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Keeping a motorcycle battery charged using a nearby car?

I am in a unique situation where I have no access to a 230v wall socket to attach a battery tender. My plan is to use my car which is parked right next to the motorcycle to keep it charged up. In the past, when my motorcycle has refused to star, all I had to do was hook it up to my car battery, not try to jump start it but just leave the two hooked up for a few minutes, disconnect and then the motorcycle would start without any issues. In this case, I am attempting to make a proper charger with trickle charging capability. I want the input to be a car battery (11v to 14.4v 32A) and the output side to be a steady 12v 750mA to 1A to keep the motorcycle juiced up. 

Do you think I'm on the right track? If yes, would someone be kind enough to help me out?

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Toga_Dan9 days ago

I had a bike which had the wire from the alternator routed near the drive chain. The chain had gradually sawed through the wire. This caused it to charge less and less. Soldering extra strands of wire in fixed it.

From all my years with cars and bike I can tell you one thing that is certain:
If a bike that is used every now and then fails to hold a charge you either have a bad battery or an electrical fault.
My bike was often not used for more than 3 weeks in a row but always started first try.
Instead of finding weird ways to charge the battery it would make more sense to fix the bike.
As for the charging by car: You are wasting lots of fuel when the engine is running and if not you will drain the battery quickly as it is "only" a starter battery.
If you do need mobile power get a small and cheap generator ;)

bwrussell11 days ago

What you were doing is a jump start, for what its worth. Turning on the car while doing this only serves to make sure the car battery stays charged.

As far as using your car as a trickle charger for longer times, probably a bad idea as you'll drain the car battery and need to jump it. You should only need a trickle charger for batteries that won't be used for extended periods, months, not days. If your motorcycle gets used on a regular basis but the battery is dying in between rides then you probably just need a new battery.

Assuming a 45 amp-hour battery you could run into trouble cranking the car after ~8 hours, or less, of running a 1A trickle charger off of it. Basically even if you only trickled it over night your car battery would be likely "dead" by morning, depending on quality, temp, time, and capacity.

+1

Better, just get a solar battery "charger", that should maintain everything safely. If you are finding you need to jump frequently, the battery needs renewing.

+1

It seems your motorcycle is failing to keep its starting battery charged by itself.

Perhaps the motorcycle has loads drawing small current, all the time. Perhaps the solution lies in a method to disconnect the battery completely, like a switch, when the motorcycle is not in use.

One of the assumptions that goes into making the, what you call, "battery tender", is that device has an infinite amount of energy available, since it is connected to the mains, what you call, "wall socket"

For a device that takes energy from a car battery, that device should worry about not taking too much energy, draining that battery to a state such that the car would not start.

Or if the device only runs when the car engine is turned on, again you don't have the same kind of near-infinite available energy as the mains, because you can't run the car engine continuously, night and day.

As you have already discovered, a few minutes of just connecting the motorcycle battery and car battery together, this is enough to charge the motorcycle battery, enough to start the motorcycle.

That might be a method that works so well that there is not much room for improvement.

Anyway, if you want some numbers for current, consider the somewhat contrived case of connecting two similarly sized batteries, through a resistor R. Moreover imagine the batteries themselves are just voltage sources, V1 and V2. Also imagine V1 is the car battery, and V2 is the motorcycle battery, and V1>V2.

In any case, current flows from the higher potential to the lower. For what I have described here, the current flowing from the car battery to the motorcycle battery, from V1 to V2, is I = (V1-V2)/R

As an example, suppose R is 10 ohm. That would give 100mA of current, per volt of difference in the voltage between the two batteries. E.g. if V1 =14.0 V, and V2 =12.0 V, then the current is (V1-V2)/R = (14-12)/10 = 0.2 A= 200 mA

Presumably that current will decrease over time, as the two battery voltages get closer together. Moreover, if the two battery voltages were exactly equal, the current would be zero.

You might wonder at the case where you just connect the batteries together directly with wire, or jumper cables. I mean, isn't the resistance of that, like almost zero? Well, as you can see, that simple formula, I = (V1-V2)/R, blows up when R=0, so there probably is some resistance in there somewhere, in the cable or in the batteries themselves, because the current in the wire is probably not infinite, rather just big.