I've been wanting a compact and portable 28 Volt power supply for quite some time to power my Locator and any future 28 Volt lights away from home. I haven't seen anything I liked or that was light weight and small. I finally decided to build my own. Way back when Power Tubes were available and I had a lot of DeWalt pack scavenged A123 cells, I briefly rigged up an 8S or 9S/6.9 Amp pack and ran the Locator. I remember the cells getting very warm and after shut down, one or two cells vented. The Locator draws about 16 Amps @ 28VDC. Fast forward to a few weeks ago. Prismatic Lipos are fairly cheap now and "relatively" safe if treated with respect – paying attention to Low Voltage Cutoff, don't pull more current than they're rated for and definitely balance charge them. Blue Lipo brand are very affordable and have a decent reputation on RC Groups forum so I decided I'd go with them. Commonly available 8-cell packs provide too much voltage and 6S packs, too little. 7-cell packs are almost "just right". Few manufacturers make 7S packs and those that do, get a lot of money for them. So I bought 3 ea. 4S/5000 mAh, 25C rated packs and waited for another 3 weeks hoping the supplier would fill their backorder of 3S/5000 25C packs. Ultimately, I had to go with 3S/5000 "30C" packs because they had no idea when the 25C's would arrive. I wire one 4S and one 3S pack together for 7S and then connect all 3 in parallel via the Anderson 4-way to get my 7S, 3P, 15 Amp pack. While I waited, I put together everything I could without having the 3S packs. All parts in-hand, I set to work.
Care and feeding of the batteries:
I parallel and balance charge the 4S's together and the 3S's together to only 4.1 Volts per cell and end up with an unloaded voltage of 28.7V. When the Locator is first started, voltage falls to 28.0. I use the PowerLab 8 charger with safe parallel adapter boards with balance taps. Charging each set of packs takes about 40 min. at 1.1C. Unfortunately, I have to take the box apart to do my charging due to the complexity of all the connections and the combination of 4S and 3S packs.
Operation:
I get just under 40 minutes of run time and am using only about 80% of the capacity of the pack due to the using a 4.1 max charge voltage instead of the normal 4.2V per cell and the fact that I have to shut down at about 25.7V (3.67 Volts per cell) because the cooling fan on the light stops running. I could get a few more minutes if I ran the cells down to a safe 3.3 Volts.
The Construction:
The 6 packs with Anderson Power Pole connectors installed.
A standard fuse holder, an Anderson Power Pole 4-way connector and 12 gauge wires. The Andersons fit the box and the clamps can be snapped in place if I come across a military vehicle with 28 Volt system.
The project box I had for quite some time was just the perfect fit for the 6 packs. The upper half of the box with the 28V, 30A power switch, self-powered voltmeter, Anderson front panel double connector and 12 awg Dean's Wet Noodle power connection wires installed. The voltmeter is switched separately so I can check PS voltage without having to turn on the PS switch.
Both halves of the box, the lower with 2 layers of adhesive Velcro strips to secure the batteries. 2 layers to raise the batteries higher than the box attachment screws so they don't puncture the Lipos – Very bad if that happens.
The front of the box with Anderson double panel and power switch installed.
Batteries fitted into the box. Just perfect with the fuse holder as a spacer.
Both halves of the box with the connections made. Lots of wires and a bit messy but it is tight quarters. Got to be very observant when making all those connections to prevent a rapid vent with flames event. I color coded as much as possible.
The finished product. It weighs 7 1/2 pounds
The Locator being powered by the power supply.
Care and feeding of the batteries:
I parallel and balance charge the 4S's together and the 3S's together to only 4.1 Volts per cell and end up with an unloaded voltage of 28.7V. When the Locator is first started, voltage falls to 28.0. I use the PowerLab 8 charger with safe parallel adapter boards with balance taps. Charging each set of packs takes about 40 min. at 1.1C. Unfortunately, I have to take the box apart to do my charging due to the complexity of all the connections and the combination of 4S and 3S packs.
Operation:
I get just under 40 minutes of run time and am using only about 80% of the capacity of the pack due to the using a 4.1 max charge voltage instead of the normal 4.2V per cell and the fact that I have to shut down at about 25.7V (3.67 Volts per cell) because the cooling fan on the light stops running. I could get a few more minutes if I ran the cells down to a safe 3.3 Volts.
The Construction:
The 6 packs with Anderson Power Pole connectors installed.
A standard fuse holder, an Anderson Power Pole 4-way connector and 12 gauge wires. The Andersons fit the box and the clamps can be snapped in place if I come across a military vehicle with 28 Volt system.
The project box I had for quite some time was just the perfect fit for the 6 packs. The upper half of the box with the 28V, 30A power switch, self-powered voltmeter, Anderson front panel double connector and 12 awg Dean's Wet Noodle power connection wires installed. The voltmeter is switched separately so I can check PS voltage without having to turn on the PS switch.
Both halves of the box, the lower with 2 layers of adhesive Velcro strips to secure the batteries. 2 layers to raise the batteries higher than the box attachment screws so they don't puncture the Lipos – Very bad if that happens.
The front of the box with Anderson double panel and power switch installed.
Batteries fitted into the box. Just perfect with the fuse holder as a spacer.
Both halves of the box with the connections made. Lots of wires and a bit messy but it is tight quarters. Got to be very observant when making all those connections to prevent a rapid vent with flames event. I color coded as much as possible.
The finished product. It weighs 7 1/2 pounds
The Locator being powered by the power supply.
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