Replacement Batteries for light

Bruce said:

I have a Night Rider 15w light, and the 6v battery needs to be replaced. I have looked around and
seen a number of posts on D-I-Y battery assembly, and it doesn't seem too difficult (though I have
seen some cautions about welding/soldering tabs onto batteries).

The current battery is a water bottle with 5 1.2v NiCAD "D" batteries connected with tabs in
series. No idea abut amp-hours, as the batteries have no labels.

I have seen at my local electronic store some 4.5Ah "D" NiCD batteries, $10CDN ea. I am guessing
these are probably the same as the batteries in the old light (at least any other comparable
lights I have seen have 4.5Ah batteries)

The store also has 2.5Ah "D" NiMH batteries for the same price. I have also seen on the web
(nimhbattery.combatteries.htmOpen ↗) NiMH "D" cells with up to 11Ah (!), for about
$35US a pair.

For a given wattage/voltage, can you compare Ah between NiMH and NiCAD? I.e., would a 4.5Ah NiMH
and a 4.5Ah NiCAD, connected to the same light, have the same run-time?

NiMH have a much longer run time than NiCd for the same size battery. They also have a flat voltage
until they're almost fully discharged -- then they drop quickly (if you have a low battery warning,
it may be wrong -- just stop using the light when it starts to get yellow). However, they require a
different charger. I seem to remember that the easiest is just a constant current source for about
5-10% charging current (225-450 ma for the 4.5 ah) and stop when the batteries begin to heat (if you
miss the stop point, the batteries lose life quickly).

Quoted message said:

If building a battery, I think I would prefer to go with a higher capacity NiMH. It seems to me
that going with an 11Ah battery over a 4.5Ah makes sense. Same size battery, but much longer
run-time. Downside is additional cost (about $25CDN per battery as compared to $10CDN per
battery), additional charge time, and possibly some additional weight

10 hours to fully recharge at a 10% charge rate (I'm pretty sure <sigh> -- it's been a while).

Quoted message said:

(don't know about this - are higher capacity batteries necessarily heavier, given the same
physical size?).

Yes. It's an energy density thing. With the same technology, a higher capacity battery has to have
more "stuff" in it 🙂. I've seen "D" size NiCd batteries that are really just "AA" packaged into a
bigger tube -- I learned to look at the amp-hour capacity.

Quoted message said:

I recognize I'd also have to get a new charger if I went with NiMH instead of NiCAD. Still, it
would work out to about $150 for presumably much longer run time (I haven't done the calculations,
but I expect an 11Ah 6v battery could run a 15W lamp for 4 hours?)

Check the net. You may be able to do better.

David

<snip>

Quoted message said:

Quoted message said:

For a given wattage/voltage, can you compare Ah between NiMH and NiCAD? I.e., would a 4.5Ah NiMH
and a 4.5Ah NiCAD, connected to the same light, have the same run-time?

NiMH have a much longer run time than NiCd for the same size battery.

By "size" do you mean amp-hour rating, i.e., do you mean a NiMH battery with a 4.5Ah rating will run
much longer than a NiCD battery with the same 4.5Ah rating? If so, any idea of the expected
difference, i.e., a NiMH of 4.5Ah will usually run for 1.5x as long as a NiCD of 4.5Ah?

<snip>

Quoted message said:

I seem to remember that the easiest is just a constant current source for about 5-10% charging
current (225-450 ma for the 4.5 ah) and stop when the batteries begin to heat (if you miss the
stop point, the batteries lose life quickly).

So... does this mean that a battery charger that is designed to charge a 4.5Ah battery won't work
properly for a 10Ah battery, or just that it will take more than twice as long as the 4.5Ah?

B.

Bruce said:

<snip>

Quoted message said:

Quoted message said:

For a given wattage/voltage, can you compare Ah between NiMH and NiCAD? I.e., would a 4.5Ah NiMH
and a 4.5Ah NiCAD, connected to the same light, have the same run-time?

NiMH have a much longer run time than NiCd for the same size battery.

Yes. It will have a higher amp-hour rating and it will last longer (making sure that you don't buy a
"AA" packaged to take the place of a "C" or "D" 🙁 ).

Quoted message said:

By "size" do you mean amp-hour rating, i.e., do you mean a NiMH battery with a 4.5Ah rating will
run much longer than a NiCD battery with the same 4.5Ah rating? If so, any idea of the expected
difference, i.e., a NiMH of 4.5Ah will usually run for 1.5x as long as a NiCD of 4.5Ah?

No. 4.5 a-h is 4.5 a-h. Since they're both a 1.2v/cell, they'd have the same energy capacity.

Quoted message said:

<snip>

Quoted message said:

I seem to remember that the easiest is just a constant current source for about 5-10% charging
current (225-450 ma for the 4.5 ah) and stop when the batteries begin to heat (if you miss the
stop point, the batteries lose life quickly).

So... does this mean that a battery charger that is designed to charge a 4.5Ah battery won't work
properly for a 10Ah battery, or just that it will take more than twice as long as the 4.5Ah?

Depends. If it's a smart charger, it may not work -- they tailor the charge current and voltage
to be more efficient and charge faster. Some smart chargers are smart enough to handle a large
range of sizes.

If it's a simple charger, up to a point, it'll just take longer. As long as the charing current is >
~1/50 a-h capacity. Somewhere lower than this (I don't know where), it's just not enough to charge
the battery.

Most simple chargers are just a wall transformer and rectifier. The wall transformer acts as the
current limiter. The ones that are "overnight" chargers (16-20 hours or more) are this really low
current (~1/50 of the a-h capacity) style and just charge continually assuming that the current is
sufficiently low not to overcharge the battery hard enough to damage it. If this is the kind that
you have, it may not fully charge a much larger battery.

Another difference is that NiMh has a higher internal discharge rate. I seem to remember that it's
somewhere around 1% / day (faster at first, then slower). So, if the battery sits for a month before
use, it'll have lost ~30% of it's charge. All batteries do this -- I don't know the rate for NiCd
except that it's slower.

David