Adventures of M7ISY

As stated in other posts, I started my amateur radio experience on a YAESU FT-818nd. A portable radio with an internal battery and an advertised max output power of 6 Watts:

Four power levels are available on the FT-818: 6 Watts, 5 Watts, 2.5 Watt, and 1.0 Watt. When using Alkaline batteries or the supplied SBR-32MH Ni-MH Battery Pack, the microprocessor, detecting internal battery use, automatically sets the power level to 2.5 Watts, which appears on the display as “L■■”. If you set the power to 6 watts, the power level icon is the same as for 2.5 Watt operation, but at 6 Watts the icon is blinking. For 1 Watt, there is one “bar” to the right of the “L” in the power icon, and for 5.0 Watts there are three “bars” displayed.

Quote from the YAESU FT-818nd user manual

According to our understanding: When operating with the internal battery, the radio automatically changes the setting to 2.5 Watts, but you are able to override this, by manually setting the output power back to 6 Watts. So that is what we tried. We managed to get the power icon blinking, which according to the manual can only happen at 6 Watts. This reassured us, that we would transmit on 6 Watts. Transmitting on 5 Watts would show the “L” with three black bars next to it. Confusing to us, we never managed to get into this setting, but assumed, that the manual might just be wrong here. (spoiler: it’s not wrong).

We researched through the internet but were not able to find any details that exceed the information in the user manual. Most websites state “when using internal batteries, power level is set to 1.5W”, but don’t mention, if this is a hard limit, or just an adjustable setting. As we anyways considered buying another battery to extend the radios runtime, we gave a 3000mAh Li-On battery a go. It has a rated voltage rage from 9V to 12.6V. With that battery we ran further tests and compared the results between the Li-On and the original NiMH battery that came with the radio.

	Original NiMH battery	Li-On Battery
Capacity	1900 mAh / 18 Wh	3000mAh / 33.3 Wh
Voltage Range	8V – 11.2V	9V – 12.6V
Nominal Voltage	9.6 Volts	11.1 Volts
Internal Resistance	high	low

Comparing the two batteries shows major differences. The higher capacity will allow the battery to last longer. The higher voltage range will run the radio at a higher voltage for a longer period of time. This will become interesting in a bit. The lower internal resistance of the battery will result in a lower voltage drop when drawing a big current from that battery.

With that new Li-On battery we apparently were able to select the 6 Watts output power on the radio. We continued experimenting and monitored the battery voltage during our tests. The FT-818 can display the battery voltage on the screen. This can be configured through the menu. For the NiMH battery we usually saw quite a significant voltage drop during transmissions, even when transmitting at lower power levels. When using the Li-On battery, the voltage drop was significantly smaller, but still present.

Our testing showed, that when using the Li-On battery we are able to transmit on full power until the battery voltage drops between a magic threshold. Once dipping below that voltage, even if this is only for a few seconds during a transmission, the radio will disable transmitting on full power again, until the voltage is stable above that threshold again (we needed to restart the radio for that, and fully recharge the battery).

This threshold was fixed at: 11.5 Volts. A Voltage that the NiMH battery never reached, which explains why we were never able to set the power level to HIGH. As the Li-On battery, when fully charged, outputs 12.6 Volts and even during high power transmissions the voltage drop is small enough to stay above 11.5 Volts. BUT: This is only true for a fully charged battery.

Soon after that, the battery discharges enough, so that the voltage under load drops between the 11.5 Volts. While the battery extended the runtime of the radio and allows high power TX in the beginning of the battery cycle, in our opinion, the threshold of 11.5V is reached after too short time, to actually enjoy high power TX. But: As the overall runtime is greatly extended, we will stay with that battery.

Conclusion

To make use of the 6 Watts output power of the YAESU FT-818nd, you need to keep the voltage above 11.5 V. This can be done by replacing the NiMH with a Li-On Battery, but will only allow TX for a short time. For the remaining (very long) runtime of the radio, TX is limited again.

Another, better option is to supply external power to the radio. This for example can happen from a car battery with a matching adapter. You also can buy LiOn-Batteries with a way higher capacity. Those should keep the Voltage above 11.5V for even longer. Just make sure to not exceed the 16V maximum input voltage (nominal voltage: 13.8V) of the radio! Using bigger lithium battery backs will at least remain some portability of the radio. If you intend to use the radio only stationary, supplying power from mains might be an option. But: be aware that you a) might need to install some filters, to not interfere. And b) to not use the charger that came with the radio. While it can output the required voltage, it can not output more than 500mA. According to the YAESU manual, during a transmission the radio requires up to 2.7 Amps. The manual requires an external power supply that can provide a current of at least 3 Amps.

We were not able to figure out, if the connector for the internal battery could also handle a voltage of up to 16V. If so, it’s a shame that the Li-On battery being sold for that radio only has a maximum voltage of 12.6V (most likely that’s three cells with 4.2 max voltage each). If the battery would use LiFePo cells (with a max voltage of 3.6 Volts per cell), using 4 cells would allow for a max battery voltage of 14.4 Volts. This would ideally allow to transmit at full power for longer, than with the Li-On battery.

For now, we stay with the LiOn battery, as portability is very important for us, but might invest in a bigger, external LiOn battery for that radio.