Testing a Battery/Alternator with a Voltmeter...

[gerpena]

easiest way to check your alternator is to have the engine idling with the headlight on... then put a couple of revs on.

If your headlights get slightly brighter, then your alternator works.

also if you put a voltmeter across the battery terminals whilst the bike is running, the volts registered should be ABOVE 12V (Possibly 6V if you've got a wierd bike)

I'm not sure all bikes will run without a battery, depends on how many coils in the alternator, and their positioning

thanks for the advice. i had the battery recharged 3 weeks ago, and the bike had run/started perfectly until yesterday.

is this evidence that the alternator is fine and the problem is the battery simply cant hold a charge for longer than 2 weeks and needs to be replaced?

i have no problem buying a new battery i would just like to make sure thats what my problem is, in addtion to ignorance that is.

[Nibblet99]

Hi,
It also depends on how you use the bike... when you start it, it takes a lot of juice out of the battery, and it takes a while for it to charge back up fully. If your last couple of rides were only 3-5 minutes, that could well be your problem.

as a general rule of thumb, batteries are disposable items that should be replaced every couple of years

2 weeks is more than possible for a bad battery to flatten in, but you can only really tell by investigation... so in short....

If your battery is over 5 years old... change that first, before even bothering to disassemble diagnose the problem

if your battery is over 2-3 years, consider changing it, it'll usually make it better at starting reliably, but is a personal choice really (unless the alternator appears to work fine. in which case you change it)

if it's less than 2 years old, only change it if the alternator tests ok, and you still get the same problem after putting it on a charger (out of the bike)


Hope this helps
Andy

[flynrider]

Correct me if I'm wrong, but I thought that if the bike was running and you pulled the positive cable off the battery and the bike died it was the alternator?

You could be wrong with some older bikes. Many of them used the battery voltage to regulate the output of the alternator. If you disconnected the battery, the regulator would spike the alternator output. I had the misfortune of finding this out the hard way, when I disconnected the battery on my '80 Kaw 750 while it was running. I immediately noticed that all of the lights got really bright. When I blipped the throttle to a couple thousand rpm, things got extremely bright just before all the lights and fuses blew. I had to replace every bulb from the headlight to the taillight.

FWIW, later model electrical systems did not do this. They would die when the battery was disconnected.

[Iain]

Yeah, the current may be too much for your multimeter/ammeter to handle. I'd suggest you find out how much current your alternator and battery supplies, and make sure your multimeter is rated for it. The specs in the repair manual for my cb350, say the alternator only puts out a max of 4 amps, and thats at 3000rpm. Thats assuming I'm reading this right. But it very well may be differnt for you, as my bike is small. But check out if the battery is fine first of course, before you potentialy blow a brand new piece of equipment.

Anyways, have fun and don't zap yourself, it kinda stings a bit, I did it two days ago.

EDIT: You may also want to make sure your battery isn't shorting out and discharging itself. Thats always a possiblity, if you had any other trouble starting it, you may just have a loose wire that touches the frame every once and a while, but once you get into a seating position on your bike, it falls away from the frame. Just a thought.

[Nibblet99]

The specs in the repair manual for my cb350, say the alternator only puts out a max of 4 amps, and thats at 3000rpm.t.

Thats because it's only a charging circuit at the end of the day, the battery can put out many more when you push the starter button. The circuit you drew would be fine, IF you bypassed the ammeter with some heavy duty, low resistance cable, during the starting of the bike

Anyways, have fun and don't zap yourself, it kinda stings a bit, I did it two days ago.

Yeah, ever had an old cable going to the sparkplug? you know when you've touched one of them whilst riding (when numb in the leg - stopping was interesting) That got replaced after I did it!

EDIT: You may also want to make sure your battery isn't shorting out and discharging itself. Thats always a possiblity, if you had any other trouble starting it, you may just have a loose wire that touches the frame every once and a while, but once you get into a seating position on your bike, it falls away from the frame. Just a thought.

Very good point, worth investigating if all else fails

[9000white]

saw a gs850 suzuki that burned out every electrical component on it when battery cable was disconnected while running.because inquiring minds want to know i researched why this happened and found out that as much as 300 volts AC is generated when cable is disconnected.it cost over $800.00 to undo that mistake.

[BuzZz]

You should have your battery load-tested. A battery can read 12-13volts at rest and still drop to almost nil when a load is drawn.

[TechTMW]

You should have your battery load-tested. A battery can read 12-13volts at rest and still drop to almost nil when a load is drawn.

Yes that's about it. Sorry I didn't reply sooner.

[WicAndThing]

I don't mean to burst a bubble here, but simply putting a volt meter on your battery will not tell you if the battery is good or bad. The only way to do that is to measure the current output of the battery under load.... a weak batter may still show the valid voltage, but fail miserably under load. That's why they have those special little machines at the auto parts stores to test your batter and alternator.

[Iain]

Thats why I say hook up the voltmeter and ammeter, and turn it on to test. Someone should write guides after all these discussions, get the general concensus on a topic, then write up a guide for future referance. It would be very useful to everyone out there wondering these same things.