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Wondered if anyone could explain this - had a few probs recently with the dash-reset/not enough charge to crank after the bike standing for about 5 days. The battery is just coming up to three years, it's had quite a few discharges (forgot to plug in the Optimate , and the alarm just drank the battery dry), so figured it was dying, so got a new one (one of the Westco high CCA ones someone recommended). The battery was reading about 11.5V (on the dash), but did read 13.5V when the bike was running, so figured it was the battery not the wiring.

New battery is great (really throws the starter better), but noticed I'm still getting the same voltage readings . The battery reads 12.5V when I turn the engine off, and is down below 12V when I check the following morning. Still shows 13.5V when the engine's running though.

So, the question is, why is the battery showing such a low voltage when there is a much higher voltage being produced when the engine is running? Is it possible that the "charge" available is appearing on the clock's volt-meter, but isn't reaching the battery at the same level? And, lastly, should I be concerned about the low voltage on the new battery?

Have managed not to do the wire mod before, looks like it may be on the cards over the winter

Thanks in advance

On my Blackbird, after a day off it it would always read around 11.5 volts, I'm guessing that this is due to the alarm and clock draining power while its not running.

On fire up if it was read at the same time it used to surge to just over 14 volts and then settle back to 13.5 on tickover. I imagine the surge voltage is due to the coils (2 fitted)

I'd only worry if it drops to far, maybe its an earthing problem if it drops a volt a day, ie afgter 5 it might read 7 or 8.

Thats my guess anyway, I'm sure someone with a lot more understanding will be along soon

the readings your getting are normal,the 13.5v your getting is the charge from the alternater when running,the 12v when stood is the battery voltage. hope that helps.

fastasfcuk wrote:the readings your getting are normal,the 13.5v your getting is the charge from the alternater when running,the 12v when stood is the battery voltage. hope that helps.

What he said

Don't make the common mistake in believing that voltage = charge in the battery.

They do roughly work together but if a battery goes below about 75% of it's rated voltage (ie 9v for a standard 12v battery) that is a good sign it is goosed.

As far as your 13.5v goes, if you want to charge a battery then the voltage into it must be greater than the voltage out in order to actually put some charge into it. You shouldn't actually be using any battery power once the bike is running - if you do it means you are using more power than the generator can supply and ultimately your battery will go flat and the engine will die because there is no spark.

As an aside, a battery can often show a full 12 volts even if it is knackered. The best way to check it is to put a multimeter across the terminals while you are trying to start it. If the voltage drops to less than about 5 or so then the battery has had it.

Yep - lead acid battery condition can only be effectively assessed under load. Only this way will you drop any voltage across it's internal resistance (which is greater for knackered batteries).

D-Rider wrote:Yep - lead acid battery condition can only be effectively assessed under load. Only this way will you drop any voltage across it's internal resistance (which is greater for knackered batteries).

And i was trying not to be technical about it

I thought you needed to be readin in excess of 14V to get the battery charging

Falcopops wrote:I thought you needed to be readin in excess of 14V to get the battery charging

no,13.5v+ is fine. so long as it's not over chargeing.

You only actually need more than whatever the battery holds, so 12.5v would do it but you'd loose that as sson as you turned the lights on.

Any more then 14v and you're going to start cooking the battery, so anyone that's done the wiring mod, watch out for the battery starting to bulge slightly, getting hot or that rotten egg smell drifting from beneath the seat (and I'm not talking about the aftermath of last nights beer and kebab intake)