No real tricks, just a few techniques that can make battery pack
building/rebuilding easier.

Don't reuse old cells or the old welded connecting strips for new battery
packs. If the old cells have leaked, the pack connector copper leads may
also be damaged if the leakage has reached the ends of the stranded copper
leads. The liquid that's leaked out of the cells can wick up into the
stranded copper leads.
Replace the pack leads with the same gage wire (or larger) if any damage is
visible.

Choose new cells of the correct physical size and reasonable capacity,
fresh, flat top cells with soldering tabs already attached. Solder the tabs
to connect the cells together, not wires or other material directly to the
cells. Some dealers will add tabs to cells for free.

Cells with tabs used to be made with the 2 tabs oriented 180 degrees apart
(looking down at the top of the cell, one tab points left, the other tab
points right).
The cells I purchased recently from 3 different sources have the tabs both
pointing in the same direction.
Lay the cells out side-by-side in a line with alternating + & - tabs
adjacent to each other, and all the tabs pointed upward.
To get the tabs to meet and overlap, make a 45 degree bend close to the
welds, which changes the direction of the tab to 90 degrees.
By making the 45* bend close to the weld, the tabs won't stick out past the
sides of the cells.

Clean the tabs and tin them with solder using flux.. as they overlap, tin
the top of one tab, and the bottom of the other. With the cells neatly hot
glue tacked together, apply an additional small amount of solder to the area
where the tinned tabs overlap, and press the tabs together with a tool until
the solder cools.
I place a small square of fiberglass tape under the joined tabs as extra
insurance so the + tabs can't short circuit to the cell case (even though
there is an insulating washer under the plastic skin).

If the original battery pack utilized a thermal protection device, one
should definiely be installed in the new pack. Many times a thermal switch
is marked with the temperature rating. Temperature rise during charging can
be detrimental to the useful life of the battery pack, and also to the
capacity of the cells.
Thermal protection switches with leads can be soldered to tabs of the cells.
The polyswitch types of devices should be spot welded directly to the cells.
Any other components (diodes, precision resistors) that were part of the
original battery pack should be checked for normal values.

Connector leads can be soldered to the appropriate tabs, and shrink tubing
over the soldered connection and the end of the lead will offer some
support/strain relief for the leads.

There are dealers that also sell the shrink wrap to cover the cells, for
making a neat and secure battery pack.

A few simple guidelines will prevent most problems.
Don't solder directly to cell terminals.
Don't allow cells to get hot while charging.
Don't overcharge battery packs. One of the best ways to prevent overcharging
is to use a manually reset timer to limit the amout of time for charging.
Don't over-discharge battery packs. When the device/tool starts to run
slower, stop using it and recharge the battery pack.

Many rechargeable consumer devices are supplied with very poor quality
chargers (just a transformer and a diode was the cheapest method).
If you can't get a reasonable service life from the battery packs, you might
consider choosing another method of charging the battery, whether that means
removing it to connect it to a better quality charger, or modifying the
device with an external connector for safely connecting a better quality
charger.

In some cases, it's just better to have more than one battery pack, that can
be easily swapped by means of an external connector.

--
Cheers,
WB
..............


"Amanda Ripanykhazov" wrote in message
...
I am hoping this question isnt too basic for this forum but I am
trying to replace some rechargeable batteries (in a Roomba: I
understand that generally only one or two start malfunctioning over
time and that it is usually the same one). It MUST be easier than I am
finding it.

I can just about solder on a connection but these rechargeable C cells
(nickel metal hydride? I am pretty sure they aren't lithium!) always
seem to have some sort of electro-welded rectangular plate connecting
the batteries which wont come off with a normal soldering gun? Or do
I just have to apply an inordinate amount of heat at exactly the
position where the ;nick' seems to be in the plate? Given the amount
of heat I seem to have applied to it already with no effect
whatsoever, to me it seems that this might damage the cell?

Is there some trick to doing this please or am i supposed to use a
dremel wheel to cut the plates off, leaving just the electro-weld to
which I solder the wire for the new cell?