The linear balancer version is a success. I assembled the PCBs, using components removed from the first prototypes to save cost. The first time I applied power, without the battery connected, everything was fine. Then I connected the battery... unfortunately in reverse. I heard a “snick” sound and then realized that I had mis-wired the battery. Too late. My moment of carelessness destroyed three components: the LTC4006, the 50mΩ sense resistor, and the bottom FET. After replacing these components everything appears to be working correctly, but with different parametrics.
The gate drive waveforms are very good (scale is 1/10x):
The above trace is the top gate waveform. The trace below is of the bottom gate.
These traces show very little ringing associated with parasitic, or stray, inductance.
Efficiency: 94.8% (without the balancer circuit operational)
Charging current: 2.04A @ 7.5V
Maximum balance current: ±178mA
Cell imbalance after charge: 33mV (a single occurrence)
Float Voltage: 8.396V
Timer Accuracy: +1.3%
BAT+ leakage current without power: 26.8µA
BATMID leakage current without power: 6.6µA
Thermistor thresholds: rising (cold) =2.24V, falling (hot) =0.561V (This translates to ~0°C to 42.5°C allowed temperature range for charging.)
I charged two 3.5A 18650 Li-Ion cells until the charger stopped. There was a 33mV imbalance between the two cells after I let them rest overnight. It’s good enough.
Installing the Charger Into the T16:
I drilled a 1/8" hole in the middle of the battery compartment for the thermistor to contact the battery. I used my soldering iron to make an opening in the heat shrink tubing to allow the thermistor to contact the battery pack. (Nevermind that I made the opening in the wrong place...I corrected it later.)
A 5/16" hole was drilled into the back of lower right side of the back of the transmitter to accommodate the 5.1x2.5mm barrel jack. A 1/8" hole was drilled next to the barrel jack for the charging LED. The three battery leads from the charger were connected to a MR-30 male connector. The LED leads were shortened, and the LED was glued into the transmitter back with hot melt glue. The thermistor was attached with hot melt glue to extend about 1/2" into the battery compartment -- hopefully to make contact with the batteries. ( A bit phallic, eh?)
The PCB was attached with a good quantity of Plumber's GOOP, and left to cure for a few hours. The entire assembly looks like this:
Finally, the three battery leads for the female MR-30 connector were soldered to the T16 daughter board as shown below:
I left enough battery lead length for easy removal/reattachment of the T16 back. Sharp eyes will note that I did not connect the JST connector to the Jumper main PCB...
I connected the two MR-30 connectors together and attached the back of the case to the T16 transmitter.
Moment of Truth:
I connected the 5Ah battery pack to the JST jack inside the battery compartment and slid the battery compartment lid over top. (A screwdriver was necessary to put enough pressure on the tongue of the lid to allow it to slide into place. We'll see how difficult it will be to remove the lid when necessary.) When I pushed the power button on the T16...nothing happened. It took me a while to find out that I had not connected the power jack from the battery to the T16. Once the power jack was connected the T16 booted and appeared to operate properly.
I turned off the T16, connected the 12V DC wall adapter and plugged it into the 125VAC wall socket. The green charging light came on. All is well. The batteries were only partially discharged. The charging LED turned off after only 15 minutes, or so. I let it top-off charge for another hour and then disconnected the wall adapter. The transmitter indicates that the battery is 8.42VDC -- pretty close. I'm done.