Battery overcharging

I’ve noticed in the last couple of months that it would take longer holding the starter button to start the engine, but didn’t do anything about it. Recently when I was fiddling with the horn, I had to turn the ignition on and off about 10times to test it. When I finished and tried to start the engine, there wasn’t enough juice left. So, I charged the battery fully with optimate overnight. Was riding fine since then for a couple of weeks (30min commute to work, and then back), though it did feel again that it would take longer and longer for the starter to turn the engine (could be in my head).

Finally, decided to check the battery with a multimeter (attached picture - engine and ignition off, the battery wasn’t charged for 12 hours). I’m no expert, but that looks wayyy too much? Also, not sure if that’s normal, but the reading actually started around 15V and then slowly (30-60seconds) climbed to the figure in the photo were it remained more or less constant. Checked as well @5000rpm according to Haynes manual, which suggests it should be 14-15.5V, mine was more like 15-16V.

The battery is at least 3 years old (could be far more, but that’s when I bought the bike, and have no service history). I have no issue just buying a new battery, but from what I read on the internet an overcharged battery is likely to be caused by a faulty rectifier-regulator. I found the diode test for the rectifier-regulator plugs in the manual, would that be sufficient to prove if it is at fault for overcharging? How likely is it that the alternator is the culprit?

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Would suggest a new battery in multimeter first … low power of measuring instruments can give incorrect readings

If engine off should be a reading of 11.8v-12.8v +/- 10%
I would tend to look for Engine on should give a max (stable)reading of 14.8v 

Would suggest a new battery in multimeter first ... low power of measuring instruments can give incorrect readings TimR

I thought that picture of a battery in the multimeter display was showing up because I was testing a battery. Heh… I jest, of course. I’ll change the battery and give it another go.

If engine off should be a reading of 11.8v-12.8v +/- 10% I would tend to look for Engine on should give a max (stable)reading of 14.8v  TimR

Ok, so you’re saying I should check the reading when the engine is in idle.

If a 12V battery is holding 16v then there is an issue .
So you need to do a non running test
And a running test to see what comparison . (Follow haynes procedure for ease)

The multimeter readings are only as good as the multimeter and I’d suspect the multimeter here. Get a second opinion from another multimeter or check the multimeter against a known in fine fettle charging system.

Check the charging system voltage at a fast idle of 1,500 - 2,000 revs, I’d be looking for a reading of between 13v to 14.5v, but hey ho what’s 0.3v between friends?

Whoops, the 16.72v is just the battery power?

That’s not right

Yep, just the battery. Even disconnected the heated grips and the usb charger from it just to be thorough. I also took the battery out, cleaned the contacts, inspected it - no leaking or bulges that I could see (it’s a yuasa maintenance free battery).

I’m game to get another multimeter (had this one for almost 10 years). A mate of mine wants to buy one to work on his bike’s electrics, I could just split the cost with him. Any suggestions on a decent one?

why not try the multi-meter on a 9v bat or 1.5v bat see if seems about right?

The only way to check the battery voltage is to completely disconnect the battery and just have the multimeter prods across the terminals.

Check the charging system voltage at a fast idle of 1,500 - 2,000 revs, I’d be looking for a reading of between 13v to 14.5v

A decent multimeter? They all probably come from wherever it is multimeters are now made, China?

Fit  a new battery to the one you have and check it against another vehicle/battery.

Kewtech or Fluke are what we use at work

Afaik, the main difference between cheap and “decent” multimeters is durability and the number of functions. I can’t imagine there’s much difference in accuracy.

There is. Also the cheap ones ‘hold’ their accuracy less well, so as their battery degrades the percentage off they are goes wildly up, whereas the better quality ones stay more accurate for longer. Having said all that, if all you’re ever doing is checking continuity and testing bike batteries or levels below 12v and can live with 5% tolerance then a cheapish one will do the job. Just change the battery often…

Changed the battery on the multimeter - lo and behold, finally some readings that make sense:

12.16V disconnected battery

11.40V ignition on, engine off

9.13V lowest during engine start

14.40V engine on, idle 1.3k rpm

~14.70V @1.7k rpm

~14.40V @5k rpm 

According to Yuasa 12.10V means the battery is charged to 50%. That would mean I lost almost half the charge in the last couple of weeks even though the bike was in constant use. Also, according to them, if the voltage drops below 9.5V while cranking the engine, the battery should be replaced. So, I guess, it’s new battery time.

That’s right. But the battery voltage should not drop even below 11 volts at normal start, unless you have starting issues.
Happy to hear it is just the battery

Full marks

The thing to do is to check the numbers with the battery fully charged just so you have a reference point to start from.

Then check the voltage drop overtime.

The numbers are all good and Yuasa would say that wouldn’t they?

New battery fitted. Bike starts up instantly without fail - pure joy. Maybe it’s in my head, but it also feels like acceleration is more consistent.

Thanks everyone for the help. TimR gets 5 points for his attention to detail and his lightning fast spot-on response.

Now check system readings again … so you have a benchmark figure to go against next time you have issues !

Yep being a over observational pedant can help sometimes … but generally im just an annoying cunt

Re more power … petrol engines rely on a spark to work … this spark is generated  through the bikes 12v system … so if its struggling to provide that power at the desired oommphhh it will have a detrimental effect even if its minor … 

Same as a car that if the alternator  belt snaps (  as long as the water  pump still being driven ) that car will keep running until the battery is drained   but the lower the voltages get the car will start to lose power cough and splutter and keep trying to go until it finally dies …