How often should I have to charge the battery on my 550K that I use for daily commuting?
Never. Or, possibly every six years when the battery wears out.
I'm in the city so RPM's rarely go over 4000.
Well, I hope you've put a 750 tach on the bike so your actual revs are higher than that.
I ride with the lights on which obviously draws juice but I have had to charge the battery every other day or else it drains causing me to kickstart. Is this normal?
Not if the bike has all stock components and all your connectors and switches are making good low resistance connections that don't waste power.
I bought the battery last year and have always used a Diehard trickle charger similar to a battery tender. Also, the cells are filled properly.
Lead acid "starting type" batteries last the longest when they are not discharged. The more they are "worked" or the charge level changed the shorter their life. This is because the acid actually removes metal from the plates as it discharges and then puts it back on during recharge. The plate separator technology determines where the metal is replaced as it does not automatically redeposit back to its original location. Over time, and cycles large gaping holes can develop in the plates, significantly reducing the plate area and battery delivery capacity. Deep cycle batteries have a special separator to limit the metal migration. Further, the plates are originally made porous to give a larger surface area when new. The plates lose this porosity with repeated recharge cycles as there is no mechanism to "foam" the plate material during a normal recharge cycle.
I don't know the charge rate of the "Diehard trickle charger" if it is more than 1.4 amps it cannot completely charge the 550 battery without a little damage each time. And, if left on beyond a 95% charge (about) then it will overheat the battery even more so.
The Tenders are the way to go as they adjust their charge rate to battery level of charge, and without extra damage beyond what normally occurs during a recharge process.
The 550 charging system makes about 150 Watts at 5000 RPM, and about 40-50 Watts at idle. The stock electrical draw is about 110-120 watts depending on what is switched on. So, somewhere between idle and 5000 RPM, the alternator will make enough power to spare the battery drain but no more. Then above that RPM, it will begin charging the battery. The recharge process is not 100% efficient, so the alternator must supply 110- 120% of the power that was removed from the battery by electric start (1800 Watts per second) and any drain on the battery while the alternator was spinning slower than the bike's power drain break point RPM.
You'll notice the 74 550 has a headlight switch. When the bike was first made, operating with the headlight on was only expected at night. The battery will maintain a charge much better without the headlight on. Legislators change the operating assumptions after the system was designed. About 1976 or so, all motorcycles had to have the headlight on while it was running. Honda did not redesign the alternator in response to legislation until the 650 model.
At any rate, your bike has a problem. My 74 550 behaves like your does. The problem with mine is the field coil that measures 7 ohms instead of the 4.9 ohms it is supposed to have. This makes my alternator put out less than the 150 Watts it should make and I need 4000 RPM to reach the charging breakpoint. I have a replacement field coil for it. But, I haven't devised a method of badge removal off the alternator cover without expecting damage to one or the other. So, in the meantime I put the lower wattage stock headlight back in it and drive without the headlight during the day and stick it on the Tender at home while it is in the garage. I also have the stock turn flasher that I use as a battery voltage monitor. If it flashes normal or fast, the battery voltage is good. If it flashes slow or not at all, I nudge up the idle RPMs with the twist grip until they flash again. I admit it's repair priority is low due to other available bikes in my collection. My other 550's don't have this problem.
Lastly, you can easily waste or lose 10% or more of the bike's actual energy useage, with extra resistance in the switch contacts and electrical connectors in the bike's power distribution system that occur over time and use. With the marginal system of the 550, this can make a 500-1000 RPM difference in the battery charging breakpoint RPM.
Cheers,
