Bike to Bike fast trickle charge

[madnance]

Last fall my battery was in bad shape and it would not start my bike. I was with friends in the middle of nowhere. One of them connected about a 14 gage wire between our bikes to do a fast trickle charge and about 10 mins later I had received enough juice to start it. Those little wires would not have carried enough current for cranking, but over 10 mins pumped enough juice into the battery to crank by itself. Hurray! I am now $175 shorter in riches but have a new battery!

 

It made me think it would be ideal to be able to do the same thing plugging between two BMW bikes using the accessory plug. Its pretty hard to get to the battery terminal on some of these. However, the accessory plugs are limited to 15 amps by fuse. This seems OK, since over 20 mins you'll deliver 15*20/60= 5 amp-hours of juice at 15 amps, which is about 1/4 of the battery's 19 a-h. It could get you out of a jam easily. You would basically connect two BMW trickle charger leads by their SAE plug.

 

But how could you control the current to 15 amps max so you don't blow the fuse or smoke the connecting wires? I'm not an electrical engineer but some current limiter on a bike-bike connection seems like a good idea for this application.

 

Has anyone tried this? Or is this a bad idea?

[Boffin]

Back in 2004, BMW roadside assistance (in the UK it is the same service as for the cars 5* ) came out to me so sort out a flat battery - I had switched to parking light without noticing. The technician pulled his 5-series estate (brake) next to my bike and plugged a lead from the car into the accessory socket. He then made me a cup of tea, and when we had finished the tea, disconnected the lead and the bike fired up.

The current is self-limiting, based on the batteries internal resistance and the voltage difference. The voltage on a flat battery without load is only a couple of volts below a good battery - I would expect no more than four or five amps to flow, though I have never measured it.

 

Andy

[bobbybob]

Be VERY careful when connecting SAE plugs between 2 different bikes. The male negative terminal from one bike will mate up perfectly with the female positive terminal from the other. DON'T do this. This will release the magic smoke (i.e. sauteed RT)

 

It will be VERY easy to reverse the polarity. Don't try it unless you are electrically knowledgeable, have a multimeter, and hand wire the SAE adapter cables to insure polarity is observed (i.e. pos-to-pos, neg-to-neg) Basically you will need to create an SAE "patch cable" to connect between the two SAE pigtail leads from each bike, so that the patch cable keeps the polarity correct. Use your multimeter to test for polarity before connecting.

 

If done correctly, the excess amps are not a problem, as stated above. In fact the more the merrier--it will take less time to get a starting charge transferred. Just don't walk off and forget about it--try to crank it every couple minutes until it starts, then disconnect immediately.

[Boffin]

Be VERY careful when connecting SAE plugs between 2 different bikes. The male negative terminal from one bike will mate up perfectly with the female positive terminal from the other. DON'T do this. This will release the magic smoke (i.e. sauteed RT)

 

It will be VERY easy to reverse the polarity. Don't try it unless you are electrically knowledgeable, have a multimeter, and hand wire the SAE adapter cables to insure polarity is observed (i.e. pos-to-pos, neg-to-neg) Basically you will need to create an SAE "patch cable" to connect between the two SAE pigtail leads from each bike, so that the patch cable keeps the polarity correct. Use your multimeter to test for polarity before connecting.

 

If done correctly, the excess amps are not a problem, as stated above. In fact the more the merrier--it will take less time to get a starting charge transferred. Just don't walk off and forget about it--try to crank it every couple minutes until it starts, then disconnect immediately.

 

I did't spot the SAE bit - I assumed the coaxial BMW accessory socket on both ends.

 

DO NOT try and crank the bike with lightweight leads connected - they WILL get red-hot and could cause fire or injury. Disconnect before cranking.

 

Andy

[bobbybob]

I agree--unplug the SAE's prior to each start attempt to be safe, esp. if the wire gauge is small. This would not be necessary if you were using typical alligator clamps to battery terminals, however, as the lack of surface contact with the clamps would limit the amps that could be transferred anyway. Done this many times using a small gauge moto jumper I made, and the worst ever happens is the cables get slightly warm to the touch.

[Selden]

Be VERY careful when connecting SAE plugs between 2 different bikes. The male negative terminal from one bike will mate up perfectly with the female positive terminal from the other. DON'T do this. This will release the magic smoke (i.e. sauteed RT.

I did this a couple of weeks ago. The Hawk and the RT were parked side by side, and I thought I was transferring the Battery Tender lead from the Hawk to the RT. Zap, poof! The fuse on the RT did its job. While replacing it, I realized that the previous owner had replaced all the fuses with 15 amps, which wasn't a good idea (surprisingly to me, the grip heater circuit only specifies a 4 amp fuse).

[madnance]

Thanks. I didn't realize that SAE connectors could be mismatched on polarity! So I need to make a patch cable. What gage wire do you suggest? For 5 ft of length and 5% loss I come up with this for max amps. Should not smoke the wire:

 

gage__Amps____mins for 5 A-hr charge

18____4.3__________71

16____8.5__________35

14___ 14___________21

12____23___________13

10____34____________9

 

I've got some SAE cable designed to hook to the battery and noticed they are 16 AWG. But the BMW trickle charger lead is 18AWG, so it does not look like a good choice incase the current could go over 4A. Something that goes BMW to BMW or optional to alligator clips would cover most needs I think.

 

Also I have a question for those who have done this: Should you turn off the donor vehicle so you are not inflating the voltage to the alternator's output on that vehicle? I enjoyed the story about the BMW guy serving tea! Did he turn off his engine?

 

Thanks

[Boffin]

I enjoyed the story about the BMW guy serving tea!

 

This is England old boy....

 

 

Did he turn off his engine?

 

Thanks

 

Yes, he did.

 

Andy

[madnance]

Praise the fuse gods Selden. I would have tried the same thing! BTW my BMW manual says the "power sockets" are fused with 15amps.

[madnance]

Thanks. I didn't realize that SAE connectors could be mismatched on polarity! So I need to make a patch cable. What gage wire do you suggest? For 5 ft(CORRECTION: 6ft) of length and 5% loss I come up with this for max amps. Should not smoke the wire:

 

gage__Amps____mins for 5 A-hr charge

18____4.3__________71

16____8.5__________35

14___ 14___________21

12____23___________13

10____34____________9

 

I've got some SAE cable designed to hook to the battery and noticed they are 16 AWG. But the BMW trickle charger lead is 18AWG, so it does not look like a good choice incase the current could go over 4A. Something that goes BMW to BMW or optional to alligator clips would cover most needs I think.

 

Also I have a question for those who have done this: Should you turn off the donor vehicle so you are not inflating the voltage to the alternator's output on that vehicle? I enjoyed the story about the BMW guy serving tea! Did he turn off his engine?

 

Thanks

[moshe_levy]

Someone beat ya to it.

 

Bob's BMW has a "Battery Life Line" in page 2 of its hard-copy catalog (download here http://www.bobsbmw.com/catalog/index.html ) - 8 foot cable with BMW plug at each end for just this purpose, p/n JC-1, @ $33.00 (printed price - may not be accurate now).

 

-MKL

[madnance]

Very cool. Thank you!

[bobbybob]

I assume the ignition switch would have to be turned on for this to work because of the Canbus system?

[JR356]

Why bother with all this,when you can get a set of good quality MC jumper cables of adequate gauge from any auto parts store for about $10-12.Even come in a carry case.

Store em under the seat or keep in a saddle bag for those just in case moments.

 

JR356

[moshe_levy]

I assume the ignition switch would have to be turned on for this to work because of the Canbus system?

 

Bob-

 

Honestly, I have no idea. The Bob's catalog was printed in early 2005 IIRC, so it's really mostly dedicated to pre-CANBUS bikes. I fully agree with JR356's suggestion for true dedicated jumper cables.

 

-MKL

[EddyQ]

I assume the ignition switch would have to be turned on for this to work because of the Canbus system?

 

Bob-

 

Honestly, I have no idea. The Bob's catalog was printed in early 2005 IIRC, so it's really mostly dedicated to pre-CANBUS bikes. I fully agree with JR356's suggestion for true dedicated jumper cables.

 

-MKL

 

BMW has a special battery charger for dealing with the CAN_BUS accessory port for charging. But, jumper cables do not have this special feature. I would be very leary of connecting my CANBUS accessory port to another source of power. Direct battery connection is much lower risk IMHO.

[elkroeger]

I rigged up a set of jumper cables for the bike out of some old lamp cord, and some small alligator clamps. It's a hassle using them, because battery access is so bad on my 11rs. I did use them a few times on my R100, and it worked like a charm - easy to attach to a car, too. just shut the car off, and don't try to start the bike with it hooked up. The idea is to let it charge for 5 minutes or so.

 

It's not a perfect jumper cable, but it's small, lightweight, and can get me out of a jam.

[madnance]

I've got those MC jumper cables, but its hit or miss if you can get to the battery terminals on the 1150 RT. On my bike and my buddies', you need to take off the side panel to access the + terminal. I've Jerry-rigged a jump once poking a screwdriver under the tank to get contact with it, and then connecting a jumper to it but it wasn't a pretty site.

 

You can buy terminal extensions that would make it accessible but the guy you are riding with may not have one. See this post:

 

http://bmwsporttouring.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=628594&page=1

 

A short 3 ft, 14 or 16 gage wire with powerlet connector both ends for BMW-BMW jumps and a female connector to alligator incase you jump from a car or non BMW bike should get you out of most jams. Powerlet has the connectors for about 30 bucks for DIY'ers.

 

Sounds like speaker wire may work too!