Topic: Battery weak after long idle 78 CB750K

[cjackel]

I was stuck on the freeway and moved less than mile
in about 1 hour. I was idling and a couple of times I let
the clutch out too fast and killed the engine. I used
the electric start and it was fine for the first few restarts. But
after doing this for a while (turning bike off while traffic
stood still) I tried to start the bike and the battery was too weak. I had
lights, but the starter would barely turn over. I used
the kick starter and the bike started fine.
After the accident was cleared, I rode for 50 miles at 60mph
and parked the bike. When I went home at the end
of work, the bike started right up with electric start.
I keep the battery topped off and on a tender,
so the battery should be ok.
My question: Did the long idling in traffic cause the
battery to discharge? Do I have a rectifier or voltage
regulator problem or is this typical of the CB750 charging
system?

[Bob Wessner]

I don't think this is unusual. The batt won't get much of a charge if the engine is running below a certian RPM (don't have my manual handy to check) but it is well above idle. On top of that, the elec. starter is a huge draw. I think the best course of action under similar circumstances would be to shut it down if you are going to be stuck for a while and use the kick starter. You need some juice left just for ignition until you can get up to speed again and start recharging the battery.

Oh, maybe to satisfy yourself there are no other problems, you can check the charge output at various RPM's against specs.

[nashvegas]

Bob's right.  There is a break point (on my 400F I am pretty sure its 3000 RPM) where you stop pulling juice and start putting it back in.  You're all good.

[Bob Wessner]

cjackel,

Here is a chart covering up through the K7 that might help. Hope you can make it out.

[eldar]

You can change regulator settings to help CONTROL the amount of draw at idle. But this is a touchy thing as you cant go too high or else you will damage your battery when it is under normal charge. SO the best thing to do is to rev your engine every so often to and this will help charge a bit. Also you can unplug your headlight to save juice as well.

Another thing is to change your tail and signal lights with led bulbs. Anything to help save power.

Bottom line though is that you will never be able to KEEP your battery charged at idle but you can lessen the power draw.

[TwoTired]

cjackel,

Here is a chart covering up through the K7 that might help. Hope you can make it out.

Just a note about that chart.  There is a printing error.  The amp numbers in the 1000 RPM column are actually negative numbers.  The minus sign was was omitted.  The battery drains at idle, as the alternator, even with full 12V field voltage, doesn't make enough power to overcome the standard electrical loads.  No amount of voltage regulator tweaking will change that, as the regulator is already applying the full 12V to the field coil whenever the battery voltage is lower than 13.5 V.  If you want more power from the alternator, you have to spin it faster.

Need I say that 3 ohm coils, higher wattage headlight, etc. will increase the idle drain on the battery?

Nah, I didn't think so.

[Bob Wessner]

Good to know, thanks.

[Uncle Ernie]

I might mention that you need to check battery fluid levels when it's on a Tender. I guess the Battery Tender won't boil the battery, but it will keep it warm enough that fluid will evaporate and kill it that way. I didn't find this out by reading a brochure or manual.