[align=center]Jump Starting and Battery Charging...[/align]
Yes you can use a car battery charger to charge a motorbike battery. BUT, you'll probably reduce the lifetime of your battery. A small motorbike battery prefers a 1amp or 2amp charger, which won't get the battery too hot. If you charge a motorbike battery with something like a >3A charger, the battery will charge quicker, but get a lot hotter. This heat does damage... hence a more appropriate charger is required for bike batteries.
You can buy small 1A chargers that can be permanently connected to the bike (when parked of course). They are known and battery tenders, or battery maintainers. These specific ones will 'maintain' the battery without overcharging. If you do a lot of short trips, these are definitely worth it! They usually come with a connector, which you can plug you bike in from under a fairing or in the boot... so its convenient. If buying one, looking for something with at least a 3-stage charging system, 1.25 to 1.5A is good. You can get away with a 750mA one, but if the battery is too flat it might not be strong enough to get the battery charging. Don't be a tight arse and spend the extra $10 to get one around the 1A to 1.5A range.
Never let a lead acid battery go flat! As a general rule, if it falls below 20% charge, its doing damage. If you've ever experienced a dead flat battery, its never the same again! If your going on a holiday or the bike isn't used for a long period, I'd recommend to charge the battery and disconnect the negative terminal. Or... get a battery maintainer as mentioned above.
Read here for more info about the battery...
Jump Starting from a Car
I've heard people say its a no-no and other say they've always done it. It comes down to how you do it that matters
A car battery is significantly more powerful, both in capacity and current supplying capabilities.
If you are going to connect the jumper leads to the car battery, keep in mind that the starter motor is going to run nice and fast with that powerful car battery supplying it. So the starter motor may get hot, and given the ample supply of current, you can crank and crank and crank.... which means it is easy to damage your starter motor by cooking it!
Normally when you jump start car_A with car_B, the engine of car_B is running. This is so more current can be supplied to car_A. car_A really needs a lot of current, as it takes a lot to crank the big engine, and it also have this whooping big battery that sucks a lot of the power (trying to recharge itself).
Now... imagine if you jumped your motorbike from a car with the engine running. The motorbike is now connected to a MASSIVE battery and a HUGH alternator with a lot of current on tap. On top of this, the car is running at ignition voltage (more like 13 to 14.5V) and is not being loaded up by the small bike being attached. The motobike battery is now getting a LOT of current in a short space of time, so will start cooking. When you crank the bike, the starter motor all of a sudden has not only plenty of current, but higher voltage than it normally would have... that means the starter motor is going to cook running faster and harder than designed... more than the bike actually needs.
This is why you don't jumper off a car with it's engine running. If it's not going to start connected to a car battery, then chances are... it's not going to start! Common thing is that all the prior starting as just flooded it. Wait a while and try again. You can also put the kill switch on, hold the throttle wide open and crank (10 second crank, 10 second rest... don't cook your starter motor). Doing this will get the air flowing through and help dry up the excess fuel. Take the kill switch off and try to start your bike again.
The only issue with trying to start a bike when its connected to a car battery, is once the bike starts, it has to supply charging current to the cars battery (extra load). Hasn't really been an issue for me, but mention it so you don't leave the jumper leads on longer than necessary once the bikes engine is running
If you really really must have the car running too... then keep the engine revs low and connect the ground lead to somewhere on the engine or frame, rather than a direct line to the battery.
Apparently the electrical noise from a running car can cause problems with the bikes electrics... so I've been told. I find this hard to believe, as the batteries are like big capacitors and shouldn't let any serious spikes occur that couldn't be handled by the bike anyway. After all, they are usually designed to withstand it. Even so, there may be some bad combinations of bikes and cars that would cause problems. So best avoid starting a bike from a running car.
So can you start a car from a bike... maybe
Particularly if its a small capacity car. But you'd probably need to hold the revs of the bike up to supply enough current. If it was me... I wouldn't attempt to start the car with the bike attached. Better to connect the bike up and use it to charge the cars battery, disconnect, then try to start the car
I've jumpered a couple of motorbikes at track days from of my track bike though
1. Never crank for too long... 10 seconds on, 10 seconds off is a good measure.
2. Never jump start from a car if the car's engine is running.
3. If your battery is low, turn off the headlights (or pull the headlight fuse). This will help significantly. Also give the battery time to rest, as when it gets hot, it's capacity to supply current drops.
4. Never let your battery run flat.
5. If you've tried cranking for a while, it's probably flooded. Give it time to dry out.
MORE INFO... for those still awake...
To understand what happens, you need to understand how a battery works, and the chemistry involved. A charged battery sits at around 12.4 to 12.6 volts. The battery can only supply so much current at once. If you try to draw too much, then the battery will drop its voltage under load. So if you started the bike, the instant current draw of the starter motor is really high, so the battery voltage dips for a short period, say down to 10.5V. If the battery was low on charge, it wouldn't be able to supply as much current and drop to maybe 9.5V. As the bike starts cranking over, the starter motor actually draws less current, and the battery voltage comes up again. Now if you tried this with a car battery, it would be like having a really charged battery, and it might only drop to 12V. So there is an extra 1.5V on the starter motor than usual (using big car battery). Thats not too much, and not about to cook anything.
Now if you connected a car, and the car was running, the cars alternator is pumping out lots of current and more like 13.5 to 14.5V. Now there is an extra 3.5V more than usual... and you can see that is now a significant difference. Hence, you stress the starter motor with a higher voltage. It would be the equivalent of having the bikes motor running, and then trying to start the motor... that never happens, and the starter motor wasn't designed to operated with the engine already running at ignition voltage.
Now if you have a car connected, and it's pumping out 14V with plenty of current to back it up... whats going to happen to the little bike battery!!! Remember I mentioned it prefers a 1A charge current. Now put a car thats pumping out 20+ amps and a car battery to back it up... the poor battery is going to be rammed with current, and its going to get hot. When the chemicals inside get too hot, they degrade.
So it can clearly be seen, that being connected to a car with its engine running is just bad karma, both for the starter motor and the battery. The car is also producing a lot of conducted electrical noise, some worse than others. So its an addition risk thats not worth taking.
If the car's ignition is off, then all you have is a battery connected to some wires that aren't powering anything... aka... its just the battery. Turning ignition on (with the engine not running), just puts a load on the car battery, and when the bike does start, puts an extra load on the bikes alternator and regulator. So I wouldn't turn the ignition on.
So... jumper from the car battery with the engine off.
Once the bike is started, promptly remove the jumpers, as the cars battery presents an extra load on the bikes alternator and regulator. But you'd need to pick the revs up a bit before it actually became a problem.
BTW... If you connect a fully charged car battery to a bike battery... what do you think would happen? One to charge the other??? Nope hehe... in order for a battery to take on charge, it takes a chemical process. This process requires the voltage potential provided to the battery to be slightly higher that the batteries voltage. A battery won't really charge until you give it at least 13V, and only charge at a descent rate when its got a good 13.8V.
Now if the bike still isn't starting... you shouldn't be tempted to start the car! If the bike won't start from a nice big battery, why would it start with a cars alternator running too! I saw this recently at a bike event, where despite my warnings, a group of people started the car too, and even revved the engine. 10 minutes later of trying to crank the poor bike... it still didn't start. I reckon the starter motor and battery weren't in good condition afterwards. The extra power just isn't required. If the engine is turning over at the normal pace and the battery connect has charge, then applying extra voltage WON'T help. It's not like the spark is going to fire the fuel mixture better than normal, or the even faster rotation is going to make the engine fire easier. If the bike is not firing, its because something else is wrong, and it can't be fixed with more current. Trying is only introducing potential to make matters worse.
SUMMARY - JUMP STARTING WITH CAR - Take 2
Keep the cars ignition off.
After starting the bike, promptly remove the jumper cables.