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NiMH Charging
Basics
NiCad and NiMH batteries are
amongst the hardest batteries to charge. Whereas with lithium ion and lead acid batteries you
can control overcharge by just setting a maximum charge voltage, the nickel based batteries
don't have a "float charge" voltage. So the charging is based on forcing current through the
battery. The voltage to do this is not fixed in stone like it is for the other batteries.
This makes these cells and batteries difficult to
charge in parallel. This is because you can't be sure that each cell or pack is the same
impedance (or resistance), and so some will take more current than others even when they are
full. This means that you need to use a separate charging circuit for each string in a
parallel pack, or balance the current in some other way, for example by using resistors of such
a resistance that it will dominate the current control.
The coulometric charging
efficiency of nickel metal hydride batteries is typically 66%, meaning that you must put 150
amp hours into the battery for every 100 amp hours you get out. The faster you charge the worse
this gets.
The minus delta V bump that
is indicative of end-of-charge is much less pronounced in NiMH than NiCad, and it is very
temperature dependent. To make matters worse, new NiMH batteries can exhibit bumps in the curve
early in the cycle, particularly when cold. Also, NiMH are sensitive to damage on overcharge
when the charge rate is over C/10. Since the delta V bump is not always easy to see, slight
overcharge is probable. For this reason PowerStream does not recommend using simple minus delta
V as a termination criterion for nickel metal hydride batteries. However, modern
algorithms have been developed to enable accurate charging without using a thermistor. These
chargers are similar to the -delta V chargers, but have special measurement techniques to
detect a full charge, usually involving some kind of pulse cycle where the voltage is measured
during the pulse and between pulses. For multicell packs, if the cells are not all at the same
state of charge, and if they are not balanced in capacity, the cells may fill up one at a time,
bluring out the end-of-charge signal. In order to balance the cells it may take several
charge-discharge cycles. Luckily, NiMH does not mind being overcharged at C/10 or less, which
allows the charger to balance the cells during the trickle charge.
As the battery reaches
end-of-charge oxygen starts to form at the electrodes, and be recombined at the catalyst. This
new chemical reaction creates heat, which can be easily measured with a thermistor. This is the
safest way to detect end-of-charge during a fast charge.
Overnight Charging
The cheapest way to charge a nickel metal hydride
battery is to charge at C/10 or below (10% of the rated capacity per hour). So a 100 mAH
battery would be charged at 10 mA for 15 hours. This method does not require an end-of-charge
sensor and ensures a full charge. Modern cells have an oxygen recycling catalyst which prevents
damage to the battery on overcharge, but this recycling cannot keep up if the charge rate is
over C/10. The minimum voltage you need to get a full charge varies with temperature--at least
1.41 volts per cell at 20 degrees C. Even though continued charging at C/10 does not cause
venting, it does warm the battery slightly. To preserve battery life the best practice is to
use a timer to prevent overcharging to continue past 13 to 15 hours. Examples of this kind of
charger are shown at
http://www.powerstream.com/NiMHWM.htm
. This charger uses a microprocessor to report the state of
charge via an LED as well as performing the timing function.
Faster Charging
Using a timer it is possible to
charge at C/3.33 for 5 hours. This is a little risky, since the battery should be fully
discharged before charging. If the battery still has 90% of its capacity when the timer starts
you would have a good chance of venting the battery. One way to ensure this doesn't happen is
to have the charger automatically discharge the battery to 1 volt per cell, then turn the
charger on for 5 hours. The advantage of this method is to eliminate any chance of battery
memory. PowerStream does not currently have such a charger, but the microprocessor board used
in the C/10 charger
http://www.powerstream.com/NiMHWM.htm
could easily be modified to do the discharge. A power dissipating
package would be needed in order to dissipate the energy from a partially charged battery in a
reasonable amount of time. Another example of a 3 hour charger is
http://wwww.powerstream.com/9vnmh.htm
. This is a very inexpensive microprocessor based 9 volt "transistor
radio" battery charger that that drops to low current when the battery voltage indicates a full
charge.
Fastest Charging
If a temperature monitor is used NiMH
batteries can be charged at rates up to 1C (in other words 100% of the battery capacity in
amp-hours for 1.5 hours). The PowerStream battery charge controller shown in
http://www.powerstream.com/product3.htm
does this, as does the battery management board shown in
http://www.powerstream.com/product5.htm.
This board also has the ability to sense voltage and current for more
sophisticated algorithms.
When terminating on temperature rise the dT/dt value should
be set at 1 to 2 degrees C per minute.
Trickle
Charging
In a standby mode you might want to keep a nickel metal hydride battery
topped up without damaging the battery. This can be done safely at a current of between 0.03 C
and .05 C. The voltage required for this is dependent on temperature, so be sure to regulate
the current in the charger.
dT/dt versus
-dV/dt
These two termination methods work well for NiCads, and are both applied
to NiMH as well. dT/dt measures the temperature rise at the end of charge. After the battery is
fully charged it starts new chemical reactions in order to absorb the unneeded current. In
nickel hydroxide style batteries this consists in generating and recombining oxygen. This
process heats the battery. The sudden increase in temperature rise can be used to terminate the
charge.
Another effect of the oxygen generation/recombination cycle is to depress the
voltage of the battery slightly. If you can detect this voltage depression you can use this
signal to terminate the charge. Of course, -dV/dt is the easiest because it doesn't require a
temperature sensor. The best method for NiMH is the dT/dt method. There are two main reasons.
With the NiMH battery the voltage depression is smaller, and harder to detect than with the
NiCad battery. This almost always ensures an overcharge, which will limit the total number of
charge/discharge cycles before battery failure. Second, a new NiMH battery has false peaks
early in the charge cycle, and so the charger will terminate too soon.
There are new
algorithms that use microprocessor control to use the -dV signal to detect the end of charge.
These can work very well and several of our chargers use this technique, which involves pulsing
the charger on and off to do the voltage measurements. This technique seems to be sensitive to
imbalance in the capacity of the cells. The dT/dt is still more reliable, especially for large
packs, but in cases when only two wires are available solutions are now available.
The Ultimate Charger
Sometimes the most important issue is the
lifetime of the batteries or the total lifetime cost of the system. In this case PowerStream is
in a good position to offer the ultimate charger because of our wide experience in
microprocessor controlled battery chargers and power supplies. Specs for the ultimate charger
are:
1. Soft start. If the temperature is above 40 degrees C or below zero degrees C
start with a C/10 charge. If the discharged battery voltage is less than 1.0 Volts/cell start
with a C/10 charge. If the discharged battery voltage is above 1.29 V/cell start with a C/10
charge.
2. Option: if the discharged battery voltage is above 1.0 Volts/cell, discharge
the battery to 1.0 V/cell then proceed to rapid charge.
3. Rapid charge at 1 C until the
temperature reaches 45 degrees C, or the dT/dt indicates full charge.
4. After
terminating the fast charge, slow charge at C/10 for 4 hours to ensure a full charge.
5.
If the voltage climbs to 1.78 V/cell without otherwise terminating, terminate.
6. If the
time on fast charge exceeds 1.5 hours without otherwise terminating, terminate the fast
charge.
7. If the battery never reaches a condition where the fast charge starts time
out the slow charge after 15 hours.
8. Fuel gauge, communication to the device being
powered, LED indicators all possible.
Custom design and manufacture of state-of-the-art battery chargers, UPS, and power supplies for OEMs in a hurry!
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