Using old walkies with Li-ion batteries

Like most hams, I have some old equipment. To be precise I own an FT-911 walkie, who uses from Ni-Cd batteries. The original battery died many years ago. Because nowadays it is difficult to get Ni-Cd batteries, I rebuild the pack using Ni-MH batteries instead.

No matter what brand of batteries you choose, or what kind of charger you use with them: they will be dead in just few months. I'm really concerned about the quality of Ni-MH batteries today. Because I really got tired of replace the battery cells, I looked for a different kind of battery. What about Li-ion batteries?

Ni-MH vs. Li-ion

The useful voltage range of a typical Ni-MH cell is 1.4 volts (fully charged) down to 1.1 volts (fully discharged). This is far from the range of a typical Li-ion / Li-polymer battery, which is 4.2 - 3.5 volts.
But if you place in series three Ni-MH batteries, you will get a useful range of 4.2 - 3.3 volts, which is quite close to the Li-ion range. My walkie's original battery used six serial connected Ni-Cd cells, which almost equals the range of two serial connected Li-ion cells, so there is a possibility!

But although useful voltage ranges are similar, batteries are quite different. Ni-Cd / Ni-MH batteries are charged using current sources, while Li-ion batteries are charged using (current limited) voltage sources. This means I would need an specific charger.

The battery

The battery I chose was a clone of the LP-E5 battery, used in many photo cameras. It is marked as 7.4 V, 1500 mAh but real life tests shown its capacity is around 1000 - 1200 mAh. Anyway it is a cheap battery and very easy to find. Another advantage is this battery fits almost perfectly into the FNB-17 case, leaving a plenty of room inside. This opened the possibility to place the charger inside the battery case, bringing a perfect solution.

The charger

Li-ion batteries are charged using current-limited voltage sources. Voltage regulators, like those from the 78xx series, or even the LM317 have a very high current limit value (in excess of 1A) and dissipate a lot of heat. Lowering its current limit will involve a lot of circuitry, so I used a switched voltage regulator.

In fact, I used one of those cheap MC34063 based car phone chargers with great success. Typical MC34063 based car chargers output 5 volts at about 500mA. To transform one of these into a two cell Li-ion charger two modifications must be made.

Fortunately all these chargers follow almost verbatim the schematics proposed in the MC34063 datasheet with very little modifications, so it will be very easy for you to find the components you need to change to make a two cell Li-ion charger with them.

MC34063 schematic proposed in the datasheet

First, you must raise the output voltage from 5 volts up to 8.2 - 8.4 volts. The output voltage is set using a pair of resistors ( R1 and R2 in the above diagram ), and because the reference voltage is 1.25 volts ( the same as the LM317 regulator ) you can calculate the values needed to get an output voltage between 8.2 and 8.4 volts using any LM317 calculator. Remember: 8.2 to 8.4 volts, but no more!

Voltages over 4.2 volts per cell will damage permanently the cells, and can produce cell explosion. 

Never, never, never, apply over 4.2 volts to a Li-ion cell.

NEVER!

You have been warned!

A good target voltage is 8.3 volts. This can be obtained with R1 = 1k and R2 = 5k6. Double check with a good voltmeter the output voltage is under 8.4 volts. If not, change one of the resistors: tolerances will vary the output voltage some tenths of volts up / down  Another possibility is to replace R2 with a 10k variable resistor and set the voltage manually to 8.2 - 8.4 volts, but no more!

As you can see in the schematics, there is an electrolytic capacitor at the output, Co. It used to be a 16 volts one, but I have seen some chargers fitted with 6.3 volts capacitors here, so replace it with a 16 volt one if this is your case.

Well, now we have a constant voltage source. Now we need to limit its intensity. The MC34063 regulator has the ability to set the maximum output current with a resistor: The one placed between pins 6 and 7 ( Rsc ). It is usually a 0.33 ohms resistor, which limits the output current to 500 - 600mA at most. This can be ok for us, but I prefer to charge the cells with a lower current, so I replaced this resistor with a 1 ohm one. In this way the output current is limited to about 300mA, giving a lower ( and safer ) charging current to the cells.

To test the new current limit you can load the charger with a huge load, o even short circuit its output with a amp meter. With Rsc = 1 ohm You should not see any current over 300 - 350mA doing this. With this current I can get a fully charged battery from a flat one in about 5 hours. Not too long and still much shorter than Ni-Cd / Ni-MH batteries.

Interfacing the charger with the battery

The first thing I tried to attach or remove the battery to the charger was a miniature relay. It worked fine, but I realized how hot a miniature relay can become when is active for some hours. The last thing you want in a battery case is hot, so I tried a different method: A MOSFET transistor:

Maybe you have noticed something strange in my circuit. The MOSFET seems to be placed in reverse direction. No, it is not. It is placed ok to prevent the battery to get discharged through the charger. So, what about during charging? During charging, the MOSFET transistor operates in the third quadrant. You can learn how a MOSFET can operate in this mode in this fantastic Power MOSFET Basics from Alpha & Omega Semiconductors.

Too long? Didn't read? No problem: When the MOSFET is active, current can circulate from drain to source and from source to drain too!. The only difference is the internal diode (in fact, the transistor's body diode). If no voltage is applied to the gate, current can flow from the charger to the battery through the diode, but the dropout voltage in this diode will prevent you to get a fully charged battery, because in this way, only about 7.8 volts ( 8.4 - 0.6 ) can be applied to the battery. When the gate is not active, the MOSFET is switched off, no current can flow from the battery to the charger, neither through the MOSFET, neither through the diode (it is reversed polarized), so the battery is effectively disconnected from the charger.

When the gate gets active, the MOSFET switches on, placing a few milliohms across the diode: The diode disappears for every practical consideration, making possible the charge of the battery.

The 22k resistor and the 1N4001 diode prevents any voltage left floating into the MOSFET gate, ensuring the MOSFET is switched off when no supply voltage is applied. The 1k resistor limits the inrush current through the MOSFET's gate.

In practice

For the charger I used one of those cheap car chargers you can find almost everywhere. I'm sure you have many of them lying around. For the MOSFET I used the first N-channel mid-power MOSFET I found in my junk box: An IRF-640, but you can use any N-channel MOSFET capable of at least 1A current and about 20V gate voltage.

The FNB-17 case with charging connector

Interfacing was easy. The battery is directly connected to the FNB-17 terminals, and the charger circuitry is placed in parallel. I placed a small coaxial connector in one corner of the back side of the FNB-17's case as you can see in the images. In this way I can charge the battery applying any voltage between 11 and 15-16 volts: It does not makes any sense to apply more than 15 or 16 volts to the charger: 13.8 volts or your car's battery is ok.

An inside view of FNB-17 case, with Li-ion battery at left, and charger at right

How it works? Pretty well indeed. Since I did the first prototype, I have done some batteries like this for my friends' IC-T81 walkies: The IC-T81 walkie is very popular between my friends, but it is a pain to run it from Ni-MH batteries. Now all of them work fine. The process is very similar, but the IC-T81 battery is much harder to open because the case is strongly glued. Another problem is the LP-5E battery does not fit inside the case, except if you remove the LP-5E case, leaving the cells alone. In this way you must be very much careful to prevent short circuits. Somewhat harder, but certainly doable.

A Li-ion IC-T81 battery.

Once you have finish with the electronics inside, you must to glue again the two case halves, and if everything went ok, you should get a nice Li-ion battery for your IC-T81.

A Li-ion IC-T81 battery while charging.

Of course this same procedure can be made to many other handhelds running from six Ni-Cd or Ni-MH cells as long as you are able to fit all elements inside the original battery case.

My battery have been running with my FT-911 now for more than a year without any issues. Drawbacks? Only one: The battery goes flat before the FT-911 displays the low battery icon. When these Li-ion batteries goes flat, the internal protection circuit is fired, disconnecting the cells from the terminals to prevent further (and fatal) discharging. So when the walkie turns off by itself, I know it's time to charge the battery. For me it is just a minor drawback compared with the enormous benefits it has.

My FT-911 while charging

Some final advices

Select a self contained two cell Li-ion battery and make sure it has the appropriate protection circuit. Do not use spare cells. Because cells will be in different state and different age, and sure they would be abused in different way, they will  have different capacities, and different voltages: They will be unbalanced. Charging serially connected unbalanced batteries is very dangerous, because one of them will be always overcharged.

Do not use any bulged or damaged Li-ion battery. Never.

Do not attempt to recover a over-discharged Li-ion battery / cell. Never. It is already damaged and you can do nothing to get it back to life.

Experimenting with Li-ion cells can be very dangerous. If you decide to build something like the one I describe here, make it at your own risk. I will not take any responsibility if something goes bad and a cell explodes causing injuries to you or your goods. Current Li-ion batteries destined to the consumer market have protection circuits to prevent this but even with them, things can become really bad.

Miguel A. Vallejo, EA4EOZ