When isn't 12.5v = 12.5v?

[Matchless]

You need a battery

As they said, you need a new battery. If you measure the voltage while the starter is turning and the battery is under maximum load it is most likely dropping quite a lot? If so, it cannot put out its full capacity. This also happens if you had let it run down completely in the past and then charged it up.

[PaulC]

Ok - thanks for all the info. Off to get a new battery.

[posplayr]

Humm....

Ok - 10pm last night battery just off tender = 13.09v.
Battery 7am this morning = 12.57v.
Turn ignition switch on - drops quickly to 12.47v and then continues to drop one digit at a time. After 1-2 minutes the last reading was down to 12.1v.

Does the fact that the battery does not stay relatively steady with the ignition "on" point to the need for a new battery?

Edited:

Battery 7am this morning = 12.57v.

OK I mis read that first number. Unless it it pretty cold (less than 40 degF) that is too low.

You are probably passing the load test becasue there is only a 0.1v drop under ignition load, but the voltage is just too low. Almost sounds like you have a bad cell?


My GS750EX Battery went from about 12.8V off to about 12.5v with the key on (there is about 8 amps draw from the coils at this point; 12V/3 ohms=4 amps times 2 = 8 amps total) and because there is nothing to replenish the charge, it will keep going down. This was on a 6 month old battery. You turn the key off and it basically should get back to 12.8V. At idle it would charge to just over 13.0V.

When the starter is on it is going to drop the voltage more, but apparently there will be enough current to the coils to fire the engine.

A new battery will at least allow you to answer that question conclusively. If it doesn't help then I am wondering how much voltage are you applying with a car battery? Did you do the same measurements when you were jumpered to the car (I hope not running)?

Posplayr

[PaulC]

If it doesn't help then I am wondering how much voltage are you applying with a car battery? Did you do the same measurements when you were jumpered to the car (I hope not running)?

Posplayr[/quote]

Did not measure when connected to car (car not running) but measured car the other day at 12.58v. Expect to lose something in jumper cables but not much. I seems that since it keeps going so low and particularly after being charged I need a new battery.

[PaulC]

New battery = same behavior/condition.

So got new battery and basically same behavior/condition.

Some more info:
Get > 13.5v @2500 rpm and just < 14.5v @ 5000 rpm - so believe that charging okay at higher rpm's. But at idle, about 1100 rpm, am only seeing 12.45-12.5v. Take out headlight fuse and at idle get 12.85-12.9v.

So a bad ground affecting the idle charging? I've grounded so many wires to the battery's negative terminal that I had to get a bigger screw to hold them on.

Run one more from the negative terminal to the frame?

[loungeguy69]

So got new battery and basically same behavior/condition.

Some more info:
Get > 13.5v @2500 rpm and just < 14.5v @ 5000 rpm - so believe that charging okay at higher rpm's. But at idle, about 1100 rpm, am only seeing 12.45-12.5v. Take out headlight fuse and at idle get 12.85-12.9v.

So a bad ground affecting the idle charging? I've grounded so many wires to the battery's negative terminal that I had to get a bigger screw to hold them on.

Run one more from the negative terminal to the frame?

With a 'clean' electrical system you should have no problem with frame grounds. Patching everything to the battery's neg terminal is a short cut. Be aware that the starter grounds to the engine thus frame.

The charging system doesn't start work until ~3000rpm. 14.5v at 5k is fine. Your other figures at idle are normal also.

Clean all the frame ground points, including R/R and battery neg to frame. Your new battery should do the job.

[posplayr]

PaulC

The stock GS will tend to discharge at idle with the lights on. On my 81 GS750, you can see a perceptible dimming of the headlight when the blinkers blink.

The charging seems pretty good and certainly within spec, since you said the same behavior with the new battery can i assume you mean slow cranking, but a car battery gives you a fast turn over?

If that is in fact the care, either you are putting alot more voltage across the battery, or somehow you have a better ground for the car battery than the motor battery. Which sounds like I would suspect that your large black ground from neg battery to engine case is some how not very good.

Please confirm that you still have a slow MC battery crank-over but with car battery is cranks faster. I thought this was one of your original issues.

Posplayr

[loungeguy69]

On a side note for anyone reading: NEVER jump a bike from a running car. The voltage/amps will kill the electrical system. A car charging system activates at a lower rpm so can send a butt-load of current across jumper cables.

A guy I sold a Yamaha to tried it and actually exploded the battery and fried the electrics.

[rustybronco]

Your charging system voltages are fine.

[Roger P.]

My bike stopped cranking altogether, so I took the terminals off of the starter solenoid and brushed them shiny clean. Now it cranks like a champ.

[PaulC]

Ok I accept that the numbers are fine but coming from the automotive world the idea that the charging doesn't adequately handle the load at idle just doesn't seem right.

Based on advice in other thread I'm going to start riding in a higher gear to keep the revs > 3K.

As well as ensuring clean contacts and grounds.

Thanks.

[drhach]

Be aware that the technology that has been traditionally applied to cars has outpaced the application to motorcycles and trucks. If you need evidence, consider how late motorcycles are to the fuel injection game as compared to cars. The same is true for anti-lock brakes, water cooling, electric start, and yes, charging systems. Part of it has to do with not wanting to add weight. I think the other part though, is that it is just easier not to do th R&D. Most riders won't complain about it. You will ask many "why" questions if you compare some features of a modern car to those of an older motorcycle. You can upgrade your system a little bit, but if the battery is charging and you aren't running electrical accessories, there may be no need.

[Sandy]

Did you try just shorting across the 2 large positive terminals on solenoid to rule out a bad contact in the solenoid, I didn't see that mentioned? Having a battery charge to just over 13 volts then fall back to high 12s after a some time is perfectly normal and it sounds like your charging system is working properly. The problem has to be in the starter, solenoid or their connections. I would remove the battery, fully charge it and then without reinstalling the battery use some jumper cables and attach directly to the starter to see how it works. Attach the positive cable at both ends first, then the negative to the battery, then the other negative end somewhere that when it arcs it isn't going damage a nice chrome part. Maybe you've got a bad rebuild on the starter, wouldn't be the first time.

[Ed Vrolijk]

[quote=loungeguy69;900491] The voltage/amps will kill the electrical system. A car charging system activates at a lower rpm so can send a butt-load of current across jumper cables.quote]

Voltage yes, current, no.
Current isn't supplied or send, but drawn. Example: overhere in Europe we have 230 volts. Most houses have 16 amp fuses, wich means you can use equipment of up to 3680 Watt on a single fuse. However you can safely use a 60 Watt lightbulb, (wich only uses 0.26 Amps), on that same fuse without exploding the bulb.

Ed

[duaneage]

Everybody grounds to the negative battery terminal when the right answer is to ground to the frame. ANd then go to the frame grounds and check them. Depending on the model there are frame grounds at the head of the frame, the rear, and a few points in the middle. The three biggest frame grounds are the coils (mounting bolts) the plugs and the starter. Hooking everything up to the battery terminal means these three components get a worse ground.

The battery starts the bike. That's it. The RR powers the bike and the battery gets charged as a byproduct. It also acts as a rather large filter to smooth the RR output (a capacitor of sorts). Batteries are electro-chemical devices that do have problems from time to time.