RE: Emcotec Powerbox vs. Powerbus Pro
Since the wind was up this weekend, and I had to WORK anyway (YUCK .. another ruined weekend) I decided to play with voltage readings and see how things panned out. I'm studying the Emcotec right now, since the plane is still on the bench. I'll get the Powerbox 40/24 in this afternoon and go over it's readings.
I'm using dual 2100 ma NiMH packs (or. . I WAS), using Deans conectors (the super large ones). Voltage drop across the connectors is .001 volts or less, so you know they are doing their job. I started noticing a problem while slammng the sticks around and watching the DC voltage level. The battery packs started with 6.65 volts each, and the output from the Emcotec unit was a steady 6.1 volts with system ON, but no movement.
I've always felt that the best way to see how efficient a planes battery power and wiring are working is to watch the voltage while you hit the Snap button at full rates. This moves all three major controls at the same time, effectively putting maximum load on the battery system. A lot of people forget that the highest current drain form a motor is not while it's moving, but when it's NOT moving, then starts to move. During that very short period the motor basically acts as a dead short while the magnetic windings absorb voltage and turn it into inductence. As soon as a full magnetic field develops, you get a steady amount of "reluctance" (basically, the resistance to current flow through a steady magnetic field. . i.e.. . full charge inductor/winding/transformer), and the amount of current flow is much lower.
As for those silly little 1-amp steady-state voltage "load" boxes. . . at best they are silly, at worst they will give you false confidence that your packs are really working properly. You get the same result by just turning the switch on and seeing what the voltage level is with the servos holding the control surface in place. What you really need is a "snapshot" of the packs under extreme load, to see how well they are reacting to amperage loads 8-10 times their capacity. If you can hit the pack with that sort of load, and realize less than .7-.8 volts of dropout. .THEN your pack is working as it should, and all the cells are operating properly. A 1-amp steady-state load tester will NOT show a cell that is starting to weaken, and which may fail 5 minutes from now. A Snapshot test WILL show this, however, since the marginal cell will cause a higher-than-normal voltage dropout as it goes offline during the snapshot test. People fail to realize that batteries have a "reaction time". . the slower they react, the more marginal they are, and the closer to failure they come. Quick Snapshot tests show these characteristics much better than .5 or 1-amp steady-state testers.
SO, you may not be able to read the current draw directly (its not REALLY necessary either) but you CAN see what that extreme amount of current is doing to your batteries and wiring. In this case, when the servos are not moving, then the Snap button is pushed and all 13 of them start to move at once, you get a REALLY big current spike. I was watching the voltage level of the battery packs, and they would go from 6.6 volts down to less than 5.7 volts, then recover to 6.6 volts when the servos stopped moving. This was read directly off the pack, before any connectors. After about 10 minutes of "snapping" the plane, the packs had depleted to only 6.2 volts, which made me uneasy, and the low point of the voltage spikes had fallen to 5.25-5.3 volts.
Of note, though, the Emcotec box handled things pretty well, and I could not tell when the regulators fell off line and bypassed the voltage that was less than their rating, and the signal trace was very smooth. The output side of the box read a steady 6.1 VDC, but during the "snaps" almost perfectly mirrored voltage readings directly off the battery packs, showing a low voltage point less than .1vdc lower than the reading directly off the batteries.
Given the large voltage "depletion" of the packs, I looked them over. They used 18 gauge wire, were rated at 2100ma, and were less than 6 months old. I had some older packs on the shelf, rated at 2150ma, so I opened them up and got rid of the 18 gauge wire and soldered in some 14 gauge "power wire" used in electric buggy applications, then soldered the wires to new Deans plugs and installed the packs. After 8 hours on .1/C charge I switched the charge rate to 1/20/C, and left them overnight (10 hours), then two hours of "trickle". "Baseline" voltage was 7.05 voltes, one hour after being taken off the charger. Switch on, servos idling, and the voltage dropped to 6.85 volts over a 10 minute "idle" period to scrub off the top-surface of the charge.
Now the "Snapping" began again. . .output from the battery packs never lost more than .5 volts, with a low point of 6.4 volts, and the "idle" voltage level returned to 6.8 volts, during the 5-minutes of snap, snap, snap, snap, snap, probably 100 snaps in a row (this REALLY draws some current, btw). At the same time, voltage from the output side of the Emcotec box showed an Idle level of 6.1 volts, and a fully loaded level of 5.85-5.9 volts, a .2-.25vdc drop under very heavy loads. It's pretty obvious that there is some voltage depletion going through the Emcotec box, but this is to be expected from any electronic unit. I don't consider 1/4 volt, under this sort of extreme loading, to be an issue, and am mildly surprised that it is not a higher dropout. Anytime you ahve a voltage regulator at work, there is going to be a substantial amount of dropout under loads, even if the circuit is bypassing the voltage because it is below the rated output of the circuit.
As for what was happening with the first set of packs? First, the 18 gauge wire was not helping matters. At about 5-8 amps of current 18 gauge wire starts to provide resistance, lowering the available voltage. With each "snap" I was probably pulling 25-30 amps of instantaneous current, if not more, as the servos started moving, then stopped at the end of the travel. I'm of the opniion that the wiring itself was just not up to the task, and provided most of the voltage dropout under heavy loads. As well, the packs had been fast-charged at a .2/c rate for only 4 hours the day before, and not trickled to "balance" the cells, so some of the cells in each pack were probably at a lower capacity level than the others, effectively reducing available power from the packs. A lot of people don't realize that a "balance" period, under trickle charge, of 3-4 hours is almost a necessity to get 100% out of a multi-cell NiCad or NiMH pack, and you can't just slam them on the charger, then yank them off it and put them in a plane right after they finish the charge cycle. This is especially true for "fast charge" rates around .5/C-C1 rates or faster. This is one reason I use a charger than never charges at faster than .2/C rate, and usually leave it to trickle for a few hours before I go flying. I also never "peak" charge. this is just my own technique, as I feel that a full charge cycle at .5 or .1/C rate, then trickle charge, will bring the packs up to full capacity more gently, and ensure a "balanced" pack after the charging cycle is finished. I'm sure Red Scholefield, or some of the other battery "gurus" could write books about this practice.
You know. . you gotta wonder. . .JR "Superswitch" wiring is WHAT gauge?? 20-22, right? And the connectors use those teensy little gold plated plugs 22 gauge "heavy duty" wiring .. and people just plug these things into their planes, directly into the receiver buss land plugs, and EXPECT to fly with no problems. . . you just gotta wonder how much voltage and current this system is actually capable of handling. . Perhaps it's okay for a Pattern sized plane, but when you get up to 30-40% or larger things get marginal REALLY quickly. I have an 80" funfly I'm finishing. . it uses 6 8411 servos and a single 5735 HiTec for the controls. Off a fully charged 2700ma NiMH pack w/14 gauge wiring, I had to run TWO JR Superswitches to the Receiver, otherwise the servos would start getting really sluggish or the receiver would drop out into PCM lock due to low voltage. I installed a single DPDT 20-Amp switch from Radio shack (40 amp capacity), with 3 power pigtails on the output side to the receiver ports, and no more problems. . . .you just gotta wonder how marginal things really are.