Scantronic Homelink 75

[james.wilson]

Interesting. Petrolhead do you see much of this in occupied homes? ie the 10 degree comment?

[9651]

Not something we've seen on some 9752's on a couple of local Churches, bet they go below that in the winter. Thing I usually find with the 713/4r is poor coverage and slow catch. I believe the Homelink use the same?

[hpotter]

yep, homelink uses 713/4 which in turn use 3.6v lithium batts. however op may have newer xcel pirs which use std aa batts, though with 30%+ better range didnt think so.

temperature does affect all batteries to different degrees though depends on application as well as construction

from energizer

Alkaline and carbon zinc batteries contain a water based electrolyte. As the battery temperature approaches the freezing point of water (32°F or 0°C)

chemical reactions within the battery slow down due to reduced ion mobility. Ion mobility defines the ability of electrons to freely move, critical to

the generation of electricity through a chemical reaction. The net result is a reduction in overall performance or run time which can be recovered as

temperatures approach typical room conditions (68°F to 78°F or 20°C to 25°C).

but perhaps more apt as we're talking wireless, from iet (appologies to author, cant remember who, where)

Over the past few years, a shift has occurred as a result of two things: the expanded

use of wireless technologies and the resulting desire to connect things that have never really been connected before.

The blessing of the good old wired connectivity days was that you generally always had a way to get power as well. Creative minds

have come up with ways of sharing wired media be it the legacy 4-20mA current-loop system or the more modern

power-over-Ethernet standard.

Cutting the data line, or making it wireless, means that we need to cut the power line as well. It means that we are left with harvesting

power from our environment or accessing power stored in batteries. The main questions I get asked when speaking with customers

about wireless sensor networking are “which wireless technology should I use?” and “what kind of range will I get?”. These invariably

come up, but the number one question is “how long will my batteries last?”.

The bad news is that the initial answer is “well, it depends”. But it’s possible to get a more accurate answer if you start to look at how

the system interacts with its power source.

Just like a tank full of water, the energy in a battery is greatest when the tank is full and warm. So, also, is the voltage. As the battery

empties, the voltage tends to decrease. The reality of battery usage is that it never really empties: the voltage drops until the cell

becomes unusable. From a physics perspective, the output voltage decreases because the internal resistance inside the battery

increases. Typically, for a 1.5V AA-size cell, we consider the battery empty when the voltage drops to around 0.9V.

Playing by the rules

It is time to look at two fundamental battery rules.

The first rule relates to temperature. Cold batteries store electricity well but, unfortunately, they don’t supply power well. Some

technologies do better than others but, in general, it is sensible to store most batteries in cold places – just make sure to warm them

up before use.

The second rule, which has a bigger impact on design, relates to the non-linearity of batteries. This is where our storage tank

metaphor breaks down. The capacity of a battery is related to the current it is being asked to provide, and this is not at all linear. High

current draw makes battery capacity smaller. For example, a battery might have a capacity of 1,000mAh for current draw of 5mA.

However, the same battery might have capacity of 500mAh for current draw of 200mA.

This is very important to understand because most people look at the capacity on a battery spec sheet and assume that the

calculation will be linear across current draws and temperature. This is a very dangerous practice because it only works when the

change in temperature and current is very small. Not only that, different battery types have their own characteristics.

Taking usage and battery type into account, let’s take a look at calculating the battery life for a simple example. In a setup where a

temperature sensor is connected to a Zigbee-based network that relays data to a display, the goal is to read the temperature out on

the display every minute. Let’s assume that each read transfers 128B of data and that the time to transfer the data will be about 200ms.

In order to preserve battery life, the radio must sleep until needed for transmitting and receiving data. For the sake of discussion, we’ll

assume that the data transmit and receive time is 200ms, and wake-up transmit and receive time is 50ms. The sleep time is whatever

is left.

As the device has to wake up and transmit 1,440 times in one day, its total waking time is 360 seconds. Next we need to get the

device current consumption. For the sake of simplicity, we assume that the sleep current, for the 23.9 hours the device is snoozing, is

11μA, while the wake current is 14mA. Using these numbers, we can calculate our daily energy consumption as 1.66mAh. The

average current will be 69μA.

Now we get to the tricky part. For an alkaline battery, we use the AA alkaline model for an E91 battery. Using these values and

plugging into the current consumption and daily current consumption, we get a capacity of 2217mAh at 0°C and 2839mAh at 20°C.

Dividing that into the daily demand, we end up with lifetimes of 1,335 and 1,710 days, respectively.

Doing the same for the lithium battery, the model gives lifetimes of 1,807 and 1,946 days respectively, suggesting that the coin cell is

a better option.

soz abt the long quote, couldnt see a way to edit contructively

[petrolhead]

Interesting. Petrolhead do you see much of this in occupied homes? ie the 10 degree comment?

Usually see this on wireless equipment fitted to wooden door frames where the door is an external one. Even when the house is nice and warm inside the door frame can get very cold on the surface it is fitted to. Only seen it once on a lithium battery and it was -14 outside, happens on much milder days with alkaline batteries, I usually switch those ones to lithium 9v batteries, The systems we fit in nursing homes run on 9v pp3s swapped by the customer when the pagers flag up low battery, So I usually label up the ones that need the much more expensive lithiums.