Modern Buddy

I have a almost new lithium battery in my Buddy 50.
I left the key on overnight and it went dead.

How do I charge it. I have a battery tender but was told not to use on a lithium..
I can foot start it......will running the engine charge the lithium.

Starting the engine would normally charge the battery, if you ride it long enough. The problem is though, lithium batteries don't like to be totally discharged. There's a high chance that your battery won't charge again, even with an approved lithium charger. Doesn't hurt to try though.

Some lithium batteries have a "safety" built in to avoid a total discharge. Electric bicycles, lawn equipment, you phone. But I've seen some cheap lithium batteries for motorcycles not recover from a complete discharge. Get the proper charger for your lithium battery and charge it on the slowest setting possible. Hope it recovers!

Battery Tender Brand has a lithium charger. I had a similar situation where the lithium battery was dead and my lithium charger wouldn't touch it. So much for "smart" chargers. What I did was connect an old AGM battery to the lithium and hope some electrons transferred. They did! It was just battery to battery, plus to plus, minus to minus. Any car or bike battery would have the same effect. After sitting all night there was enough voltage to allow the smart charger to do its thing. It has been holding 13+ volts ever since.