Re: High voltage NiMH??
- From: ASAAR <caught@xxxxxx>
- Date: Tue, 08 Aug 2006 18:25:42 -0400
On Tue, 8 Aug 2006 13:27:38 -0500, m Ransley wrote:
My sony charger charges NiMh to 1.45v, try a different charger, I dought
they can go to 1v as another poster states, Nicads are dead at 1.2v,
potentialy ruined at 1v if it was a high load discharge from a drill. I
havn`t checked my Nimh when fully discharged but they show 75% depleted
at 1.2v. I also dont know their state if let sit one day. It may just be
your charger, Sanyo and Panasonic are the best cells made.
You're battery ignorance is off the charts. Please go to any
battery manufacturer's website (Energizer, RayOVac, Sanyo, etc.) and
check the voltage discharge curves. Voltage measurements under
no-load are nearly meaningless. Under anything from a slight to a
very large load, both NiCads and NiMH batteries are close to being
fully charged if they show 1.2 volts. Most of the energy that they
deliver will be over a voltage range (under load) of about 1.25v. to
slightly more than 1.0 volts. Claiming that either NiCad or NiMH
batteries would be potentially ruined at 1 volt is total nonsense.
Energizer's Battery Application Manuals for NiCad and NiMH batteries
state:
[From the NICKEL CADMIUM Application Manual]
The capacity rating of Energizer nickel-cadmium cells and batteries is based upon output in
discharge at the 1 hour rate to an endpoint of 1.0V/cell for all cylindrical cells. If current is
withdrawn at faster rates than these standards, capacity is decreased.
Except in the case of complete discharge, neither cell condition nor state of charge can be
determined by open circuit voltage.
[From the NICKEL-METAL HYDRIDE Application Manual]
Nickel-metal hydride cells are essentially an extension of the proven sealed nickelcadmium
cell technology with the substitution of a hydrogen-absorbing negative
electrode for the cadmium-based electrode. While this substitution increases the cell
electrical capacity (measured in ampere-hours) for a given weight and volume and
eliminates the cadmium which raises toxicity concerns, the remainder of the nickelmetal
hydride cell is quite similar to the nickel-cadmium product. Many application
parameters are little changed between the two cell types, and replacement of nickelcadmium
cells in a battery with nickel-metal hydride cells usually involves few significant
design issues.
A typical discharge profile for a cell discharged at the 5-hour rate (the 0.2C rate) is
shown in Figure 9. The initial drop from an open-circuit voltage of approximately 1.4
volts to the 1.2 volt plateau occurs rapidly.
To prevent the potential for irreversible harm to the cell caused by cell reversal in
discharge, removal of the load from the cell(s) prior to total discharge is highly
recommended. The typical voltage profile for a cell carried through a total discharge
involves a dual plateau voltage profile as indicated in Figure 14. The voltage plateaus
are caused by the discharge of first the positive electrode and then the residual capacity
in the negative. At the point both electrodes are reversed, substantial hydrogen gas
evolution occurs, which may result in cell venting as well as irreversible structural
damage to the electrodes. It should be noted that the nickel-metal hydride cell, because
it uses a negative electrode that absorbs hydrogen, might actually be somewhat less
susceptible to long-term damage from cell reversal than the sealed nickel-cadmium cell.
[Note: Figure 14 shows that the first voltage plateau is - 0.3 volts
and the second plateau is reached at about - 1.8 volts. To state
that they're potentially ruined when reduced to + 1.0 volts (whether
under moderate or very high loads) is absurd. Most battery chargers
that have cell discharge circuits place batteries under load until
each cell reaches 1.0 volts. A small number discharge cells below
1.0 volts, but it isn't necessary, as by the time the voltage has
dropped to 1.0 volts there's practically no energy remaining in the
cell.]
The key to avoiding harm to the cell is to terminate the discharge at the point where
essentially all capacity has been obtained from the cell, but prior to reaching the second
plateau where damage may occur. Two issues complicate the selection of the proper
voltage for discharge termination: high-rate discharges and multiple-cell effects in
batteries.
[Note: This indicates that to avoid harming the cell, its discharge
should be terminated while the cell voltage lies somewhere between a
high of + 1.0 volts and a low of - 1.8 volts. This is *not* very
difficult to comply with.]
Voltage Cutoff at High Rates
Normally discharge cutoff is based on voltage drops with a value of 0.9 volts per cell (75
percent of the 1.2 volt per cell nominal mid-point voltage) often being used. As can be
seen in Figure 11, 0.9 volts is an excellent value for most medium to long-term
discharge applications (<1C).
.
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