Re: The SF M6-R: a regulated rechargeable SF M6 (H
I performed the resistor swap-out and tested the charging setup with the same wallwarts I use for the Tiger85 harness mods and it was a disaster! When they say "15VDC at 833 mA" they are . . . well . . . lying. First of all there is a significant AC ripple. Second, even the PEAK of the ripple is lower than 15 volts at 800 mA, and third, even the RMS voltage at 550 mA is less than 15 volts. So, the short of it is that this $5 wallwart will NOT work as a supply for the charging circuit.
When I tested it, it failed to stop fast charging and I finally had to disconnect the battery pack to prevent damage. So next, I decided that maybe it was because the pack had been fully charged just the day before, so I discharged it, and then started it charging with the M6-R charger + 15VDC wallwart. An hour and a half later it had prematurely ended the fast-charge. Oh well, the applications notes specifically warn you about these problems if the MAX712 is not adequately powered, AND they discuss the nominal ratings and ripple of wallwarts, and what the real output is like. So I was warned. Next I reconnected the 18VDC regulated supply and finished the fast-charge cycle. The circuit then worked flawlessly and terminated fast charge well before the pack was too warm.
But I still believe that the 18VDC regulated linear supply which I am currently using is too much of an overvoltage, so I am thinking of using a variable high-quality bench supply to see if a TRUE 15VDC source would be adequate, because I found a 15VDC switching power supply for $15. (The 18VDC regulated linear supply I am using right now is $10--but it weighs something on the order of 2 or 3 pounds, which really adds to the freight charges).
But even with the extra over-voltage the transistor junction temperature is well below it's maximum (150C) and I do like this monster linear supply--plus it's cheap.
I haven't done any more work on brightnorms M6-R pack, but I plan on finishing it this weekend, along with a couple other tasks on my priority list.
Oh, and a word on the zero delta-V MAX712 vs. the negative delta-V MAX713: the 712 only charges NiMH, while the 713 can do both NiMH and NiCd. However, the circuitry of the 712 keeps the peak cell temperature of the battery pack cooler than the 713 does, and by a significant margin. It works quite well, actually, even when charging a pack that is already pretty well at full charge. Some chargers will seriously overheat a battery pack in this state--I'm thinking of my Hitec CG-340 specifically, whose instructions warn the user against trying to charge a full pack.
So this is a very nice feature, and along with the transition to trickle charge after fast charge, means that you can always keep your pack nicely topped up, even on a daily basis if you wish.
