When isn't 12.5v = 12.5v?

[PaulC]

Where to start?

My bike is kept most times on a battery tender and the battery reads a solid/steady 12.5v when I take it off. I've recently put a new stator and R/R on and the battery reads in the low 13v when bike is running. Also did the coil relay mod. The bike does not crank well at all but it does fire up quickly.

I went for a ride yesterday for over an hour. Stopped to get a drink, checked the battery before I shut it down and it read 13.1v. Came back 5 minutes later and battery reads 12.5v and bike cranks really slow but finally catches.

So due to the problems with the electrical problems I've been having I've had to jump the bike from my car battery which reads 12.5v. But when I do this the starter cranks a lot quicker.

So when is 12.5v not equal to 12.5v? So is it the difference in the "reserve" power (cold cranking amps)?

I took the battery to Batteries Plus and they put a load on it and said it was fine, but maybe it's time for a new one since it's almost 2 years old?

[rustybronco]

When it's preceded by a minus sign...

now you get to figure out why your charging system voltage is in the low 13's.
as for the slow cranking speed, it sounds like the battery is bad.

[lecroy]

So when is 12.5v not equal to 12.5v?

Never.

Put the DVM in volts mode. Clip the negative lead to the negative lead of battery. Put the positive lead to the battery positve. Read the voltage. Hit the starter switch and read the voltage. How much does it drop?

Next, move the positive lead to the output side of the starter solenoid. Then hit the starter and read the voltage. How much drop?

Next, move the positive to the starter lug (located under the cover). Again, hit the starter and read the voltage.

If the drop is small, move the positive lead to the case ground of the starter, hit the starter and read the voltage.

The starter motor is starting to fail or you have a solenoid or wiring problem (assuming the battery really is good).

[bonanzadave]

Whats your battery voltage at 2k rpm ? If its less than 14vdc youre not charging. Have you looked at the stator papers ? Maybe start by cleaning the RR connectors and attaching the RR ground to the - battery term. Dont trust a battery load test. And yes, Current (amps) is what cranks youre starter. A two year old battery should be fine unless you let it go completely dead.

[posplayr]

Voltages seem low

Dunno, But I would think the battery tender should charge to about 12.8-13V minimum.

At 4000 RPM you should see about 14.0V across the battery (I dont remember the exact specs off hand) if properly charging.

Sound like you have a few things as potential issues

I'm kinda assuming that the load test was done right and maybe neither your bike or tender is charging right.

Where are you measuring the voltage?

Posplayr

[jbs80106]

My thoughts...

The fact that it cranks poorly is a possible symptom of a dirty starter or worn brushes. The difference in my cranking was night and day after replacing the brush assembly.

However, that said, it seems to crank stronger if you jump it. How about starting with the trickle charger plugged in?

Check voltage at the starter underneath little rubber cover. Should have same voltage there as at the battery when you push start button.

Jumping off your car battery may be kicking more current down the line than your bike battery....

[PaulC]

Ok - so more info --

Have the R/R grounded to battery negative terminal.
Seeing 13+v at idle, rev it to 4k see 14+.
Am measuring at the battery.
Starter was professionaly rebuilt a year ago.

But - presuming that whatever may be wrong with the electrical system is constant - the question is why does the car battery crank the starter faster?

[posplayr]

Starting to sould like the battery

If all you are doing is jumpering a car battery across the moto battery and it cranks faster, then I would say you moto battery is weak.

If you are connecting the car battery is some other way, it might be connections.

12.5 volts is a little low to be sitting quiesent.

When you just turn the key on, without cranking how low does the battery go? The coils draw a few amps and that will drop yoiur battery further ; sound like you are dropping all the way down to 12V which is further sign the battery is on the way out. Have you check the level?

Your other numbers sound right on.

Posplayr

[PaulC]

I'm sure I've check but will re-check tonight the voltage just turning key.

[BrianMcDonald]

But - presuming that whatever may be wrong with the electrical system is constant - the question is why does the car battery crank the starter faster?

The power delivered to the starter motor is based on the current flow (measured in amperes) and voltage (measured in Volts). The motor is going to turn this electrical power into real work by cranking over your engine. Your on-board battery may be able to produce 12.5V just like your car battery, but if it can't provide as many amps there's less power available to run your starter. The number of amps it can put out is based on the structure of the battery itself and the resistance of the electrical circuit path between the source (battery) and the load (starter). Car batteries are almost always rated for more amperage (Goldwings not withstanding) and thus can provide much more power to the motorcycle starter, all other things being equal.

That higher current available from a car battery can, if assisted by a running car alternator, melt some of the wiring harness on your bike (*). This is why you should never jump a motorcycle from a running car.


(*) That bit was old anyway. It needed replacing.

[posplayr]

The voltage test with the ignition only

Is a mini load test for the battery. If I recall correctly I get about a 0.25V voltage drop at the battery supplying the coil power. It would be more if you have the lights on.

A load test simply looks for how much the battery voltage drops when you put a load on the battery. A well charged battery doesn't drop as much voltage as a bad battery would supplying a given current.

Basically the battery has internal resistance (i.e. voltage drop related to supplying current) and the more the internal resistance the worse the state of the battery.

While this is a ignition test is sort of uncalibrated , you should stay above 12V in any event and preferably above 12.5V. Since it is hard to measure battery voltage while you are cranking the engine (due to the voltage variations during cranking), the mini load test is preferable.

If you are getting more drop than 0.25v and less than 12.25v at the battery then you most likely need a new battery.

Posplayr

[Clone]

Also you have to check the voltage you are getting across the posts of the battery when the bike is spinning at 4-5000rpm because it sounds like your charging system is not keeping up, especially if you have to boost the bike and keep putting it on a charger. I would suspect the stator is on the way out, also check the power output between the yellow wires of the stator.

[lucabond]

My 2 cents, have you cleaned your grounds? From the battery to engine and starter to ground. Poor ground makes it had for current to flow. Wouldn't hurt to add a new ground from the battery to ground.
Good luck.

[PaulC]

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?

[drhach]

You need a battery