Alternator/battery choice

I'm just starting to convert my old 750 race bike into my next road bike. At the moment it runs total loss from the battery ( it has no alternator). I'm trying to choose which alternator to buy and one of the things I'm reading is that with a high output alternator you need a 14 AH battery.
Well, there's not much free space on this bike and the battery is a 9AH hidden behind the gearbox on a little tray. There's no room there for a bigger battery there, so would I be OK running the 180 watt alternator with that battery and is it sufficient for normal running with a headlight on all the time?

pommie john said:

I'm just starting to convert my old 750 race bike into my next road bike. At the moment it runs total loss from the battery ( it has no alternator). I'm trying to choose which alternator to buy and one of the things I'm reading is that with a high output alternator you need a 14 AH battery.
Well, there's not much free space on this bike and the battery is a 9AH hidden behind the gearbox on a little tray. There's no room there for a bigger battery there, so would I be OK running the 180 watt alternator with that battery and is it sufficient for normal running with a headlight on all the time?

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Lot's of electrical misinformation everywhere... a big battery is not needed if you choose a high output alternator. Using the age old water analogy to explain electricity, the battery is merely the water tank. A high output alternator will not overfill it, the regulator will valve off the inlet pipe, so to speak.


The 45W headlamp, taillamp at 5W, Boyer at 29W, a total usage of 89W, or about half of your alternater output.
(standard 180W) The 9AH battery has ample reserve for a significant period of idling/stop-n-go.


JFWIW, I run the standard single phase Lucas,with zener and rectifier and the (45W) Halogen lamp is on whenever the wheels are turning. 9AH battery, never had to top up the battery. With the headlight on main beam, the voltmeter will read 14.X as soon as 2000 rpm is reached. I have a Bosch headlamp unit, but here is a similar setup: http://www.ebay.com/itm/TRIUMPH-NORTON- ... 3a&vxp=mtr

As for high output alts, you saw the Sparx recall, right? Am I the only one who thinks "Sparx" is a good name for electrical equipment, much like "Sinker" is for a boat? :shock:

I am going through the same loop myself for my cafe racer project.

I will be running a BT-H magneto (so don't actually need a power circuit to run the bike)

However, I will be fitting a high power headlight, indicators and modern electronic speedo and tacho.

I have gone for the three phase high power sparx kit (which is supplied with rotor, stator and matched rectifier)
The output of this is 210 watts - so will be more than enough for anything I want to do now, and into the future.

I am mating this to a tiny Li-Ion battery from Shorai, which I hope to mount up under the hump of the seat.

Just fit what ever battery or even no battery as long as you've an alternator to carry the loads once started. Ms Peel did most of her past life with 210 Sparks/Podtronics and a shirt pocket size 1.2 ah gel cell alarm battery, so lot of storage room left and easier to lift back up off my groundings. A too small wet lead battery could boil off on long runs on low beam or no beam is about the worse you could expect. Too big a battery would just be extra mass and never reach a full top off to preserve its full life expectancy. My 1.2 ah was ~ 1" thick, 2.5" wide and 5" long and maybe a lb or so heavy. Only time it let me down was in rain in big city stop and go tedium, after 30 miles tender throttle on fissioned axle, so if I shut down I could not just kick start the Boyer again. Once rolling free of inner city jungle it charged right back in a few min.