XJ Charging System vs Lithium Batteries

Hi all,

I have had my 1982 XJ650LJ (Seca Turbo) since 2018 and its been great fun to ride. The lead-acid battery i bought when I first got the bike was getting old and having a hard time starting the bike if I let it sit for too long, so I thought I would upgrade. I did some research and debated for a while but ended up buying an antigravity lithium battery for it. I installed it and went for a ride.

Not long into the ride it blew the 20amp charging fuse and I knew I had a problem. I read a handful of threads about lithium batteries in these bikes and found that although an aftermarket regulator/rectifier was recommended, most people seemed to have no issues with their stock unit charging the lithium battery. More research turned up that these aftermarket R/Rs are functionally the same as the stock unit but with the voltage setpoint adjusted down about 0.5v to suit the lithium cells better.

So after some digging on my bike, I find that the stock charging system is charging my lithium battery up to about 15.2 volts. With my old lead acid installed, it charges right at 14.5 volts. Is there something wrong with my charging system? Or is this just an incompatable battery for the stock XJ system? I did check voltage at the brown sense wire for the R/R and found it to be about 0.8 volts less than the battery voltage, so its possible this is causing the higher charging voltage. But I'm having a hard time understanding why the R/R charges at 14.5 with my old battery and suddenly charges at 15.2 with the new one.

Any ideas? Suggestions? I might just buy another new lead-acid and be done with this but if I can easily make the lithium work I would like to keep it.

 

I'll guess the Lithium has a different (higher?) internal resistance..

 

Lithium batteries require a higher charging voltage, this often plays havoc with the electronics on older bikes which where not designed to cope with those higher voltages. There indeed are proper regulators to feed the lithium battery the voltage it needs but as stated, the other electronics are not designed to work with them.

Moral of the story, stick to a lead acid battery, go with an AGM as your best choice.

 

I understand this sentiment, and probably will end up getting another lead-acid battery...

But I'm just wondering if somebody can explain conceptually why it works like this. In my mind the brown wire going into the R/R is the sense wire and the R/R adjusts the alternator field voltage to achieve 14.5v +/- 1v on this brown wire. This is true on my bike with the lead-acid battery installed.

This is also true with the lithium battery installed except that when the brown wire is set at 14.5 volts by the R/R the battery voltage is 15.2 v. Why the difference?

 

Looks like a100man has the best answer on that one. If the regulator is connecting and disconnecting based on resistance and not voltage then a lower resistance battery would run up to a higher voltage since the resistance of a battery gets lower as it's state of charge increases.