| They will charge just fine. It will take longer, since the NiMH packs | usually have a much higher milli-amp hour capacity. | | If your new pack is double the capacity, it will take twice as long to | charge it, assuming both were fully discharged. That is the trick.
The concern is that since the self-discharge rate of the NiMH cells is higher than NiCd cells, you may be charging slower than the self discharge rate. I do not believe this to be true, and believe your `twice the capicity, twice as long' rule of thumb to be just fine. Sure, it might take a little longer than that, but it's not going to hurt if your battery is only 90% charged when you have 3x the original capacity.
Still, taking 48 hours to do a full charge is excessive, and if you're serious about the hobby, you should have some sort of peak charger. Some day, you're going to want to fly, and didn't charge your batteries the night before. Do you skip flying entirely, or charge for just a few hours and hope that's enough, or don't charge at all and hope there's enough capacity left? None of these answers are ideal, and having a peak charger gives you a good fourth option -- top off your battery, probably in well under an hour. And you don't have to spend much money at all.
| After you have plotted your voltage, you should be able to read your pack | when you get back from flying, and be able to say how much you have drained | your pack.
This is likely to be very inaccurate with NiCd and NiMH cells. However, with NiMH cells that will power your gear all day long, you probably have enough extra juice that it won't bite you.
As for if the Sirius charger will do, it probably will, but if it's old it might not be able to detect the smaller peaks of NiMH cells vs NiCd cells. To be sure, contact Sirius. Or see if the instructions say it supports NiMH cells.
Your peak charger might not be able to charge through your TX charge jack (if there's a diode in there) -- some can, some can't. If it doesn't see the battery at all, remove the battery plug and charge through that. You can also short out the diode (which defeats it's purpose, which is to protect you from mistakes) or replace it with a poly-fuse, but that might not be worth the trouble.