The type of fixed installations noted here often used battery chargers with very significant ripple content on the output (ie unfiltered DC). The ones that I'm familiar with used an older type of technology involving SCRs (and associated circuitry) to detect and control when to automatically switch to float charge. The peak voltage did not change, only the average voltage which was waveform dependant. Some simply used a small basic charger to float charge the battery and relied on the engines alternator (where one existed) to recharge the battery following a start.

The installations I worked on pre-dated the modern switch-mode smart charger with its pure DC output as well as modern VRLA type batteries. It was standard maintenance procedure to check and replenish the electrolyte levels (with demineralised water) when visiting these unattended sites.

The best results that I obtained in regard to reliability and longevity of batteries in this class of operation were achieved using two large six volt truck batteries connected in series. This configuration gave a reliable start in in conditions where the ambient temperature was as low as minus ten degrees C. The worst in terms of longevity was using 12v batteries in parallel. This will probably start an argument but my experience here was practical not theoretical.

Any data sheets for AGM and Gel batteries that I have checked strongly recommend against using chargers with a high ripple voltage content. In other words for a variety of reasons el-cheapo chargers should not be used with AGM or Gel batteries or probably any battery where the electrolyte cannot be checked and topped up.