Measuring the charge in NiMh Batteries

I have a couple of Visionstick NiMh batteries where the amount of charge
taken seems to be a bit hit and miss , sometimes the battery will last
20 minutes others an hour - same lights etc.Using supplied chargers
which are claimed to be of the smart type.

Is their any sort of meter I can use to measure how successful the
charge has been?

Riding on rural roads without lights is a bit dickie!

Any help appreciated.

Red wrote:

Is their any sort of meter I can use to measure how successful the
charge has been?

Voltmeter
Do Nimh have a Voltage Vs Charge curve?
If so, this should tell you.

On 2006-05-19, Terry Collins wrote:
Red wrote:

Is their any sort of meter I can use to measure how successful the
charge has been?

Voltmeter
Do Nimh have a Voltage Vs Charge curve?
If so, this should tell you.

They do. AIUI, the charge curve for an NiMH battery is very similar to,
if not identical to, the charge curve for a NiCd battery. When I had a
Palm Pilot, I used NiMH batteries rather than constantly forking out for
AA (or was it AAA?) batteries; I used a third party app to set the Palm
to the NiCd charge settings. Did the job very nicely indeed.

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On Fri, 19 May 2006 05:29:15 +0800, Red wrote:

Is their any sort of meter I can use to measure how successful the
charge has been?

The voltage change is very small, so such meters tend to be unreliable. I
charge mine with a current which is low enough not to damage the battery
when fully charged ( capacity/10 mA), and pick it up to check for the
slight warmth which indicates that all the power fed to it is being
converted to heat (wasted) i.e. it's fully charged.

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Home page: http://members.westnet.com.au/mvw

On Fri, 19 May 2006 10:32:03 +1000, Terry Collins wrote:

Voltmeter

Will give a reasonable approximations.

Do Nimh have a Voltage Vs Charge curve?

Pretty much. A nominal 1.2V cell has a real end point of around 1.4V,
sometimes a touch higher.

Carefully use a multimeter on the end of the connector cable. A full
charge should be around 7V, while there will be something happening down
to about 5.8V. Beneath that and the batteries are very close to end of
charge.

--
Dave Hughes |
There's no point in being grown up if you can't be childish sometimes.
- Dr. Who

Red wrote:
I have a couple of Visionstick NiMh batteries where the amount of charge
taken seems to be a bit hit and miss , sometimes the battery will last
20 minutes others an hour - same lights etc.Using supplied chargers
which are claimed to be of the smart type.

Is their any sort of meter I can use to measure how successful the
charge has been?

Riding on rural roads without lights is a bit dickie!

Any help appreciated.


I like a meter on my charger so I can watch the amps. Also I carry a
spare battery just to be on the safe side. I built a charger from a kit
from Silicon Chip magazine and it was very unreliable.

This is a good charger
http://www.master-instruments.com.au...el/MW7168.html

Friday

Terry Collins wrote:
Red wrote:


Is their any sort of meter I can use to measure how successful the
charge has been?


Voltmeter
Do Nimh have a Voltage Vs Charge curve?
If so, this should tell you.

The voltage doesn't tell you much, in fact it drops off as full charge
is reached which is how the delta peak detection works. If you watch an
amp meter and it reads one amp and it's on for an hour or so then you
can be pretty sure that an amp/hour has gone into it.

The delta peak chargers stop charging every second or so and measure the
voltage, this keeps happening until the voltage it measured is
slightly lower than the previous voltage it measured. You can see the
amp indicator pulsing as it charges, measures, and then charges again.
Once it's detected the peak voltage it gives a little bit more of a
timed charge just to give it that final topping up and then it stops.
They usually also have a temperature sensor that detects a sudden rise
in the battery temperature, as a sudden rise indicates it's fully
charged and any excess charge will cook the battery.

With nicad cells the peak is easy to detect but with NiMh it's sometimes
missed by the charger so the temperature sensor is a good backup to
prevent overcharging.

Friday

On Sat, 20 May 2006 01:26:11 +0000, Friday wrote:

The voltage doesn't tell you much, in fact it drops off as full charge
is reached which is how the delta peak detection works.

True, but overstated. If it's 1.4-1.5 volts/cell you know it's got at
least a 95% charge in it. The voltage drop you're talking about is more
like 0.05V, so it drops from 1.5 to 1.45 V. That's going to be about the
same as the fluctuation on a cheap multimeter, so not hugely worrying.

--
Dave Hughes |
"Love is a snowmobile racing across the tundra and then suddenly it
flips over, pinning you underneath. At night, the ice weasels come."
- Matt Groening