Early in the riding year, my Sprint stopped charging. There are essentially three (and a bit I will mention later) parts to this. The battery, the regulator/rectifier (RR) and the 'alternator' which comprised a ring of magnets in the form of a shallow cylinder (the rotor) mounted on the left hand end of the crankshaft, spinning inside a ring of interconnected coils (the stator) attached to the left hand crankcase cover. The rotor/stator produces a three phase output of about 22V per phase, rectified and regulated to something between 13.5V and 14.5V (by the RR) and sent to the battery.

Investigation showed that one of the 3 phases wasn't producing anything. Removing the stator showed at least one coil nastily blacked and clearly burned out. I'd bought a new battery last year - not a top brand but adequate I assumed. It seems rotor burn-out is not uncommon on many bikes and certainly not that uncommon on my Triumph Sprint ST. So, not knowing what the cause was, I decided to replace the RR with an 'uprated' MOSFET type RR at the same time as the rotor. A quite pleasant afternoon's work and all seemed fine with the system charging well. But only about 150 miles later the bloody thing stopped charging again. This time leaving me a few miles from home with all the electrics (including speedo and tacho) dying and finally the engine failing. Got home by using a jump-start pack - which are brilliant devices!

The stator had failed again. Resisting the desire to set fire to the two wheeled monster, I decided to have one last go at a fix. As mentioned, there is only the stator, RR and battery in the whole circuit - and, possibly the wiring loom connection from RR to battery. So, I decide to replace the battery, RR and stator AND directly connect the RR to the battery, cutting out the loom. But, not wanting to just end up with the same issue again (and again, and again...) I thought I'd also investigate RR alternatives

The standard type of regulator used in motorcycle charging circuits is a so-called 'shunt' regulator. This effectively provides a path from the source (in this case the rectified current from the stator) to ground via a variable resistance, in effect diverting what is needed to the load (battery). So, from the load, current is effectively 'shunted' away to ground. Cheap and easily designed for high loads, it does, in effect, mean that the excess current just flows away to earth keeping the source current high all the time. The output DC voltage remains essentially constant.

A series type regulator, also known as a series pass regulator, works by only taking the current it actually needs for the load from the source. The output DC voltage is inherently more variable.

So, a series type regulator should, on average, take far less current from the stator and therefore keep it cooler and less liable to burn out.

However, it is not easy to find a series rectifier/regulator in the UK. The manufacturer with the best reputation seems to be ShinDengen, with its SH775 or updated SH847. However, the only reliable suppliers of these seem to be based in the US - with long postage times. There are loads of suspiciously cheap SH478 'replacements' available on eBay BUT caveat emptor, apparently these cheap knock-offs are actually shunt type RRs that have the same form factor and will obviously do the same job (rectify and regulate) as the genuine article but obviously don't work in the same way and keep full current drain on the stator.

I really didn't want to wait half the summer for a US sourced item, so kept poking around on the Interweb thingy. Finally I found a company called mtp-racing in Germany. They do an SH847 lookalike (imaginatively called an SH847AA) but with a proper 'series' regulator description.

"In contrast to the normal shunt controller, this controller can have a power of approx. Have 650-690 watts (55-60 amps) without sending the excess electricity back into the alternator, thus protecting the alternator (ideal for models with thermal alternator problems) such as Ducati , Aprilia etc. in addition, a series regulator has been proven to reduce fuel consumption. This regulator can be used for all 3 phases as well as for 2 phases alternators."

So, unless they are really fibbing on a public internet website - they do seem to be a well established company - this should be what I was looking for. So I risked it! It did take a day longer to arrive than suggested, but COVID-19 might be the reason and the firm was very responsive to my emails. Fingers crossed. This sin't a direct one-for-one form factor replacement and it turned out to be quite abugger fitting it. In the end I replaced a bolt fitting with a stud and nut. Hopefully it'll all hold together.

I also sourced a stator from Wemoto. This turned into a mini saga because despite entering the correct details of my bike into the website and checking the published dimensions, the part that arrived was the wrong size (diameter too small and thickness too big). However Wemoto were very responsive to by querying this. A long story short but it seems their supplier is sending the wrong item. So, Wemoto sent me a different (and correct dimensioned) stator from another manufacturer (Electrex World) refunding my postage and the small difference in price. I've no problem whatsoever with customer service like that.

Finally I bought a 'proper' Yuasa battery - no cheap knockoffs this time.

So, another, less pleasant, afternoon and I'm back up running again. All charging nicely, 14V at 3000RPM. Time will tell and I'll update this post later.

Oh, one other thing, the stator/rotor is apparently oil cooled but it's impossible to find actual detail of how this actually works, and forum requests have only returned replies that indicate this is not a problem - hope that's true.