Internal LiFePO4 batteries for the KX3

When operating my KX3 portable, I normally use an external power pack. However, sometimes I want a minimalist “grab & go” setup allowing me to set up quickly and operate for a reasonable time without much gear. Also, for extended operation, I have several commercial power banks that have protective circuits triggered by the KX3’s startup. By starting with internal batteries, then switching in an external source, I am able to use these high-capacity sources.

The KX3 provides for internal NiMH batteries but I haven’t been satisfied with them. With no real charge monitoring system, it is hard to know when and how long to charge them. Also, I prefer to charge and maintain batteries individually rather than in series, even though it means removing batteries for charging. Moreover, given the compromise antennas I sometimes use, I wanted to be able to operate at 10W, and the low voltage of the 8-cell NiMH arrangement does not provide enough voltage.

To solve this, I turned to my self-described “nutty engineer buddy” Jim Richards. Jim is an electronics and software engineer who retired from a major computer company after serving as a “solutions architect.” He and I have been friends since college; in fact, I was the best man at his wedding (and they are still married 55 years later!).

Jim suggested rewiring the existing battery holders into two parallel packs using 3.2v AA LiFePO4 batteries. These batteries are more stable and safer than Lith-Ion batteries, provide a fairly consistent voltage, and four in series give 12+ volts, enough to power the KX3 at 10W.

Jim’s design includes what he calls a simple “current summer” using diodes to allow the current from the two parallel packs to be added together without wasting energy equalizing each other. The small internal resistance of the diodes is combined with the practice of starting both sources at equal charge. Within a short time, if one source voltage is higher than the other, the voltage drops to equal the other source and their currents are then combined. The diodes block the packs from cross feeding, thus preventing wasted energy equalizing each other.

The design requires rewiring the existing battery holders as in the diagram below.

In short, you will need to undo the series wiring of the holders and re-wire them in parallel, using the same wires (you may have to add a bit to the positive side), two diodes and appropriate shrink wrap (see photo below). That is, in the positive (+) line of each 4-bay holder, insert a 1N5819 Schottky diode to help form a self-leveling 4S2P configuration.

The rewired battery pack. Diodes are inside the red shrink tubing.

Here are the steps:

1. Remove the batteries, unplug the pack and remove the battery holder. (To easily remove the batteries, I made a tool by sanding the end of a bamboo chop stick into a screwdriver-style wedge.)

2. Cut the original shrink tubing off everything except that over the two parallel wires from the black connector.

3. In the original configuration, the black (negative) wire from the left-hand holder is connected to the red wire from the right-hand unit to make a series circuit. Cut the negative wire coming from the right-hand unit at a length that will allow it to be soldered to the black wire from the left unit.

4. Slip a piece of shrink tubing over the wire from the left, then solder the two wires together plus the black wire from the connector. Slide the shrink tubing over the whole connection and apply heat to shrink.

5. Now, do the same with the red wire, inserting the two diodes as in the diagram, though you may probably will need to add a short piece of red wire between where the diodes join together and where the two parallel red wires from the connector join. Be sure to slip from the left a length of shrink tubing that will cover the diodes and connections. You will also need a second short piece of shrink tubing over the joint with the two parallel red wires from the connector

When replacing the screws securing the battery holders, the KX3 assembly manual emphasizes the importance of tightening the heads enough that they are below the surface of the plastic to avoid damaging the covering on the batteries. This is especially important for lith-based batteries. Jim suggests that a better option is to purchase 4 #2-56 x ¼”slotted drive flat head machine screws to replace the original metal battery holder screws. [i]

There are a number of choices for batteries. I have been using HyperPS B087R9H66Q

ones from Amazon. Amazon B087BKV56C are another possibility. Charge the batteries with a charger like Amazon B087BKVHBW that charges cells individually with LEDs to indicate when each cell is charged. (Note that the optional internal charger for the KX3 cannot be used.) It is a good idea to mark the sets of batteries with a Sharpie pen so that you can keep the batteries in a set together in the same pack.

To avoid depleting the batteries too far, I’d suggest setting the minimum battery voltage in the KX3 to 10v. If you are operating at 10W, when the power drops to 5W you will be nearing that point.

Note that I have only used this setup on CW, and the intermittent nature of keying yields the best battery run time between charges. It has not been tested for extended operation at high output levels or on digital modes.

Jim provides more information about this modification and the use of these batteries here.

I’ve been pleased by the capacity of these batteries and have enjoyed a number of ragchews at 10W on a single charge.

[i] I got my nylon screws on eBay from Product Components Corp. Email: sales@producxt-components.com