In article jMktf.667649$_o.390375@attbi_s71, bw wrote:

[Comments which follow are my own opinions only. Other people may
disagree. Your mileage may vary. Void where prohibited.]

I need to get a new battery for my Kenwood TH-G71 hand held and am thinking
about going with an after market NiMH type instead of the OEM NiCad.

NiMH has some advantages and some disadvantages over NiCD. Biggest
and most obvious advantage is greater capacity. Biggest disadvantage
is a somewhat higher self-discharge rate - NiMH batteries can lose
20-30% of their charge per month just sitting on the shelf un-used,
while NiCd batteries can hold their charge quite a bit longer.

For this reason, NiMH is probably not the best choice for
batteries/packs which will spend weeks or months sitting in your ARES
"go-pack" - they'll probably have self-discharged just when you need
'em.

I'm told that some of the new NiMH batteries have a greatly reduced
self-discharge rate, but can't recommend specific types or brands.

Two questions:

Are there any recommendations on where to purchase the NiMH battery?

I've been happy with the couple of purchases that I made from
Batteries America (http://www.batteriesamerica.com/). We bought a
bunch of the 6-AA-cell battery holders for the Icom W32, and the
fit/finish/function of the cases seems as good as Icom's own product
at a rather lower cost.

They have several different packs for the TH-G71 available. You get
your choice of NiCd or NiMH, with several different combinations of
battery voltage and battery capacity. If you'd be satisfied with a
maximum transmit power of 3 watts you could go for the NiMH 7.2V 1800
mAh pack, or for full transmit power get any of three 9.6V packs (600
mAh NiCd, or 1100 or 1450 mAh NiMH).

They can also sell you cells, or complete insert sets, if you want to
try cracking open your existing battery pack and rebuilding it.

Other vendors may also provide you with a similar range of choices.

Can I use the original charger on the new battery?

I'm just using the original BC-17 wall charger and the manual says to charge
the stock PB-39 NiCad for 15 hours. How do I know how long to charge the
NiMH battery and/or how do I know when it's fully charged?

As a general rule, you _can_ use a charger designed for NiCd batteries
with NiMH batteries, but may not get the best results (in terms of
total capacity and/or number of charge/discharge cycles).

The PB-39 sounds like a standard "trickle" charger, with a low and
coarsely-regulated charge current. You can probably use this, and
just multiply the original charge-time specification by the ratio of
the packs' capacities. If the stock pack took 15 hours and was in the
range of 600 mAh, then an 1100 mAh pack would take almost 30 hours.

For fast-charging, it's best to have a charger which "understands" the
charge-cutoff behavior of NiMH batteries. A fast-charger for NiCd
batteries will usually charge until the battery starts to heat up...
this can be detected because the battery's terminal voltage stops
rising, and actually starts falling a bit. The charger detects this
"negative delta V" condition and shuts off the charge (or switches
back to a trickle-charge).

This method will actually overcharge a NiMH battery. NiMH is happier
with a "zero delta V" shutoff... the charger switches off the
fast-charge when the battery voltage stops rising, before it begins
falling.

Getting the best out of a NiMH battery seems to be a someone
controversial topic... not unlike the question of charging gel cells,
AGM batteries, and so forth. Lots of opinions, not a lot of
completely solid answers. The best information I've been able to find
about NiMH suggests:

- The best recharge regime is a fast-charge (at C/2 rate or so), with
several different mechanisms used to detect full charge and shut
off the fast-charge current: zero-delta-V, a temperature-rise
sensor, and a timer for safety.

- Following the fast-charge with a few hours of low-current trickle
charging can be beneficial in cramming the last little bit of
capacity into the battery.

- NiMH batteries may not deliver their full advertised capacity until
they've been through several cycles of "full charge, nearly full
discharge."

- Like NiCds, it's best not to run NiMH batteries all the way down
to zero - they don't like the resulting "reverse charging"
impact on their weaker cells. When they're drained to 1.0 volts
per cell, consider them fully discharged!

- If you're going to store NiMH batteries for an extended period,
discharge them to the 1.0 volt level first. Recharge 'em before
you use 'em.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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