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Author Topic: Battery dying but battery seems to be good  (Read 2583 times)

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Offline luwys

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Battery dying but battery seems to be good
« on: December 28, 2018, 11:26:31 PM »
Hey guys, i am new to this forum and hoping to gain some knowledge on my bike that i am having trouble with. I have a 81’ honda cb750k and it runs beautifully but am having issues battery wise i bought a new battery for the bike as the one it had before was dead and with the new battery the bike cranked over nicely and rode great for a while but a day or two later after a good ride i parked the bike to get some lunch and when i came back the battery was dead. I left nothing on so i was a bit confused i got the battery recharged and everything seemed great battery held charge but when i put it on the bike and had a good ride and parked it the bike died on me again. Im not sure where to start my bike has this weird 2 kill switch side thing so i am not sure if there would be an issue there i hope someone could enlighten me
Thank you!


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Offline Stev-o

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Re: Battery dying but battery seems to be good
« Reply #1 on: December 29, 2018, 10:07:56 AM »
First off,  welcome.   I believe your bike is a DOHC and this website is devoted to SOH. Models but this check will work on all bikes:

Get your voltmeter and set it to DC.    Start your bike and let it idle, check voltage at battery, should be 12-13V.   Then hold a fast idle at 2500 rpm, check again, voltage should go up if your system is charging, I suspect that it is not. 

Report back and good luck!
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'74 "Big Bang" Honda 750K [836].....'76 Honda 550F.....K3 Park Racer!......and a Bomber!............plus plus plus.........

Offline innovativems

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Re: Battery dying but battery seems to be good
« Reply #2 on: December 29, 2018, 10:41:26 AM »
The dohc Honda’s are notorious for rotors and rotor brushes going bad which lead to charging issues. 


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2007 KTM 990 adv
2003 Honda RC51
2000 Yamaha YSR50
1978 Honda Express
1975 Honda CB400f
1974 Honda CB350f
1974 Kawi H2
1974 Honda CB550f
1971 Kawi h1
1973 Kawi h1
1974 gt 550
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Offline luwys

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Re: Battery dying but battery seems to be good
« Reply #3 on: December 29, 2018, 03:18:16 PM »
Okay so i put the volt meter before turning on the switch and it was at about 12.47 and then when i turned the switch it went down when i cranked the bike over and running  and began reving to 25000-3000 the voltage read from11.67-12.09 any ideas on what is going on here?


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Offline innovativems

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Re: Battery dying but battery seems to be good
« Reply #4 on: December 29, 2018, 04:10:20 PM »
Yeah your charging system isn’t working.  Next step, test the ac output of your stator


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2007 KTM 990 adv
2003 Honda RC51
2000 Yamaha YSR50
1978 Honda Express
1975 Honda CB400f
1974 Honda CB350f
1974 Kawi H2
1974 Honda CB550f
1971 Kawi h1
1973 Kawi h1
1974 gt 550
1974 gt550

Offline luwys

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Re: Battery dying but battery seems to be good
« Reply #5 on: December 29, 2018, 04:58:00 PM »
What should i look for ?


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Offline Stev-o

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Re: Battery dying but battery seems to be good
« Reply #6 on: December 30, 2018, 10:16:53 AM »
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'74 "Big Bang" Honda 750K [836].....'76 Honda 550F.....K3 Park Racer!......and a Bomber!............plus plus plus.........

Offline luwys

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Re: Battery dying but battery seems to be good
« Reply #7 on: January 09, 2019, 12:05:13 PM »
Hey guys so i got the new rotor in there and i tested it and it tested good i turned over the bike and the battery isnt charging up past 12 when i rev it up to 2500-3000 rpm do any of you guys know what to look into ?


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Offline luwys

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Re: Battery dying but battery seems to be good
« Reply #8 on: January 09, 2019, 12:10:23 PM »
Let mw rephrase its charging up to about 12.3-12.4 on the multimeter when i rev the bike 2500-3000 is this normal or is there something else i need to fix


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Offline calj737

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Re: Battery dying but battery seems to be good
« Reply #9 on: January 09, 2019, 12:16:23 PM »
Sounds low. What is the voltage with the rpms held at 4,000?
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"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline luwys

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Re: Battery dying but battery seems to be good
« Reply #10 on: January 09, 2019, 12:20:03 PM »
Its reading 11.94


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Offline calj737

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Re: Battery dying but battery seems to be good
« Reply #11 on: January 09, 2019, 02:12:21 PM »
You’re not charging.
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'74 550 Build http://forums.sohc4.net/index.php?topic=126401.0
'73 500 Build http://forums.sohc4.net/index.php?topic=132935.0

"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline luwys

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Re: Battery dying but battery seems to be good
« Reply #12 on: January 09, 2019, 02:13:22 PM »
What should i look for?


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Offline Stev-o

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Re: Battery dying but battery seems to be good
« Reply #13 on: January 09, 2019, 02:57:13 PM »
Here is a very good diagnostic "how to".

Click here....

http://www.oregonmotorcycleparts.com/FAQ.html
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'74 "Big Bang" Honda 750K [836].....'76 Honda 550F.....K3 Park Racer!......and a Bomber!............plus plus plus.........
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