Recently I was researching an alternative to Sealed Lead Acid Batteries (SLAB), the cause of this research…back pain. While SLAB’s have performed ok for my car portable use, I was reminded just how heavy a 50AH SLAB is after straining my back putting it back on the shelf. So having seen a number of posts and videos on the Internet about the weight advantage I needed to give alternatives some serious thought.
I suspect like me, many of you will have heard of Lithium batteries but what I hadn’t realised is that there are different types and specifications. I was looking for three things in a new battery; appropriate voltage for amateur radio use (13 to 14 volts), low voltage sag (little voltage drop when under load), and a high number of recharge cycles (reduced cost of ownership).
I hope you agree that Lithium Iron Phosphate (LiFePO4) came out as the most suitable. I was also fortunate that while away on a DX’pedition to the Isle of Barra with Bob (M0KLO) he kindly lent me his KX3 and a small radio model style 13v LiFePO4 4.2AH battery pack to try. It was certainly compact in size, light weight, and an ability to maintain voltage during transmit but the total capacity of 4.2AH I felt was a bit small for what I had in mind.
A battery of about 15AH seemed to be about the size suitable for my needs based on my initial transmit tests with a Yaesu FT-891 “field” radio which suggested about 8 amps on transmit was a good target if I wanted 3 hours of operating time on a single charge. Remember that this is not an exact science as less current is drawn when listening as to transmitting and whether you use CW, Data, or SSB.
A search on the Internet provided a range of options from which I produced a shortlist:
Top or Bottom Balance (YouTube)
How to Bottom Balance a battery pack (YouTube)
To achieve bottom balance I discharged the individual cells to 2.7 volts each and measured the voltage variation between the cells (after a settling period of 24hr) using the ISDT Battery Checker via the balance leads to achieve a variation between cells of only a couple of mV. I then charged the cells to 3.4 volts per cell using the charger's (ISDT) upper storage charge setting of 3.4v with a charge current of 1/10th the pack capacity i.e 15AH divided by 10 = 1.5 Amps. The cell voltage variation at 3.4v across the pack was only 8mV. I found that if the individual cell charge voltage was increased to 3.6v, the cell voltage variation will also increase to 100+ mV. If the charge current is increased for example to 5A then the cell voltage variation will increase at top of charge. This in itself is not a problem, and is predicable, but monitor the voltage to make sure no individual cell goes beyond the 3.6v manufacturer specification.
If the ISDT charger is used to perform a discharge the balance leads are connected and the ISDT unit will not let a cell go below 2.8v. When used with my radio, I use a small voltage monitor connected to the balance leads which cycles continually providing both a pack and individual cell voltages during use. The voltage monitor has an audible alarm set to 2.7v, so if any cell reaches this lower limit an alarm sounds and I stop transmitting and disconnect the battery to prevent further discharge. After use, I charge the pack with a storage charge and store in a dry cool place in my garage.
I hope the article proves useful.
73 Glen G0SBN/P