New battery or worse?

I've decided to treat the old girl (the bike not the wife) to a new battery.
I've experienced rough running and sometimes a failure to even turn over.

However, on doing a test with my trusty meter I've found 12.4 volts at the battery with engine off
BUT, only 12.1 with the engine running at about 2000 rpm.

I'm fearing the worst with a reg/rec failure. Please tell me it isn't so!

 

I had that sort of reading with a 990 which had a failed reg/rec. Quite surprised it was about £70.

 

you need to make sure your making power in to the reg/rec 1st. dont forget its ac voltage between generator and reg/rec

 

yep there should be 3 wires coming from the stator , 3 phase ac. just put the meter in ac mode and run the probes across any 2 connections and see what you get .. not sure what the exact voltage is meant to be on your model but average voltages for bikes is around 50-70 volts at around 5k, usually somewhere around 20v at idle .. if your getting that across all 3 phases then sounds like the reg/rec has gone poof

 

What's a "reg/rec"??

 

When the motor runs the alternator makes electricity to charge the battery & power the lights, ignition etc. It makes more than you need & the excess is released as heat which is why the regulator/rectifier is within the airstream & finned to keep it cool. Tis about the size of an average persons hand so not like that of Stars 'n Stripes. Btw this simpistic version was for my benefit not yours.

 

The "rec" bit converts alternating current (AC) from the alternator to direct current (DC) which the battery and other electrical components on the bike uses, hence rectifier
.
The "reg" bit makes sure there are no surges or over voltage conditions, hence regulator.

As Les says, any excess energy is released as heat which must be dissipated to prevent the reg/rec from burning out, literally.

I think!

 

And I thought rec was for rectum.

 

Why is it converting AC to DC?, a battery is DC what is AC on the bike? Do alternators discharge an AC?
I always check a battery when engine not started no ignition, with a Meter set to DC, then start engine, and check the battery again, if the reading is higher when engine is running by a approx 1.5 to 2 volts DC, the battery is charging. I assume bikes are the same?

 

on cars i believe there converted at the alternators where bikes are not both will have 12v dc at battery

 

Just to add confusion. A Generator makes electricity. An Alternator make AC electricity (Often a generator with a rectifier enclosed) or shall we talk Dynamos.

 

better still windscreen wipers that work on vacum

 

as far as im aware nearly all charging system on recent 70's onwards cars and bikes are ac rectified to dc voltage... there may be the odd one or 2 that aren't , that use a dc generator but quite uncommon I would think . It is quite cheap and fairly straight forward to build a robust , relilable ac generator/alternator, hence why I guess they have been in use so long. building one that generates a smooth dc output is alittle more complicated and I guess isn't cost effective, altho I dare say with modern technology it prob now is and its just become one of those cant be bothered to change the design etc.. I wont bore you with the physics and hows , but basically bunch of magnets spin round some wire wound coils, thus generating an ac current, in cars the rectifier (part that converts ac to dc voltage) and the regulator ( regulates a steady , smoothish voltage in a specified range) are built into the alternator housing itself. mainly as cars have more room for these parts, and it can be easily belt driven. also the alternators in cars generally have a finned section on the rotator shaft to aid in cooling the reg/rec unit. so it has active cooling ... as opposed to a motorbike where asthetics are quite important, having an unwieldy alternator hanging off a mounting with a belt attached to a pulley isn't such a great idea. so you have a bunch of magnets epoxied onto a flywheel arrangement, which is affixed straight on the end of the crank. the coils are fixed to one side casing and the magnets spin round them creating the ac current... down to space issues and I assume costing, the reg/rec unit is generally separate and as les said about the size of your hand. the components inside are quite small but they have to dissipate so much wasted electricity in the form of heat that it needs a large surface area to achieve this. usually cast aluminium, the body of the rectifier unit is placed somewhere there is sufficient air movement to keep it cool.

 

He he, just knew this thread would get complicated.

 

lol from the sounds of it at 12.1 v I don't think the bike would be getting very far before conking out....

 

With duct tape?

 

He's tried that!!

 

But I'm obviously not trying hard enough!!

The bike definitely goes in it's present condition, but it's getting it started that's the problem.
I'm pretty sure it's a knackered battery, but I've got to wait until Tuesday now as I was sold the wrong type.
Why is life soooooo complicated?