External battery for electric start

[Excess Impulse]

With the arrival of winter and riding season a few months off, I'm hoping to use the time to do some maintenance and upgrades to a couple of my bikes. I have a 1980 XS400 that I have enjoyed working on as my first ever build, transforming it from a cruiser to what might be referred to as a scrambler. It has been a very enjoyable and fun experience, and without this forum, I would have never attempted it, as all my information and ideas have come from what others here have done and helped me with along the way. As you can see from the photos, I have removed (among many other things) the original battery and am running a very small battery that would never be able to run the starter, so it is kick start only. I still have the starter in the bike. When the weather is warm and I ride it often, it starts without much trouble, but since last fall I have had difficulty starting it, and I haven't had it running since September. I don't want to put a bigger battery in it, but it sure would be nice to have the electric start for the times I can't kick start it. I had this idea to have an external battery, and have drawn up a rough sketch. I'm no electrical expert but it seems to me this should work, I could have the starter solenoid on a quick disconnect and just plug and play with the original harness and starter switch still on the bike, and unhook it as soon as it gets running. The only things that would stay on the bike would be the starter with the quick disconnect plug on the positive wire. Can someone who knows tell me if this is going to work or not? Do I need a fuse anywhere? Ideas, suggestions, improvements? Thanks for taking the time to look.

 

[BBS360]

Reminds me of the external starter motors they use for some races.

You would need to connect the bigger battery's ground to the starter motor's ground. It needs to be the same #6 cable. Maybe find some welding cable for both. It will be much more flexible and handle repeated use better.
Could maybe find a two-prong plug with a sufficient ampere rating for the positive and ground.

I can see why you want to do this but it does not seem ideal. What happens if you leave the starter battery at home and then can't kick it when you're away?
Make sure all the starter solenoid terminals are thoroughly covered so there's no possibility of shocks or shorts.
It's only 12V but there's a lot of current available in those batteries.

 

[sandmanred]

+1 on ground from the battery to the frame. Check the cold cranking amps on the external battery. It's probably 5-6 times that of the one that would normally be with the bike. You might want to fuse the main lead with something close to what your stock battery is a capable of. Or if you're always at home when you do this maybe just build/buy yourself one of these. If you make it the shape of the top picture it will tip up for storage.

 

[Excess Impulse]

+1 on ground from the battery to the frame. Check the cold cranking amps on the external battery. It's probably 5-6 times that of the one that would normally be with the bike. You might want to fuse the main lead with something close to what your stock battery is a capable of. Or if you're always at home when you do this maybe just build/buy yourself one of these. If you make it the shape of the top picture it will tip up for storage.

That's pretty cool, a little more elaborate than I was thinking but I like it!

 

[Excess Impulse]

Reminds me of the external starter motors they use for some races.

You would need to connect the bigger battery's ground to the starter motor's ground. It needs to be the same #6 cable. Maybe find some welding cable for both. It will be much more flexible and handle repeated use better.
Could maybe find a two-prong plug with a sufficient ampere rating for the positive and ground.

I can see why you want to do this but it does not seem ideal. What happens if you leave the starter battery at home and then can't kick it when you're away?
Make sure all the starter solenoid terminals are thoroughly covered so there's no possibility of shocks or shorts.
It's only 12V but there's a lot of current available in those batteries.

I was thinking the bike's starter motor was grounded to the frame through the mounting bolts? So running a ground cable from the remote battery to the bike's frame should work? I will have to remove the starter if I do this and make another cable anyway as I was being lazy when I stripped the bike down and just cut the positive cable. The remote battery will be the one in my car and I would just use clamps like a jumper cable has to connect to the terminals, so a fuse is probably a good idea. I have seen the quick disconnect connectors on amazon and ebay. The reasoning behind the remote battery is even in the summer if I don't start the bike for a week or so it becomes hard to start. Once it has been running it starts pretty easy, even a day or so later, it just doesn't like to sit! I use Stabil 360 in all my bikes so it's not bad gas. I wasn't sure if I needed to ground the start solenoid as it won't be mounted to the bike, but it looks as though it grounds through the bike harness. Also not sure if it would fry my little battery in the bike. Thank you for the replies.