The batteries in my STX-3500C are about to croak. The battery packs are no longer available as far as I can tell. Can I build new packs replacing the NiCAD batteries with NiMH rechargeables? If I use the charging circuit within the panel that uses cresnet power, it's a slow charge that takes about 20 hours. Will this be OK or will I have to stick with NiCAD?