CB750K battery draining! HELP!
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CB750K battery draining! HELP!

This is a discussion on CB750K battery draining! HELP! within the Site Help and Support forums, part of the Caferacer.net Forums category; Hello everyone!, New to the forum, have tried everything i can possibly think to fix this issue, you guys are my last hope. It all ...

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Thread: CB750K battery draining! HELP!

  1. #1
    Junior Member Nanahan's Avatar
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    CB750K battery draining! HELP!

    Hello everyone!,

    New to the forum, have tried everything i can possibly think to fix this issue, you guys are my last hope.

    It all started when i was heading home one day and my battery exploded! It was a lead acid battery.
    I replaced it with a NEW lithium ion battery with a battery tracker that records data on an app. On the ride home the motorcycle died on me,(through the app i noticed i was getting 18V to the battery) so i assumed the rectifier/regulator went bad, so i replaced it with one that works with lithium batteries. When I charge the battery at home with a special charger for lithium batteries the battery holds its charge. the battery is a Antigravity 12v Lithium 8 cell.

    This is what happens now.
    The motorcycle is able to run when i have the ignition in ON (headlight off), not IGNITION (headlight on).
    Once i turn the ignition key to IGNITION and the headlight turns on, the battery rapidly drains. and if i accelerate, the voltage on the battery doesn't rise. I already checked the field coil (7.2 ohms)and stator (not grounded, reads 1.0 OHMS between leads minus 0.4 ohms between the voltmeter leads). I have replaced the stator and field coil and rectifier/regulator connectors with new ones.

    The motorcycle is a 1976 honda CB750K SOHC

    Is there something im missing to check? I'd greatly appreciate your feedback.
    The stator cover is currently off right now, is there anything i should check before i reinstall it?

  2. #2
    Senior Member woodsman's Avatar
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    I don't know a lot about Li batteries in bikes. What I have read, in my understanding, is changes are required to the bikes electrical system before you hook up the battery. 18vdc charging may have fucked that battery, they won't take overcharging, it's actually dangerous.

    It sounds to me like you had problems with your charging system initially, pumped 18vdc into the battery 'til it cried Uncle. Then threw in an Li battery to solve the problem.

  3. #3
    Senior Member TrialsRider's Avatar
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    Your original battery either blew up because it froze, shorted to ground, or it boiled dry. Pretty much had to be one of those :|


    Always begin your electrical trouble-shooting with a wiring diagram :|
    http://manuals.sohc4.net/cb750k/tech...ce/WD750K6.pdf

    I see a 3 phase alternator connected to a rectifier plus an additional field coil that is directly connected to a regulator.

    Logic would say test the 3 yellow wires for both continuity (there should be a known resistance between each yellow wire and ground) that will tell you if any of the coils is shorted.
    Next you crank the engine over and measure the voltage output from each of those 3 yellow wires (typically that will be AC volts in the vicinity of ~17 Volts)

    Next you test that 4th. field coil (white wire according to the diagram) again test the resistance of the coil as well as the voltage output which is headed for the regulator.
    If we are all good so far we know the problem is Not the alternator.


    Disconnect your horn, they draw a lot more power then one would expect and they are prone to causing an electrical short. (if the horn works good now it's probably ok)
    Last edited by TrialsRider; 12-02-2019 at 08:58 AM.

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  5. #4
    Senior Member TrialsRider's Avatar
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    Read me: CB750 ch8- charging system.pdf

    add: normal operating voltage for your battery is 13.3-14.5V
    according to the web :| battery should never see 14.7 or it might go poof!
    Last edited by TrialsRider; 12-02-2019 at 01:23 PM.

  6. #5
    Senior Member DesmoDog's Avatar
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    Vintage bike with vintage charging system running lithium battery is a bad idea. Also, 18v is too much for a lithium battery.

    Replace the voltage regulator with a modern one (mosfet) AND add a voltage meter.

    This is from the FAQ on EarhtX's website. Their batteries have built in protection BTW.

    Can I use a lithium battery in an older vehicle (before 1986)?

    NO. Unless you have replaced the charging system with a new modern permanent magnet generator and rectifier regulator. It is very important to only use a lithium battery in a vehicle that has a modern charging system that will regulate the voltage between 13.8-14.6V. Older bikes/vehicles that have not been updated are not advised to use a lithium battery as the voltage will swing too high and too low at times and damage the battery.
    Find the whole thing here:
    https://earthxbatteries.com/faqs

    If you went cheap and bought a battery with no protection, and are running it on a system that's pumping 18v into it, it WILL melt down in spectacular fashion.

    The regulator on my 851 went, and I started pumping massive volts into the EarthX battery I had. It overpowered the protection and the battery melted down. As I understand it, an over simplified explanation is with the excess voltage the battery begins plating it's own internals until it shorts itself out. You basically connect the negative and positive terminals internally. This is not a fun situation to be in as once it starts there's not a damn thing you can do until all the smoke leaks out. And that smoke REEKS by the way.

    In my case the bike stuttered, then died. I pulled off to the side of the road and as I was removing the seat to check things out, I noticed a little wisp of smoke. Then things got exciting as the smoking and arcing (I could see the battery self destructing behind the fairing) increased as I tried to pull the fairing before anything started burning. Fun fact - you can't pull the fairing off an 851 with the factory tool kit. Luckily this happened right in front of a motorhead's house who looked out the window and thought "That doesn't look like steam" and came out to help/supply tools.

    This is what it looks like after all the arcing stops but the melted stuff is still smoking;

    http://teamyikes.com/851/EarthX.mov

    That link probably won't work because my website isn't secure anymore, but trust me, it's not something you want to have happen.
    Last edited by DesmoDog; 12-02-2019 at 06:29 PM.
    -Craig
    The Mighty Monza Jr. Thread: https://www.caferacer.net/forum/proje...r-project.html

  7. #6
    Senior Member TrialsRider's Avatar
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    Is probably not a great thing that you know what the toxic white smoke smells like :| I recommend that maybe the next time you run away.

  8. #7
    Senior Member DesmoDog's Avatar
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    I could smell it for a couple days after that. Probably not the healthiest experience of my life. And the stench was ingrained in the plastic parts afterwards too. I pulled the battery box off and let it sit outside, plus scrubbed it off with a stiff brush. Doesn't smell anymore but the plastic parts are scarred for life.

    The scary part is these batteries are used in airplanes. I'm guessing they aren't allowed to be installed in the passenger compartment. That would be a VERY bad day...
    -Craig
    The Mighty Monza Jr. Thread: https://www.caferacer.net/forum/proje...r-project.html

  9. #8
    Senior Member Geeto67's Avatar
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    Quote Originally Posted by DesmoDog View Post
    Vintage bike with vintage charging system running lithium battery is a bad idea.
    you know, it's funny, This weekend I was over at someone's house looking at their VW powered motorcycle because I am in the process of working on one myself. He was running a $100 Amazon (MMG) Lithium Ion Battery to start the bike. I asked him about his experience with the battery and he's had it a while now and it cranks the VW engine over easily (300CCA) and he is running the old VW 12v generator (post 1968 Type 4 engine), points ignition, and what looked like the stock VW reg/rect. The battery says int he spec sheet do not charge past 15V, and the stock VW generator would be 30amps and 14.2-14.5 V max.

    Since I need a small battery with 300 CCA minimum, I was thinking about picking one of these up until I saw your comment about old vehicles and lithium ion batteries. I am curious, is it just because the electrical systems in old bikes are failure prone to age or something else? as long as charging is within parameters, it should be fine, right?
    TrialsRider likes this.
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  10. #9
    Senior Member TrialsRider's Avatar
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    Lithium battery technology is closer to a capacitor then to a lead acid storage battery.
    Optimum operating temperature for lithium ion is +5 to +45 degrees C, they don't work or store so well in sub-zero.

    ... something to think about: Lead acid batteries are water cooled, dry chemical batteries are not, a fully charged lead acid battery will freeze at -70C




    "Figure 1: Discharge voltage of an 18650 Li-ion cell at 3A and various temperatures.
    Cell type: Panasonic NRC18650PD, 2.8Ah nominal, LiNiCoAlO2 (NCA)
    Source: Technische Universität München (TUM)"

  11. #10
    Senior Member DesmoDog's Avatar
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    Quote Originally Posted by Geeto67 View Post
    Since I need a small battery with 300 CCA minimum, I was thinking about picking one of these up until I saw your comment about old vehicles and lithium ion batteries. I am curious, is it just because the electrical systems in old bikes are failure prone to age or something else? as long as charging is within parameters, it should be fine, right?
    As I understand things, it's because the older charging systems didn't regulate the voltage all that well even when they are working. Any time it starts plating it's internals it's cumulative damage - even if the voltage drops, the damage is there to build from the next time (<- That's entirely my idea, no one told me this). I would think if it doesn't see too much voltage or too much discharge you'd be fine. But I'm no expert on this stuff, just an "experienced" user. For how much a modern regulator cost it wasn't a big deal to add it (especially since my original was truly fuckticated). (That's a technical term for FUBAR'D) I'd also add a volt meter as mentioned. I'm modifiying the one on my 851 to plug into the SAE connector for the battery charger. I tried hooking it up to a feed from the dash, but the voltage drop was too much for my liking. (30 year old bike...)

    FWIW I replaced the battery that melted down in my 851 with another EarthX lithium battery. But I also added a volt meter and replaced the regulator. And the ECU. And a bulb or two, And a relay. And the fuel pump. Were they all related to the over charging? Maybe - though I the reason I had to replace the ECU is some sort of over voltage protection dealie had literally fallen off the circuit board and was rattling around in the box. Coincidence? Doubtful. All of which says more about 30 year old Ducati regulators than batteries I suppose but still.

    I'm still a big fan of the technology, it was just a relatively expensive learning curve.
    -Craig
    The Mighty Monza Jr. Thread: https://www.caferacer.net/forum/proje...r-project.html

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