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LED Headlamp Bulb - Law Change

Blog Date 22 March 2021

From the UK Government's own website 

https://www.gov.uk/government/publications/special-notice-01-21-headlamp-conversions/mot-special-notice-01-21-headlamp-conversions

In short - Up to 22 March 2021 it was an MOT failure to fit an LED or HID bulb to an existing headlamp unit that was not LED or HID. From 22 March 2021 this will not apply to motorcycles - AS LONG AS - the replacement LED or HID bulb still creates the correct illumination pattern and doesn't dazzle other road users. Also note this only applies to motorcycles and not cars, trucks etc.

It's a curious one this. 

Your motorcycle was not allowed to go on sale until that model had been homologated (inspected and approved). Part of this homologation process is to ensure the headlight works as it should and produces a beam pattern acceptable for use on UK roads. The primary concern is that on dip beam a correctly adjusted headlight should not dazzle oncoming traffic. 

If your bike was fitted with a standard halogen bulb when sold, it would have been homologated with a standard halogen bulb. I used to labour under the notion that I could replace that bulb with any other bulb, including non halogen bulbs like LED and HID. No. Because that setup, that configuration has not been tested and approved and could possibly lead to your vehicle being a menace to other road users.

BUT BUT BUT!! LEDs in particular suit motorcycles. They are more visible, particularly during daylight when incandescent halogen's "yellowish" colour can merge into the sunlight. They also use less power and with many motorcycles having feeble alternators low power consumption LEDs can reduce the electrical stress.

The headlight on the bike has a yellowish hue to it
Regular bulbs aren't really bright white.

In a rare sensible move the rules have been changed it seems. You can now fit an LED or HID bulb to your motorcycle AS LONG AS IT DOESN'T DAZZLE!!! This can be checked by the MOT centre using the beam tester that's been in use for eons. 

As riders we have all been not just dazzled but temporarily blinded by ill-adjusted and/or illegally upgraded headlights. Before you rush out and fit a 10 billion candle bulb capable of illuminating most of The Norfolk Broads - think about other road users. Just because you're hacked off with eyeball melting HIDs in your rearview mirror doesn't mean you have to be a plonker too.

Other than the risk of, shall we say, less scrupulous riders dazzling us I think this is good news. If Sharon's crackling on her headlight is due to heat from the incandescent headlight then a suitably sized, shaped and configured LED could be the cure. 125's can and do struggle to power even the 35watt bulbs, LEDs could enhance the smaller bike's visibility and reduce the load on both the alternator and the regulator/rectifier. 

With LEDs the beam pattern will be altered by the shape and design of the bulb. Take a moment to look at a regular bulb, notice the dip filament is atop a tiny dished plate which directs the light only one way. Note how far the dip filament is from the mount. Note the blackened tip. These things matter as the direction of the light and position of the filament changes how the light is bounced off the shaped reflector and thus the beam pattern. Not any old LED bulb will do then. The bulb needs to emulate these positions and directions to ensure it works with a reflector correctly. I'll need to tread carefully when buying a bulb to ensure a good beam pattern. 

A H4 bulb with the dip filament circled
The shape, dimensions and the tiny dished plate are all critical.


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Reader's Comments

Upt'North ¹ said :-
These are supposed to be good Ed, if you can find any.
Not a LED'er myself with two H4's burning bright it seems well enough illuminated anyway.
I do use Osram Ultra Life's which do what the name suggests.
Upt'North.

https://www.amazon.com/KATANA-H4-9003-Headlight-Bulbs/dp/B07JM421VC/ref=sr_1_44?...
22/03/2021 12:46:55 UTC
Borsuk said :-
I fitted one to the Honley, made a vast improvement but then again so would Wee Willie Winkies candle lamp. 35 watt bayonet fitting lamps do not do illuminate much. I have seen an LED on a French Himalayan owners blog. He was quite impressed with the improvement and all you had to do was rotate the lamp to adjust the beam pattern. He did have to make adjustments to the securing bolts of the headlamp unit but it was just a matter of reducing the length, they are about twice the length they need to be.
22/03/2021 13:26:49 UTC
Snod said :-
Obligatory link to an article that explains why these will never be any good in most reflector headlights..

Also, aren't bright LEDs as watt hungry as normal incandescent bulbs? Or have they got better?
https://www.danielsternlighting.com/tech/bulbs/Hid/conversions/conversions.html...
22/03/2021 14:21:59 UTC
Bob said :-
I put an HID conversion in a Fazer 600, it did give a huge improvement in light output, this was one of the type where there is single light producing element and a solenoid to physically move it in and out for high and dip beam.
It worked well but then I became paranoid about it failing - with a traditional filament bulb if high beam blows you can quickly flip to dip or vice versa - that wouldn't have been possible with the HID kit I had. So I took it out again.
These dual element LED types look more likely to me....
22/03/2021 14:37:14 UTC
Upt'North ¹ said :-
Snod,
Thanks for posting. Interesting read.
I wonder if the law change will lead to better LED alternatives.
Although the UK market must be fairly small.
Upt'North.

23/03/2021 08:57:18 UTC
Ren - The Ed¹ said :-
I think I'd have to experiment... but... at around £60 for a pair of LEDs (pair because marketed at cars) it could be an expensive experiment! As Snod's link points out though it is a complicated subject. The "proper" solution is to fot a light designed for LED bulbs. I'd be happy on my scruffy 125 to rip, cut, hack and bodge and new unit in place but I doubt Sharon would, and the sight of my own blood makes me queasy.
24/03/2021 09:05:22 UTC
CrazyFrog said :-
I normally replace the standard H4 50/55W build with an Osram Night Breaker 50/55W halogen. These are touted as being twice as bright as a normal halogen, and whilst I've no scientific way of confirming this, they do make a big improvement and are, of course, fully MOT legal.
24/03/2021 18:11:33 UTC
Bogger said :-
My current bike has, Ahem, an LED headlamp fitted as standard don't cha know.

Bogger
24/03/2021 20:44:54 UTC
Upt'North ¹ said :-
Bogger,
Is it just fitted with an LED bulb at the factory in the LED inspired surround or are these things sealed units. I'm guessing the former in which case are you now able to buy replacement bulbs from the manufacturers?
Upt'North.
25/03/2021 08:14:20 UTC
Ren - The Ed¹ said :-
The headlight on the CB500X is LED. It's a unit, a non-serviceable, non bulb-changeable, if it breaks you're-screwedable unit. Most LEDs claim a service life of 50,000 hours. Taking an average speed of say 20mph that would be 1,000,000 miles. Yeah. Right. I'll believe that when I see it. If the unit loses an LED I suspect mine will be getting some aftermarket unit grafted in. The aftermarket unit can't be any worse than the OEM one on main beam.
25/03/2021 08:25:10 UTC
Upt'North ¹ said :-
Interesting Ed, so we've gone back to the 70's and fitted sealed beam units, so full circle as normal.
Do you know how much these things cost?
I'm guessing, again, there will be businesses cropping up just about everywhere to take these things apart and refurb accordingly. It's happened with ABS, Dash Modules, ECU's etc.
Upt'North.
25/03/2021 08:43:35 UTC
Upt'North ¹ said :-
On a further note Ed me mucka.
It'd be a hoot if it failed on an overseas jaunt.
Upt'North.
25/03/2021 08:44:46 UTC
Upt'North ¹ said :-
Actually just read your post, I know, I know.
So it's crappious giganticus anyway...
Upt'North.
25/03/2021 08:46:14 UTC
Ren - The Ed¹ said :-
The unit has several "bulbs" (it seems wrong to call LED bulbs but habit) so if one failed the trip could still continue I suppose. But if the whole unit failed then yes I'd be up the creek without a paddle. As like as not I'd bodge on a rechargeable torch and stick to daytime riding.

The dip beam on the 500 is ok, ish. The main beam is odd. You *CAN* see there is more light coming out of the unit and with a wider and higher spread. BUT, weirdly, it doesn't seem to ILLUMINATE the road ahead much more than dip. It's as though we have more light but the extra light is ineffectual.
25/03/2021 09:49:09 UTC
Upt'North ¹ said :-
That does surprise me Ed, you'd have thought with a clean piece of paper they could do better. Surely it would be just as easy to make an effective light instead of an ineffective one. I would think if there are multiple bulbs (?) then one being out may not be a MOT fail. I know the led rear lights can have units out and still pass.
Upt'North.
25/03/2021 16:25:51 UTC
Bogger said :-
I serviced the Super Cub today ( a rare day off work ) and had a look at the LED headlamp. Mine also has multiple LED's and not a single unit. It'll cost a fortune to replace, if/when it stops working.

Bogger
25/03/2021 21:14:24 UTC
Upt'North ¹ said :-
I'm suddenly very content with my two H4's.
£8 every two or three years and a little plastic removal sounds surprisingly appealing.
Upt'North.
25/03/2021 23:23:48 UTC
Ren - The Ed¹ said :-
The multiple bulbs in the headlight are not like the tail lights Upt'. In mine there's 4 or 5 powerful units and if one went out I would question if it would affect the beam pattern. A taillight is there merely to be seen and with say 15 little red LEDs then one or two being out ain't an issue.

It boils down to how long do the LEDs *REALLY* last? While your 2 easy to replace H4s are cheap if you were to break your headlight unit while wrestling the bulbs in and out, that would be several hundred quid from Honda. There's pros and cons in both systems.

The proper solution is to have a simple bracket that takes an off-the-shelf unit that can be replaced quick, easy and cheap. £27 off Fleabay...
https://www.ebay.co.uk/itm/353195067929...
26/03/2021 09:20:53 UTC
Upt'North ¹ said :-
I have no experience of LED's in an automotive setting but in domestic applications there life seems very varied with factors of type of bulb(?) and heat being possible reasons for early failure.
I like the idea of a cheap throw away unit but life ain't usually that simple. And what about the planet.
Upt'North.
26/03/2021 09:43:50 UTC
Upt'North ¹ said :-
Plus they're all stuck in the Suez Canal.
Upt'North.
26/03/2021 09:44:22 UTC
Upt'North ¹ said :-
Did ya know that,
unlike halogen or HID bulbs that will simply stop working or burn up, LED headlights react or degrade in a certain way. As the diodes or the circuitry burn up, the maximum brightness of the LED bulb will tend to get lower. They will still light up, but the light output will be dimmer than usual.
So they may last forever (maybe) but you won't be able to be seen or indeed see.
Every day's a school day.
Upt'North.


26/03/2021 09:51:04 UTC
Ren - The Ed¹ said :-
I did not know that LEDs lost their mojo over time, I guess every day is indeed a school day. Man I hated school though.

I've been asked to pop ever to Suez with the 125 as it's the only vehicle with enough power to pull the wedged dinghy out and yet light enough to run along the river bank. I suppose I'll have to remove the top box again to give them access to the luggage rack as a tying point. Last time I did this I stretched the 125's chain, but they refused to pay for that as they said "it was already knackered". Cheeky swines. I'll just add it on to my hourly rate this time - pffffft.

I wonder though, if I fit an LED that uses say 20 watts as opposed to the 35 watt incandescent it currently has, I'll keep the extra 15 watts? Might make all the difference at the top end.
26/03/2021 13:55:29 UTC
nab301 said :-
Checking online it looks like the LED 500X headlight is over £400 stg, the super cub LED headlight is over £200 and while I was at it the LED rear light unit on my 250 V Strom is near £200... The standard 500X filament headlight is about £200 but the OE H4 headlight bulb is listed @ £40 odd !!.
Nigel
26/03/2021 16:08:05 UTC
Upt'North ¹ said :-
If we've already discussed this then please ignore,
Why do OEM bulbs seem to last forever and replacements seem to have a much shorter life.
I don't buy cheapo bulbs so why do they fail quicker than OEM?
It could all be in the mind of course.
Upt'North.

26/03/2021 17:03:41 UTC
Jim said :-
Don’t know the answer, Upt’, but my experience is the same. Once you replace the bulbs that came with the car, it becomes a more regular chore.
26/03/2021 19:33:13 UTC
Ren - The Ed¹ said :-
I wonder if something degrades to the point where it takes out the OEM bulb and remains degraded thus shortening the life of the replacement bulb.

Electrical connectors corroding? Reggy-reccy aging? Rubber light mounts going hard? Heck even suspension linkages wearing will make the ride harsher on the bulb.
27/03/2021 06:14:09 UTC
Bogger said :-
Mr Ed bloke said "I wonder though, if I fit an LED that uses say 20 watts as opposed to the 35 watt incandescent it currently has, I'll keep the extra 15 watts? Might make all the difference at the top end".

The only thing that will be incandescent, is you, with rage. As you will have spent money ( shock, horror ) for no speed benefit what so ever. Personally, I think you should give it a try.

Bogger
27/03/2021 09:44:39 UTC
Bogger said :-
With regard to bulbs not lasting. Why would the manufacturer want them to last?? They most certainly want to sell you another one. So they make them so they don't last. There's a profit to be had.

Bogger
27/03/2021 09:47:17 UTC
Ren - The Ed¹ said :-
Bogger - the power limit on a learner legal 125 is 11kw, 11,000 watts. If I save 15 watts this could provide an extra 0.14% of raw arm wrenching mind bending thrilling performance.

I've known plenty of motorcyclists spend a small fortune to gain less than 1%.
27/03/2021 14:02:46 UTC
Ian Soady¹ said :-
This (see link) is an interesting article on light bulbs and planned obsolescence....
https://spectrum.ieee.org/tech-history/dawn-of-electronics/the-great-lightbulb-c...
27/03/2021 16:19:46 UTC
Upt'North ¹ said :-
Interesting indeed Ian, and reiterates Bogger's point nicely.
However it doesn't tell us why OEM bulbs last longer, if indeed they do.
I'm with you Ed, Reg/Rect/Corroded connections etc. So it's not the bulbs it's sloppy maintenance.
I have noticed that some automotive shopping emporiums sell specific motorcycle bulbs, never bought any as they are very expensive, but would they last longer?
Upt'North or Mr Sloppy.
27/03/2021 16:39:46 UTC
nab301 said :-
Nice idea Ren on power output but correct me if I'm wrong , while most cars use field coil regulation in charging circuits most current motorcycles ( with the exception of most BMW's and I'm sure some others) just shunt excess current/ voltage to earth, so my understanding is that the alternator always works at maximum capacity meaning no power gains if you reduce electrical circuit loads ? I'll sponsor you a dyno session with the CBF125 if you want to run it with and without the alternator connected!
Going slightly off topic a friend of mine was a budget club (car) motorsport participant back in the 80's . I'm not sure why, but he was offered a drive for one meeting in a current 1.3 Vauxhall Astra ( compared to his own ageing self prepared 1.3 Fiat 128 3p) He qualified mid field and was looking forward to the race , just before heading to the grid the bonnet was opened and some work was carried out , he wasn't told what . His lap times in the race were apparently consistently quicker than qualifying. He discovered later that the alternator belt had been removed reducing the electrical load on the engine... ( obviously there were some mechanical losses too driving the alternator and before anyone asks, the alternator belt drives just that on those engines and the battery was fine for a short 15min race.)
Nigel
27/03/2021 20:49:58 UTC
Bogger said :-
OEM light bulbs last longer to get them through the warranty period. Manufacturers do not want the associated labour costs of bulb replacement. Labour costs would far outweigh the cost of a bulb. However, the bulb and the labour costs for replacement 'under warranty' would be passed back to the bulb manufacturer. That's why the bulb manufacturers supply a 'better' OEM bulb. The cost to the vehicle manufacturer in this process would be the administering of the warranty claim. It's all about covering your arses to make more profit.

Bogger
27/03/2021 22:10:11 UTC
Bob said :-
Correct about the permanent magnet alternator, except for a lot of Yamahas with the stacked alternator, most bikes have this system.
At the end of the day 15W = 0.02HP, you'd be doing well to feel that.....
28/03/2021 09:31:05 UTC
Ren - The Ed¹ said :-
My knowledge of motorcycle alternators *COULD* (read - likely) be wrong. I need to find out whether or not the "spare" electricity from a motorcycle alternator is in the form of current or voltage. I thought if the drain of the lights, ECU, FI etc was for example 5 amps at 12v the alternator would provide 5 amps as soon as it could (maybe 1500rpm) then the voltage would rise. The reggy reccy would then trim off the excess voltage as the revs rise.

So at 1500rpm for example - 5 amps at 12v, at 5000rpm it would be still 5 amps but at 32 volts and the reggy reccy has to trim off 20 volts at 5 amps leaving 12 volts at 5 amps. If the power drain rose to say 7 amps then the same would apply having to trim off 20 volts at 7 amps instead. Thus the more amps used by the motorcycle the more amps and voltage that needs to be trimmed. Thus the more load the more heat has to be dumped by the reggy reccy.

I fear this simply hightlights my lack of knowledge and I am braced to be put right on this.
29/03/2021 14:11:46 UTC
Upt'North ¹ said :-
Ed, you've made my head hurt, you naughty boy.
This is what I understood but like you I am no electrical genius, infact you could cut that statement short.

The flow of direct current from the alternator windings is controlled by a regulator unit. This then stops excess current from trying to destroy the vehicle’s battery – which if it wasn't regulated would go caboom.

The more I study the less I realise I know. And I've got C and G's, South Cheshire College don't ya know. Perhaps I should go back, naaaahhhh, it'll be a Tesco.
Upt'North.
30/03/2021 09:57:12 UTC
Ren - The Ed¹ said :-
"The flow of direct current from the alternator windings is controlled by a regulator unit." - Eerrrr not quite It's an alternator because it produces an alternating current. The reggy reccy "rectifier" part converts the alternating current into a not-quite-perfect-but-good-enough DC (direct current) for the battery. The "regulator" part of the reggy reccy limits the voltage to about 14 volts (a bit more than 12v to "push" the electrons into the 12v battery).

If the wires from the alternator were connected to nothing at all, at the bare ends of the unconnected wires would be voltage but no current as no angry pixies are moving. That potential difference aka voltage would rise the faster the alternator spins (as would the cycle rate ie the rate as which the potential changes polarity).

But I'm still not 100% wise as to what happens when a load is put in the alternator...
30/03/2021 12:32:21 UTC
Borsuk said :-
Most of the lights in my house are LED.

When I first moved in the previous owners had 50 watt halogens every where. 30 of them in total and 12 of those in the kitchen if I remember right. I replaced them all with 3 watt led,s for about the same lighting. More expensive but but a lot less power used. I have never got the claimed x thousand hours out of them but they were cheapo Chinese and the reduction in heat in the kitchen in Summer was worth it. We have led flood lights at work which have been switched on almost continuously for 6 years now so they seem to be lasting.

And before Ren the Ed to the power of minus 100 makes any comments. I am not and have not been anywhere near the Seuz canal in years never mind this week.


01/04/2021 00:24:25 UTC
Upt'North ¹ said :-
Borsuk, you know we were thinkin it, don't ya.
I expected to see a RE strapped to the deck. Oh the shame of it.
The longevity of LED's as far as I am aware is fairly proportional to the dissipation of heat. Hence the fans and heat sinks that these things have in automotive fitments. Like you my home is full of the things and overall they do a good job but it can be difficult to find the correct warmth and power required. The only bulbs which fail way to quickly are the G9 replacements which live inside sealed enclosures and increased heat as a result. The many H4 replacements are going strong after 7 years more or less continual use.
Are you sure it wasn't you?
Upt'North.
01/04/2021 09:18:19 UTC
Ren - The Ed¹ said :-
I heard a recording from when the Ever Given grounded itself. A voice with a thick Glaswegian accent can be clearly heard shouting "Och nooo, ####! @@@@!! I meant the OTHER left..."
01/04/2021 10:08:35 UTC
Bob said :-
I've just fitted these:
https://www.amazon.co.uk/gp/product/B08DRF7TY3/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1
To my two KLX250 regular riders.

What a transformation! It's a life changing technology.
No longer do oncoming cars fail to dip their lights, either because the driver doesn't notice me or increasingly these days because the automatic dimming system doesn't see my 55W halogen headlight bulb.
Now I can give any non-dimmer a blast of the headlight of death and they soon dip theirs!
More importantly it's so bright that now my eyes don't need to adjust to the dark before I can see where I'm going, which means that if I do get dazzled by an on-comer I can still see where I'm going.
The beam pattern is very good, with a sharply defined dip cut off and a good letterbox high beam throw.
The bulbs fitted straight in, with no modification.
I measured the current, my old "150% brighter halogen" bulb drew 4.1A, this LED bulb draws just 1.5A and it appears to me at least 3 times as bright. That's very good news for any small bike owner, since small bike = small alternator.
That 2.6A refund will run my electric jacket next winter! On the KLX the headlight is on when the ignition is on, in the past when servicing I've been pulling the lighting fuse so that I don't run down the battery - now I don't need to . Also my heated grips wouldn't work unless the engine was running, because the standard headlight bulb pulled so much current that the battery voltage was dragged down so low that the grips went into power saving mode, now they don't - a further example of the reduced load from the new bulb.
Importantly the high and the dip circuits are completely separate COB devices, I do have a little worry about the reliability of these aftermarket solutions and for that reason I never fitted an HID to a bike because there is no backup circuit if the emitting element fails. In this LED bulb if the high beam COB fails, the low beam is still available to switch to (in a panic) and vice versa.

MOT time tomorrow - I'll find out if that law change works....
16/09/2021 13:05:12 UTC
Bob said :-
And a little on alternators.
The permanent magnet alternators on most bikes produces a rising potential difference (voltage in layman's terms) between each of the three phase outputs (normally yellow wires) and ground.
Each of the yellow wires is connected to two diodes which are arranged so that whether the voltage is positive or negative current can flow into the system, this is called full wave rectification.

The next stage is the regulation of the voltage.
What you have after the full wave rectification stage is a mess of interleaved half sinusoidal waves, positive with respect to ground but of varying voltage and frequency. The higher the RPM the higher both the voltage and frequency are.
This mess of waves is caused by the multiple phases of the windings, typically 3, so it can't be connected directly to the battery or any other components.
The job of the regulation stage is to look at the voltage in the electrical system and then to switch through varying amounts of these half sinusoids to maintain the voltage at the set point (typically 14 to 14.5 V). The regulator usually has a sense wire which is black, the ground is often green and the output is red. The black sense wire is usually connected near to, but not directly to the battery and this allows the regulator to see an accurate measure of what the voltage in the system is.
As the electrical load on the system varies, so does the proportion of the sinusoidal waves that get switched through - more load means more wave is needed to maintain the loom voltage.
You may have heard the term "switch mode supply", your mobile phone charger is one of those, well the modern motorcycle reg/rec works in a similar way. It's done this way because it is the most efficient. Passing current through semiconductors causes heat loss, these modern switching supplies dissipate the minimum amount of power because they only pass the current they need to pass. If there is no load they pass no current.
*** THEY DO NOT SHUNT UNWANTED CURRENT TO GROUND ***
If the semiconductor is turned off then no current flows, that's it, the voltage on the alternator coil will increase due to back EMF but it's designed for that (and yes there will be a slight amount of heat generated due to parasitic losses in the coil).

In the old days the reg/rec was a diode bridge and then often a linear power regulator, some times in the form of a zener diode. These old devices dumped unwanted power in the form of heat, they were crap. I think that many people are confusing the old system with the new.

The game has moved on, years ago there simply weren't the fast switching high current low loss MOSFET devices available to do this job - now there are, so that's how we do it today.

16/09/2021 19:36:25 UTC
Bob said :-
MOT passed, with the LED headlamp bulb in place.
The tester commented on how good the beam pattern was.
Result!
17/09/2021 18:01:58 UTC
Ren - The Ed¹ said :-
Great news Bob! And thanks for the lowdown on reggy reccys. So... bike has no lights on and only a minor current draw for the ecu say. The MOSFETS see the voltage at 14v drop to 13.8v so they add just a smidgen more of the many sine waves to the charge until 14v is returned. The rest of the wave isn't "burnt off" but merely disconnected from the charge so the alternator coils simply feels less load, less drag, less turning resistance.

Then I put my lights on, heated grips, phone on to charge and my 10 billion candle power spot lights. The MOSFETS see the charge drop from 14v to say 10v and jump into action. The allow more of the sine waves through to the charge until 14v is restored. This will cause more drag, more turning resistance in the alternator and that is all.

Am I starting to get a grip of this?
18/09/2021 19:11:53 UTC
Bob said :-
Yes, that's exactly what happens.
The battery is important as it smooths out the spikey high frequency bits of sine wave and helps maintain a steady DC voltage level.
Depending on the alternator, reg/rec and battery there may still be some fluctuations (referred to as "ripple") on the battery side - you can check if this is the case by putting your multimeter to AC and testing across the battery when the engine is running. If the voltage is pure DC the meter will show 0V AC, but often there is some AC there.
If you have an oscilloscope that will show what's going on through the charging system - it's interesting.

20/09/2021 13:50:35 UTC
Bob said :-
..Except to say that the MOSFETS are just switching devices, there is a controller circuit which monitors the voltage and switches the MOSFETS accordingly.
20/09/2021 13:54:06 UTC
Ren - The Ed¹ said :-
So there's an extremely basic volt meter inside the reggy reccy that's watching the voltage. When low, switch the MOSFETS on for a bit longer, when high switch them on for less time. I know that's over-simplified but I only have a simple brain.

I am glad you've explained this. I incorrectly thought the purpose of the heat sink fins on a reggy reccy was to dump the excess power. They are merely there to cool the very fast switching MOSFETS and maybe a little of the basic volt meter.
20/09/2021 20:22:34 UTC
Bob said :-
MOSFETS revolutionised electronics when they came along.
Before that we had "traditional" transistors, NPN and PNP are types you may have heard of.
The problem with those traditional transistors is that when current is flowing there is a forward volt drop created by the fact that they are basically made from two diodes connected nose to nose. So if we're passing say, 10A of current and the forward voltage drop is 0.7V (a typical value) then we will dissipate 10*0.7 = 7W of power.
Doing the same job with a MOSFET there is no forward voltage drop, but there is a resistance to current flow which is called RDSon. A typical RDSon value might be 0.016 Ohm, so for the same 10A the power dissipation would be 10*10*0.016 = 1.6W.
So the MOSFET dissipates just 23% of the power that the traditional transistor would dissipate. Also that forward voltage drop of 0.7V means that everything is 0.7V lower than it otherwise would be, so if your target voltage is 14.5V you need to be at 15.2V on the upstream side of the transistor.
21/09/2021 11:00:00 UTC

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