Batteries for portable radio operation are always an interesting subject for discussion. I have been asked many times which batteries I use. My battery of choice for portable operation is the LiPo (Lithium-ion polymer ) battery.
I find the 4 cell 5 Amp variety suits my situation very well. The operating voltage can be up to 16.5 volts with no load when fully charged. However the stated nominal operating voltage is 14.8V Most operators would be concerned these voltages may cause damage to sensitive 13.8v radio equipment. This is correct in some circumstances. I operate a Yaesu Ft-857 for SOTA activations, this is a radio designed for in vehicle operation where voltages may exceed 14.5v. The specifications for the FT-857 state the normal supply voltage is 13.8v +/- 15%. This equates to an operating voltage range of 11.73-15.87V. Even better for the FT-817 being specified at 8-16volts . Ok so I hear you say ” Your 16.5v battery is outside that range!” This is true, however by making use of a full wave bridge rectifier in circuit (I use a MDA3504, 35Amp 400piv variety) the voltage will be lowered by 1.2-1.4v. (remember silicon junction theory 101) There are two advantages, a lower voltage and polarity protection. Ensure you wire the rectifier correctly in circuit, the Battery wires are connected to the “AC” input terminals on the rectifier, Do not be concerned which battery wire connect to what “AC” terminal it makes no difference. However it is critical the Radio positive (RED) wire is connected to the + (positive) terminal of the rectifier and the Radio negative (BLACK) wire is connected to the – (negative) terminal of the rectifier. I recommend using an in line fuse between the rectifier and radio for protection. Once the bridge rectifier is in circuit the no load voltage is 15.1V from a fully charged LiPo battery. There are some heat losses via the rectifier however mine is mounted on a small heatsink and becomes luke warm at 100w output on SSB. I would not recommend a full duty cycle mode such as SSTV or FM for long periods at this output level.
How long does this battery last on the FT-857? That depends on what output power is used. For my SOTA activations I set the output to 30% , (approximately 30w) at this level I have successfully activated 3 summits of about 20 contacts each and still had plenty of power left. There are precautions with LiPo batteries, they can be easily damaged by incorrect charging and balancing. They have also been known to catch fire under certain circumstances. I have not experienced any such issues with the batteries I use, however I follow the documented procedures for this type of battery. It is always a good idea to monitor the individual cell and combined voltage when using the battery. There are many voltage monitoring devices available for purchase, I use a fairly simple unit which emits a loud screech when the cell voltage reaches a pre set level. Mine is set to sound the warning when any cell reaches 3.5Volts.
This setting will ensure the alarm sounds when the battery reaches less than 14 volts. The limit can be set lower, however I prefer to err on the side of caution maintaining the battery condition since replacement cost is not cheap. There are several brands and styles of LiPo battery available, I use the Turnigy brand, they have a very good reputation especially with the Radio Control crowd. I prefer to use the hard case variety, they are a little more robust out in the field. There are lower voltage (12.3v) 3 cell LiPo batteries available for those who want to connect directly to the radio equipment. Hobbyking is a good source for these batteries. I tend to avoid ebay as a source of cheap batteries, usually these batteries are cheap because they are older stock and have not been regularly charged so have degraded over time. Dealers who have a high turnover of stock do not have this problem.
As can be seen here the battery voltage indicated is 16.4v and the display on the radio is 14.9v after the rectifier. Transmitting at full power (100w) reduces the voltage to 13.2v on peaks. The LiPo battery is able to provide bursts of high current when required, in some cases up to 30 times the charge current to 150A. Charging can be easily managed with the many charges available on line. The charger I use allows an input voltage of 13.8volt perfectly suited to charging batteries whilst driving between SOTA summits.
Radio operating at full power with 13.2volts indicated. The battery recovers quickly after transmitting making it ideal for portable operation. In comparison to Sealed lead Acid batteries the LiPo battery is about quarter the weight for the same current, perfect for lugging to the top of summits! As always ensure you read the instructions and follow the precautions for using LiPo batteries with your equipment.