Last edited 01/04/2003
Batteries - some thoughts
A work-in-progess. Call in to see how it's going or tell me your own thoughts on the subject.
So there it is. You've built your first Trail Friendly Radio, and its as light as a feather. But then you attach the batteries - and it seems like it weighs a ton.
For TRF users the batteries are a heavy drain on our energy resources especially when we are taking them on a long hike. They can easily weight more than the radio that they supply. So, what can be done?
Are batteries necessary?
There a few alternatives to batteries for a TFR. There are some technoligies that can augment the power such as solar panels and even small wind generators but in reality, we seem to be stuck with batteries. I note that there has been some work done in the UK on clockwork radios and I look forward to seeing the first clockwork TRF!
What sort of batteries?
There are a number of things that you need to be taken into account when choosing batteries. First you need to calculate how much capacity you need for your adventure. Let's assume that your TFR runs on 12 Volts and consumes 30mA on receive and 230mA on key down. You would like to be out for 5 nights and use the radio for about an hour a night. During that hour of operation, you will spend 40 minutes receiving and 20 minutes transmitting.
You will need a battery capacity of:
( IRX(mA) x Time(fraction of an hour per day) x days of operation) + (ITX(mA) x Time(fraction of an hour per day) x days of operation)
which for our example is (30 x 0.66 x 5) + (230 x 0.33 x 5) mAH = 478mAH
Now it may be tempting to choose the smallest batteries that will give 500mAH but beware - there are several other factors to consider yet!
Perhaps you want to find out how long you will be able to operate with a specific battery I have made a little spreadsheet for that which you may find helpful!
Different battery technologies
There are many different battery technologies around but only a few are actually worthy of consideration for Adventure Radio - these are the ones that are readily available. Batteries fall inot two classes, Primary or Secondary. Nothing complex here though. Primary batterys can't be recharged and Secondary batteries can.
Ten years ago the standard Primary battery was thge Leclanche cell. It's a Zinc-carbon battery and has the advantage of being cheap and having a resonablely low self-discahrge rate - so it can hang about in the shop for a few weeks and still be fine when you buy it. However it has some disadvantages too. These are that the output voltage falls as the battery is used, the output drops dramatically as the temperature drops below about 10 degrees Centigrade, the Zinc outer case is disolved from the inside as the battery is used. So when it goes flat holes may appear in the Zinc allowing the midly acidic electrolyte to leak into your radio.
Alkaline batteries (Zinc-mangenese dioxide) have largely replaced Zinc-carbon batteries for most uses these days. They do not have any of the problems of the Zinc-carbon battery. In comparison, their only disadvangae is a higher unit-cost. The have 5 to 6 times the life of a Zn-C battery, a very low internal discharge rate (good shelf life) and good performance at lower tempoeratures, down to perhaps 5 degrees Centigrade.
The most common secondary battery around is the NiCd (Nickel-cadmium)
Some useful battery links for you to be looking at...
Choosing the right battery type and size
What does battery capacity actually mean?
The effects of temperature
Should the batteries be inside the radio or separate?
Charging in the field
Designing your radio to make the best use of the available battery power.