Batteries to the Field
I have seen many questions on just what is the best battery for use in the field. The answer depends on a number of factors. But first one needs the data necessary to reach the right conclusion. That conclusion will depend on your circumstances. However the information on this page and the information in some of the links I have provided should help you reach a decison with a fair degree of confidence that it is the best decison.
The first step is to compare the various types of batteries. Those most frequently used are the alkaline batteries, NiCad batteries, NiMH batteries, and Sealed Lead Acid(SLA) Batteries. The table below summerizes the important features of each. As I am concerned primerally with opertation where you must "lug" the batteries to some "remote" location I have restriced the non-SLA batteries to the the popular AA size. If you are hiking or backpacking weight is an important considertion. The data can be readily extended to othersize batteries. I will return later to how just carrying the battery from your car to the picnic table or using them in the shack might effect the battery choice. The information in this table came from the Eveready and the Power Sonic websites. I choose these not because their batteries are better than their compeditors but because the data was easily obtainable from their websites. The characteristics of the other manufacturer's batteries are very similar.
|Discharge Rate||Weigth for|
|Alkaline e2 AA|
The first step is to understand some of the data in the table. The manufacturers don't make it easy to compare the various batteries. This is at least in part because each battery was orininally designed for a different purpose.
The first key piece of information is the recognition that battery capacity is only defined in terms of the load on the battery and how low you are willing to let the voltage of the battery reach. The effect of the load is seen best with the information for the sealed lead acid batteries. Look at the date for the PS-1212. When the current to the load is 145 mAh the battery capacity is 2900 mAh. However when the load increases to 450 mAh the capacity declines to 2250 mAh. We have lost over 20 percent of the battery capacity. From this it is clear we must know what load we will place on the battery in order to make the right choice. We should also know how long we want to use the battery in the field.
The next step I want to take is to make some pairwise comparions between the batteries in the table. This will allow us to come to a quicker conclusion on the "best" choice. I have summerized these comparisons in the outline below.
This is a fairly easy comparison. Both batteres are rated at the same voltages when charged and discharged. The NiMH can be run for 5 hours at 320 mA while the high capacity NiCad can be run for 5 hours at only 145 mA. The only disadvantgae of the NiMH batteries is that a 12 volt pack will weight a few more ounces. The clear choice here is the NiMH batteries.
This comparison is a bit harder. But weight is a significant factor as the SLA batteries are much heavier than the NiMH batteries. The best comparison is perhaps with the PS-1220 SLA battery run at 370 mA. The capacity here is about 1850 mAh. For the weight involved I can carry three ten cell sets of the NiMH batteries each with a capacity of 1600 mAh for a total capacity of 4800 mAh and run them at 320 mA. I think the NiMH batteries win this test.
There is no data in the table that can help in this comparison. Both types of batteries are rated the same. The manufacturers don't do a very good job of selling the "new technology" batteries. You are forced to review the perfomance charts at their web sites and view some very small graphs an attempt to make out which is better. What they all tell you is the e2 type batteries will outperform the older alkaline batteries when you run them at the higher current levels. They don't attach any numbers to these claims. When I look at the charts I frankly don't see a lot of differences. So give the price differences I see in the stores I have been unwilling to try these batteies.
This leaves us with a choice between the alkaline, or e2 if you prefer, and the NiMH batteries. I have concluded that the choice at this point will depend on a number factors which are specific to the operation at hand. But the most important question at this point is what is the relationship between the capacity of the alkaline batteries and the amount of load on the the batteries. The NiMH batteries hold up much better under heavy loads. But the alkaline are better if the load is not so big. The question we seek to answer is at what point, at what load, are the two types of batteries equal? Is it at a 200 mA load, at a 300 mA load, or is it at another level. Then the next question is how much current does the rig you will use draw? The answer to these two questions will put you close to deciding which type of battery is better for your usage. Unfortuneatly the answer to those questions are not easy to come by.
I have assembled the table below to provide some data on the current draw of a few of the popular rigs. For a rig like the K1 if I assume I transmit 50% of the time and that I have a 50% duty cycle then I'm running at about 155 mA(=0.75*57+0.25*450). So looking at the first table I know I can get at least 1600 mAh from the NiMH batteries and something less than 2850 from the alkaline batteries. So the question becomes what is the loss in the alkaline batteries capacity as a result of running them at the much higher level of 155 mA rather than at the 25 mA that the rating information is given for. If I do the same calculation for the SST, I find I get an average current of 76 mA. It may well turn out that alkaline batteries are best for the SST, but with the higher current draw the NiMH batteries are better for use with the K1 under similar operating conditions.
|Rig||Current in mA|
|11.1 Volts||13.9 Volts||11.1 Volts||13.9 Volts|
To choose the right battery I have concluded that there are so many other factors that come into play that basing the answer on just the numbers above is not a wise course of action. Experimentation is necessary to get to the right answer. For that reason I have run a number of test that put the alkaline batteries against the NiMH batteries. The following table provides links to a description of each test. Below the table I have provided a discussion of some of the factors which complicate battery comparisons. Some of those are addressed in the testing. Others you will have to take into consideration in making your own choice on which battery type to use.
Alkaline vs NiMH
8 Cell Pack
Usage was spread over 3-4 four days with mostly contest type QSOs. Not a lot of activity. This test came out as a close tie. Total operating time was about 25 hours. Alkaline
Intensity of Use
This test gets at how much does spreading the battery usage over several day impact performance. The answer was it didn't. QSOs vs Hours This page compares the number of QSOs vs the number of hours the batteries lasted for both alkaline and NiMH batteries
Thus far running 500 mwatts most of the time I have not been able to get the alkaline batteries to outperform the NiMH. Given that the NiMH will better handle the higher current drain that occurs when the rig is run at higher power levels I have concluded that the best choice is the NiMH batteries. I do need to run some tests at the higher power levels to comfirm that conclusion. The only exception to that would be when you much be concerned with the self-discarge of the NiMH batteries. If you are in the backwoods for a week or two that can become a signicant negative feature of those batteries.
AE5X has also been testing batteries in the field. You should check out his results.
There are quite a number of other issues that should also be considered when making the battery choice. These include:
This all needs to then be factored into the tradeoff between the load and the storage capacity of the batteries you will be using.
- How low are you willing or able to discharge the batteries?
- What is the output power of your rig as a function of battery voltage?
- How much power do you wish to run?
- Do you use a 10 cell or an 8 cell battery pack?
- What is the self-discharge rate of the batteries?
- What is the cost comparison?
How Low Can the Battery Go?
Or When is the Battery Dead?
Each battery design has a different level at which the battery is considered to be fully discharged. Go back the table above and review those numbers. The SLA batteries can be taken down to 10.5 volts. But even this is not a firm figure. Reviewing some of the renewable energy sites that sell these batteries I find that they recommend not discharging these batteries below the 30% level (11.75 volts). If you check out the solar power energy sites you will fine that the charge controlers that have a low battery load disconnect activate that feature when the batteries drop to about 11.5 volts.
Now consider the alkaline batteries. I put a set of eight of those in my K1 and ran the rig at 500 mwatts. After several days of use when I get ready use the rig I took a few measurements. The voltage on the battery says 11.2 volts. I turn the K1 on and K1 reading is 10.6 volts. I get a 0.6 volt drop for two reasons. First I lose 0.4 volts because the K1 has a diode in line to protect the rig. Second. I lose 0.2 because there is a 57 mA load on the battery. I key the transmitter and the voltage reading drops to 9.7 volts. I get this because I now have a 450 mA load on the battery. For my purposes the 9.7 volts is the reading I need to pay attention to. The rig wants to see more than 8 volts for tranmitting a clean signal, so I can safely use them. However if all I'm going to do is listen on the bands the voltage I care about is the 10.6 volts. So depending on how I'm going to use the rig I have two different standards on when the batteries are dead. In addition, if you look at the table the batteries are good to 0.8 volts per cell. But if I declare them dead when the 8 cell pack is at 8 Volts that is 1.0 volts per cell.
What all this means is that when the chart tells me I get 2850 mAh from the battery if I run it at 25 mA down to 0.8 volts I lose some of that capacity when I don't go all the way to 0.8 volts. But how much do I lose? Well that depends on what I declare to be the point at which the batteries can no longer be used for my purposes.
How Much Power are You Running?
When making a determination as to when the batteries are dead you must also ask how much power do I want to run. The power output of the the DSW rigs drops as the battery voltage drops. At 12 volts you can get 2 watts out, but at 9 volts that can drop to 500 mWatts. If you want at least a watt out you will be using less of the capacity of the battery pack than if you keep it up till the battery pack drops to 8 volts. This is critial as the NiMH have a flat discharge curve. They hold the voltage until close to the end of their capacity. So you will be putting out the higher power longer. The alkaline batteries show a steady decline in voltage. This will make them look worst as the power output of the rig will steadily drop as the you use the batteries - something that will not be as pronounced with the NiMH batteries.
When statisticians make comparisons they like to hold as many things constant as they can. With some rigs the power output drops as the voltage of the battery pack drops. This makes it much harder to make a clean comparion as you find you are comparing battery life, but the amount of power out durring the life cycle of a set of alkaline batteries is not the same as that for a set of NiMH batteries. So in a way you are comparing apples and oranges. However, if that is the type of rig you are going to use in the field, then you want to know how the alkaline batteries compare to the NiMH batteries in that rig. So do the tests with the rig you will be using under the conditons you will be using it - if you can.
Eight or Ten Batteries?
The question of how much power you run brings up the issue of how many batteries to use. To get 12 volts you must use a set of 10 NiMH batteris. A reasonable person would also consider using a set of 10 alkaline batteries. The reason for this is the lower voltage cutoff for the alkaline batteries of 0.8 votts. Many rigs do not function below 11 volts, some are good down to 9.5 to 10 volts and only a few are good down to 8 volts. With 8 batteries you could only discharge the 8 battery pack down to 1 volt per cell when the rig cuts out at 8 volts, effectively wasting some of the battery capacity. The situation is even worst for a rig that does not operate below 10 volts.
However using 10 batteries does not mean they will last longer. The specifications for the EMTECH NW40 illustrated the situtation. At 12 volts the rig uses 52 mA on receive and 1140 mA on transmit. At 11 volts the rig uses 48 mA on receive and 970 mA on transmit. This shows that if you increase the supply voltage you also end up using more current. So by moving from 8 to 10 batteries you are able to use more of the capacity of the batteries however you also need more capacity to run the same number of hours. As a result it is not clear that you gain more operating time by using 10 batteries with this rig. What you do gain is that ability to run the rig at a higher power level. Unfortuneatly with this particular rig you are running 4.8 watts at 12 volts, so increasing the voltage too much means you are no longer runnin QRP. This again shows that to decide on 8 vs 10 batteries you need to know your rig. How much power does it put out and how much current does it draw at various voltages?
A caution must be inserted here. A ten cell alkaline battery pack with fresh batteries comes in at a bit over 15 volts. Many rigs are only rated up to 15 volts. So you should exercise some caution. One solution is to use a dummy cell for the tenth cell until the voltage drop a bit.
Self-Discharge is the term used to describe that fact that all batteries loose some of their capacity as they sit on the shelf unused. This problem does not exist for all practical purposes with the alkaline batteries. But it is important for most rechargeable batteries. The NiMH seem to discharge a more quickly than other rechargeable batteries. I did a quick test and concluded that they loose about 5-10% of thier capacity sitting for only one week. So if you are taking these types of batteries out on and extended backpacking trip this self-discharge should be taken into consideration. For uses like field day, QRPTTF, and similar contest charging the batteries the day before the contest is sufficient.
If cost is the only factor there is no question that the NiMH batteries with the ability to be recharged several hundered times are the best choice.