After a little investigative playing around, I found out this much about the little cheap HF float chargers. I have been using one for several years, and it has worked flawlessly on batteries that are in good condition, keeping their voltage at 13.2.
In this....... or was it another thread, we got to discussing the standing voltage of a good condition liquid lead acid battery being 12.6-7.
After finding my Bullie battery standing at 12.38,......meaning that it is not what it used to be, but still starting the bike ok, I connected a same brand new HF cheap float charger to it, and after monitoring it for a few days, I found the charger maxing out at 12.82, which is hardly a hi enough voltage to keep a battery fully charged. I first suspected a defective charger, but today I connecter same charger to a known good battery w/ a standing voltage of 12.6, and it soon maxed out at 13.22 volts just as it should. To know for sure, I then connected a more expensive Shumacher 2 stage float charger to the s-40 Bullie battery, and it turned yellow while charging 14.6 volts for about 40 minutes, then turned green (float mode) at 13.32 volts.
Moral of the story, the little HF charger is a fine charger, as I and many others are using them to maintain batteries for very long periods of time. But if a battery is not in good condition (not holding 12.6 volts, the HF charger will not hold the 13.2 volts for which it is rated, and therefore will not fully charge the battery.
Besides 2 HF cheap float chargers, I have 2 of the Shumacher 2 stage float chargers, and also a 2 stage "Battery Tender" float charger also. The Shumacher and the Battery Tender are more expensive chargers, and are probably better regulated, because unlike the HF charger, they both hold float voltage of 13.2-4 no matter the condition of the battery. All comes back to you get what you pay for sometimes