1946 Model 18 battery recommendations

For my last 2 battery replacements I have used Westco ES13-6. These are AGM with a good AmpHour rating and are a snug fit inside the larger of the 2 replica rubber battery cases available from Lucas. Connection is by vertical spade terminals, 2 sizes 6.25mm (+) and 5mm (-). To run the cables under the cover the connections have to be turned 90 degrees. I achieved this by modifying piggyback crimp terminals. Westco batteries are available from Tayna (and others).

... witha 4AH alram type battery on my ES2. I'd probably not use it if I was dependent on coil ignition but with a magneto seems fine to me. I don't ride at night though.....

https://www.toolstation.com/sealed-lead-acid-battery/p94217

It easily fits in a replica case with plenty of padding. I know Al Os doesn't like them but they work for me. Standard dynamo setup with RB107 regulator.

I hide a Cyclon battery away...Cheap and available...I've used more expensive AGMs which are easier to position, but they don't seem to last any longer.

I'm using a pair of 6v AGM batteries wired in parallel in the 16H because  when I fitted them I hadn't decided if I was going to change to 12v.  Another bike has 6v Cyclone cell battery which happily survived a nearly 8 year layup without being charged.

As others have said a Cyclon  battery  is a good bet,,  they hold the charge from one season to the next  and last  very well.  I have one on the Rudge,  its never needed any attention .Something to do with the  unusual construction.

.." Is a battery rated at 4 Ah able to take the charging rate from the standard dynamo?  I have replaced the voltage regulator with a new solid state regulator, if that makes a difference. "

In my experience yes although I have the short 45 watt dynamo rather than the long 60 watt version.

I run a lead-acid Yuasa 6N6 battery,= (intended for a Honda step-thru or similar), inside a replica rubber case.  It's 6V, 6 amp hour, and holds enough reserve to run the lights for an hour or so, should you suffer a dynamo failure...(just a dirty commutator, in my case). The Yuasa works OK, lasts, and is not expensive.

I previously had a pair of Cyclons, with maybe 8 amp hours' capacity.  The intention was that they could be wired in parallel for 6V, or in series for 12V if I ever got round to converting my bike.  During the year and a half between buying the batteries and getting the bike going, the Cyclons got discharged and were never happy after that. So, back to Yuasa.

/4.5 amp hr sealed/non spill lead acid batteries. Coincidently, I am just putting 2 onto my 1949 ES2. I fasten the two together with cut down inner tube and place in a replica T shaped battery box, bought from Armours from memory...pack out for a comfortable fit. The batteries are connected together to give 6v. Keep a ventilation space/tube etc

I have used the same on my 1937 model 50 for 5 or 6 years. Despite this bike not being regularly used,I have only had to charge the batteries once in that time, and that was when I deliberately ran them down to test how long I could keep a Paul Goff LED headlamp going when wired direct to the said batteries. (I seem to recall that I ran a headlight this way on my unlit 1935 Inter, battery in my tank bag) to a NOC rally, probably Holland. The headlamp lasted at least 5 hours, plus.

EAW of Woking sell them, about a tenner per battery. EAW are online and also at Kempton and Ardingly autojumbles. AEW also list the T shaped case.  LED's have come a long way, you get what you pay for. I can and do ride at night with Paul Goff headlight bulbs.