Review: Tenergy Centura Low Self-Discharge Rechargeable 9V Battery

November 23, 2010

Ever since I tested the first AA low self-discharge NiMH rechargeable battery, I’ve been waiting for someone to produce one in the 9V (PP3) size. It seems like everyone is producing the AA size, and some manufacturers are also making the smaller AAA, but no one has taken the trouble to make a 9V battery. Until now.

This lack of interest on the part of manufacturers in producing a 9V battery is probably due to the relatively few applications for this battery. Those of you over the age of about 35 may remember these referred to as “transistor radio batteries”, but improvements in technology have rendered them obsolete for that purpose. However, they are still used, and I have two devices that would benefit from a rechargeable 9V battery: an active-noise-reduction (ANR) aviation headset, and the remote control for my gas fireplace.

So, I was quite happy when I received an e-mail one day from Tenergy, asking if I’d like to review their new Centura line of line of low self-discharge AA, AAA, C, D, and 9V batteries. So far I’ve tested the AA and AAA sizes (included in my low self-discharge battery comparison), and the 9V size described here.

First Glance

The Tenergy Centura 9V battery is, of course, the same size and shape as any other 9V battery. Its rated capacity is 200mAh, which is about half the capacity of a disposable 9V alkaline battery, and slightly below average among rechargeable 9V NiMH batteries.

Like any NiMH (or NiCd) 9V battery, the Tenergy Centura doesn’t actually produce 9 Volts. This is because NiMH and NiCd batteries must be made up from individual NiMH or NiCd cells, each of which produces 1.2 Volts. Thus, the voltage of the entire battery must be a multiple of 1.2V.

A disposable alkaline 9V battery is made up of six 1.5V alkaline cells, giving a total of 9V. Many “9V” rechargeable batteries are similarly made of from six 1.2V NiMH cells, giving a total of only 7.2V. Some devices designed to operate from 9V batteries will not work with such a low voltage.

I was glad to find that Tenergy’s 9V battery produces 8.4V, meaning that it is made up from seven cells instead of only six. Of course, that means that each cell must be slightly smaller, thus giving the slightly lower than average capacity. However, the higher voltage means that in many applications, the usable capacity will be higher (that is, the capacity before the voltage drops too low to operate the device).

Test Results

I tested the Tenergy 9V batteries using my BattMan II computer controlled battery manager. Both charging and discharging were done at a current of 140 mA, which is the lowest available current setting. This is probably a higher current than the batteries are intended for, although not high enough to overheat and damage them, so it is a good test of their abilities.

As with my series of AA tests, the 9V batteries were tested in four stages:

• Fresh out of the package (i.e. how pre-charged they are).
• Immediately after being recharged.
• One week after being recharged.
• Seven weeks after being recharged.

At each stage, the capacity (in mAh), energy capacity (in mWh), average voltage under load (in V), and internal resistance (in Ω) were measured. The results reported below are the average of the two batteries tested (although they were very close in all respects).

Fresh From the Package

This test determines how the battery performs fresh from the package. One benefit of low self-discharge technology is that the manufacturer can pre-charge the battery, so that it has some usable capacity as soon as you buy it. This is probably the least important test result, because there are too many variables other than the ability of the battery to retain a charge. These include the conditions encountered during shipping, and the amount of time the battery has been on the shelf in the store. Nonetheless, it is interesting to know.

Measurement	Value
Measured Capacity @ 140 mA	128 mAh
Total Energy @ 140 mA	1060 mWh
Average Voltage Under Load	8.26 V
Internal DC Resistance	1.56 Ω

The 128mAh delivered is only about 64% of the rated capacity, but this is also typical for most AA low self-discharge batteries. To simplify pre-charging (by not requiring banks of “smart” chargers), most manufacturers only pre-charge to about 70%.

Immediately after Recharging

Any rechargeable battery, whether low self-discharge or “regular”, will perform best immediately after it has been recharged. This is the condition under which manufacturers rate the battery’s capacity.

Measurement	Value
Measured Capacity @ 140 mA	199 mAh
Total Energy @ 140 mA	1713 mWh
Average Voltage Under Load	8.62 V
Internal DC Resistance	1.20 Ω

I was very surprised with these results for two reasons. First, the delivered capacity was almost exactly the same as the rated capacity, despite the fact that I was drawing much more current than the standard C/5 rate used when rating batteries (40mA for a 200mAh battery). Second, the average voltage under load was a very respectable 8.62V, meaning that equipment designed for a 9V battery should not have any issues with this battery. At a more reasonable 40mA discharge, you can expect the average voltage to be about 8.74V.

One Week after Recharging

The “one-week” test is the first one that gives some idea of the battery’s low self-discharge characteristics. Here are the results:

Measurement	Value
Measured Capacity @ 140 mA	170 mAh / 85.4 %
Total Energy @ 140 mA	1429 mWh
Average Voltage Under Load	8.43 V
Internal DC Resistance	1.49 Ω

At first these results disappointed me a bit, since an 85% charge retention after only one week is worse than the seven-week retention of most of the AA batteries that I’ve tested. I suspected that most of this loss occured in the first few hours and that the ongoing rate of loss wouldn’t be this high.

Seven Weeks after Recharging

This is the real low self-discharge test. After seven weeks, many normal NiMH batteries have almost no usable capacity remaining. The 9V Centuras performed very well here:

Measurement	Value
Measured Capacity @ 140 mA	161 mAh / 81.1 %
Total Energy @ 140 mA	1347 mWh
Average Voltage Under Load	8.37 V
Internal DC Resistance	1.38 Ω

Although they were down to 81% after seven weeks, they still retained 95% of the charge they had after one week. Thus my guess that the rate of loss would decrease was borne out.

Longer Term Predictions

Batteries lose charge approximately exponentially, meaning that over any fixed length period of time, the percentage loss is about the same. Given two data points, one can estimate the loss rate, and from that predict the capacity over longer time periods.

Based on the 1-week and 7-week results, this battery can be expected to provide about 135mA after six months of storage, and about 110mA after a year. Keep in mind that these figures are for the overly-high discharge rate I used, so results in real applications should be even better.

Charging

Tenergy produces several smart chargers suitable for their 9V Centura battery. In theory, any smart 9V NiMH charger will work, so long as it is not one of the super-fast (under an hour) ones. Do not use a dumb charger, as leaving the battery connected to it after charging has completed will damage the battery.

Applications

I mentioned the two applications that I personally have for these batteries, and that is how I intend to test them next.

My aviation headset (a low cost model converted to ANR with a retrofit kit) gives me about 15 hours of operation from a 9V alkaline battery. That works out to a couple of batteries per year. I would expect to get about half that time from the Tenergy 9V battery, but since it is rechargeable, there are no batteries to buy or throw out.

Our gas fireplace remote control is unusual in two ways: it is ultrasonic instead of infra-red, and it uses a 9V battery instead of the two AA or AAA batteries used by other remotes. It too uses a few batteries per year, so the Centuras should be a good choice there too.

One place I will not be using this battery is in our smoke detectors. Although I should replace the batteries once per year, I rarely remember to do so, meaning the rechargeable battery could actually run down before I get around to recharging it. Our smoke alarms are dual-powered (AC and DC), so they only drain the battery during a power failure, hence alkaline batteries with their long shelf life last for years at a time.

Where to Buy Them

Tenergy batteries are available in North America from Amazon.com:

• One Centura 9V Battery
• Package of Four 9V Batteries
• Package of Four 9V Batteries with 2-bay Smart Charger
• Package of Eight 9V Batteries with 4-bay Smart Charger
• Package of Twenty 9V Batteries with 10-bay Smart Charger
• Two-bay Smart Charger Only

                    

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