Self-discharge life of Ni-MH batteries for photographic flash applications

Update – I have completely switched to Eneloop batteries.  That is all I use for all of my gear for normal conditions.  If I am going to extreme (below 0 F) temperatures, I’ll used Energizer Ultimate Lithium.

Overview

Nickel-metal hydride (Ni-MH) batteries are very popular with photographers for use in photographic flash equipment.  The batteries are readily available at virtually any store in the western world and work well with camera equipment.  They provide excellent rush current compared to traditional alkaline batteries and both of those are far superior to old heavy-duty or carbon batteries.  Neither alkaline or especially carbon batteries were designed for high-demand current devices such as modern photographic flashes and digital cameras.  Also, Ni-MH batteries are rechargeable and can be used a seemingly unlimited number of times.  In comparison, alkaline batteries are primary cells and once exhausted, must be thrown away.

For all the advantages of Ni-MH batteries, there are several disadvantages.  The first is that they have a much higher self-discharge rate than any other type of battery.  This may not seem that significant until one tries to use batteries charged last month, only to discover that the Ni-MH batteries have discharged to the point of being unusable.  It is very important to make sure the batteries are charged fairly regularly and certainly the evening or day before they are to be relied upon.  More than once, this photographer has let the stockpile of batteries go for a while and tried to use them in a rush only to discover that regular maintenance and upkeep is necessary with Ni-MH batteries.  Some manufacturers suggest not allowing the cells to dip below 1.0V, otherwise permanent damage might occur.  Always stay on top of your Ni-MH.  There are some hybrid batteries out there that help avoid this issue but they are beyond the scope of this article.

Secondary issues with Ni-MH batteries are the primary cell voltage, resistance to cold and hot temperatures, recharge time and required recharge technology.

The impetus of this lab testing and measurement series was several of the batteries refused to power up Nikon SB-800 flashes on a regular basis.  It was unknown if the batteries had just been sitting around that long or if there was something else going on.  We began a regular cycle of charging the batteries just before use.  However, we lost confidence in the ability to just grab a battery off the shelf that had been charged in the last week and use it without concern.

Products tested

We tested batteries from three different manufacturers to analyze what the differences and similarities were. Our first test battery is from Panasonic, a 2100mAh model #HHR-210AAB.  Panasonic was one of the original commercial developers of the Ni-MH battery.  The specific batteries tested were ordered through http://www.digikey.com.  These typically cannot be found at regular consumer stores and must be ordered online, usually through a distributor like Digikey.  The second battery is the Duracell 2050mAh Ni-MH battery, model #DC1500 NiMH.  This battery can be purchased at any major store in the United States.  The third battery is the Energizer 2500mAh battery, model #NH15-AA. This battery can also be purchased at any major store in the United States.  Both the Duracell and the Energizer batteries were purchased over the span of one year at different stores.  This effectively eliminates the possibility of any battery batch problems associated with purchasing all of the samples at an individual store at the same time.

We took several samples of each battery that had been used for 3 years in the same conditions and the same number of uses.  Also, they have received the same amount of care and maintenance, recharging and usage.  We used a 1-hour charger from Duracell meant for Ni-MH and Ni-Cd batteries, model #CEF80NC.  This charger charges each cell independently, which is important because it take into account the differences in charging speeds of each individual cell, rather than taking the traditional 2-cell approach common in most Ni-Cd chargers.

Data

The first batteries tested for self-discharge were the Energizer batteries.  Six batteries were randomly chosen for this test.  Each cell was individually charged on the Duracell charger and allowed to stay in the fully charged state long enough for the battery to cool down.  This helped negate any possible coupling between the battery cooling down and the self-discharge rate.  The test was run at a constant 20 degrees C.

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Figure 1: Graph of Energizer self-discharge
As can be seen in figure 1, while cell #6 discharged slowly over a period of 100’s of hours, the rest of the batteries discharged quite rapidly, becoming useless within 4 days.  These results confirmed what we suspected – there is something wrong with several of our cells.

The second batteries tested for self-discharge were the Duracell batteries.  Two batteries were randomly chosen for this test (as we only had 2 batteries to sample from).  The charging and handling characteristics were the same as for the Energizer Ni-MH batteries.

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Figure 2: Graph of Duracell self-discharge
As can be seen in figure 2, the self-discharge rate of the available Duracell batteries matches the discharge curve suggested by the manufacturers.  Although the discharge looks very steep, the voltage drop over a period of 500 hours was only slightly worse than the good Energizer cells.  This might be due to the fact that these Duracell batteries have a smaller charge storage capability (2050mAH vs 2500mAh), suggesting they are of slightly older manufacturing methods.  Whatever the difference is, it can be seen that these batteries hold their charge over time.

The third batteries tested for self-discharge were the Panasonic batteries.  Four batteries were randomly chosen for this test.  The charging and handling characteristics were the same as for the Energizer and Duracell Ni-MH batteries.


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Figure 3: Graph of Panasonic self-discharge
As can be seen in figure 3, the self-discharge curve of the Panasonic batteries is excellent compared to that of the Duracell and certainly that of the Energizer batteries.  The data from the Panasonic batteries suggests that battery capacity is not the determining factor of self-discharge rate. These Panasonic batteries seem to hold their charge very consistently for a long period of time and do not show any failed cells.

Figure 4 shows the relative comparison between the different battery manufacturers in an average computed from all the different time samples.  Also, the worst and best case performers from the Energizer battery samples were included into the data set for a comparison against the average performance of the Energizer, Duracell and Panasonic batteries.

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Figure 4: Graph of the average performance and individual samples
Discussion

As can be seen from the curves in figure 4, the Panasonic batteries outperformed the Energizer and Duracell batteries easily.  Even the performance of the best Energizer cell does not match the average performance of a Panasonic cell.  Thus, the best Panasonic cell from this series of tests can easily outperform the best Energizer cell.  The two Duracell batteries seemed to survive the test of sitting on the shelf decently but still not nearly as well as the Panasonic batteries.  This test does not suggest that the Panasonic batteries will outperform the higher rated Energizer batteries in high power use tests in an Nikon SB-800 flash.  That is material for another article.  The relevant point here is that the Panasonic batteries can be charged and left on a shelf for 500+ hours (20 days) and still be used as though the had just been charged.

From these measurements, we have concluded that the major consumer manufacturer brand batteries just do not compare in performance to the industrial manufacturer batteries.  Although ordering batteries over the Internet is a little more time-consuming than purchasing batteries at a local store, the life time self-discharge performance is clearly in the Panasonic battery manufacturer’s corner.