This typical caravan bulb is described as
MR16 21 LED Spotlight style Halogen replacement LED bulbs are suitable for both 12v and 24v DC systems whilst maintaining consistent brightness during charging and load fluctuations.
and
Specifications for MR16-21L
- MR16 21 LED Spotlight style bulb
- 21 x SMD2835 (new type) LEDs
- Lumens: 353L (Warm White) or 365L (Cool White)
- Colour Temperature: 2730K WW or 6440K CW
- Wattage: 2.8W
- Voltage: 10v - 30v DC
- Current draw (at 12.7v): 240ma (0.24A)
- Approximate Equivalent to 35W Tungsten bulb
- 120 degree wide angle beam
- Dimensions: 50mm diameter by 45mm deep
- EMI free (No electrical interference)
- Features: Internally PROTECTED against Voltage Spikes and FUSED for extra safety
- This is very different to a MR16 used from an AC supply, the main thing of course is it is DC, but also 130 lumen per watt, where the AC types can be as low at 75 lumen per watt. So may be in the same MR16 package design, but nothing like the bulb we would use from a switched mode power supply in our houses.
Firms like Bedazzled specialise in lighting for boats and caravans, you will not get these bulbs off the shelf in Tesco. The voltage is the main problem for a bulb designed for houses, they are designed for 12 volt, not 11.5 to 14.8 volt found in caravans and boats, even the old halogen bulbs were designed to take 14.8 volt from the stage charger found in many boats and caravans.
BS 7671:2008 said:
Under normal service conditions the voltage at the terminals of any fixed current-Using equipment should be greater
than the lower limit corresponding to the British Standard relevant to the equipment. Where the equipment is not the
subject of a British Standard, the voltage at the terminals should be such as not to impair the safe functioning of that
equipment. In the absence of precise data a voltage drop of 0.8 V from the power supply to the equipment may be
allowed.
If a supply is obtained from a generator or from a low voltage supply via a transformer/rectifier unit, the extra-low.
voltage at the output terminals of the supply unit should be maintained between 11 V minimum and 14 V maximum with applied loads varying from 0.5 A minimum up to the maximum rated load of the supply unit. Over the same
load range, alternating voltage ripple should not exceed 1.2 V peak to peak.
It would seem the writers do not know how a stage charger works, or even the old dynamo. The typical stage charger would work in three stages, 1) The max output of the charger, until the voltage reached a set figure, 2) Then hold at that set voltage until the current drops to a set figure 2) hold at a lower voltage either for set time, or until the voltage dips below a set figure to restart the charging sequence. For a lead acid battery flooded the voltage was normally 14.8 volt, and for valve regulated lead acid 14.4 volt, also know as absorbed glass mat. The current for a 300 Ah battery set would be around 3.8 amp, which can cause a problem as if lights take over 3.8 amp then the rate will never drop, so often also a timer so should that happen the volts will still drop.
The better system was to pulse charge, and the decay rate after each pulse is used to work out charge rate, this can mean an even higher charge voltage, in the main used with alternator to battery charging units, which can charge at 120 amp, where the boat has two alternators feeding one battery bank.
This is mainly boats, the problem is a lead acid battery is slow to charge, the batteries used with solar panels and the like can be recharged in 1.5 hours, but the lead acid normally will take 8 hours or more to complete the charge, so the narrow boat people would look for a shore charging point once a month to condition the battery, in other words fully recharge it.
The old Lucas dynamo two bobbin regulator was set to 16 volt open circuit voltage, charging the battery it would never hit that voltage, but I still have the Lucas workshop manuals, showing how to set up the RB106 and RB108 regulators.
