Building Batteries

(Bruce) may have said:

One question, however: these batteries have some kind of doohicky in line between a couple of the
batteries; I think it is a resistor, or diode, of some kind (though I know little about how to
identify such things). Any ideas on what this might be or what it does?

Very likely a thermal fuse. Many laptop battery packs have a thermal protection device that trips if
the pack gets overheated. Most will self-reset after they cool. It's typically in series with the
cells, and physically located in the middle of the pack.

By the way, 8 cells would be a 9.6V pack, probably from a radio controlled toy or a cordless
power tool.

Quoted message said:

I have read in some places that a "smart" battery pack is made by having some kind of thingy that
cuts the circuit at a certain temperature, to keep the battery from overcharging (as the easiest
way to tell that a NiMH battery is fully charged is apparently that it begins to heat up). Could
this be what this is?

Yeesh, no, some smart NiMH packs include a thermal sensor to allow the charge controller to throttle
the charging when they get hot, but the cells can also heat up when being drained, so temp is *not*
the thing that tells you when they're fully charged. On the other hand, when charging cells in
series, as is the procedure with a laptop pack, the cells will not get hot at the same time; the
weakest cell will overheat first, and when that happens, the smart thing to do is stop before the
cell is damaged. Charging cells in parallel is preferable, but you can't have them connected in
parallel for charging and at the same time connected in series to power the device, so laptop makers
use the easy way out despite the fact that it's not the best.

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(Bruce) said:

I am looking at putting together my own battery packs for my bike lights. I have come across some
surplus NiMH battery packs that look as though they came from a laptop; 8 x 1.2v
slightly-larger-than-AA cells, conneted by tabs. Very Cheap. Hoping they might have some life
left in them.

Could be a 5/4 AA cell or perhaps an F cell. If you really want to know, get a copy of Digi-Key's
PDF catalog (www.digikey.com). It gives dimensions of cell sizes you never heard of.

BTW, a NiMH F cell is good for about 3.5 amp hours, pretty respectable. It's a common size in laptop
packs and you can get them cheap on ebay (sometimes).

Quoted message said:

I was thinking I could pull off 3 of the batteries in these packs, and I would have a 6V battery I
could use for my lights.

5 cells may give short bulb life, you may need to use 4. At full charge cells are more than
1.25 volts.

Quoted message said:

One question, however: these batteries have some kind of doohicky in line between a couple of the
batteries; I think it is a resistor, or diode, of some kind (though I know little about how to
identify such things). Any ideas on what this might be or what it does?

Probably a fuse for short circuit protection. Bad Things happen when you short a NiCad or NiMH cell.
Use it with your homemade re-pack. How to tell? Get a cheap digital multimeter and ask a friend how
to use it.

It's probably not a diode. Contacts on battery packs are usually made so you can't apply
reverse polarity.

Quoted message said:

I have read in some places that a "smart" battery pack is made by having some kind of thingy that
cuts the circuit at a certain temperature, to keep the battery from overcharging (as the easiest
way to tell that a NiMH battery is fully charged is apparently that it begins to heat up). Could
this be what this is?

It's not the best way to do it. By the time the pack heats enough some damage has already been done.

Most smart chargers measure voltage across the pack as it charges. With NiCad batteries the voltage
drops when the pack is fully charged. With NiMH the voltage goes steady. You can try it yourself
with a dumb charger and a multimeter.

Some chargers also have a thermistor in the pack and use it as a last-resort shutoff. These are easy
to spot from the extra set of contacts - the thermistor needs its own connection.

Quoted message said:

B.

If you want the charger to beat all check the Maha MH-C777+. It does NiCad, NiMH and lithium packs,
measures pack capacity and can charge just about anything ever made. Two thumbs up.

Jim

The R/C (radio controlled) car/airplane crowd has PLENTY of experience with building custom battery
packs. There are really cheap connectors, and soldering lugs (to connect the batteries) with lots
of tutorials on the internet. Cells are available for really cheap to quite expensive for some
realllly killer cells. Obviously you can customize the voltage, and you could customize the shape.
Might be a worth while option? Check some online hobby stores just for fun.. "tower hobbies" is one
that I can remember.

Mike mikebeauchamp.commikebeauchamp.comOpen ↗

"Bruce" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

I am looking at putting together my own battery packs for my bike lights. I have come across some
surplus NiMH battery packs that look as though they came from a laptop; 8 x 1.2v
slightly-larger-than-AA cells, conneted by tabs. Very Cheap. Hoping they might have some life left
in them.

I was thinking I could pull off 3 of the batteries in these packs, and I would have a 6V battery I
could use for my lights.

One question, however: these batteries have some kind of doohicky in line between a couple of the
batteries; I think it is a resistor, or diode, of some kind (though I know little about how to
identify such things). Any ideas on what this might be or what it does?

I have read in some places that a "smart" battery pack is made by having some kind of thingy that
cuts the circuit at a certain temperature, to keep the battery from overcharging (as the easiest
way to tell that a NiMH battery is fully charged is apparently that it begins to heat up). Could
this be what this is?

B.

Bruce said:

I am looking at putting together my own battery packs for my bike lights. I have come across some
surplus NiMH battery packs that look as though they came from a laptop; 8 x 1.2v
slightly-larger-than-AA cells, conneted by tabs. Very Cheap. Hoping they might have some life
left in them.

I was thinking I could pull off 3 of the batteries in these packs, and I would have a 6V battery I
could use for my lights. One question, however: these batteries have some kind of doohicky in line
between a couple of the batteries; I think it is a resistor, or diode, of some kind (though I know
little about how to identify such things). Any ideas on what this might be or what it does?

I'd guess it's a fuse, perhaps a thermal fuse. There has to be something to break the circuit in
case of a short. These battery packs hold a lot of power and are capable of delivering large
current. Shorting the circuit is a serious issue. If you're building your own pack you do need to
make sure that it's properly fused.

Quoted message said:

I have read in some places that a "smart" battery pack is made by having some kind of thingy that
cuts the circuit at a certain temperature, to keep the battery from overcharging (as the easiest
way to tell that a NiMH battery is fully charged is apparently that it begins to heat up). Could
this be what this is?

You're talking about a thermistor. Yes it's part of a good smart-charging system. It senses the
temperature of the pack, but the idea behind a good NiMH smart-charger isn't the absolute
temperature, it's the *change* in temperature over time. As the batteries approach full charge, the
temperature begins to rise in a predictable curve. So the charger knows when to cut back to a
maintenance charge before damage is done to the battery. The charger needs feedback from the
thermistor, so if your doohickey only connects between one battery and the next without any other
leads, it's probably not a thermistor.

Quoted message said:

B.

Go to Topica.com and join the BikeCurrent discussion group

Werehatrack said:

that tells you when they're fully charged. On the other hand, when charging cells in series, as is
the procedure with a laptop pack, the cells will not get hot at the same time; the weakest cell
will overheat first, and when that happens, the smart thing to do is stop before the cell is
damaged. Charging cells in parallel is preferable, but you can't have them connected in parallel
for charging and at the same time connected in series to power the device, so laptop makers use
the easy way out despite the fact that it's not the best.

Charging in series has the advantage that you charge all the cells, against the dirty-contact
problem in parallel charing, that one or more often are not charged at all.

Charging _individually_ but not in parallel, as in some of the newer Maha chargers, gets them all
charged best. Any lights that don't go on show which contacts need cleaning right away; as in
charging in series, where the light not going on says that _some_ contacts need cleaning, but not
which ones. (Caig DeOxit vial with brush is perfect for contact cleaning; I can't imagine trying to
keep my army of IC recorder AAA's charged

I was thinking that not fully charging the stronger cells was an advantage actually, since what
kills a pack is the stronger cells reverse-charging the weaker ones when they die (so don't let the
pack run down near empty!) and it would be good not to charge the stronger cells up fully - but I
don't really know that the weaker cells charge first (I think the opposite - they're the laggards),
and the stronger cells in any case start with a partial charge that the weaker cells don't have.
It's hard to say, whether charging a pack is better done in series or parallel. As individual cells,
you wan't them done individually.
--
Ron Hardin [email hidden]

On the internet, nobody knows you're a jerk.