NUE-PSK Digital Modem

Battery Life Study

So just how long will those two 9V batteries last in the modem?



Note that the capacity of standard alkaline 9V batteries range from about 400mAhr to 625mAhr, depending on manufacturer. In addition, the terminal voltage of alkaline batteries drops nearly linearly as they are depleted. If one uses these in the NUE-PSK modem, and leave the backlight turned ON, the initial discharge rate with fresh batteries will be about 60mA. As the terminal voltage of the batteries drops to 12, the discharge rate increases to about 85mA. Further depletion of the battery voltage causes additional increases in the discharge rate to about 100mA when the terminal voltage of the two batteries in series drops to about 9V.

Note also that rechargeable 9V batteries typically have a much lower capacity -- less than 250mAhr.

If we assume a rate of 60mA, a 500mAhr battery could conceivably run the modem for a bit over 8 hours. However, at a discharge rate of 100mA, the best that could be expected would be 5 hours. Therefore, I would expect that something close to 5 hours for fresh batteries could be achieved.

Here is a chart showing the modem power requirement versus terminal voltage of the battery. Note that the current is significantly less when the backlight is turned off. If you are operating with internal 9V batteries, this is the recommended mode. If you want to run with the backlight turned ON, it is recommended that you run with an external Gel cell.

In addition, there are 9V lithium cells available (Ultraife) that have a much higher capacity, if internal batteries are required, especially if the backlight will be left ON.


73, Milt W8NUE



James, n5gui writes ...


My system continues to operate at a displayed voltage below 6 volts which represents an external voltage of about 7.2 volts. I have modified my system to include a 1N4001 in the external power line for reverse polarity protection, which may account for the difference.

At some voltage below that, the system seems to get unresponsive to the select button, rotary encoder, and keyboard, but I am not quite sure where because I cannot monitor the voltage display and test these functions at the same time.

If I let the internal batteries "dwindle" near the end of their life, I can see the battery voltage display dim out below 6 volts, with 5.1 - 5.3 being the last readable value the few times I have tried it.

I cannot give you numbers on battery life. The two fresh alkaline batteries I put in after I performed the battery modification (prior to that I had used a single battery in the "lower" slot ) will still run the unit with backlight for 5 - 10 minutes. That has been six weeks of miscellaneous testing by two HAMs, but no continuous operation.



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Page last updated:  April 19, 2008