NiMH Failure and Cold Recharging

We’re looking at data from a weather station. The device has a battery pack of three AA NiMH rechargeable batteries that are recharged by a simple diode and solar panel charging circuit. The solar panel powers the device during sunny days, and the battery pack powers the device at night and when overcast.

The following chart shows the regular rise and fall of the sun via the solar panel voltage. It also shows the steady decline of air temperature, albeit slightly warm for Chicagoland in October.

Flipped solar panel and dead battery Chicago 2010

Flipped solar panel and dead battery Chicago 2010.

The weather station’s solar panel sat outside the device’s case, as the station had not yet been installed in a weatherproof box. Apparently it was windy late October 27 or early October 28, because the small solar panel blew off the weather station into the tall grass below. This is illustrated by the drop in solar voltage in the days that followed, and the decline ❶ of the battery voltage because the solar panel wasn’t receiving enough light to adequately recharge the battery pack.

As I approached the weather station, I noticed the solar panel hanging down. More importantly, I noticed that the device was powered off, which was a surprise since the battery pack should last four months without being recharged (ignoring self-discharge).

2450 mAh batteries / 0.8 mA power consumption = 3062.5 hours
3062.5 hours / 24 hours in a day = 127 days
127 days / average 30 days per month = 4.233 months

Fortunately, the flash chip does not require power to retain its contents. So, I brought the device inside and downloaded the data. The final entry was on October 31 at 6:07 AM.

The battery pack voltage was 3.61 V, which averages approximately 1.2 V per NiMH cell. That appears to be perfectly healthy. The weather station can continue operating with a battery pack voltage as low as 2.9 V.

So, why did the device power off? Perhaps freezing temperatures ❅ were to blame? Maybe repeated daily trickle-charging harmed the battery capacity?

Individual Cell Voltage

I opened the battery pack to recharge each cell in a “professional” recharger. The two Sanyo Eneloop NiMH cells measured 1.4 volts each (fully charged). However, I was shocked (another pun, sorry) that the Energizer NiMH cell had only 0.9 volts.

The probably with a 0.9 V NiMH cell is not necessarily the low voltage. It indicates that the cell is depleted and can’t supply very much current either. Since the three cells are in series, the failure of the Energizer cell prevented the entire battery pack from being able to continue to supply current.

I let the Energizer battery warm up, measured the voltage (still 0.9 V), recharged it, and tested it by powering a small motor for a while. It seemed fine in these limited tests. What’s the deal?

  1. Bad luck (defective cell, old stock)?
  2. Inappropriate charging by my circuit?
  3. Worn-down battery from prior usage (Wii and Xbox 360, primarily)?
  4. Freezing?

I loaded up the battery pack with fresh, Lenmar R2G NiMH cells and returned the weather station to service. Let’s see if we can find evidence as to the cause of the Energizer cell failure.

Trickle Charging

My charging circuit consists of a small solar panel (43 mA peak, 5.5 V peak) passing through a 1N5817 diode (drops maybe 0.2 V). Even if the panel could supply 43 mA above 4.2 V (1.4 V fully charged cells x 3), that would be a charge rate of only 0.019 C (2300 mAh low end AA / 43 mA). That’s even below Panasonic’s ultra-conservative trickle charge suggestion of 0.033 C.

Frankly, this isn’t a trickle charger anyway; it is an intermittent charger because it only charges during sunny days. In Chicago, annual daylight varies from between 9 hours to 15 hours, not all of which has enough sunlight to power the solar panel voltage above the battery pack. So, this should be considered a 33% to 50% duty-cycle low-current charger.

It doesn’t seem like the charger is the source of the problem. I’m not claiming this charge method will result in optimal longevity; I’ saying it won’t kill a battery used in a low-drain device in four months. The evidence will accumulate over the years by watching the longevity of the newly-installed batteries.

Charging NiMH Batteries in Freezing Conditions

According to the manufacturer’s datasheets, NiMH (nickel-metal hydride) batteries should not or can not be recharged below 32 °F (0 °C). Apparently this is because water is part of the chemical reaction, and the water would be frozen in the electrolyte as opposed to being in a mobile liquid form.

As for discharging, NiMH batteries have significantly reduced capacity below freezing. According to the Energizer manual, the cell capacity is about 50% at 14 °F (-10 °C) and 20% at -4 °F (-20 °C).

Let’s take a look at December and see if the weather station continues to operate. Here is a graph from a little more than a week of cold temperatures:

Temperature versus NiMH battery recharging

Temperature versus NiMH battery recharging.

The air temperature barely got above freezing for a short time on most days. There are even a few days when the temperature remained below freezing entirely.

Interestingly, the clear weather station box heated up to as much a 90 °F for short periods in the sun. Well, technically the temperature sensor inside the box heated up. Because the sensor is exposed to sunlight through the clear lid (rather than hidden underneath a circuit board), we don’t know the true temperature of the batteries or circuit board.

Notice that the battery pack is in fact recharging when the solar voltage reaches or exceeds the battery voltage. (It was snowy or cloudy on some days, not permitting enough solar power for recharging.) Also, the battery pack continues to operate the device throughout the night, even when temperatures dropped down to -3 °F on the early hours of December 15.

Although the battery pack is working, the data confirms that the battery capacity or voltage is reduced by the cold weather. Notice the steeper slope ❶ of the battery discharge when the air was below 0 °F, compared to the less steep slope ❷ when the air was around 20 °F.

Charging When Frozen?

December 14 had the coldest daylight period. Let’s examine that graph more closely to determine if battery charging is occurring when the box (and therefore batteries) is below freezing.

Temperature versus NiMH battery recharging 14 Dec

Temperature versus NiMH battery recharging 14 Dec.

It is difficult to tell. On the one hand, the battery line appears to rise only when the box temperature is above freezing. On the other hand, it is doubtful that the previously frozen battery pack is actually as warm as the exposed thermistor that measures the box temperature. Perhaps portions of the battery electrolyte aren’t completely frozen due to the overnight discharging that powers the device?

The charging doesn’t seem to be an illusion caused by the solar panel voltage temporarily lifting the device load, as the battery pack retains most of its increased voltage even after the solar voltage drops.

I guess I’m going to need to bury a thermistor in the battery pack and increase the sample rate to once per second to get better data.

Another Energizer Failure

Another Energizer NiMH 2450 mAh cell experienced an abrupt failure! But this time it occurred at room temperature and it had never been charged by my circuit. Ah ha!

A quick search of the web reveals complaints from some other consumers, with similar symptoms. Also, the Energizer datasheet for the 2450 mAh version shows the product is obsolete.

So, it appears that the problem in this case was a bad battery. I bought some fresh Energizer 2300 mAh batteries to give them a fair shake. I'll let you know what I find out.

The next set of data looks at the temperature under the ground, compared with air temperature and sunlight.