[hpv] how much voltage should a "6V" NiMH battery have?

[Carl Etnier]

I don't know how what the voltage per cell is for NiMH, and I'd like
to know what the fully charged voltage for my NiteRider NiMH battery
rated at 6V should be. I need to top it off before a ride on the
weekend, and I don't want to overcharge.

Anyone have the figures for measured voltage at full charge and empty
for a "6V" NiMH battery? I could probably calculate back from any
other common voltage rating, if you have that.

Also, what is the voltage profile over burn time for NiMH? Does it
decline sort of linearly, like alkaline batteries, or stay fairly
steady and then drop off suddenly at the end, like NiCds? (From what
I've seen of the performance, it seems more like NiCds, but
NiteRider has put so much in the way of electronics between the lamp
and the battery, I don't know whether I'm seeing a constant battery
voltage or a declining battery voltage producing a light of a
constant brightness.)

Thanks!

Cheers,
Carl
carl....@itf.nlh.no
A kinetic Yankee usually in King Harald's port (Oslo, Norway)
In the US (mostly Madison, Wisconsin) 2 August 1999 - summer 2000

[Larry Menzel]

Carl,

I don't know if this fully answers your question, but it should be helpful.

Ni-MH (Nickel Metal Hydride) batteries are fast becoming the most popular
choice of digital camera users, photgraphers,cellular phone and laptop
users, and many others because these batteries have a much higher capacity
and are virtually memory free. These batteries do not need to be discharged
every time before recharging. Ni-MH batteries can be damaged from heat by
overcharging but this is easily avoided by using a high quality,
microprocessor controlled battery charger or a charger designed for Ni-MH
batteries and charging as directed.

As far as voltage is concerned, a 6 volt battery should put out 6 volts.
The real factor is the amp/hour rating of the batteries, and the larger the
rating the longer they will last. The voltage is simply an indicator of
how much power the battery is putting out, and determines the bulb you
should be using. Many of the more expensive systems are 12 volt. The
lower the voltage and the higher the output (amp/hour) ratings the longer
the batteries will last.

Without going into all the math, a NiMH battery will discharge from it's
peak stored energy potential, in this case about 1.4 volts per cell,
delivering a fairly level output until it's cut-off point is reached,
usually around 0.8 volts per cell, at which time it will cease to perform.
Running NiCad or LiIon or NiMH batteries past their cut-off, i.e. leaving
them on once they've started to quit, will result in damage to the
cells...to the point that they may not be rechargeable. Always stop using
your battery pack shortly after the light starts to dim significantly.
Trickle charging is preferred (12-14 hours) to fast charging and will
result in greater battery life and more recharges per pack.

All NiMH ( Nickel Metal Hydride ) batteries as well as all Nicad Batteries
are all 1.2 Volt for consumer use. Once charged they average 1.3 to 1.4V .
So you can be assured that these are the batteries that everyone is using
in their digital cameras and other electronic equipment. The NiMH
batteries are always shipped uncharged and must be fully charged before
their first use.

Standard Charging: Charging at 0.1C (% of capacity) standard rate for 14-16
hours will greatly enhance the NiMH battery's service life. NiMH batteries
can withstand overcharge at 0.1C rate indefinitely. Charging at or below
0.1C rate is suitable across a temperature range from 0 degrees C to +45
degrees C.

Fast Charging: All NiMH series can be fast charged at rates from 0.5C to
1C. Fast charge termination measures should be employed. These include:
Temperature Cut-Off (TCO), Rate-of-temperature rise (dT/dt), negative Delta
V (-dV). A timer control set at 105% of nominal capacity can be used as an
additional protection. For maximum capacity, a trickle charge can be
applied after fast charging. Most suppliers recommennd top-off trickle
charging at 0.1C for 2 hours or 0.2C for 1 hour. Fast charging can be done
at temperatures from +10oC to +45oC.