnt waterproof, but not everyone needs full waterproofing. "E" alludes to it but, you can zip tie the end of a length of tube making it like a sock and stuff 2 packlets in it for decent protection.

As for the non soldered connections, you can solder them if you wish. I've never had Tamiya connectors come loose nor these connectors (http://dealextreme.com/details.dx/sku.10788) or ones like them (Deans, Powerpole). A bit of tape if you are extra paranoid.

I should mention that you can go upscale and use these PC mount real 18650 holders (http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=393250&k=18650). They are pricey and probably need some mods, but should work even better.

I didnt claim it was pretty, just functional, inexpensive and came with various advantages. YMMV

I'm not talking about the Tamiya connector coming loose, I'm talking about the non-soldered batteries breaking contact with the battery contacts when the pack is hit. I've had that happen a lot on non-soldered battery packs.

I agree that individually charging cells is a better solution, but personally I'm willing to replace the pack a little more often for both the convenience of not having to take multiple pairs of batteries out and change them into a charger 2 at a time (or to buy bigger or more single-cell chargers) and also to make sure that my light won't cut out right when I need it, as I'm heading in to a rough patch on the road and the pack is really getting vibrated hard.

With multiple parallel packs it's VERY unlikely that they'll ALL fail momentarily at the same time from the same bump. ;)

Also, there's no reason in the world that you can't just waterproof the whole thing and hook it up to a single charger... Bonus being that if the pack started to have reduced capacity early you could tear it down, test the individual cells and rebuild it, reusing whatever cells were good, replacing bad ones.

With multiple parallel packs it's VERY unlikely that they'll ALL fail momentarily at the same time from the same bump. ;)

The MagicShine (for example) is extremely sensitive to momentary dropouts. It will switch itself off when there is any interruption, or a sudden voltage drop.

I've not tried homebrewing battery holders. There don't seem to be any real commercial products. The rumor circulating around the forums is that this happens because 18650 batteries are temperamental and potentially hazardous, and therefore should be restricted to commercial battery pack manufacturers. This sounds like pure bull to me (since when are Chinese manufacturers so concerned about product safety that they pass up the opportunity to make a buck? Not likely).

The best solution seems to be the TurboFerret: http://www.turboferret.co.uk/Index.htm

This battery holder seems robust enough, but of course I have not tried it.