7 Cell Nimh Voltage

[Marine Farinha]

Ihave a battery pack of NiMH batteries. It is ten cells with 1.2V, 4000mAh each, put together in series. So rated voltage is 12V. After charging, i.e. when the charging device says that it is finished, I measure 14.3V.

NiMH cells start at about 1.5 V right when fully charged, drop to about 1.2 V most of their discharge life, and are pretty much empty at 900 mV. Stopping there is usually safe. 800 mV is where you definitely want to stop to avoid damage. There is so little energy left at that point that there is no benefit in draining the cell further anyway.

You may think therefore that your 10 cell pack can therefore be discharged to 9 or 8 V, but unfortunately not. There will always be some imbalance between cells. If you can measure individual cells, you can go until the lowest cell hits 800 mV, but then stop right away. The cell with the least capacity will get there first. Once it does, its internal resistance goes up and further current causes the voltage to drop rapidly, causing permanent damage.

10 NiMH cells really should not be put in series without a way to at least measure individual cells. If you designed the battery pack, then you need to fix this. If someone else did, then they are not trustworthy and it would be a good idea to dump them and find someone that knows what they are doing. With 10 cells, it's hard to pick a reasonable stopping point because possible imbalance between cells could be significant, especially after a few charge/discharge cycles. Maybe use 1.1 V average per cell, but this is really not a good way to deal with a 10-cell pack.

You will have the same problem with charging. You will have to use relatively low charge current, like maybe C/4 until you think the first cell is near full, then maybe a C/10 or so to trickle charge for a couple of hours so the other cells hopefully catch up. Again, the right answer is to not get yourself into this mess in the first place. Packs with this many cells need to have individual cells measured at least, and the best way is to have some charge balancing circuitry. This shunts some charge current around the full cells so that they don't get overcharged while the less full cells catch up. Of course this requires measuring individual cells to know when to enable the shunts per cell.

LOL at the criticisms of the wisdom of putting these in series. While completely accurate when expressing the ideal, there is middle ground. Say hello to my '06 Honda Civic Hybrid battery pack - 132 D cells in series comprised of eleven 12-cell welded assemblies. No cell voltage monitoring capability.

I did 3 charge/discharge cycles going progressively lower until I hit the aforementioned 100V. I also let the pack rest and recover to 112V or so before I again took it down to 100V (took only a couple minutes, repeated a few times). I never let the pack out of my sight or took my eye of the voltmeter for more than a few second during the whole process.

My efforts have paid off. An essentially non-functional battery (for last 1.5 years - provided almost no electric-assist and triggered a battery recalibration every five minutes) has been restored to function properly. This pack has 132k miles on it and it's lived for 8 years in AZ heat.

You do not want to leave the batteries flat for extended periods of time otherwise a corrosion that insulates the plates starts and reversing the polarity from fast series cell discharges can cause damage.

In addition lifetime can be reduced faster with temperature rise and thermal sensors to cut-out the charger are the cheap solution to chargers. Heat management and temperature monitor as well as V "empty" thresholds are the features of intelligent chargers.

NiMH cells have a value of 1.2V for most of their discharge cycle. Fully charged they are about 1.4V. Since you are getting slightly more I'd recommend checking that they are all at the same potential and slightly discharging the ones above 1.4V before your next charge. I'd recommend not discharging below 1V though I've gone lower a number of times by accident and there was no permanent damage.

For really low current applications you can probably go down to about 0.8, you do have to draw the line somewhere though and I'd suggest not lower than this when using multiple cells in series since the danger exists for some of the cells to be charged in reverse by the others. This damages the affected cells and future charge cycles become unreliable when using delta V charge termination.

They are typically used as a substitute for similarly shaped non-rechargeable alkaline batteries, as they feature a slightly lower but generally compatible cell voltage and are less prone to leaking.[7][8]

Work on NiMH batteries began at the Battelle-Geneva Research Center following the technology's invention in 1967. It was based on sintered Ti2Ni+TiNi+x alloys and NiOOH electrodes. Development was sponsored over nearly two decades by Daimler-Benz and by Volkswagen AG within Deutsche Automobilgesellschaft, now a subsidiary of Daimler AG. The batteries' specific energy reached 50 Wh/kg (180 kJ/kg), specific power up to 1000 W/kg and a life of 500 charge cycles (at 100% depth of discharge). Patent applications were filed in European countries (priority: Switzerland), the United States, and Japan. The patents transferred to Daimler-Benz.[9]

Interest grew in the 1970s with the commercialisation of the nickel-hydrogen battery for satellite applications. Hydride technology promised an alternative, less bulky way to store the hydrogen. Research carried out by Philips Laboratories and France's CNRS developed new high-energy hybrid alloys incorporating rare-earth metals for the negative electrode. However, these suffered from alloy instability in alkaline electrolyte and consequently insufficient cycle life. In 1987, Willems and Buschow demonstrated a successful battery based on this approach (using a mixture of La0.8Nd0.2Ni2.5Co2.4Si0.1), which kept 84% of its charge capacity after 4000 charge-discharge cycles. More economically viable alloys using mischmetal instead of lanthanum were soon developed. Modern NiMH cells were based on this design.[10] The first consumer-grade NiMH cells became commercially available in 1989.[11]

About 22% of portable rechargeable batteries sold in Japan in 2010 were NiMH.[16] In Switzerland in 2009, the equivalent statistic was approximately 60%.[17] This percentage has fallen over time due to the increase in manufacture of lithium-ion batteries: in 2000, almost half of all portable rechargeable batteries sold in Japan were NiMH.[16]

In 2015 BASF produced a modified microstructure that helped make NiMH batteries more durable, in turn allowing changes to the cell design that saved considerable weight, allowing the specific energy to reach 140 watt-hours per kilogram.[18]

The reactions proceed left to right during charge and the opposite during discharge. The metal M in the negative electrode of a NiMH cell is an intermetallic compound. Many different compounds have been developed for this application, but those in current use fall into two classes. The most common is AB5, where A is a rare-earth mixture of lanthanum, cerium, neodymium, praseodymium, and B is nickel, cobalt, manganese, or aluminium. Some cells use higher-capacity negative electrode materials based on AB2 compounds, where A is titanium or vanadium, and B is zirconium or nickel, modified with chromium, cobalt, iron, or manganese.[19]

NiMH cells have an alkaline electrolyte, usually potassium hydroxide. The positive electrode is nickel hydroxide, and the negative electrode is hydrogen in the form of an interstitial metal hydride.[20] Hydrophilic polyolefin nonwovens are used for separation.[21]

NiMH batteries of bipolar design (bipolar batteries) are being developed because they offer some advantages for applications as storage systems for electric vehicles. The solid polymer membrane gel separator could be useful for such applications in bipolar design. In other words, this design can help to avoid short-circuits occurring in liquid-electrolyte systems.[22]

Duracell further suggests that a trickle charge at C/300 can be used for batteries that must be kept in a fully charged state.[24] Some chargers do this after the charge cycle, to offset natural self-discharge. A similar approach is suggested by Energizer,[20] which indicates that self-catalysis can recombine gas formed at the electrodes for charge rates up to C/10. This leads to cell heating. The company recommends C/30 or C/40 for indefinite applications where long life is important. This is the approach taken in emergency lighting applications, where the design remains essentially the same as in older NiCd units, except for an increase in the trickle-charging resistor value.[citation needed]

Panasonic's handbook recommends that NiMH batteries on standby be charged by a lower duty cycle approach, where a pulse of a higher current is used whenever the battery's voltage drops below 1.3 V. This can extend battery life and use less energy.[23]

To prevent cell damage, fast chargers must terminate their charge cycle before overcharging occurs. One method is to monitor the change of voltage with time. When the battery is fully charged, the voltage across its terminals drops slightly. The charger can detect this and stop charging. This method is often used with nickel-cadmium cells, which display a large voltage drop at full charge. However, the voltage drop is much less pronounced for NiMH and can be non-existent at low charge rates, which can make the approach unreliable.[24]

The temperature-change method is similar in principle to the ΔV method. Because the charging voltage is nearly constant, constant-current charging delivers energy at a near-constant rate. When the cell is not fully charged, most of this energy is converted to chemical energy. However, when the cell reaches full charge, most of the charging energy is converted to heat. This increases the rate of change of battery temperature, which can be detected by a sensor such as a thermistor. Both Panasonic and Duracell suggest a maximal rate of temperature increase of 1 C per minute. Using a temperature sensor allows an absolute temperature cutoff, which Duracell suggests at 60 C.[24] With both the ΔT and the ΔV charging methods, both manufacturers recommend a further period of trickle charging to follow the initial rapid charge.[citation needed]

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