Technical Discussion Topic:
"Batteries and the Micro908"
by Joe Everhart, N2CX
December 12, 2004
Welcome to the wonderful world of battery
First, a "cell" is a single AA cell and strictly speaking a battery is multiple cells - in the Micro908 a holder with 8 cells. I'll try to be consistent but bear with me if I slip and mix terminology.
The original design for the Micro908 had a battery holder to provide the operator with a means of using the device for short periods without tethering it to an external power source. We did not provide any charging provisions so that the user had the flexibility to use whatever batteries he chose. For very short duration use, 6 alkaline AA cells are the economical choice. Those with deeper pockets could use premium AA Lithium cells (at about $2.50 per cell!). Those desiring to use rechargeable batteries could use either LiCd or LiMH cells. Our original thought was that allowing recharging of the battery pack inside the unit could prove risky for those using the non-rechargeable alkaline or Lithium cells.
Unfortunately removing cells to recharge them requires taking the case apart. It was decided that we could add a simple charger without major circuit changes. Along with that we added terminals for a charger jumper so that non-rechargeable cells could be used by those who wanted this option.
Now battery chargering is a topic that takes up entire books for a single battery chemistry and we wanted to keep things as simple as possible. So we decided that providing a very basic trickle charger was a reasonable way to go. More about that later.
The very simplest trickle charger is simply a resistor connected between a battery to be charged and a DC source with a higher voltage than the battery. The resistor limits the current to a safe level. Add a couple of isolation diodes to allow either battey or external power operation of other electronic circuits and you have the scheme we use in the Micro908.
Now for some details on charging batteries this way. The most important consideration is safety, both to the equipment user and to the batteries. In short, we don't want a charging circuit that is likely to blow anything up.
Second we want the charger to operate under a variety of circumstances. We can suggest to the user that only NiMH cells be used for power, but since the more common NiCd AA cells will fit in the same holder, you can be sure that someone will use them. NiCd cells generally have much lower ampere-hour capacities than NiMH so their charging currents have to be lowered accordingly. If this is not done, the cells will overheat very quickly and in the worst case explode causing at the very least a caustic mess inside the Micro908.
Ideally the charger would be able to rapidly bring even NiMH cells up to snuff but this means that the charging current would have to be several times as high as for the NiCd types. As stated above, safety dictates otherwise.
Then comes the issue of external power. We intended the Micro908 to be operated from the usual "12V" power source. Unfortunately this means a wide range of voltages. A fully charged Lead-acid or gel cell will have a terminal voltage of up to 13.5 to 13.8 volts though normally it is the lower number when not being charged. It is often convenient to use one of the ubiquitous "wall-wart" type power supplies, hopefully one rated for 12V output. Indeed those with internal regulators do output close to 12 V. But those without regulators might give 12V with their full load, but up to 16 volts with a light load! The Micro908 includes a regulator to handle this range, but accommodating it in the charger would add a good deal of complexity. For simplicity's sake we did not design a universal charger.
OK, enough background, what are the characteristics of the charger we used? Simply stated, it is intended to provide a reasonably safe trickle charge for 8 NiMh AA cells with an ampere-hour capacity of 1600 mA-H or more when an external power supply of 12 to 13.8 V is used. In addition, it will provide a trickle charge for NiCd cells with a rating of about 1000 mA-H or more. Cells with lower ratings will be charged at a rate somewhat above trickle charge levels to they may have a shortened life expectancy. However the charge rate should not be high enough to cause catastrophic failure.
Here are some nominal numbers for the charging circuit used. NiMh cells and the 8-cell pack have the following voltages:
Fully discharged: 1.0V/cell or 8.0V/pack
"Nominal" operating voltage: 1.2V/cell or 9.6V/pack
Full charge voltage: 1.4V/cell or 11.2V/pack
With a 47 ohm resistor used at R47, nominal charging currents should be:
(Vext - Vd - Vbatt) / 47 ohms
where Vext is the external charging voltage. Vd
is the drop across isolation diode D9 Vbatt is the battery pack voltage
This calculates to charging currents under those conditions:
Fully discharged: (13.5-0.3-8)/47=111ma
"Nominal" cell voltage: (13.5-0.3-9.6)/47=77ma
Full charge: (13.5-0.3-11.2)=43ma
So it is obvious that the actual charging current
depends on the charge state of the cells being used. If they are fully
discharged, a full charge will take a long time. However if they are not
fully discharged an overnight charge will probably be adequate.
Now why didn't we design in a universal do-everything charger?
The answer is that the charger was a circuit addition that was added as a convenience to the Micro908 user. We wanted to free him (or her) from the bother of opening up the unit to pull out batteries for charging by adding a usable charger.
Adding truly universal charging circuitry would add to the complexity of the 908 and would add at least $20 or so to the cost. Finally it would have delayed getting the unit in everyone's hands, something that was absolutely not a good idea.
BTW you might ask how I use my unit, power-wise. I generally power it from a regulated 12V wall-wart or "12V" gel cell in the shack. When I use it in the field I normally use an external 2-A-H gel cell or a 10-cell NiMH pack that otherwise powers my QRP rig. For those occasions where portability is very important, I use internal 1800 mA-H MiMH cells and limit their use to no more than an hour so they do not run down.
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Page last updated: Dec 12, 2004