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battery theory

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inreb

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In the interest of putting my full ignorance on display. After coffee, jumped on the bike, no headlight and only the weakest protest at the starter relay. Not good as I prefer to peel out in a stylish fashion. Managed to get a bump start. Ran fine c/w headlight fully operational. At home, again no start, no headlight. Voltage checks in at 12.82. Should start. This is where my ignorance comes in. Couldn't get proper jumper cables on so used 16ga. with alligator clips to jump to 13+ volt good battery. Headlight comes on and a quick stab at starter button reveals a willingness in that dept.
So, how can a battery read 12.82v and not even show a tiny bit of headlight?
Since I'm mostly a parts changer I head over for a Pacific Rim Power Sonic battery replacement. Sales guy up sells me a Deka AGM. Initially on guard, my defences drop when he starts ripping chinese quality. We're on the same page. He hands me the Deka, already charged and ready to go. Hmm.
At home the Deka shows 12.49v. I'm pretty sure that's not good. My mind starts reeling out of control. What now? Back to my supplier, don't want to install. I go in miffed but soft. He assures me everything is cool, these things can sit on the shelf 2 yrs. unlike the chinese garbage. I go home, install battery and she fires right up. What's a load test?:cool:
 
Bad cell?
 
It was the current (amperage) that was "bad". As a battery ages the internal resistance increases lowering the current flowing from the cells while still showing decent voltage.

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inreb said:
In the interest of putting my full ignorance on display. After coffee, jumped on the bike, no headlight and only the weakest protest at the starter relay. Not good as I prefer to peel out in a stylish fashion. Managed to get a bump start. Ran fine c/w headlight fully operational. At home, again no start, no headlight. Voltage checks in at 12.82. Should start. This is where my ignorance comes in. Couldn't get proper jumper cables on so used 16ga. with alligator clips to jump to 13+ volt good battery. Headlight comes on and a quick stab at starter button reveals a willingness in that dept.
So, how can a battery read 12.82v and not even show a tiny bit of headlight?
Click to expand...

Did you measure the terminal voltage on the battery when you turned on the ignition (i.e. turned on the headlight)? Or when you pressed the starter button?

Theory: A battery is not just a pure voltage and current source. The electrical model includes an ideal voltage source followed by a serial internal resistance. That resistance is always present though it can vary in its effective amount. With a fresh, perfect battery when you press the starter and your headlight dims a bit what you're seeing is that resistance; as current flows out of the battery through that resistance, you see a voltage drop across the resistance which is reflected as a voltage drop across the battery terminals. The more current you flow, the more drop you see. If you pull virtually no current you see virtually no voltage drop (V = I x R).

So: One failure mode of a dead battery is to develop a very high effective internal resistance. When you measure its terminal voltage with a DVM (which draws nano- or microamps) you might well see 12.5 or 13V but as soon as you try to draw any current out of it the high resistance causes a drop across the terminals. So a light won't show anything and a starter won't turn. At this time, you'll see a very low voltage across the battery terminals.

The upshot is that measuring the terminal voltage of a battery can tell you something about the state of the battery, it's not necessarily giving you the whole picture of battery health. This is why part of testing a battery is putting a load on it and seeing who well it delivers current and holds the terminal voltage.
 
Bigdawg is right. Electric motors are typically amperage driven. Lights need both, if you have a 12V 36W headlight, that means it need 3A. A better indicator of battery size is the Ah (amp-hours) and standby voltage ratings. This is why you can't run a bike off 6 AAA batteries from the dollar store.

Edit: somehow ninja'd by a much longer post...
 
Your descriptions are very vivid. Next time put on a gopro so we can see it in the flesh
 
RockerGuy said:
Your descriptions are very vivid. Next time put on a gopro so we can see it in the flesh
Click to expand...
He saves that GoPro for his dinner invites.

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Lots of batteries show over 12 volts but hold no amperage.

It's toast.

Buy a new battery.
 
Thanx to the cogent responders and spitballers alike. I love yous equally.
 
lots of terminals will pass the voltage,, but not the current ... and when you install a new battery, the terminal gets cleaned up, and the vehicle works again,, and the old battery is assumed to be junk........ if you put the old one back in.. that is a load test... if you are sure the terminal connections are good.. and the voltage is good with no load.. and then with load ,, the voltage drops off.. then the battery is junk

often I need a battery that will get me thru.. so I stop at the local recycler.. and test any battery that looks like it will physically fit.. I find lots of very good batteries in the scrap.. they charge me $20 ....

recently.. a touring biker had the dreaded clickketyy click ... turned out that the battery terminal bolt was too long.. it tightened up ,, bottomed out... before it really tightened the cable terminal to the battery terminal.. but had been working .. until that poor connection eventually crusted up the battery/ cable connection... so we happened to find a shorter battery terminal bolt.. sanded the connections clean.. added some electric grease,, and it was back to working again...
 
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inreb said:
no headlight and only the weakest protest at the starter relay. Voltage checks in at 12.82.

used 16ga. with alligator clips to jump to 13+ volt good battery. Headlight comes on and a quick stab at starter button reveals a willingness in that dept.

So, how can a battery read 12.82v and not even show a tiny bit of headlight?

At home the Deka shows 12.49v. install battery and she fires right up.

What's a load test?:cool:
Click to expand...

assuming my wild guess is correct ,, that it was a bad connection.. I would guess that when you tested the battery .. your voltmeter leads touched the battery,, not the cable.. If you had touched the meter leads to the battery only..took that reading,,. then to the bike cables only and measured a different voltage... especially while the headlight was on.. then you may have detected the problem right away.

again , assuming my wild guess is correct ,, that it was a bad connection.. when you put the jumper cables on to the bike cables,, you bypassed the bad connection between the bike battery and the bike cable ...
 
I had a similar problem with my car years back.
I went through 2 batteries in a month. The second time the mechanic checked the alternator.
It was the problem. It was on its last legs and was turning out 14 volts.
This turned the 12 volt battery into a 14 volt battery which was the wrong voltage to start the car.
Once boosted, the car would start and run, but wouldn't start again.
Thereafter when I have battery trouble, I always check the alternator first.
 
clean the terminals on a new battery as well, I've watched guys get no contact on a new battery as the manufacturer often puts a wax like coating on them the prevents corrosion.
 
statsman said:
I had a similar problem with my car years back.
I went through 2 batteries in a month. The second time the mechanic checked the alternator.
It was the problem. It was on its last legs and was turning out 14 volts.
This turned the 12 volt battery into a 14 volt battery which was the wrong voltage to start the car.
Once boosted, the car would start and run, but wouldn't start again.
Thereafter when I have battery trouble, I always check the alternator first.
Click to expand...

Around 14Vdc is normal output for an alternator. Alternators typically fail either in the diode bridge or the regulator. If the regulator fails, the output voltage can go quite high (>16V) as engine revs climb which can damage the car's electronics and even boil/overheat the battery electrolyte. A bad diode bridge may allow the battery to discharge when the engine's not running (making it appear that the battery is no good) and may also cause excessive AC ripple in the car's electrical system which can affect ECUs and even inject noise/whine into the audio system.
 
Already mentioned here, but essentially check and clean terminals if necessary, and then measure both the voltage with the battery at rest, and the voltage when attempting to start. If memory serves, if it drops below 10v while cranking then a cell is toast in the battery, even if it shows in the high 11s or 12s when at rest.
 
Think about it from this perspective - here's a pic of an 12 volt A23 battery, somewhere between AA and C in battery size.

hqdefault.jpg


As you can see, it's basically 8 button cell batteries tied together in a neat wrapper. Like our motorcycle batteries, it's also 12 volts....but of course it's not going to start a motorcycle.

As others have mentioned, amps matter just as much as volts, and it sounds like your battery is no longer doing it's thing in the amps department. It's not uncommon to see these sorts of situations where the voltage remains in the normal department however, while the amperage capability falls on it's face.

TLDR version: Check your battery terminals are tight and clean, if everything good in that regard, remove battery and have it tested for free at Canadian Tire. Battery probably bad. Replace.
 
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Roadghost said:
Testing battery:

Press start button. Observe whether bike turns over.

If no, get new battery.
Click to expand...

Not ideal. Lots of things can screw up that sort of "test":

- Loose positive cable at battery or starter.
- Loose negative cable at battery or frame
- Wonky starter relay
- Wonky start button
- Failing starter

I could come up with more. Lots of stories of people out there who couldn't get their bike to start, ran to replace the battery...only to get the same result afterwards...issue wasn't the battery afterall, $100+ wasted. Moral of the story, always confirm a part is bad before replacing - the throwing parts at it method of diagnostics is very expensive.

The only 100% reliable test is to remove the battery and have it load tested.
 
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Appreciate all the positive and negative pov's. I am up and running, ergo, in a celebratory ride mood. K-W Bugout 10 here I come!!
 
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