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Tuesday, 14 July 2015

Which AA Batteries to use in Ltl Acorn Wildlife/Trail/Security Cameras

Fig 1:   Battery Cells Tested
One of the most frequently occurring reasons for users having problems with their trail cameras is in their choice of battery cell. Just collecting a pack of batteries on the way out of the supermarket is not the way to do it because all AA cells are not equal.

This post identifies some of the different cell types and brands; and my test results show the best choice for the purpose.

Trail cameras with night time infra-red LED arrays require a power source capable of delivering a relatively high current and the extent to which a
battery can achieve this is determined by its chemistry. The pertinent factors are the amps per hour rating (mAh), the cell's internal resistance and its voltage drop (discharge curve) when under load.

Fig 2:  Typical amp hour ratings for AA battery cells.
This chart was borrowed from diskdepot

Any rechargeable battery with an amp hour (mAh) rating of less than 2400 mAh is not suitable.

Figure 2 shows typical amp hour ratings for AA batteries and on the face of it Alkaline cells look good but under high load the voltage drops steeply and their subsequent infield life is much shorter than NiMH and Lithium-iron disulfide (Li/FeS2)



Primary (disposable) batteries.


Zinc–carbon with a 400 to 900 mAh rating, Zinc chloride at 1000 to 1500 mAh and Alkaline with a rating of 1700 to 3000 mAh deliver a potential of 1.5 volts but the voltage drop under load is the limiting factor (see fig 3). 

Warning: The zinc chloride cells are usually marketed as heavy-dutyextra-heavy-duty, or even super-heavy-duty batteries, and offer about twice the service life of general purpose zinc–carbon cells, or up to four times in continuous-use or high-drain applications. Regardless of what is printed on the case, zinc batteries are low power and totally unsuitable for use in digital cameras.

Alkaline with their higher amp hour rating will provide reasonable performance for cameras with the smaller infra-red LED arrays (5210 and 6210) but with the later large LED array models (5310 and 6310) infield life is much shorter when shooting videos at night.


Fig 3:  Discharge curves for AA size Lithium, NiMH and
Alkaline showing voltage drop under load.
Borrowed from Paul Allen Engineering
Lithium-iron disulfide (Li/FeS2) is a primary (disposable) battery which has a nominal voltage potential of 1.5 volts. 

The off load voltage of Energizer Ultimate Lithium is about 1.8 volts but drops under load to below 1.5 volts. 

After its initial drop it exhibits a shallow discharge curve (see figure 3) and is able to deliver its rated power for an extensive period.
See also figures 6.



Secondary (rechargeable) batteries. 


Nickel Cadmium (NiCd) cells at capacities of 600 to 1000 mAh are not suitable.
Standard Nickel–metal hydride (NiMH) cells have good capacity (600 to 2900 mAh) but self discharge at about 30% per month which makes them unsuitable for long periods of deployment.

Low self discharge (LSD) Nickel–metal hydride (NiMH) cells are a recent development epitomized by the Eneloop brand (originally Sanyo but now Panasonic) which have excellent characteristics and performance; and although they have a nominal cell voltage of 1.25 volts they exhibit an almost flat discharge curve above 1.2 volts until they are fully discharged as shown by the red line in figures 3, 4 and 5. 

The big difference from standard NiMh cells is the Eneloop's low self discharge rate and Panasonic claim 85% of charge remaining after 1 year for the Eneloop Pro 2500 mAh; which means that a trail camera drawing 0.4 mA on standby would still be capable of operating well in excess of 6 months, whereas standard NiMh would be fully discharged within 2 months of standby mode. 


Internal resistance and voltage drop under load.


Fig 4:  Discharge comparison between Alkaline and
Eneloop AA cells under a 500 mA load.
Borrowed from a PDF by Roy Lewallen

Fig 5:  Discharge comparison between Alkaline and
Eneloop AA cells under a 1000 mA load.
Borrowed from a PDF by Roy Lewallen

Figures 4 and 5 provide a graphic comparison of the discharge curves for Alkaline and NiMh when subjected to  500 mA and 1000 mA loads respectively.

Note the greater voltage drop and reduced time in figure 5.

Acorn cameras will operate with battery voltages down to approximately 4.3 volts which is 1.07 volts per cell. At that point the camera will effectively shut down from a working point of view. There will however be a very low current draw in the order of micro amps, as if it were in standby mode. See battery leakage below.

So the crucial point in the voltage drop curves is where the lines drop below 1.1 volts and the graphs illustrate the relative operating times for continuous current drain at 0.5 and 1 amp. 

The 0.5 amp drain roughly approximates to day time operation while the 1 amp drain approximates night time operation. Note that the time in minutes shown in figures 4 and 5 is for a cell under continuous load in a test situation and not for a camera in the field. During normal operation a camera presents a pulsed load in small doses and deployment time extends to weeks or months depending on how often the camera is triggered and whether it is set to take images or videos or both. 


Internal Resistance

Fig 6:  Discharge curves for Energizer Ultimate Lithium
showing voltage drop under various loads.
Borrowed from www.lygte-info.dk
The main reason for the difference in performance between Alkaline and NiMh is the cell's internal resistance. Battery cells with large internal resistance show poor performance in supplying high current pulses. This is because current is decreased with higher resistance. Current equals voltage divided by resistance (i=v/r). So the higher the internal resistance, the lower the current output ability. Low internal resistance batteries are much better at supplying high current pulses. You can read a full explanation of this at Learning about Electronics.
Internal resistance increases as the battery discharges. Therefore, a typical alkaline AA battery may start out with an internal resistance of 0.15Ω but may increase to 0.75Ω when 90 percent discharged.


Note: Internal resistance measurement of a battery cell is a complex issue which cannot be undertaken with an ohm meter. For digital applications a special meter is used to inject a 1000 hertz signal which excites the cell and calculates the resistance using ohms law. You can read more about this at Battery University.

Fig 7:  Discharge curves for Panasonic Eneloop AA
showing voltage drop under various loads.
Borrowed from a Panasonic data sheet

Working at low temperatures.


Internal resistance also increases at lower temperatures but I don't have data for sub zero conditions.

Typical operating temp' range by battery type:
Lithium     -40° C to +60° C suitable for very low temperature operation.
NiMh LSD   -20° C to +50° C good high drain performance with low self discharge.
Alkaline    -18° C to 55° C  but with more than a 50% drop in performance at  0° C with a 250 mA discharge making them unsuitable for reliable sub zero use in digital cameras.



Battery Leakage and Corrosion.


Fig 8:  Battery leakage in an Ltl Acorn 6210
Leaking cells is another thing to bear in mind when deciding which type of battery to use and is a risk when using Alkaline cells especially if they are discharged and left in the camera.

Even when the camera is switched off it is still trying to draw 300 micro-amps from the battery. Alkaline batteries in particular do not like this and will eventually start to leak which very often causes irreparable damage.

Figures 8 and 9 illustrate the sort of damage that can be caused, in this case to the ribbon cable and control panel in an Ltl Acorn 6210.
Fig 9: Control panel PCB in a 6210 damaged by
battery leakage.


The following explanation is borrowed from Wikipedia

Alkaline batteries are prone to leaking potassium hydroxide, a caustic agent that can cause respiratory, eye and skin irritation. This can be reduced by not attempting to recharge disposable alkaline cells, not mixing different battery types in the same device, replacing all of the batteries at the same time, storing in a dry place, and removing batteries for storage of devices.

All batteries gradually self-discharge (whether installed in a device or not) and dead batteries will eventually leak. Extremely high temperatures can also cause batteries to rupture and leak (such as in a car during summer).

The reason for leaks is that as batteries discharge — either through usage or gradual self-discharge — the chemistry of the cells changes and some hydrogen gas is generated. This out-gassing increases pressure in the battery. Eventually, the excess pressure either ruptures the insulating seals at the end of the battery, or the outer metal canister, or both. In addition, as the battery ages, its steel outer canister may gradually corrode or rust, which can further contribute to containment failure.

Once a leak has formed due to corrosion of the outer steel shell, potassium hydroxide absorbs carbon dioxide from the air to form a feathery crystalline structure of potassium carbonate that grows and spreads out from the battery over time, following along metal electrodes to circuit boards where it commences oxidation of copper tracks and other components, leading to permanent circuitry damage.

The leaking crystalline growths can also emerge from seams around battery covers to form a furry coating outside the device, that corrodes any objects in contact with the leaking device.



Test batteries.


Typical open circuit potential and internal resistance for fully charged cells measured at 1KHz at 20C:

1 Energizer Ultimate Lithium Li/FeS2.........1.82 volts.......0.277Ω
2 Eneloop Pro NiMh LSD..........................1.38 volts.......0.023Ω
3 Duracell preCharged NiMh LSD...............1.37 volts.......0.032Ω
4 Vapex Instant  NiMh LSD ......................1.37 volts.......0.028Ω
5 Maplin Alkaline......................................1.61 volts.......0.196Ω
6 Uniross Alkaline.....................................1.60 volts...... 0.144Ω


Test conditions.

Ltl Acorn 6310WMC
Test camera: Ltl Acorn 6310WMC Wildlife/Trail Camera

Purpose of test: 
The purpose of my tests was to establish which batteries performed best in an extremely high current demand situation. To achieve this I used an Ltl Acorn 6310 trail camera which was set to take 60 second videos with a 50% duty cycle in a blacked out space so that the camera was shooting in night mode.
Maximum current drawn under these conditions is approximately 800 mA.

Battery:
The test camera has tubes for 12 cells in total comprising 3 independent battery circuits each of 4 cells in series. Only one set of 4 cells was used for each test so the operation times shown in the chart will be much longer when a full set of batteries is used.

Camera settings:
Mode - Video only | Video size - 1080p | Video length - 60s | Interval - 0s | Sense level - Off |
Time lapse - 2 minutes | Side PIR - Off |

Test Duration:
For each test the camera was set to ON and left until the battery was discharged to a point where the camera would no longer operate.


Test results.

Summary.


The tests results above are organised high to low for the number of video files on the card at the end of the test but for Duracell and Vapex if you take into account the number of truncated files caused by the camera's power saving function, then the Vapex performed slightly better in this test.

Your choice of battery cell is going to depend on the circumstances the camera is used under. If you're only making casual and occasional use of the camera then Alkaline cells are an easy single use option for short deployments. My choice is Maplin Extra Long Life which are a reliable, good performing cell at a competitive price.
For longer deployment and more serious applications such as prolonged use at sub zero temperatures they are not the answer.
Energizer Lithium or similar are the obvious choice for longest infield life (priced from around 5.00 GBP to over 11.00 GBP for a pack of four cells) but from a value for money and an environmentally friendly point of view the only sensible choice has to be NiMH Low Self Discharge (LSD) cells. These vary dramatically in price and performance with Panasonic Eneloop Pro currently the best and most expensive at 10.99 GBP for a pack of four cells. Least costly are the Vapex Instant at 5.99 GBP for a pack of four cells.

Note: Normal NiMH are not suitable because they self discharge at a rate of about 30% per month whereas Pre-charged LSD NiMH will retain 70 to 80% of charge at 1 year.

Note: NiMH cells need to have a minimum  rating of 2400mAh.



Performance.


Energizer Ultimate Lithium: Best for long infield life (but only single use) and probably the only choice for temperatures down to -40° C or up to +60° C.

NiMH Low self discharge: Good infield life and can be recharged for up to between 500 and a 1000 charge cycles depending on manufacturers claims and what you believe. Chemistry is capable of operating at temperatures between -20° C and +50° C but is likely to vary between brands.

Alkaline: Short infield life and single use at temperatures between -5° C and  +55° C. Alkaline cells also present a high risk of leakage under certain conditions as mentioned above.


Cost effectiveness based on lowest UK prices, 100 deployments using 4 cells and shooting 1 minute videos at night as per the above tests:

Energizer Ultimate Lithium: 280.00 GBP for 467 hours of video.

NiMH Low self discharge: From 5.28 to 13.00 GBP for 284 to 353 hours of video.

Alkaline: 56.00 GBP for 43 hours of video

Infield time will vary widely depending on activity levels, how you have the camera set up and assuming a full set of 12 battery cells could be as long as a year or even more.

It's obvious to me that providing the Vapex Instant cells maintain their performance for a reasonable number of charge cycles (the manufacturers specify 1000 cycles) then they present by far the best value for money overall. Needless to say I will be commenting about them in the future, especially if they fall below expectations.

Note: The Vapex Instant cells do not work to full performance straight out of the pack but reach their full potential after four to six discharge/charge cycles.
Reference:
Eneloop Pro 2500 mAh - Data Sheet
Duracell preCharged Duralock 2400 mAh - Data Sheet
Vapex Instant 2500 mAh - Data Sheet
Energizer Ultimate Lithium - Data Sheet
Energizer Technical Bulletin - Battery Internal Resistance
Learning about Electronics - Battery Internal Resistance
Wikipedia - Internal Resistance
Battery Universioty


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This web site is about the wildlife, particularly the mammals, of the Glen Affric National Nature Reserve area in the north west Highlands of Scotland, UK; and the equipment I use to search for them, which is chiefly trail cameras.

I provide a technical support and parts service for the Ltl Acorn range of cameras and the income from this provides for the upkeep of this site and the purchase of cameras for my own surveying.

I hope you find the site useful and informative; and please contact me if you have any questions that I haven't already covered.