I agree you can probably get by just using your DC to DC converter just the way it is, though you won’t get the maximum charge or life out of the battery. If preventing electrolyte boil-off and the production of hydrogen gas is a big issue for you, or you are trying to maximize the life and efficiency of the battery – you might want to look into a more sophisticated system. A “totally correct” battery charger for lead-acid batteries will typically have 4 (sometimes 5) stages of charge. The first stage (“overcharge”) only programs in when the battery starts out with less than 50% of full charge. The charge current is limited in overcharge state to avoid boil-off of electrolyte or damage to the battery. Between 50-90% charge conditions, the battery can safely accept a charge at a faster rate, and the charger will go into the second stage (“bulk charge”). In the bulk stage, the current is set to the maximum that is safe for the battery design and the rating of the charger. At 90% charge, the charger will switch over to the “absorption” mode to top off the battery – charging at a constant voltage of between 13.8 and 15.5VDC. The ideal voltage varies with the temperature of the battery, and the most sophisticated chargers will have a battery temperature probe. A simpler charger will usually just be set somewhere around 14.5VDC in this stage. At 98-100% charge, the charger will shift to “float” mode – with the voltage lowered down to around 13.5VDC for flooded lead-acid batteries, and 13.2VDC for AGM types – again, to avoid boil-off. Some chargers add a 5th stage in between the absorption and float stages, where the voltage is raised around a volt above the normal float level for an hour or two, to equalize the charge in all the cells. This may sound overly complex – but the programming is just built into the control IC in the charger – and to the user, just plug it in and off you go. Hope this isn’t TMI.
