NiMH in a Midland

[KK4JUG]

A neighbor just gave me a Midland 75-785 portable CB. He says it works great but I haven't tried it yet. Anyway, it takes a pickup truck load of AA batteries (9 to be exact). The unit is switchable from Ni-cad batteries to Alkaline but there's no mention of Li-ion or NiMH. I'm guessing the radio was made not too long after Marconi died.

Will NiMH batteries provide enough power to run the radio a decent length of time. The radio has a built-in Ni-cad charger and using that is out of the question.

 

[mmckenna]

Nickel Cadmium batteries usually ran at about 1.2 volts per cell.
Alkaline batteries usually run at a 1.5 volts per cell.
The switch is probably there to adjust the low battery warning, or something similar.

Many decades ago I had a Radio Shack CB walkie talkie. It took something like 8 AA alkaline batteries to provide 12 volts DC. It had 2 "dummy" batteries that just passed through voltage. If you were running NiCd batteries, you had to remove the 2 dummy batteries and install a total of 10 AA Nickel Cadmium batteries to get 12 volts DC.

Since NiMH batteries run about 1.4 volts per cell, you should be OK with running them. Might show slightly low on the battery level meter, if it has one, but that shouldn't be an issue. Leaving the switch in the alkaline setting would probably be fine.

I've never seen a Lithium Ion AA battery that did 1.5 volts, they are always in the 3.7 volt range, so you don't want to use those without some way over lowering the voltage to the correct level.

 

[RFBOSS]

The switch does not have anything to do with the battery meter.

It is part of the charging circuit.

This radio has a DC input jack that will accept 13.8 volts. It is what is called a closed circuit jack in that it interrupts the battery circuit when a plug is inserted. This is so that alkaline batteries are not inadvertently trying to be charged, not a good thing.

When the alkaline/NiCd switch is in the nicad position, the batteries are supplied with charging current.

Both NiMH and NiCd have virtually the same voltage.

But NiMH cells have much higher mAh ratings. A typical nicad AA cell was about 600 mAh at a 20 hour rate.

At typical NiMH AA cell (depending on the cell) can range from around 1500 mAh to as much as 2500 mAh at a 20 hour rate. This means that a fully charge group of NiMH cells will give you much longer operating time than the typical nicad cad cells.

 

[KK4JUG]

Good information, guys. Thanks.