last updated 28 Jul 2003
Choosing the best batteries for the job can seem daunting, with all the different choices available - and there is a lot of conflicting information on the web! Forget about normal use-once (primary) cells unless money really is no object and you don't care a jot about the waste. They are handy to keep as a backup though, admittedly, and if all else fails away from home it's very useful to be able to pop into a nearby store to buy some. Yes they store more power than rechargeables (secondary cells), but a good set of rechargeables (plus a backup set for heavy usage) should perform well enough, providing greater than half the capacity of alkalines. It costs next to nothing to charge up a set of NiMH or nicads.
Most PMR446 sets use either normal AA size cells, or the smaller (half the weight) AAA cells which although still nominally feature the same 1.5 voltage only have about 43 percent of the power capacity of AA cells - you may wish to consider this when deciding which radio to buy.
Alkaline (Zinc-Manganese Dioxide) cells give the full 1.5 Volts but are use-once. Long shelf life - several years, less than 0.01 percent per day self-discharge. Up to about 2800 mAH capacity for AA and 1200mAH for AAA - although despite this high rating they can have trouble delivering a highish current for very long, but this shouldn't be a problem with typical radio use. No toxic metals are involved since using mercury was banned a few decades ago. The discharge curve of an alkaline battery ranges from 1.5 to 0.9 Volts, and when used with a heavy load the voltage will drop quickly to somewhere around 1V.
Rechargable Alkaline Manganese cells are a recent development, specially made to be recharged a few tens of times with special chargers. The 1800mAH capacities fall to around that of NiCd after very few charges however, thus spoiling any initial short-lived advantage. Their low self-discharge rate makes these more suitable for infrequently used low current equipment (not handheld radios).
Zinc Chloride cells were the popular use-once choice before alkalines caught on, again with the full nominal 1.5 Volts under load but only half the power capacity (same as rechargeables). Cheap, but not really worth it.
Lithium (Li) 1.7 Volt AA cells are sold as being 30 percent lighter than alkalines with 3 times the life (similar capacity but better current delivery) and again a long 10 year shelf life. Usually very expensive for what they are, and not very environmentally friendly.
Lithium Ion (Li-Ion) cells are 3.6 Volts nominally, and so are not suitable for making AA size cells. Li-Ion powered devices usually have custom battery packs. Very good characteristics though, regarding shelf life and capacity (twice that of nicad). No metallic Lithium is involved, so safety is OK.
Nickel Cadmium (NiCd) known as 'nicads' were the first rechargeables to catch on.
1.255V per cell, or 1.24V under load (often rounded to 1.2V), but keeps this voltage until
80-90 percent discharged.
Can be recharged 100s to 1000s of times, but prone to a 'memory effect'
which limits the power storage. This can happen if a cell is recharged before being fully used
(or to a certain optimal 1.0 Volts point - complete discharge is not advised) - the cell then only gives
the capacity that had been used before recharging. Some argue that this doesn't really happen,
cell damage due to abuse is more likely the cause... overcharging... a battery should not be hotter than
body-warm after charging... a nicad should not be trickle charged for more than two days.
Nickel Metal Hydride (NiMH) caught on in the late 1990s as a superior replacement to nicads, with the same 1.24 Volts under load. The capacity could be two or three times that of a nicad typically, depending on the types involved, but the technology allows at least 40 percent more. Some web pages cite much less memory effect and some say that there is NO memory effect at all which I find more believable - just do top-up charges whenever you like. Any 'memory effect' is probably just overcharge damage. Some say that NiMH cells can be recharged more times than NiCd, others say less, but this may vary by manufacturer and may well have improved a lot in recent times in favour of NiMH. NiMH cells lose more charge per day, estimates range from 0.8 percent per day (25 percent per month) to 1.2 percent per day (40 percent per month). Apart from this I find these NiMH cells are much better all around - the ideal solution for most purposes (radio purposes - NiCads are still useful for RC models as NiCads can deliver a higher peak current at any given momemt than NiMH). AA capacities varied at first from a rated 1100 to 1650mAh, 550 to 600mAh for AAA, recently advertised new ones are claimed to be 2200mAh (AA) and 800mAh (AAA).
A different technique is required to recharge an NiMH cell compared to a nicad, although some recent chargers can cope with either type, don't use an older NiCd charger for NiMH cells. Although both NiCd and NiMH can both be charged with constant current at C/10 (i.e. a tenth of the overall capacity, 150mA for a 1500mAh cell) the behaviour of the two types is different during charging. A proper NiMH charger needs to detect a full charge far more accurately than for nicads. A slow charger that simply times the charge is more likely to be satisfactory than using a fast nicad charger for NiMH cells where overcharging is almost certain.
In summary, rechargables are the best solution, and given that replacing a set of batteries every year or two isn't going to break the bank, the superior capacity of NiMH wins the day. Be careful to buy a good charger, as a poor one may overcharge or overheat the cells leading to damage often interpreted as memory effect. Only use fast chargers if you're really impatient, and to lengthen the life of your batteries only use a charger that is 'intelligent' enough to shut off when the batteries reach full charge or get too warm. A chip-controlled technique called 'negative delta V' is a good idea - this monitors the cell voltage and detects changes in voltage (delta V) and spots when a negative change occurs at full charge. Otherwise I'd recommend a slower 14-hour 'intelligent' charger which is less likely to cook your cells!
You might find that a perfectly acceptable overnight charger is so much cheaper than a good fancy one,
that it's not worth spending more - buying more batteries over the
likely lifespan of the expensive charger will cost about the same in the long run. Or you may find
that the amount of charge held over the lifespan of your cells is more important to you - if a better
charger gives you more use of your radio each day as well as making the cells last longer before
replacement. If your cells don't last quite the full day in your radio you'll want to find the
highest capacity cells available and look after them well.
Power Loss with RechargablesI always thought that these radios would have circuitry regulated such that maximum power is obtained regardless of battery type.
Jerry : "When I got my Cobra Microtalks I emailed Cobra tech support and asked if there was a difference in output power between alkaline and NiCad/NiMh. They said there was no difference in output."
Randy : "This statement makes sense! With a voltage regulator between the batteries and the transmit/receive circuit, small voltage differences should make no difference at all!"
However, I guess it's too much to expect this for all radios. Some users have found that there is a small loss of power and range with rechargables.
If the radio is not regulated, then using nicads/nimhs instead of ordinary batteries will result in less TX power. Each rechargable has a nominal voltage of approximately 1.24V whereas standard primary cells (zinc/alkaline) are 1.5V; so the voltage is 'only' 82.5% of what it would be with alkaline. With the supply voltage on the RF power amp determining the power output (following a squared rule such that half the voltage gives a quarter of the power), the result is roughly 340mW instead of 500mW. This reduction by 68 per cent is a loss of 1.7dB (which is only 28 percent of a 6dB S-unit), reducing maximum range IN FREESPACE to 82.5 per cent of what it would have been with the full 500mW - not really likely to be a big problem for most users down here on planet Earth. Note that power versus range (in space) has a square involved again, with 4 times the power needed for 2 times the range, so if the range (in space) is proportional to the square root of the power, a power reduction of 68pc becomes a range (in space) reduction of 82.5pc (which is the same percentage reduction as the voltage). For more details of why freespace range differs from conditions down here on Earth, please read the technical page.
Corresponding results were obtained by a forum user "+27dBm" :
Bear in mind that power is proportional to the square of voltage, and these results are fairly close to what we expect. For 3.3V (73.3 percent of 4.5), 73.3 per cent squared is 0.537 - close to half, equating to 270mW - 3.18V would be needed for precisely half the power. With 6.0V being 1.33 times 4.5V, 1.333 squared is 1.777, equating to 889mW (+29.5dBm) - in fact 5.808V would be needed to give 833mW, so perhaps the voltages used in these tests were not quite exact or the limit was being reached.
What this means in effect, is that at any given distance from an alkaline-powered TX the signal will be 1.7dB lower in strength if the TX is changed to rechargables. The advantage that alkalines give you is very marginal, in the very short area where you MIGHT have further coverage, the signal will only be 1.7dB above the break-up level at best.
Vince : "The final outputs on my Cobra FRS-250's and Tekk ProSport+ are indeed connected directly to the battery and the output power will fluctuate with battery voltage, if I'm reading the schematic correctly.
Of course there's a voltage regulator but it sources the microcontroller, transmit driver, and entire receive section.
The output power should be proportional to the square of the battery voltage because: 1) I don't see any means (on the schematic) to do gain control on the transmit side, 2) the final output is a standard common-emitter BJT with no emitter degeneration to control the gain (other than supply voltage).
The receive side appears to be regulated so there's no reason 4.8V versus 6V would make a difference, at least on my radios. It certainly could be the Rayleigh Fading that Randy mentioned (strong in one spot, move 1 foot and lots of static).
Basically, if my radios did 500 mW at 6V, it'd be putting out 320 mW at 4.8V. And, yes, I think Cobra tech support is not telling the truth, either intentionally or through ignorance (fairly common with tech support folks!).
Draw your own conclusions!
Further readingSome notes for more info...
"When a NiCd cell is charged it reaches a pronounced peak voltage near the end of the charge cycle. NiMH cells on the other hand exhibit a much more subtle, thus harder to determine, voltage peak."
"Technically, NiMH cells should be charged at constant voltage and the charge terminated when the voltage "peak" levels off, BEFORE it starts to drop off. Nicads should be charged at constant current and disconnected when the voltage starts to drop after levelling off."
Techniques of charging sealed nickel-metal-hydride batteries
Constant-current, no feedback:
Constant-current, temperature feedback:
Constant current, voltage feedback:
The recommended charging methods for both NiCds and NiMH though similiar, are not the same; NiCd batteries can be charged at a C/10 rate indefinetly without damage, and this is the recommended charge technique. NiMH on the other hand are also normally charged at the C/10 rate but NiMH are more susceptible to damage than NiCd cells. They can be charged at this rate for about 16 hours but then the charge must be terminated or reduced to C/40 which may be maintained indefinitely. Note that neither of these methods requires a charge termination technique beyond timing. If however you want to Fast Charge them you generally must use some form of charge termination technique to actually determine the state of charge or you may damage the cell. Fast Charge rates are typically >1C for <3hr. With the RS 5 hour charger the charging currents are such that it would not be considered fast charge unless the cells being charged had capacities of less than .185 Amp-Hrs for NiCds or about .3 Amp-Hrs for NiMH cells. These are pretty small AA cells so I suspect that RS did not intend this to be a Fast Charge device, and so probably did not put in any charge termination method other than the timer.
In practice the charger is charging at about a C/4 rate if you charge 1200 mAh NiMH cells and the charger is in the NI-MH mode. This is neither Fast Charge nor the recommended C/10 rate but somewhere in between. At the rate it charges, completely depleted 1200 mAh NiMH cells should require about 6.5 hours to fully charge. Note that because the batteries are not 100% efficient in converting charge current to stored charge you must deliver more energy during charging than you will get out. The charger shuts down in 8 hours so you will be charging for 1.5 hours more than required for a full charge if you let the charger terminate on its own. This "probably" won't harm the batteries too much... but. There's always a but in engineering. Most digicams (at least my Oly 500L) do not actually deplete the batteries before the camera quits working. I don't know how much of the usable energy is actually extracted, but I bet it's only about 50% or so. This means you will probably significantly overcharge your batteries if you pop them in the charger and let them charge for 8 hours. You have probably noticed that some chargers discharge the battery first, ostensibly to ward off the dreaded memory effect, but actually to set the battery to a known state of discharge so that charging can be terminated via timer without damage to the battery. Wish I'd thought all this out before I bought the charger!
Anyway, it happens that as the battery nears full charge there is a pronounced increase in its temperature. This is quite noticeable just by feel. I let them charge for a couple of hours then check their temp every 1/2 hour or so. When they start to feel really warm, they're charged.
If in fact the NiCd setting charges at constant current for 8 hours (probably) a completely depleted 1200 mAh NiMH cell would only be about 80% charged at the end of the charge cycle.