Lights, capacitors, action.

[Groily]

5v at the battery = 'pretty flat'.

Dropping to 4v at the battery at idle suggest that there is some discharge through the regulator at idling speed - if so the ammeter will show that.
It is a common characteristic of many regulators which do not cut out early enough on descending engine rpm.

Ideally, as the ammeter drops to Zero with falling revs, the cut-out should operate crisply, thus avoiding drainage from battery to dynamo as dynamo output voltage drops lower than battery voltage.
An already-flat battery will lose more voltage in front of your eyes if there's any sort of drain on it.

As to 'Why?' . . . If the bike is run 'lights on' in traffic at low rpm for much of the time, the battery may well discharge quite quickly. A small one will be affected much faster than a bigger one.
If the lights come on brightly with revs and the ammeter shows a charge, then the dynamo is supporting loads so is very probably fine. And your 7 - 7.5v is also right for the output from the regulator at revs. 7.2v is the magic number for charging a 6v battery.

Options in mitigation include using LEDs (for the headlamp in particular) to reduce current demand; wattage at the headlight of not more than 35W if a higher-rated one is fitted (original would have been 30/24W); running in daytime with the headlight 'off' (or with an LED running lamp instead); a bigger battery; longer runs away from traffic and / or keeping the revs up!

You' should be quite safe using the meter on the 10A scale to measure charge between the battery feed wire and the battery live terminal as JS said. It's not going to be higher than that, even if the battery is very flat and nothing at all is switched on.

Sounds to me as if the charging system is working OK, but riding conditions and maybe battery capacity are working against it, combined perhaps with a regulator that isn't cutting out as swiftly as we'd like.

[clive]

I have an Alton alternator on my G3 rigid which is rated to produce 120 watts at full speed, however I live in Ealing with 20 mph speed limit on every road in the Borough and quite a few speed cameras. I found that running around on short journeys with idyling in the traffic was flattening the 12 volt AGM battery pretty quickly if I rode with the headlight on. My solution was to fit LED sidelights (I have one in the headlight and two modern torpedo lights) Between them they are probably better in daylight than the headlight bulb. Slight reduction in appearance but the bike is a bitsa anyway. Rear light remains a normal bulb but no problem bumbling around at 20 now. I imagine short journeys in traffic with a dynamo are very unlikely to keep a small capacity battery charged.

[Groily]

with 20 mph speed limit on every road in the Borough and quite a few speed cameras.

Sympathies Clive. It must be sorely irritating.The usual 'one size fits all' - none so deaf as those who will not hear.
It's hard to keep anything to such low speed continuously I find. A lot of our French villages now have similar, with road humps (some viciously 'vertical take off' and front spoiler-scrunching on cars). This makes it noisy and clattery for vans and trailers, tractors and trucks, and forces constant unnecessary gear changes. Often, it's first gear on almost any bike, with associated noisiness getting there. Some undesirables make a lot of racket to underline the point.
Having low limits near schools and playgrounds and so on was entirely reasonable, but now they're almost universal despite often deserted streets, and one is - seriously! - often overtaken by enthusiastic cyclists . . .
.

[The Vandal]

Dropping to 4v at the battery at idle suggest that there is some discharge through the regulator at idling speed - if so the ammeter will show that.
It is a common characteristic of many regulators which do not cut out early enough on descending engine rpm.

Had a quick fiddle today and something I did made the ammeter needle move to discharge but it seems stuck there, battery connected or disconnected, engine running or not, revved or ticking over, it didn't seem to want to move from there.

Makes me think it might not be in the best shape but who knows.

You' should be quite safe using the meter on the 10A scale to measure charge between the battery feed wire and the battery live terminal as JS said. It's not going to be higher than that, even if the battery is very flat and nothing at all is switched on.

Sounds to me as if the charging system is working OK, but riding conditions and maybe battery capacity are working against it, combined perhaps with a regulator that isn't cutting out as swiftly as we'd like.

I made up some wires and tried connecting the meter in line with the positive battery terminal but I couldn't get any readings at all. I'm a bit clueless with this stuff not really sure where I should be plugging things in or what the dial on the meter should be set to though.

I'd probably quite like to get shot of the modern reg and fit something more original, I remember I tried to update the electrics on my Enfield when I owned it and was a total headache, the older stuff probably took a bit more setting up but worked better once it was set up.

[Groily]

On that meter, you need to connect the leads so that the black stays plugged into the socket marked 'common' and the red goes into Amps and Ohms etc (not in the Volts socket), then switch to the 10A scale, and then put probes between the end of the battery live wire and the battery live terminal (in your case negative as your earlier pic showed Positive Earth).

If your ammeter isn't working - it sounds pretty dodgy to me - then you could put the meter across the two terminals on the back of the ammeter and see what that says (same scale, same connections of the red and black leads). You could, also, just bridge the two terminals on the ammeter with a short bit of wire, or simply put both sides on the same terminal post. This will remove any fault there from the equation, so that any tests you do with the meter at the battery end are not going to be compromised by a defective main circuit.
There are also little green / red sensors available these days from people like Paul Goff, which can replace ammeters and provide a good indication of what's happening.

I wouldn't revert to a mechanical regulator myself. More of a fiddle and frankly less reliable than an electronic one unless really carefully set up by 'One Who Knows'. There aren't too many of them about, although there are one or two in some Sections of this Club - if a person happens to live in the right area to take advantage of that expertise and generosity of spirit! The Lucas instructions are good, but you still need to be very precise and very careful at every stage. "That'll Do" rarely does for long! The one you have ought to be OK, even if there is a small discharge at very low rpm due to a poor cut-out function. Many of them are made like that.

I don't think there's a whole lot wrong with your electrickery to be honest, as the key elements of the charging side seem to work. If you could see what was happening at different speeds via a working ammeter or 'charge / no charge' sensor, I think it would be good. It's the not being able to see what's going on that creates the stress!

[The Vandal]

On that meter, you need to connect the leads so that the black stays plugged into the socket marked 'common' and the red goes into Amps and Ohms etc (not in the Volts socket), then switch to the 10A scale, and then put probes between the end of the battery live wire and the battery live terminal (in your case negative as your earlier pic showed Positive Earth).

Excuse my stupidty, the "10A scale" being the bit down the bottom of the dial that says 20m above a line with 10A in white text in a red rectangle underneath it?

I think I know what I've done wrong and why I wasn't getting any reading, I was working on the positive side of the battery. I thought positive and live were the same thing. lol.

If your ammeter isn't working - it sounds pretty dodgy to me - then you could put the meter across the two terminals on the back of the ammeter and see what that says (same scale, same connections of the red and black leads). You could, also, just bridge the two terminals on the ammeter with a short bit of wire, or simply put both sides on the same terminal post. This will remove any fault there from the equation, so that any tests you do with the meter at the battery end are not going to be compromised by a defective main circuit.
There are also little green / red sensors available these days from people like Paul Goff, which can replace ammeters and provide a good indication of what's happening.

A new ammeter isn't too expensive on the spares section so maybe best that I just grab one of them. I did notice previously though that orange and brown from my reg join together to become a purple and black wire and instead of goiung straight to the ammeter that goes to the headlight switch and from there there is a piuggyback over to the ammeter so maybe I'll try rerouting that...hmmm wiring diagram I've found in the manual seems to suggest this configuration is correct though.

I wouldn't revert to a mechanical regulator myself. More of a fiddle and frankly less reliable than an electronic one unless really carefully set up by 'One Who Knows'. There aren't too many of them about, although there are one or two in some Sections of this Club - if a person happens to live in the right area to take advantage of that expertise and generosity of spirit! The Lucas instructions are good, but you still need to be very precise and very careful at every stage. "That'll Do" rarely does for long! The one you have ought to be OK, even if there is a small discharge at very low rpm due to a poor cut-out function. Many of them are made like that.

I don't think there's a whole lot wrong with your electrickery to be honest, as the key elements of the charging side seem to work. If you could see what was happening at different speeds via a working ammeter or 'charge / no charge' sensor, I think it would be good. It's the not being able to see what's going on that creates the stress!

Fair enough, will persevere with the Dyno Tec one, new ammeter certainly seems like a good idea.

[Groily]

oh, and maybe check the leaflet that came with the meter to be certain on scales - but yes, the 10A one, with the leads connected for Amps. Not sure about the 20m thing.
Positive is only Live if you're Negative Earth. You aren't, I think.
Live wire from Regulator goes to ammeter and switch, or switch and ammeter, in whichever order. Same thing as long as it's to the right terminals. Green and Yellow come from dynamo. Dunno about your colours upstream, but in many cases for 6v operation there are 2 wires to double up from the Regulator and connect to SW and Ammeter, but only one - with one suppressed - if going 12v.

[jackstringer]

The 20m is for the 20mA range. Depending on the connections you use at the front will determine the if it's for 20mA or 10A. If you are not getting a reading there is a chance the fuse if blown on your meter...says a man sat here with 4 multimeters with blown fuses and they are used by people with Phds I'm electical pixies.

I have a similar issue with my G3, where the ammeter isnt showing much in the way of a reading. I plan to have a tinker at some point as I suspect the new regulator isnt working the same way the old ones does and I am just not seeing the current.

[clive]

Generally i would expect with a charged battery and a good electronic regulator that the ammeter would only show minimal if any charge even at at speed. This is largely true even when i am running with LED sidelights. The way i check all is working satisfactorily is to turn on the headlight when on tickover. This should then show discharge which goes back to slight charge when you get up to speed. If the ammeter does not go to discharge when the headlight goes on its probably faulty. If the ammeter works but does not clear the discharge with headlight on and a bit of speed suspect the dynamo or regulator. Test the dynamo by disconnecting the wires putting a bare wire between two points that the wires connect to. From this bare wire take a wire to the connections on a bulb and earth the rest of the bulb. Rev the bike slightly and the bulb should light showing the dynamo can deal with load. ( its worth using an old bulb or a 12v one as the unregulated dynamo can supply quite a voltage and burn out the bulb. Dont rev too much.)
If the bulb lights ok then suspect the regulator. Saves all the faff with multimeters that have blown their fuses.

[The Vandal]

oh, and maybe check the leaflet that came with the meter..

You under-estimate the extent of my stupidity.