Powering the FT-857D
*** update to this post 21 Feb 2014*** I am no longer using the 4S LiPo arrangement to power the 857D on mountain peaks, instead I now use 4S LiFePO4 batteries. I have retained the LiPo battery packs for use in the radio shack until their end of life cycle. When comparing the capacity of a LiFePO4 to a 12 volt SLA the weight saving factor is a key reason to move away from SLAs.
The purpose of this post is to share details with you about how I power the FT857D for a SOTA activation. I do not endorse any of the products shown below, you can make up your own mind on what is best for you. When deciding what type of battery you are using now or will use in the future, you might like to consider the physical size (how much space it takes up in the pack), the battery weight and should I substitute battery weight for water and food. 1 litre of water is 1 Kg, am I better to carry more or less water? Read on…
For my first SOTA activation at Booroomba Rocks I took far too much extra equipment such as an SWR meter, a small Z match and one spare SLA battery. Not having any previous experience you tend to be risk averse and have redundant gear in the pack. My pack today is likely to be 3 to 4 kg lighter, that’s plenty of surplus capacity for more water and food.
So what batteries did I use for the FT-857D? I carried two 9 Ah SLA batteries in the bottom of my pack with a combined weight of 5.4 kg. Yes I didn’t know better and would have undertaken a successful activation with one 9 Ah SLA battery. One 12 volt 9 Ah SLAB will last for 1.5 hours on QRP, beyond that the VFO loses stability. I can also hear you say why were you using SLABs? Well I had the SLABs in my shack and didn’t see any reason for using a different arrangement. Some weeks later I became aware of LiPo batteries through a thread on the SOTA Yahoo site and through reading the SOTA blogs of VK3YY, VK3ZPF and VK3WAM, all three had made the transition to LiPo batteries in one form or another. If you are not convinced about the merits of battery weight scroll to the bottom, but come back and read on.
The power requirements for the FT857D are vastly different to the FT-817. The 857D manual states the recommended power supply voltage be no more than 15% of 13.8 volts. That means the 857D can cope with a max DC supply at 15.8 volts. Okay so where can I get a LiPo rated at 15.8 volts? You can’t, but you can buy a 4 Cell LiP0 (4S) which when charged is 16.8 volts, but that’s too high for the 857D? A single LiPo cell when charged is 4.2 volts while the safe discharge voltage is 3.3 volts. The operating range of a 4S LiPo is 16.8 to 13.2 volts.
How do you deal with the higher voltage? You can use diodes in series to drop the voltage however you need to consider the possibility of using more than 5 watts and what that may do to the diodes 😦 . When operating VHF DX at 35 to 40 watts the current drain is 6 to 7 amps. You can build a DC regulator based on a LM317 with high current pass transistors but your regulator needs a box and will consume space in your pack :(.
What did I do? I purchased a variable DC-DC buck regulator rated at 12 amps from an eBay store. Remember if you use a 4S LiPo at full charge state, you need the regulator in series until the LiPo voltage drops to a safe working voltage say 15.5 volts. At that point and at a convenient time you can remove the regulator and feed the 857D directly from the LiPo.
How do you check the LiPo voltage and know when to remove the regulator? I use a Watts Up meter in series between the battery and the regulator. The Watts Up meter is a very useful tool, it displays the battery voltage, load current and Ah spent. Ah spent is valuable to know if you will be using the battery at the next summit. Deduct Ah used from the battery capacity and you can work out how long the battery will last. This will help you decide if you need to carry a spare battery for the next summit and so on.
How do you know when a single cell has discharged to 3.3 volts. You can monitor the Watts Up meter by keeping an eye on the battery voltage as it gets close to 13.2 volts. This requires constant attention to the meter when you are distracted by signal reports and writing station details in the log. There just isn’t time when you are dealing with a pile up. Fortunately a genius has done the hard work and invented a LiPo battery alarm.
Finally you will need some form of charging device to charge the LiPo. Best option is to buy a balanced charger from a reliable source. I purchased the LiPos and a balanced charger from Hobby King (online) via the Australian store. If you are thinking of not using a balanced charger search YouTube for exploding LiPo batteries.
Below are pictures and details of the components I use to power the FT857D and monitor the battery state.
From the top: 2 Nano-tech 4S 5Ah LiPo, DC-DC buck regulator, LiPo battery alarm and the Watts Up meter
This is a balanced LiPo charger rated at 7 amps. The main leads of the battery are connected to the charger output while the battery balanced plug is plugged into the 4S balanced port on the right hand side charger. The charger has ports for 2S to 8S. The charger is an intelligent device and will reduce the charge current as the battery voltage approaches 16.8 volts. Once the battery voltage = 16.8 volts the charger switches the charge circuit off. The charger has other options for time limited charge periods or Ah capacity limits. Not shown in the picture is a LiPo charge bag used to cover the LiPo while charging or being stored when not in use.
The input supply to the balanced charger is from 6 SLAs in parallel used in the shack. The SLAs are charged by a 10 watt solar panel.
I use a Watts Up meter to monitor the state of the battery and amp hours used or spent. The Watts Up meter was purchased from Powerwerx along with 50 sets of 30 amp Anderson Powerpole connectors.
Watts Up meter connected to the LiPo at the Source port. The regulator or radio is connected to the Load port.
The 12 amp variable regulator was purchased from an eBay store. Search for DC-DC buck regulator. I deliberately chose the 12 amp model so I can run 40 to 50 watts for VHF-DX. If you don’t intend running QRO then consider a lower current rating. I haven’t experienced any RF interference from the regulator.
LiPo Battery Alarm
The alarm unit is an important addition to the SOTA station. The unit plugs into the battery balanced charger plug and cycles through each cell displaying the cell voltage and the sum of all cells. The length of the cycle is variable depending on the number of cells in the battery. In my case I am using a 4S therefore the unit cycles through Cell 1, Cell 2, Cell 3, Cell 4 then displays the sum of the cells. The alarm (very loud) is triggered when an individual cell’s voltage drops to 3.3 volts. If the alarm is triggered you should disconnect the LiPo and replace it with a charged battery. If you ignore the alarm then do so at the risk of permanently damaging the cell. Considering the cost of a LiPo, I would not ignore the alarm. The alarm unit costs $2.00 AUS, I found the item on eBay by searching LiPo battery alarms. The model number is AOK-BL8S.
The alarm unit is designed to work with 2S through to 8S LiPos. My battery is a 4S therefore the first 4 pins are connect to the balanced plug.
The pictures below tell the story on battery weight. If you are carrying SLAs in your pack, think about the weight saving and your capacity to take more water and food on a long SOTA activation?
I stopped using SLA batteries on 10 March 2013 after the activation of Mt Tennent VK1/AC-025 and haven’t looked back since.