B...zorg is probably closest to the answer - it depends. I am currently looking at this issue for a new B407 which came equipped with the standard 17 amp hour NICAD. I am considering whether to stay with this battery, change this one out for a 28 amp hour NICAD (optional on the B407) or switch to a SVRLA battery, the only one of which is STC'd currently for the B407 is the Concord RG407 27 amp hour battery.
In favor of the NICAD 28 amp hour is that it can produce the highest starting current (Ipp) of the three (by about 30%) which should mean faster acceleration on the start (provided the battery is properly maintained) with lower probability of a hot or hung start (especially important when the acft is cold soaked). The NICAD life also wins; I am aware of NICADs in B407s that are still going after 13 years with no cell replacements, but with by-the-book maintenance. Re NICAD maintenence, Marathon Norco has a line of NICADs they call "Micro Maintenence" or "M^3". The claim is substantially lower maintenence than standard NICADs, and greater energy density. See
Micro Maintenance Batteries - MNAI . Not many helicopters approved for this battery yet. Marathon also permits an extended maintenance interval for standard NICADs under some circumstamces - see link for details.
The typical SVRLA is junk (for turbine engine starting) after 2-3 years, but these also cost about one third of a new NICAD of equivalent capacity. It is my understanding that SVRLA batteries are also subject to thermal runaway but that this is less likely than in a NICAD.
Teledyne Gill is said to be working on a new type of SVRLA battery with longer life and higher starting power, but it is not yet available with an STC for the B407.
I have used both NICADs and SVRLA batteries in lightly used airplanes and helicopters for years and have never had a problem with either, however, I have arbitrarily replaced the SVRLA after two years no matter what (all have lasted at least this long - in fact, I sometimes transfer these batteries to less demanding use like starting the acft tug, and they sometimes last another five years even when neglected). I use a Power Products Activator 282 charger/maintainer which can be set for either type battery to keep the batteries fully charged. This is a professional grade aviation product that can be easily carried in the helicopter (weight less than 2 lbs). Sells for about $450 U.S. See
Power Products, Inc. - Our Products for more info. The only drawback with this product is that the max charging rate is about 2 amps, so if the battery is low and you are in a hurry, this wont solve the problem. It is really intended to be connected all the time to maintain the battery at full charge, however it will charge a flat battery in the fullness of time (overnight for example). For faster charging, I use a Power Vamp PS30M 30 amp power supply. See
powervamp Group - Product Detail: PS30M . Also a professional grade product. However, this is not a set it and forget it unit. It is a power supply useful for charging batteries, or powering avionics in the hangar. It should be monitored when charging a battery. Not intended to be used for engine starting. The Power Vamp Cool Spool 29 is a great unit for starting. See
powervamp Group - Product Detail: Coolspool 29 .
I use each of these products and find them to be of high quality and a good value for the money.