MFJ-9720 and 4s3p pack

Lead acid batteries are heavy and don’t have the power density of lithium-ion chemistry batteries. I wanted a battery pack that could power a radio on the go and didn’t weigh anywhere near the weight of my lead acid batteries. I have a couple 12V 14Ah batteries. 12 volts * 14 amp hours = 168 watt hours. I wanted an equivalent battery that was much lighter. My lead acid battery weighs in at a whopping 9 lbs 9 oz.

Lead Crystal Battery

Secondly many amatuer radios are actually meant to run from 13.8 volts and not 12. They were designed for 12v vehicle systems which with an alternator are usually at 13.8v -14v. As you can see from the excerpt from the manual for my MFJ-9420 20M SSB transceiver below it can take 12 – 15 volts and it requires 13.8 for full power output.

Excerpt from MFJ-9720 channel radio

A lot of people are using LiFePo4 cells which have a nominal voltage of 3.2v per cells. Four of these in series will equal 12.8 volts. So these comes very close to equivalent to a 12v lead acid which is actually 12.8 volts when fully charged. The LiFePo4 cells have a maximum charge voltage of 3.65 volts which would mean the battery can be 14.6 volts, but this would quickly sag underload down to 12.8 volts, so neither LiFePo4 or lead acid really meet the 13.8 volt really needed for full power from the radio.

18650 Lithium-Ion cells have a nominal voltage of 3.7 volts and have a max charge voltage of 4.2 volts. Four of these in series equals 14.8 volts with a max charge of 16.8 volts. This exceeds are voltage requirement of 13.8 volts, but can exceed the max of 15 volts for the radio. So add some voltage regulation and we can take that voltage and drop it down to a steady 13.8 volts.

We also need a way to create a pack of cells. There are number of ways you can achieve this with nickel bus bars and a spot welder or solder cells together. I decided I wanted to create a pack I could if I needed to disassemble to remove replace a bad cell. So I decided to go with the Vruzend DIY battery kit.

So next I needed some cells. In the spirit of DIY and to save some dollars I decided to harvest some cells from bad laptop batteries. If the battery has a bad cell in it the whole battery will be “bad”. I got hold of some bad laptop batteries and disassembled them and then charged and tested the cells. Most of them tested good and I had enough to build a 12 cell pack. The Sanyo cells I harvested that were good all tested to a little over 2300 mAh of capacity. Running four cells in series and three in parallel will get us a 14.8v 6900 mAh pack or 102 watt hours.

After assembling the pack together the next thing we need is a Battery Management System. The BMS will keep the cells charges balanced and keep the cells from being overcharged or over discharged. I found this BMS on Amazon for $3.58.

4s3p pack with BMS
4s3p Pack with BMS installed.

I wired up the pack and connected balance leads to the battery and then placed an Anderson powerpole connector on the power leads from the battery. After getting the pack wired up, I wrapped it in high temp Kapton tape to protect it.

The completed battery weighs in at only 1 lbs. 12 oz. compared to the 9lbs. 9 oz. of our lead acid battery. Alot easy to bring out for portable ops. No we just need some voltage regulation to bring our voltage down 13.8v for our radio. So I went on Amazon and picked up a pack of six high efficiency DC to DC buck convertors. Six of them were $10.98. They can handle up to 3 amps. The MFJ-9720 will draw up to 2 amps on transmit and 50-100mA on receive so within the range this little buck convertor can handle.

DC-to-DC Buck convertor

I had some small little project boxes so I mounted the buck convertor along with two small voltmeters inside the project box. I soldered a set of leads to the input along with a voltmeter. On the other end of the input leads I soldered another Anderson powerpole to connect to the battery. I then solder leads and the second voltmeter to the output of the buck convertor. I put a 5.5mm OD coaxial jack to match the plug on the back of the radio. I plugged Anderson powerpole connector from the buck convertor into the battery. Now I can see the battery voltage on the first voltmeter and the output on the second one. Adjust the trimpot on the buck convertor to get the voltage output to 13.8 volts and then we’re done.

Buck convertor inside of enclosure.
Voltage set
Voltage regulator with voltmeters
4s3p running MFJ-9720
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