Silverace - Dotty Spotty - Issue 10: A Brief Primer on Ni-Mh Battery Technology

26 October 2000

This issue is a brief primer on Ni-Mh technology often used in today's rechargable batteries that are used in digicams. More information can be obtained from the various scientific and engineering resources available at libraries and sources online, where this information was collected from.

A comprehensive overview of all types of battery technologies can be found at www.ncjrs.com, article # 172868, and Duracell OEM web site.

Ni-Mh battery technology is a recent development that came after the tried and true Ni-Cd technology in response to smaller electonic devices that required longer life and smaller packaging. These devices, such as the cell phone and digicams, arrived just about the same time Ni-Mh technology first arrived on the market for the consumer in the late 1980s.

While expensive at first, like all new technologies, increased use and improvements have brought costs of a set of 4 Ni-Mh AA batteries down to around $10 USD.

Versus Ni-Cd technology, Ni-Mh batteries have several advantages.

1. Greater capacity. Around 30-40% longer run times vs. Ni-Cd due to a greater energy density.

2. Long life without significant memory effect. Unlike Ni-Cds which have the infamous memory effect problem, where partially depleted cells that were recharged were only able to discharge to that partially depleted level not further, Ni-Mh batteries are mostly immune to these effects, allowing users to recharge at any time they please.

Brief chemical reaction for those interested:

MH + NiOOH -> M + Ni(OH)2 during discharge
M + Ni(OH)2 -> MH + NiOOH during charging

Naturally, like all chemical reactions, the effect is not 100% reversable, and eventually, the chemicals become depleted and the cell eventually unusable. This occurs with all rechargable battery technologies in use today, and just like a car battery, Ni-Mh cells have a limited shelf and lifespan and must be replaced.

Voltage of a charged cell usually ranged from 1.2-1.3 volts, dropping to around 1.0 volts when depleted.

The voltage curve stays relatively flat during the entire discharge cycle, allowing most electronics to run safely during this period without the need for additional voltage compensators (which are usually needed with alkaline AA batteries due to their downwards sloping discharge curve, unless you simply accept a dismally short operating life).

Because of higher energy density vs. Ni-Cd batteries, run times are typically 30-40% longer with Ni-Mh batteries.

Like most rechargable technologies, the total useable capacity of a Ni-Mh cell depends directly upon the discharge rate and temperature.

Low temperatures (eg. 0 degrees C) will result in lower useable capacity (ie. total run time) than higher temperatures (eg. 20 degrees C). Run times will drop by up to a half hour or so (assuming typical 20C run time in a device is two hours vs. 0C temperature run times).

Use in a device that draws lots of energy will decrease usable run times even more.

Similarly, output voltage may drop by 5-10% between these two temperatures.

On the other hand, long-term shelf storage of a charged battery is best at cold temperatures if you plan to store charged batteries for a month or two. The freezer at 0C will be a better place than room temperature for maintaining total charge over long period.

You can tell when a Ni-Mh battery is nearing full discharge by a noticable increase in impendance during the last 20% or so of capacity used. This is due directly to the decreasing amount of chemicals in the battery in useable form that allows for electron creation.

Unlike Ni-Cd batteries, Ni-Mh batteries are relatively resistant to the memory effect, where batteries possess a decreased capacity due to recharging before 100% depeleted.

Any minor memory effects that are retained in Ni-Mh batteries can easily be removed by simply exercising the batteries through several full (100%) charge/discharge cycles.

While Ni-Cd batteries do remain relatively cool during their charge cycle up to about 80% capacity (the chemical reaction is a cool one; but during the latter 20%, it also changes to release heat), Ni-Mh batteries will be warm through the entire charge cycle because the chemical reaction during charging produces heat. The latter 20% charge cycle of both battery types will, in general, result in signficantly higher heat release than the initial 80%. Temperatures, depending on cells, condition, rate of charge, etc, can easily reach 60C in Ni-Mh batteries during the last 20% of the charge cycles. However, typical allowed temperatures only go up to around 50C max. in most home charger systems.

Naturally, most modern microprocessor controlled chargers will monitor the temperature in case of overcharging/overtemperature conditions and ramp down the charge input accoridngly until temperatures return to acceptable ranges.

Interesingly, the charge cycle is more efficient at lower temperatures rather than those hotter than room temperature, so those in cold environments benefit accordingly - but only at very slow charge rates (eg. those 10 hour overnight chargers) and maybe only 5-10% more capacity at most.

Above ~40C, a fast charger actually more efficient and provides greater total capacity vs. a slow charger (which just cooks the chemicals too long). However, both at these high temperatures result in significantly lower total capacities than at lower temperatures.

In terms of lifespan, a low-drain device discharging Ni-Mh batteries to about 100% depletion will result in a ballpark of about 500 charges, at which time the battery will deliver at least 80% of its rated capacity.

Note that this does not mean it cannot be used further, only that if you start off with a 1600mAh battery, after 500 such cycles, it will act like a 1280mAh battery in terms of total run time.

While one method of lengthening lifepsan is to discharge the batteries to a lesser degree (eg. use only 20% of capacity before recharging), this obviously does not work in real life situations in digicams. Ideas of thousands and thousands of charges will not apply in these use situations.

Another is to ensure that the battery remains as close to room temperature as possible during the charge cycle. This reduces the strain on the chemical reactions inside the battery - hot temperatures caused by fast charging do reduce lifespans (although not signficantly as I'll note later), as do overcharging (a moot issue with today's smart microchip controlled chargers that automatically ramp down the input charge as the battery nears full capacity).

Let's now look at fast chargers. For today's rapid chargers, suppose you buy a 1 hour fast charger and push about 1-1.3A (yes, Amps, not the typical 500-700mA used in 2-3 hour chargers) through a Ni-Mh AA battery.

To wear the battery down to 90% total charge capacity vs. new, and assuming you don't bother completely discharging the battery every time to 0% (eg. discharge to about 20% remaining like most digicam users), you would have to run the battery through about 500 charge/discharge cycles before the total capacity drops by about 10%.

On a 1600mAh battery new, that would result in a 1440mAh equivalent after 500 such cycles.

Continuing until we reach 80% total charge capacity, you would have to run the battery for another 300 or so cycles (total of about 800 cycles) before that occurs.

On a 1600mAh battery new, that would result in a 1280mAh battery equivalent after 800 such cycles.

While these numbers are approximate, and calculated based on figures from older 1200-1300mAh Ni-Mh battery performance, you can expect the latest 1600mAh batteries to perform as well, if not better due to continued improvements in Ni-Mh battery chemistry and technologies involved.

Naturally, what does this mean for the average digicam user who uses a much slower 2-3 hour charger and a digicam that depletes the batteries an hour or two slower? Much longer useable lifespans of Ni-Mh batteries without any sort of conditioning or deep discharging -- in other words, even if you have a charger with deep discharging capabilities, you may never need it at all before the battery expires due to other reasons.

Like all rechargable batteries, Ni-Mh batteries do have a shelflife of 3-5 years before they begin to naturally decay to the point where capacity is significantly reduced and/or chemistry becomes unusable.

You simply replace them at that point.

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A very important note to make on Ni-Mh batteries is that there is a distinct tradeoff between slow and fast charging and maximum lifespan and total capacity!

Ni-Mh batteries possess the longest lifespan when slow charged (10+ hours). However, due the chemistry involved, the total capacity will be less.

A rapid charged battery (1-3 hours) will possess a shorter lifespan, but much higher capacity.

Given that a rapidly charged AA battery of about 1200-1300mAh (from above) will last about 500-800 charge/discharge cycles when charged in a very rapid 1 hour (1Amp+) charger, and that the typical use of these batteries in a digicam or other modern electronic devices requires higher capacities for longer runtimes, the best approach for most people will be to rapid charge Ni-Mh batteries as close to room temperature as possible.

Those using Ni-Mh batteries in a TV/VCR remote controller, clock or other low-drain device would achieve longer lifespans with a 8+ hour overnight charger, but the battery chemistry may just decay during the passing years of use to the point where you may not even be able to use that extra time in time.

In the end, Ni-Mh AAs are only ~$10 USD for a set of 4 that last 500-800 long cycles rapidly charged.