Ironhead Battery is fully charged but bike only starts by jump starting it

[Tymanskie]

Hello I am new at working on motorcycles and I received a 1980 xlh sportster 1000 several years ago. I finally got around to messing around with it. Put new plugs, liquids, and battery in it and got it running. I’ve been riding it for about 3 weeks now never having an issue starting it at all. But this morning when I went to turn the key nothing happened, no headlight nothing. Push the start button nothing. So I assumed it was the battery. I charged the battery and tried again. Still nothing.i checked voltage on battery while it was still connected and it read out 12.4 volts.i Had to get home so I thought I’d try and hook it up the jumper cables from a car and the bike started right up no problem. Got home shut it off and same thing I have no power. But battery is still charged. Does anyone know what could be happening? Please any advice will help

[shanneba]

Check through this on electical problems-

http://sportsterpedia.com/doku.php/t...place_to_start

[doodah man]

check for loose and or corroded battery terminals first. it sounds like they are unable to pass the correct amount of current, and that is why it starts up when jumpered to the car battery - much higher current/cranking amps.

[ironmanx]

Even though a battery may show 12.4 volts it may have a collapsed cell meaning the CCA will not be enough to crank it over. A load test will show this.
New battery time I think.

[Jollly Rogers]

Depending where the cables were attached for the jump, a bad connection could have been bypassed to allow the start with a good battery in the bike but isolated from the system by the high resistance connection.
Joe

[sportsterdoc]

Originally Posted by Jollly Rogers

Depending where the cables were attached for the jump, a bad connection could have been bypassed to allow the start with a good battery in the bike but isolated from the system by the high resistance connection.
Joe

+1
1. Check both ends of both battery cables
2. What was battery voltage while attempting to start?
3. What is battery voltage with + probe on battery + and - probe on engine?
3a. Key off?
3b. Key on?
3c. While cranking?

[seajay]

Fully charged battery is 12.8 volts or higher.

[Moon Wolf]

Some batteries that test good will actually go dead under load as Ironmanx suggests. We used to call the a load short, although the terminology isn't accurate. Almost any auto parts store can do a load test on your battery.

[sportsterdoc]

A basic battery load tester simply connects a resistor and reads voltage drop.
Which is why the battery first needs to be fully charged.

Step 3, below is a field test, using the starter as the load:

1. KEY OFF battery voltage, at the battery: Normal is 12.8 (AGM, whereas a lead-acid is 12.6).
If just taken off a charger, voltage may be 13 point something. The battery needs to sit for a while (or turn on the key for ~3 seconds, then turn off) before a reading is useful.

In the case of a bad cell, the other cells overcharge and give an almost normal reading, until load is applied.
• If lower, then battery is discharged and needs charging or
• Battery has a bad cell

2. KEY ON battery voltage (headlight on): Normal is not under about 12.5 initially, but will decline the longer the headlight is on, without the motor running. Under no load, 12.5 volts indicates severe discharge, but under headlight load, it is just voltage drop.
• If lower, then battery is discharged and needs charging or
• Battery has a bad cell or
• Battery cable(s) have high resistance

3. KEY ON battery voltage while cranking: Normal is not under 10.5 volts
• If lower, then battery is discharged and needs charging or
• Battery has a bad cell or
• Battery cable(s) have high resistance or
• Starter is drawing too much current
• Solenoid contacts have high resistance