NiMH Charging

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Ted - 23 Dec 2007 22:11 GMT

I have been told that the slow chargers that come with radios will not
charge NiMH packs sufficiently. Is this correct? If it is, will a
Sirius Peak charger suffice? If not, what do you use? I want to
replace my Futaba NiCD transmitter pack with a NiMH pack but won't do
it if I have buy new charging gear.

Ted

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The Natural Philosopher - 24 Dec 2007 01:00 GMT

> I have been told that the slow chargers that come with radios will not
> charge NiMH packs sufficiently. Is this correct? If it is, will a
[quoted text clipped - 3 lines]
>
> Ted

Sounds like total bollocks to me.

My Futaba trasnmitters charge just fine on Nimh cells and the stock charger.

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Morgans - 24 Dec 2007 04:08 GMT

>I have been told that the slow chargers that come with radios will not
> charge NiMH packs sufficiently. Is this correct? If it is, will a
> Sirius Peak charger suffice? If not, what do you use? I want to
> replace my Futaba NiCD transmitter pack with a NiMH pack but won't do
> it if I have buy new charging gear.

They will charge just fine. It will take longer, since the NiMH packs
usually have a much higher milli-amp hour capacity.

If your new pack is double the capacity, it will take twice as long to
charge it, assuming both were fully discharged. That is the trick.

You should do a little charting, if you want to find out how long to charge
with your slow, dumb charger.

Make up a resistor jumper wire, or use a car tail light bulb, and check how
many amps (or parts of amps thereof) it drains on a pack, using a voltage
tester that will read amps. Take your pack and fully charge it, then start
discharging it and recording the voltage every 5 minutes. When it is down
to 1.1 volts per cell, or when the graph connecting the voltages starts a
rapid plunge downward, quit. Multiply amps times hours divided by 100, and
you have your milliamp hours.

After you have plotted your voltage, you should be able to read your pack
when you get back from flying, and be able to say how much you have drained
your pack. You will need to use the same resistor jumper that you plotted
your graph with, while you read the voltage, though. Then, after you read
how many amps your charger puts into your pack, you should be able to
calculate how long it will take to charge your pack back to full capacity.
Add a few percent extra, for inefficiency of the charging process, though.

Or you could spend a few bucks on a peak detecting charger, and know you
fully charged your packs, and do it a lot faster. Your choice! :-)

I hope I have not cludged this up, too badly; especially about how to
calculate the amp hours. You will get the idea that you have the wrong
formula, if your 2200 milliamp hour pack calculates to 22,000 milliamp hour
capacity, that I messed up the math, somewhere! <g>

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Jim in NC

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Doug McLaren - 24 Dec 2007 07:37 GMT

| They will charge just fine. It will take longer, since the NiMH packs
| usually have a much higher milli-amp hour capacity.
|
| If your new pack is double the capacity, it will take twice as long to
| charge it, assuming both were fully discharged. That is the trick.

The concern is that since the self-discharge rate of the NiMH cells is
higher than NiCd cells, you may be charging slower than the self
discharge rate. I do not believe this to be true, and believe your
`twice the capicity, twice as long' rule of thumb to be just fine.
Sure, it might take a little longer than that, but it's not going to
hurt if your battery is only 90% charged when you have 3x the original
capacity.

Still, taking 48 hours to do a full charge is excessive, and if you're
serious about the hobby, you should have some sort of peak charger.
Some day, you're going to want to fly, and didn't charge your
batteries the night before. Do you skip flying entirely, or charge
for just a few hours and hope that's enough, or don't charge at all
and hope there's enough capacity left? None of these answers are
ideal, and having a peak charger gives you a good fourth option -- top
off your battery, probably in well under an hour. And you don't have
to spend much money at all.

| After you have plotted your voltage, you should be able to read your pack
| when you get back from flying, and be able to say how much you have drained
| your pack.

This is likely to be very inaccurate with NiCd and NiMH cells.
However, with NiMH cells that will power your gear all day long, you
probably have enough extra juice that it won't bite you.

As for if the Sirius charger will do, it probably will, but if it's
old it might not be able to detect the smaller peaks of NiMH cells vs
NiCd cells. To be sure, contact Sirius. Or see if the instructions
say it supports NiMH cells.

Your peak charger might not be able to charge through your TX charge
jack (if there's a diode in there) -- some can, some can't. If it
doesn't see the battery at all, remove the battery plug and charge
through that. You can also short out the diode (which defeats it's
purpose, which is to protect you from mistakes) or replace it with a
poly-fuse, but that might not be worth the trouble.

Signature

Doug McLaren, dougmc@frenzied.us "I can't drive....88.5 kph"

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Six_O'Clock_High - 24 Dec 2007 22:26 GMT

> | They will charge just fine. It will take longer, since the NiMH packs
> | usually have a much higher milli-amp hour capacity.
[quoted text clipped - 5 lines]
> higher than NiCd cells, you may be charging slower than the self
> discharge rate.

Ah . . uh . . NiMH packs self discharge at a dramatically lower rate when
compared to NiCd's. Something like 1% versus 10%...

But that is just what someone told me a few years ago. However, it does
seem to fit the results I get with my Orbit...

Jim
AMA 1428

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Doug McLaren - 25 Dec 2007 01:11 GMT

| > The concern is that since the self-discharge rate of the NiMH cells is
| > higher than NiCd cells, you may be charging slower than the self
| > discharge rate.
|
| Ah . . uh . . NiMH packs self discharge at a dramatically lower rate when
| compared to NiCd's. Something like 1% versus 10%...

No, it's quite obvious that in general NiMH cells self-discharge
faster than Nicads, about 2-3x as fast. But even so, charging at a
C/30 rate is plenty fast to charge them up in 48 or so hours.

The new Eneloop and similar batteries are very nice however -- very
low self discharge rates!

| But that is just what someone told me a few years ago. However, it
| does seem to fit the results I get with my Orbit...

If you leave your fully charged TX with NiCd cells for a month or two,
it'll still work for a few hours. If it had NiMH cells (and not
Eneloops), it might work for a few minutes at most ...

Signature

Doug McLaren, dougmc@frenzied.us
Honesty is the best policy - when there is money in it. --Mark Twain

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Morgans - 25 Dec 2007 01:26 GMT

> If you leave your fully charged TX with NiCd cells for a month or two,
> it'll still work for a few hours. If it had NiMH cells (and not
> Eneloops), it might work for a few minutes at most ...

That isn't a problem, though. I don't know anyone that goes out to fly,
without first charging their equipment!

With a quick charger, you can charge at least one up, while you are driving
to the field.

Planning 45 minutes ahead, and charging with a quick charger is not a
problem, for most people.

Or....

You could always go for setting your stuff up on a timer, so everything is
always topped off.

If you connect some extension cords to a 7 day timer, and set your slow
chargers up so that the timer runs for about 2 hours per week, then
everything will always be ready.

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Jim in NC

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Robert Roland - 24 Dec 2007 13:08 GMT

>I have been told that the slow chargers that come with radios will not
>charge NiMH packs sufficiently. Is this correct?

No, it is not. However, NiMH is a little more sensitive to overcharge
than NiCd, so if you leave them on the charger for too long, they will
deteriorate a little quicker than NiCd.

>If it is, will a
>Sirius Peak charger suffice?

A peak charger that is designed for NiMH will work well.

>If not, what do you use?

I use one of these:
http://www.foamworks.co.nz/product_info.php?cPath=37_59&products_id=576

>I want to
>replace my Futaba NiCD transmitter pack with a NiMH pack but won't do
>it if I have buy new charging gear.

I am very pleased with my charger. Earlier, I had to remember to put
the batteries on charge the night before I was going to fly. Now, I
have batteries that can be charged quickly (most batteries these days
can), so I just stick them on the charger in the morning, and they are
ready before I am done with breakfast.

The only downside with this charger is that I want another one, so
that I can charge the Tx and Rx packs at the same time. I usually have
enough time to do one after the other, though.

As an added bonus, you can discharge your batteries once in while to
test them to see if they are in good shape. If your radio has a
built-in diode to protect against incorrect polarity, you will need to
bypass that if you want to discharge the battery through the charge
connector.

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RoRo

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Anyolmouse - 25 Dec 2007 15:21 GMT

| I have been told that the slow chargers that come with radios will not
| charge NiMH packs sufficiently. Is this correct? If it is, will a
[quoted text clipped - 3 lines]
|
| Ted

I bought some new NiMh batteries last spring that has a membrane in them
that slows down the discharge rate considerably. In fact, I just checked
them, and with eleven minutes of discharge (flight time) on them last
September, they still are showing 10.5 Vdc in my Eclipse transmitter. I
had the eight cell pack made up at Batteries Plus out of 2100 Mah cell
(new technology) Ray-O-Vacs. In the past using NiCads I had to charge
the batteries half way through a one day sailplane contest. Not any more
with these batteries. I can fly the whole weekend and still have plenty
of battery power left.

I used an old Ace bench charger at C/10 rate for an initial 24 hour
charge and have been using Towers FFC since then. A longer charge time
is definitely needed if you use a NiCad charger. Also, a NiCad peak
charger is verboten according to Ray-O-Vac. Only use a peak charger
charger that has a NiMh capability.

BTW, you can use your NiCad charger(s) that comes with your Radio
package but it is not the recommended way to go. Also, cycling the
batteries like NiCads is not necessary or recommended for the NiMh packs
that I have.

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Anyolmouse

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Model Forum / Radio Controlled / Air Models / December 2007

NiMH Charging