Date: Thu, 12 May 2005 23:05:57 +1000 From: Raymond <[EMAIL PROTECTED]> Subject: RE: [LIB] Rebuilding a battery pack
Also, what are these "Saft Primary Lithium Thionyl Chloride Batteries" seen in
http://www.slavin4u.com/lithium.html and can they be substituted? Asking
because there is a 3400 mAh version for sale on the above page and it's
dimensions are just a tad smaller than the Panasonic CGR17670HC/CGR17670HG (the
original battery). But I assume the replacements must be Li-Ion and Li-Ion
only, right?
Raymond should know more about this. All I know is that he was told that the charging circuit in the 100/110 can deal with charging cells that are not greatly larger in capacity than the originals. How much larger would define the high end limit I think is something no one has determined yet.
Whoever does the testing will want to put the battery pack under a container that can deal safely with an explosion if the cells aren't happy with their setup.
Note that a "Primary Cell" is a non-rechargable cell (like your usual zinc-carbon or alkaline cells). Rechargable cells (such as NiCAD, NiMH, Li[Ion|Poly], etc.) are known as secondary cells (a battery being made up of a pile of cells).
From what I understand (and the usual disclaimers apply about me supplying this advice in good faith but not being responsible for anything that happens, exercise due caution, I'm not a professional battery tech, etc.), you can "safely" replace the cells with similar cells of higher capacity. In the general sense, LiIon and LiPoly cells are charged with a constant current (generally 0.7C where C is the AH rating - an 0.7C charge current on a 1500mAh pack will be 1.05A for instance) up until the cell voltage reaches a certain point (3.7 volts or thereabouts), after which it's constant voltage until the cell draws no more current. The cells should NOT get hot whilst charging (well, OK, a little warm for those with higher internal resistances). If the charger is smart, it'll simply hold the constant current part longer (because a higher capacity cell will need longer at a constant charge current to get to the switchover voltage), after which it'll trickle the rest of the charge in and be done. It'll take longer than a charger designed for the higher capacity cell but it should be OK.
Now I say "safely" and "general sense" because whilst chemically the cells are similar, physically they can be quite different. The exact point at which the charger should switch from constant current to constant current moves around a little for instance, as does the safe charge current. For instance, I've got a low resistance 2.2AH 7.4V LiPoly pack here for my model helicopter which is rated at a massive 10C discharge - I can draw 22 amps continuously out of this until it flattens - but I can't safely charge it at more than 0.5C (it's designed for model gliders where you may want to blow all the charge in a few minutes to get the glider into the air real quick). To be safe, unless you know what you're doing (and have the gear to monitor the battery), try to get as similar a cell as possible to the originals (maybe slightly higher capacity but don't, say, put LiPoly cells in there ... not that they'd fit). Also make sure the thermal fuses and temperature sensor are there, they may be what saves you from an exploding pack.
The point is I want to do this operation once, and then do it with the best
available parts. I want the best cells I can get (for a reasonable price) so I
don't need to open up my battery pack again in a looong time.
PS. I managed to get the pack open with just a few slight scratches on the
visible parts of the battery pack (the part that goes hiding into the Libretto
is pretty scuffed though because I used too much brute force but that doesn't
matter :) I stopped to think for a second and used a very slow technique on the
visible parts of the pack; I slid a very very small screwdriver in the seam and
used a small hammer to tap it along the seam so that I never had to jerk the
screwdriver in or out of the seam... I just tapped it slowly and gently so that
the glue came off and the seam opened in little steps... I really don't know
how to describe it but if someone is interested I could try to take a few
staged pics of the method I used.
I had a lot of luck cracking the edge by pressing a weak point with a wide blade screwdriver until I heard the glue crack. At that point I was able to use my fingernails to start opening an edge. Then I found that I could insert a small 1/4 inch strip of plastic cut from a credit card into the opening, and then just pull it along the side to expand the opening. I did have to take a small flat watch screwdriver to one or more of the corners though. In the end there was only one very small scratch that's hardly noticible.
Whatever you do, do NOT poke anything metal sideways into the seam - puncturing the cells is the LAST thing you want to do!
Whenever I muck around with LiIon, be it a pack that I've just built or a charger I've just built (or purchased), the first 2 or 3 times I charge them, I do it outside, on brick paving, with a barrier made of 2 layers of bricks between it and the house and set up such that if anything does explode, it won't hit anything easily flammable (the vegetable patch!). I also keep a temperature probe on the battery (many DMMs nowadays come with them, even the real cheap ones) to make sure the temperature isn't going to go mad on me. I know it's overkill but it's better to be safe than sorry!
Good luck!
- Raymond
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