would I be right in thinking that to charge a 12v 7 ah sealed lead acid battery from a cigarette lighter socket on a canal boat, I would need to wire a bulb into the positive lead of the connecting wire (series) to limit the current?
Charge a 12v battery from a cigar lighter
charginglead-acid
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You will probably need an intelligent charger (micro controller based with a custom program to handle inputs from V&A meters, solar charge controller, & a power regulator).
Common methods of determining the SOC of a 12V(nominal) battery aren't very accurate. Most often people use a generic table that shows V & SOC %. The problem with that data set is that it applies to a new/good battery and as a battery ages & deteriorates the internal battery resistance increases, charge capacity decreases, & charge rate decreases (i.e. the charge rate of an aged battery is slower at a specific voltage than a new battery)--so you can't rely on it as a reliable method to charge a battery with solar energy while the engine is off & power is being drawn from the battery.
If you use a micro controller based smart charger with a good program, you should be able to do the following to determine the maximum safe voltage to apply to the battery while the engine is off: 1. measure the battery V 2. measure the A draw of the system 3. To maintain the battery's current SOC, your power system must produce as much power as the system is consuming. Your program would calculate the optimal charge A & V to provide sufficient charge current to offset the power consumption rate. 4. To add a net charge increase in the battery, your computer program will have to provide a higher charge rate than the charge consumption rate (you can decide that rate or make it a user configurable parameter).
Note: The regulated charging system of a running vehicle provides sufficient power to charge the battery & maintain other electrical loads. Your smart charger will have to emulate the same power availability while the engine is off (which might mean you'll need quite a bit of PV surface area (depending on loads).
Actually SLA batteries have a vent... so the name "sealed" is a bit of a misnomer. VRLA (valve-regulated lead-acid battery) is actually a name for the same tech.
Practically every UPS (uninterruptible power supply) I know of has one [or more] SLA[s] inside, so it's generally safe for indoor use. Here's a snippet from an APC white paper on the issue:
Valve regulated lead acid (VRLA) batteries [...] do not require special battery rooms and are suitable for use in an office environment. Air changes designed for human occupancy normally exceed the requirements for VRLA [...] ventilation. Vented (flooded) batteries, which release hydrogen gas continuously, require a dedicated battery room with ventilation separate from the rest of the building.
And bit later in the paper the difference in gas output is quantified as 60 times less for VRLA:
VRLA batteries are considered to be “sealed” because they normally do not allow for the addition or loss of liquid. A vented battery can give off sixty times more gas than a VRLA battery in normal use.
And the reason for this is that in a "sealed"/VRLA battery:
hydrogen recombines under pressure with oxygen into water inside the battery. Gas can only escape when internal pressure exceeds the rating of the pressure valve.
The fact that they're pressurized explains why in extreme cases of misuse or abuse they end up seriously bulged. The plastic container is actually designed to cope with that scenario.
Also, small SLAs (almost certainly the one you have) use gel as electrolyte suspender so won't spill liquid[s] even if cracked. The larger ones use a glass mat instead (gel is rather expensive).
Best Answer
No, I don't think you'd need any sort of current limiting.
However, you won't get any useful charge into the battery unless the vessel batteries are also being charged, either by the engine alternator, or from a shore-power charger.