LiFePO4 Portable Power Kit

LiFePO4 Portable Power Kit

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).

Lithium ChemistryNominal Voltage
(4 cells in series)
Recharge Cycles
Lithium Titanate9.6v3000-7000
Lithium Nickel Cobalt Aluminium Oxide 14.4v500
Lithium Cobalt Oxide14.4v500-1000
Lithium Manganese Oxide14.8v300-700
Lithium Nickel Manganese Cobalt Oxide14.8v1000-2000
Lithium Iron Phosphate (LiFePO4)13.2v1000-2000

 

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:

  1. “radio control” style soft battery packs to make up a battery of 17AH. Various comments on the Internet suggested quality control of these packs is variable. Costs today (Mar’2018) for a 8.4AH pack is £ 52.83 + p/p. Due to the manufacturing process if one of the internal cells develops a fault the whole battery is a right off. Requires a balance charger to maintain cells. https://hobbyking.com/en_us/zippy-flightmax-8400mah-4s2p-30c-lifepo4-pack-xt90.html

 

  1. Electric Golf Cart suppliers have a good selection of LiFePO4 batteries in capacities from 15-40AH available with chargers and 3-5 year guarantees. Mainly sealed units with an internal Battery Management System (BMS), while this is perhaps convenient it has the drawback that if a cell or the BMS develops a fault the battery may be a right off. Generally supplied with a simple charger. Costs depending on capacity and warranty term but are generally £150  to £300 per unit. NB: Always double check golf cart batteries, are they definitely LiFePO4?www.topcaddy.co.uk/category/batteries/lithium-batteries/

 

  1. Electric Bikes commonly use LiFePO4 packs of various sizes (8,10,12,15AH), individual cells can be purchased and made into a pack. All of the required components can be purchased online or from an electric bike components supplier. Costs increase as cells capacity increases. For 4x15AH cells + cable bits to make 13.2 battery (Mar’2018) approximately £100. Simple chargers are available similar to the ones provided by the Golf Cart suppliers, but would recommend radio model style balance chargers suitable for LiFePO4 batteries.
    LiFePO4 (UK) Battery supplier
    http://eclipsebikes.com/index.php?cPath=25_10
    ISDT T6 LiFePO4 charger
     https://hobbyking.com/en_us/isdt-t6-lite-600w-charger.html?___store=en_us
    ISDT Battery Checker
    https://hobbyking.com/en_us/bg-8s-smart-battery-checker.html

 

I decided on option 3, if there was a problem with an individual cell I could replace it at minimum cost and it allowed me to take control of the management of the battery pack and individual cells. As a self-build I could also choose on different form-factors depending on requirements and components. It is also a simple task to increase the capacity of the pack by putting another one in parallel if needed at a later date. I also purchased a radio control style charger (ISDT T6).  This charger provides greater control of charging and also includes storage charge and discharge options and very importantly it allows charging of cells without a balance lead connected as I would be Bottom Balancing. Note that some LiFePO4 chargers will refuse to work without a balance lead connected. I also purchased the ISDT Battery Checker for more precise measurement of individual cell voltages on charge, storage, and discharge.

The rationale for bottom balancing is that I want the cells to converge to the same state of charge when discharged, before use and I charge the whole battery as one, rather than have the charger bulk charging the cells and top balancing them when using balance leads. I’ve included the following YouTube links for the background to bottom balance and not using a BMS. 

Bottom or Top Balancing
https://youtu.be/0KSFitqvap0

One example of how to Bottom Balance a battery pack
https://youtu.be/J2WvQre8sAQ

To achieve bottom balance I discharged the individual cells to 2.7 volts each and measured the voltage variation (after a settling period of 24hr) between the cells using the ISDT Battery Checker via the balance leads to achieve a variation between cells of a couple of mV. I then charged the cells to 3.4 volts per cell using the chargers (ISDT T6) upper storage charge setting of 3.4v and 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 8mV. I’ve found that if the individual cell charge voltage is increased to 3.6v, the cell voltage difference will also increase to 100+ mV. Also 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 predicted however monitor the voltage to make sure no individual cell goes beyond the 3.6v manufacturers specification.

My final choice was for 2×2 rather than the 4×1 cell pack, mainly because it fits neatly into a box that I subsequently purchased and it also fits better the compartment underneath the boot-floor of my car. The red/black leads with PowerPoles fitted are for the high current connection to the radio, and are also used when charging and discharging, the white leads are the low current balance leads that are used for voltage monitoring. If the ISDT T6 Lite is used to perform a discharge the balance leads are connected and the T6 will not let a cell go below 2.8v. While using the battery pack with my radio a small voltmeter is connected to the balance leads which cycles continually indicating pack and individual cell voltages during use. The audible alarm is set to 2.7v, if any cell reaches this lower limit an alarm sounds and I stop transmitting and disconnect the battery to prevent further discharge. After use the pack gets a storage charge and stored in a dry and cool place in the garage. 

Enjoy your radio
Glen G0SBN/P

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