AGM is what I use, Li-Ion needs a different voltage regulation which AGM does not and you get the advantage of no leaks and so far long life.  Li-Ion are at risk of spontaneous ignition.

My 1959 99 is fitted with a Varta sealed 12v 6AH 50(a) YTX7L-BS. Its been on the bike at least 4 years and there have been no problems. I think this maybe broadly equivalent to what it was fitted originally as don't want to complicate things with more modern items given my lack of knowledge in case I cause problems and it seems to have worked. The bike has stood inevitable given today's pandemic without use for about 9 months yet only a slight drop in charge. I think it was 12.5. Topped it up last week on charger as it happens and it is holding 12.85 volts. Not sure of the cost of the batteries you propose but replaced my Royal Enfield battery although magneto model with the same battery for about £30 earlier this year as this was about 7 years old given this experience. Trust this helps. Regards, Hugh   

Sorry Lionel, but why pay more for a big battery and lug the extra weight around? 

It isn't as if there is a huge electrical load on a 99 and it is a kickstart only bike so to my mind a big battery isn't required either, unless you want to park it with the side lights on all night.

Provided that the wiring and alternator are in good order, then the alternator will more than balance the loads put on it. A 7ah battery should be perfectly fine on the bike. 

Regards, George. 

(Ps: I have a 7ah on my 650 and have no issues due to battery size)

Why pay more for a big battery? I'm a pessimist; if the charging system goes down you've got more range to get somewhere convenient for repairs. (On the other hand, I'm a real pessimist; you also have the ability to travel further away from sources of help … )

If you run a  basic charge control system a bigger battery   is able to  cope with   the  high's and lows  a bit better.   Mine has enough guts to power a compressor  to pump tyres and  toast me  with a short blast of a 70 watt  jacket.   Once warmed up  25 watts   is enough. The rest gets used on electronic ignition,indicators, sat nav, chain oiler and heated grips. I usually run daytime lights . Don't think the zener gets a look in,but the  rectifier  has a big heat sink and i carry a spare.

An equivalent to the 2MC capacitor you can remove the fuse after the battery fails and continue home. 

Hi John,  You are going to have to expand on that ,  I'm left behind !   I think the 2MC is still availiable , I still use one and yes,my bike starts and runs without a battery , A couple of miles up the road I realized  it was still on the bench !!. The ampmeter went beserk.

Thinking about charging issues,  My rectifier is probably working close to its max, is there a HD version or can 2 units be used in some way?.  Alternatively I could adopt the easy rider position and warm my boots on it.

Robert, any BSA wiring diagram for 1970 has the 2MC capacitor as an option, its also in the Commando wiring diagrams at least for 72 to 74. Do not fit a NOS 2MC as they are destroyed by age, modern capacitors do not. If your battery fails by going high resistance you have to pull the fuse for the capacitor to work, if you do not then the battery sucks all the electrons for itself.

Starting in 1968 Triumph and BSA sold only “road” bikes with the battery/coil ignition. If you wanted to ride off-road or race you were forced to buy a road legal motorcycle, remove the lights and otherwise prepare it for competition.
Part of that preparation included removal of the battery and battery tray. The battery, which was
required for proper functioning of the battery/coil ignition, was replaced with the Lucas version of
the ”battery eliminator”, otherwise known as the 2MC capacitor. The 2MC was a large electrolytic
capacitor that connected to the harness in the same manner as the battery, but weighed about
1/100th of the battery.
Starting in 1968, Norton P11’s also came from the factory with a battery and a 2MC capacitor
fitted. Again, the P11 was a competition machine and most P11’s ended up with the battery
removed. Then, starting about 1970, all Commandos came fitted with a battery and the 2MC. By
installing both, Norton was able to hold the system voltage at high levels even though the battery
may have started to age and slip below 12V.

I also have a couple of reasonably modern reg/recs  that I intended to fit to my 99 and as a replacement for one on the Ducati thats fried its alternator, however  i'm hearing that some replacement alternators are failing after a short life and  there are newer designs of  regulator that are better  . So I'm not fixing  whats not broken,and waiting for  clarification.

I bought an AGM battery for the car by mistake ,the main dealers said it would invalidate the car warranty as the charge system was not compatible . Don't think I fully believed them.

Lionel can you advise what the original spec  for the battery to be fitted to a 1959 99 if 6ah is too puny was as have been looking but can't find anything. Thanks. Hugh

C

I’ve had no problems with AGM batteries and would recommend them over conventional sealed lead acid. The main advantage is they seem to last longer with needing a recharge. I use a standard Yuasa 12v battery charger. I have Motobatt AGMs on all my bikes including the Commando and Dommie.

Hi Alan, I did melt one, the replacement has a decent heat sink .