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Greg TeiberApr 2, 2018 (Updated Dec 17, 2019)CommentShare

Your Motorcycle Battery Lies About Voltage!

Every bike on the market today has a 12-volt battery buuut … that’s not really the truth. Its power isn’t fully charged unless somewhere in the area of 12.5 volts after its been off the charger for a few hours. Here’s where things get a bit funny.

You threw your battery on the charger Wednesday night and here you are Friday evening, beer-in-hand as you prepare to warm up the bike for an oil change. Before you pull the charger off, you check and find a charge of 12.3 volts. “Meh, it should be ok”.

I mean, it’s a 12-volt system anyway but when you disconnect the charger and turn over the motor, the starter barely cranks. You keep fighting. The engine sputters a little. Maybe it barks to life but the headlight is dim at idle. Now your bike won’t idle well and doesn’t want to keep running unless you rev the RPMs slightly high.

Before you go chasing those carburation or injection issues (long story short) …

It’s Time To Replace The Battery

If you’re unlucky, it’s time to go over the charging system but what gives? The tender said the unit was full! The technical answer is: Your battery developed a short in a cell over the winter. The rest of the story gets a bit into electronics and chemistry. We’ll come back to that but before we continue, here’s what you need to do to prevent tears this year.

Motorcycle Battery

  1. Visually inspect the unit. If it’s a flooded cell, make sure none are low on water. One or two flooded? Spend the $40 on a new battery. If all cells are a little low, top it off and keep going.
  2. Trickle charge that battery for a couple of days.
  3. Disconnect it from the charger and let it sit for a few hours.
  4. Check the voltmeter. Is it over 12.4 volts? If so, you’re likely good for the year.

That’s all the vital battery information you need for one season.

Why Do Batteries Read So Many Different Voltages?

Batteries are chemical storage of electricity, resistors and capacitors. All of those factor into a wide range of voltages you might experience with a modern 12-volt system. On a 6-volt unit, a single shorted cell would knock you down to ~4v, making it obvious your battery is toast.

When charging, you need to raise the voltage higher while the cell is at rest to push electricity into it. That’s why, when charging a battery, you might see as much as 14v. Even when float charging you’ll often see 13+ volts.

Capacitors, at their simplest level, are just a bunch of parallel plates. Amusingly, so are lead-acid batteries. When you charge one, they’ll “fill up” on a few extra volts, holding onto that for a while once you’ve taken it off the charger.

This is why you need to let voltage settle before trusting a voltmeters reading.

Batteries Are Also Resistors

If you put your voltmeter across the battery terminals and turn the bike on, you’ll see the voltage drop right to 11v or less as soon as the headlight and coils activate. If the battery had no resistance, you’d see the full 12.4v across the terminals but the battery has some internal resistance which becomes part of the circuit as soon as you turn the bike on.

When you turn the bike off, why doesn’t the voltage come all the way back up again? That’s chemistry rearing its spikey head. The battery is a chemical cell and that chemical process takes time. So, if you leave the unit alone for a while, the battery’s voltage will climb back up. If you only had the bike on for a few moments, it’ll recover most of its voltage.

Hopefully this saves you some frustration. If you’re lucky and without issues, you’ve at least learned a little about how your batteries work. Enjoy an easy-start riding season!

What Electrical Solutions Have You Employed?

There are a lot of battery related truths in motorcycling. What issues have you tackled? How did you go about it and why? Your input is invited. Post an article!

This is the end!
Greg Teiber

About Greg Teiber

This person contributes motorcycle articles and information without registering.

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