What kind of battery charger can I use?
First, a word about discharging batteries: The most common reason for a battery to be discharged is from taking a lot of short, low-rpm trips. It takes about 10-20 minutes of running over 3-4000 rpm to recharge from a start. That means that if you go take a bunch of 5 minute trips down to the store (start, run 5 minutes, stop, start, run 5, stop, start, run 5, etc.) you can wear down the charge on your battery quickly. If you're taking longer trips, charging should be covered.
Not riding the bike for a long period is the most common way to kill a battery, so you should use a maintenance charger in such cases. This article covers two different kinds of these.
For installation help, see Installing a battery tender.
Newer-style electronic chargers
There's a new breed of battery charger out there. The tried and true Battery Tender and its knockoffs have a fixed charging "map" (2-step for the Jr, 3-step for the Plus) and only put out one level of charge. The new kids on the block claim an 8-step "map" (though it's likely 4) with the ability to select different amps and different voltages, and they can adjust those steps to better charge a battery. It's kinda like the difference between a cell phone from a few years ago and today's smart phones; they do the same basic function, but the new ones have many more things that they can do as well.
The 2 biggest players in this field are Noco and Ctek. They both have several price levels from $30 to $60. This review covers the $60 option from Noco, model G3500, which is the largest one they make that can still do a 1 amp/hr charge. It provides charge levels of 3.5 amps at 6v or 12v (for cars/trucks) and .9 amps at 6v or 12v (for motorcycles). On top of that, those are split into "normal" and "cold (below freezing)/agm" options. This gives you the ability to charge nearly any vehicle battery, excepting those with lithium batteries.
Even with all that, the biggest gain that the new breed of charger has to offer is smart battery monitoring. What this means is that where the Battery Tender types give out a constant trickle (which can and does wear on the battery, even sometimes cooking the fluid out of a maintenance-type battery over the course of a few months), the new kind of chargers stop the trickle and just monitor voltage until it drops below 12v, where they then resume trickle charging. In effect, it "pulses", saving wear on both the charger and the battery.
There is one more feature of the G3500. It has what Noco has dubbed Recovery mode. It can recover deeply discharged and sulfated batteries. Both are common conditions, especially with older bikes and batteries, so it's a useful feature, especially if you deal with a lot of different vehicles. This is not a feature of their lowest priced G750, but it is on the mid-price G1100.
All this doesn't come without a negative or two, though. Price is clearly the first one; the low and middle price options are $5-6 more than the Battery Tender Jr and Plus models - maybe a bit more with some off brands. Also, the quick-connects aren't the standard SAE 2-pin. That means you have to special order a pigtail, at $6 each, vs $2-3 for the standard ones. In the photo, the orange parts are the Noco connector. A standard SAE is down by the thumb.
If you want to save some cash, you can always cut up your brand new pigtail and splice it to an existing SAE pigtail. This would be the way to go if you already have multiple bikes with the standard connector in place.
There are some points that make this charger worth the extra money:
Older-style trickle chargers
Note: With any trickle charger, you only need to hook it up every couple weeks to keep the battery fully charged. It's not necessary to leave the charger on all the time, and in fact you shouldn't.
The Battery Tender Plus model is a commonly-used charger. It comes with a harness that you can permanently mount to the bike, so you can plug the bike in when you get home and be sure to have a fresh battery in the morning. It has a smart circuitry system that will not charge the battery any more once the battery is fully charged, to prevent damage.
The Battery Tender Jr is cheaper than the Plus model. It's basically the same as the regular Tender, but with a lower output. It's generally sufficient for any motorcycle application, and if you don't mind leaving it overnight or so, a drained car battery is not outside its capabilities. The Jr does get hotter if you're going to do that, but not plastic-melting hot. The Jr model will only charge one model at a time, which is usually not a problem. If you get the Plus model, you can charge two batteries at once.
Battery Tender was the first, and remains the most popular, product of its type. There are many places now selling similar items, though. Wal-Mart, Harbor Freight, and many auto accessories stores sell trickle chargers, usually at a cost that's less than a Battery Tender.
Shown here is the Sears version. Custom stand not included.
You don't want a charger that has too much charging capacity. The Battery Tender Plus puts out 1.25 amps/hour. Any more than that is asking for trouble. The Battery Tender Jr is only 750 mA (.75 amps). These itty bitty motorcycle batteries are very easy to cook. Look for a one amp or smaller charger. They should be readily available.