Low battery voltage when starting?

Ok so I just installed a new Datel digital voltmeter on my 1150RT and it reads
13.7 volts while riding
12.6 volts shut down
8 volts aprox. when cranking over
I read on here somewhere that it shouldn't drop below 10v when cranking up and should be replaced if it does?
So should I replace it? It did give me a problem (wouldn't start) a few weeks ago but I hadn't ridden it for a couple of weeks since then have been putting it on battery tender 2-3 days a week and It holds a charge, steady 12v for 3 days or so, off the tender just sitting. So haven't had a problem since but I'm headed to DVD in January and don't want to find out there that I should have replaced it.

Thanks Bill

How old is the battery? If its holding a charge I wouldnt replace it if it were mine. But I would carry jumpers!

Your voltages look healthy to me. 12.6V says that it's fully charged and 13.7 says that proper charging voltage is there. Dropping to 8V as opposed to 10V when cranking is incidental. Where you wire the voltmeter in and the guage wire you use can affect the readings a bit.

If it were 6V when cranking, I'd check for reasons and prolly replace the battery. Oh, and I always carry jumpers.

Battery is original so about 3 years old. Good suggestion on the jumper cables will be purchasing soon.
Thanks Bill

Muneio said:

8 volts aprox. when cranking over.

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That voltage is kind of difficult to read without a min/max hold feature. The voltage varies while cranking due to the varying load as the engine turns over...more load during compression stroke.

8V is low. I remember having a stalled starter motor, a Valeo with a dropped magnet which was evident when I measured the voltage across the battery while trying to turn over the motor (it didn't)....in the 8V range if I remember...indicating massive current draw.

Never tired this, but I've been told if you have a good battery that if you can keep the bike from starting (eg, disconnect plug wires) the voltage should drop when you hit starter, but then bounce back up even while the starter is still engaged.
My 1150 GS will drop to about 9 v when I hit the starter.

I'd take a look at where the VM is connected, just to see. 8V at the starter itself isn't so bad - I went thru some measurements recently, and saw 8-9 V at the starter while cranking my R1100RS. I checked the current draw, and the starters on both my bike and DW's R1150R Rockster drew about 250A peak while cranking. 5V of drop with 250 A of current indicates 20 mohms of resistance in the cables from battery to starter. I'd believe those cables were 2 hundredths of an ohm.

So couldnt you get better starting then by installing better (less R) cables?

guitardad said:

I'd take a look at where the VM is connected, just to see. 8V at the starter itself isn't so bad - I went thru some measurements recently, and saw 8-9 V at the starter while cranking my R1100RS. I checked the current draw, and the starters on both my bike and DW's R1150R Rockster drew about 250A peak while cranking. 5V of drop with 250 A of current indicates 20 mohms of resistance in the cables from battery to starter. I'd believe those cables were 2 hundredths of an ohm.

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You better not drop 5V in that short length of cable. My guess would be a 0.5V drop in the cable.

When you are showing 8V at the starter terminal, you battery isn't at 12.8V, its probably at 8.5V.

You forgot to take into account the internal resistance of the battery which is probably about 15 milliohms. No, don't try to measure this with an ohm-meter.

Yeah, when I was cranking I was only measuring voltage at the starter. What surprised me was the amount of current. The 250 A was measured with a Fluke clamp-on meter that had an inrush setting, so that was the max current it drew at any point. But still, I never thought it would be that many amps!

guitardad said:

Yeah, when I was cranking I was only measuring voltage at the starter. What surprised me was the amount of current. The 250 A was measured with a Fluke clamp-on meter that had an inrush setting, so that was the max current it drew at any point. But still, I never thought it would be that many amps!

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Well the current draw is going to vary due to the varying load on the starter as it goes through the cycles of the engine. Plus thick oil on a cold morning will have an effect as well.

I would have expected about 100 A. The starter on a Dash 8 we were testing was drawing 1100 A at initial start-up. Wow!

Izat what "cold cranking amps" on a battery is refering too? Like you see "500 cold cranking amps".

jingdog said:

Izat what "cold cranking amps" on a battery is refering too? Like you see "500 cold cranking amps".

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Just some battery terms...

Cold Cranking Amps

Cold Cranking Amps is a rating used in the battery industry to define a battery's ability to start an engine in cold temperatures. The rating is the number of amps a new, fully charged battery can deliver at 0??F (-18??C) for 30 seconds, while maintaining a voltage of at least 7.2 volts, for a 12 volt battery. The higher the CCA rating, the greater the starting power of the battery.

Reserve Capacity

Reserve Capacity, (RC) is a battery industry rating, defining a battery's ability to power a vehicle with an inoperative alternator or fan belt. The rating is the number of minutes a battery at 80 degrees F can be discharged at 25 amps and maintain a voltage of 10.5 volts for a 12 volt battery. The higher the reserve rating, the longer your vehicle can operate should your alternator or fan belt fail.