Upgrading a radioshack pro106 scanner from niCd to liion?

[Beamin]

I love this scanner because its the only way to pick up the police radio in my state, sadly SDR's can support the two channels at once that are required, they should invent one that does. Anyways, it has the 4 AA (2X2) square battery pack in the back with the option to charge nicd or nimh (There is a setting in the scanner to do this, but don't see why that would matter).
So how hard would it be to:
 A:convert to lion that I would need to recharge the battery separately through a hobby charger (supports all types of RC plane/drone/car batteries) or
 B: some how use the charging feature built into the scanner. As far as size goes to get the same mAh how big would the lion be ? And how big to cram in the circuitry?
 Since its six volts I would need two cells at ~4volts and then reduce the voltage? I think I used to run it off a 9volt AC adapter, but I don't want to try that again incase I'm mistaken and it gets damaged. Do they make an off the shelf substitute? I know you don't see AA lion sizes because some idiot like me would accidentally stick it in a nicd charger, but you would think that with the small size to mAh ratio of liions they could fit protection and charger chip/buck converter all in one package. Or at least a four AA square sized drop in substitute since many devices use that. I'll probably go with option A at first and if that works i will start trying to make a little PCB with surface mount to make it self contained.

[Audioguru again]

A lion is a wild animal. A Lithium-ion (Li-ion) is a battery type.
One cell is 4.2V when fully charged, 3.7V when half discharged and is 3.2V when something MUST detect Low-Battery and MUST disconnect it.
Two cells in series are 8.4V, 7.4V and 6.4V as above. It also MUST detect and disconnect when the voltage becomes 6.4V. A low-dropout voltage regulator or converter can produce 6V.
A li-ion battery has a short life if it is stored at a full charge. Right now I am measuring and discharging my 2-cells Li-Ion batteries for my RC model airplanes to a 7.4V storage charge.
I am glad to hear that you know about using a proper Li-ion charger.

A 2-cells Li-ion hobby battery will have more mAh for the same size as 4 AA cells. Hobby batteries are made in many sizes and you might find one that fits.
A voltage converter circuit probably will not fit inside the radio but 3 rectifier diodes in series will probably fit and will reduce the 8.4V down to about 6V to 6.3V when the Li-ion battery is fully charged.

Some Li-ion batteries (but not hobby batteries) have a "protection circuit" inside. It can be the charger and low voltage disconnect circuits. 

[magic]

B is somewhere between tricky to impossible, depending on how the charger works, which we know nothing about. But there are inexpensive lithium charger modules which could perhaps replace the original thing.

Two cells in series need to be disconnected when either of them is outside of the allowed operating conditions, not just the sum of them

[Beamin]

A lion is a wild animal. A Lithium-ion (Li-ion) is a battery type.

ROAR!!

One cell is 4.2V when fully charged, 3.7V when half discharged and is 3.2V when something MUST detect Low-Battery and MUST disconnect it.
Two cells in series are 8.4V, 7.4V and 6.4V as above. It also MUST detect and disconnect when the voltage becomes 6.4V. A low-dropout voltage regulator or converter can produce 6V.
A li-ion battery has a short life if it is stored at a full charge. Right now I am measuring and discharging my 2-cells Li-Ion batteries for my RC model airplanes to a 7.4V storage charge.
I am glad to hear that you know about using a proper Li-ion charger.

A 2-cells Li-ion hobby battery will have more mAh for the same size as 4 AA cells. Hobby batteries are made in many sizes and you might find one that fits.
A voltage converter circuit probably will not fit inside the radio but 3 rectifier diodes in series will probably fit and will reduce the 8.4V down to about 6V to 6.3V when the Li-ion battery is fully charged.

Some Li-ion batteries (but not hobby batteries) have a "protection circuit" inside. It can be the charger and low voltage disconnect circuits.

So when you use diodes to drop the voltage they will give off the same heat as if you used resistors? Whats the formula to figure that out? Do you size the diodes like resistors where each one would be equivalent to a quarter watt for the standard black cylindrical ones with the silver band often used in regulators? Why does a diode always drop 0.6 volts where a resistor's drop will always vary based on the starting voltage?

Wouldn't option b work if you treated the charging circuitry as a constant voltage source? I think the radio just trickle charges the cells when its plugged into ac.

I think I almost asked who what where when and why all in the same post!

[james_s]

Any linear device that drops the voltage does so by converting the excess voltage into heat. This means that whatever device you use it will produce the same amount of heat for the same voltage drop with the same load current, it follows Ohms law. Diodes drop a relatively constant 0.6V or so, resistors drop a voltage proportional to the load current.