Topic: CB550 Rebuild Charging Issue

[PatrickZ]

Hey there good people of SOHC!

I wanted to pick your collective brain power on corrective actions for my new CB550 Rebuild into a sweet (yet unfortunately not yet well functioning) cafe racer. Bike was a 1975 CB550E, and I'm aware of their marginal charging system.

The Bike mods:

• Replaced reg/rec with Rick's combined solid state unit 14_100 for lithium batteries
• New lithium battery AGM, 29WH (2.4 Ah) capacity
• Lights replaced with LEDs
• Installed motogadget mblue unit, mostly new harnesses
• Existing alternator
• Dyna 3ohm coils

Symptoms:
Bike dies after 20-30 min of city riding (stop / go), 30-40 min of highway riding (at higher rpms). I wired in a voltmeter LED while riding, and even while rev'ing above3-4K RPM, I observe the following: I can maintain 12-13v in the first 10 min, then 11.9-12.8v in next 10 min, and then 10.5 - 12.2v even at high RPM, then the bike dies.

Testing:
I ran a modified version of TwoTired's test (accounting for my mods). My rested battery measured 12.5v (2 hours uncharged). With LED lights off, I ran RPM & voltage test:
Bike on 12.5
Idle 12.3
2500 13v
3000 13.1v
4000 13.3v
5000 13.4v

I should be seeing voltages >14v at 4000+ RPM but I'm not.

Reg / Rec
Could not do diode test. White/Green resistance confirmed at 4.9 ohms.

Alternator
Visual inspection seems OK. White/Green resistance at 5.0 ohms. 3 yellow wires resistance at 0.9-1ohm. This is higher than 0.35 specified by manual, but I'm also doing it with an extra foot of harness in there because I'm measuring it up by the reg/rec.

Hypothesis:
The demand on the electrical system is higher than the supply can accommodate, even at higher RPMs. The primary cause is likely the 3 ohm coils, which I should probably replace with 5 ohm coils. Alternatively, or as a contributor, the R/R could be set at too low a limit and clipping the voltage at <13.5V. Finally, I could have some wear in the alternator or its connections that are contributing by reducing its function.

My questions to the group are:

• Could the alternator be contributing given its higher resistance? Or is that resistance increase too minor? 
• Is the rectifier limiting the charging at too low a voltage and causing this issue? Why am I not getting 14V at 4000 RPM? 
• Or is it truly the coils primarily that are the issue?

Thanks in advance for your help!
Patrick

[scottly]

Disconnect the white field wire from the regulator and measure the resistance from the wire to the battery negative terminal.

[PatrickZ]

I'm getting 0L / open circuit between the field coil side of the white wire and battery negative terminal.

Getting 107 kohms from RR side of the white wire to battery negative terminal.

Thanks!

[scottly]

I'm getting 0L / open circuit between the field coil side of the white wire and battery negative terminal.

This is a common mistake with custom wiring. On the stock wiring harness, all solid green wires are connected to ground (batttery negative). Rick's has a diagram that shows the green reg wire tied to the field green wire, not to ground. You should read the same 5 ohms as across the white and green field wires.

[PatrickZ]

Thanks Scott.

I wired in a new GND wire in the connector to get both the field coil and RR greens to ground. Now reading 5 ohms from field coil white to battery gnd.

Ran the bike up to 5000 rpm and still only seeing 13.3V at the battery though.

Thoughts?

[scottly]

With the engine not running, key on, measure the voltage between the reg black wire and battery plus. This should be very low.

[PatrickZ]

Hey Scott,

Unfortunately I can't unplug that wire from either end, I'd have to cut into it. I'd prefer to avoid and minimize resplicing, unless I have to take the RR out. What's your thinking, it's the RR and not the extra draw from the coils?

[scottly]

Is the black reg wire spliced to the harness? My thinking is there is a voltage drop through the M-unit feeding the reg. Just for grins, jump the white field coil wire directly to battery plus, and check the charging voltage with the engine running. Note that this test bypasses the regulator, so watch the voltage closely and shut it off if it goes over 14V, and disconnect the jumper.

[bryanj]

I wrote a comprehensive charging circuit test its in the faq section on the UK site, see if that helps.
Personaly i dont like lithium batteries, seems to me that a lot of people post problems

[PatrickZ]

Is the black reg wire spliced to the harness? My thinking is there is a voltage drop through the M-unit feeding the reg. Just for grins, jump the white field coil wire directly to battery plus, and check the charging voltage with the engine running. Note that this test bypasses the regulator, so watch the voltage closely and shut it off if it goes over 14V, and disconnect the jumper.

Scott,
Jumped the wire to battery positive. Voltage still reading 12.5-12.8v even at 4000 rpm. So I think we can eliminate the RR and Motogadget as the source of the voltage drop.

[nicks2319]

Here's a link to the commonly shared charging system checklist

https://gist.githubusercontent.com/nicholasserra/1687632607928119f1a5/raw/c84f6eaca655a41be8d7451d9ac1d6f12f420384/gistfile1.txt

[scottly]

So it's lower with the reg bypassed?? If the battery is run down, it will take time to recharge to full voltage, so testing should be done with a fully charged battery. You mentioned LED lights in the first post; does this include the head-light? Do you have a Dyna ignition, or just the Dyna 3 ohm coils with points?

[PatrickZ]

You beat me to it Scott.

While diagnosing the supply side, I've also been measuring the demand side as well. I built a power budget in Excel and went through the bike verifying that I'm seeing the expected demand.

The field coil, Dyna ignition & coils and LEDs all check out as expected. And then I got to the headlight. It still had the 90w halogen (which came with the new headlight)  that I'd forgotten to swap out with the 15w LED bulb.

With the extra 90w load from the light being on, I can't imagine the charging system had a chance.

I still can't get the voltage above 13.5V at higher RPM. But I'm going to try riding it tomorrow around the neighborhood and see if I still see the rapid battery depletion going on.

[scottly]

Something wasn't quite adding up.

[PatrickZ]

Finally got a 15w LED in the mail. Swapped it out today and went for a ride. Consistently seeing charging at 14.5v at 4k rpm, so everything looking like it's working beautifully!