I am going to use my car charger, with a 12v lamp in series between the positive charge cable and positive battery post. That will reduce the charging voltage to a safe level.
Yes, a properly sized lamp will do the job, but the voltage drop across the lamp will decrease as the battery voltage approaches the charger voltage and the lamp resistance will therefore also decrease. You need to make sure that the charger voltage remains low enough to prevent overcharging if the battery remains connected for months at a time.
During the winter in Dayton, OH, I would remove our motorcycle batteries and bring them into our heated basement for storage and as-needed charging. It was an inconvenience to re-install the batteries, but if weather permitting wife and I decided to "suit up" and go for a ride, that is what we did. We were always properly dressed for motorcycle riding and almost always wore our helmets. Do it right and you can tolerate even the coldest of days for a few hours, even if your motorcycle battery doesn't!
I used to have a Variac connected to the primary of a really large low-voltage transformer that in turn was connected to a large, convection air-cooled, silicon bridge rectifier. This combo could be used to start my vintage 1968 Mercury Cougar with its small V8 engine, delivering a hundred amperes or more to the starter motor... I didn't have a shunt to measure the starter current at that time in the 1980s when I acquired those parts.
Later on, after ruining a 50-0-50 ampere D'Arsonval meter movement equipped with an internal current shunt, I removed the meter and the Variac and substituted a length of nichrome wire in series with the positive output and used the transformer/rectifier combination as a battery charger for 12 V DC car batteries. The nichrome served as a crude current-limiter, but I never did try to use it to measure the charging current. Nichrome, like tungsten light bulbs, has a positive temperature coefficient of resistance, so as it gets hot the resistance increases. Good thing, too, or electric heaters and light bulbs would quickly "run away" when connected to a constant-voltage power source such as the usual house wiring.
Anyhoo, this lash-up was not a good thing to be left unattended. I always monitored the battery terminal voltage by periodically disconnecting the charging wire and waiting for the voltage at the battery terminals to stabilize. If it was anywhere near 12 V after waiting five or ten minutes, I would try to start my car with the newly recharged battery. If the car started, I checked the battery terminal voltage to make sure the alternator had taken over the charging function. If the alternator appeared to be working, I rolled my "charger" back into the garage to be used another day. I figured the car alternator circuit was "smart" enough to finish charging the battery... assuming the battery hadn't been ruined by the deep discharge cycle it had just experienced.
It seems to me that anyone can sometimes forget to turn something off that isn't turned off by the ignition key... so, the next morning they get up and discover their car won't start. That's why I kept the kluge charger around, even though I have a AAA road-side service subscription. Most of the time we had two vehicles and I could use the battery in one of them to jump-start the other if the "dead" battery still had even a little charge left in it. Now that we are down to one vehicle, I guess I will resurrect the kluge charger, which I disassembled because I wanted to equip it with larger wheels purchased years ago for that purpose but never installed.
The suggestions given above regarding charging circuits appear to be good ones, but since I don't ever see the need to "trickle charge" a car battery, like Grandfather did to make sure his Dodge would start on a cold West Virginia or eastern Tennessee morning, I doubt I will make any "improvements" to my beast of a battery charger. Best way to take care of your motorcycle battery is to keep it warm and recharge only as necessary.
Use a hydrometer to make sure the cells are okay and remember to adjust the hydrometer readings for electrolyte temperature.
