over months this type can cook a battery DRY. aka no control
Agreed, not the same. Battery Tender is a float charger, meaning it only throws current to the battery if it senses it needs to. Trickle chargers constantly charge so the battery never rests
Absolutely right. Here's a better description from the Deltran (Battery Tender company):
1. How is the Battery Tender® Jr. battery charger different from a trickle charger?
The Battery Tender® Junior (BTJR) battery charger has microprocessor controlled power electronic circuitry which enables it to preform and safely control a number of sophisticated charging functions, well beyond the capability of inexpensive trickle chargers. Some legacy marketing literature refers to the BTJR as a “Trickle Charger with a Brain”. That description was based in context on two parts, first the relatively low output current, and second, the reduced level of charge control sophistication on earlier BTJR models relative to the BT Plus. Since mid-2006 the only major functional difference between the BTJR and the BT Plus is the maximum amplitude of the charger current, 0.75 and 1.25 amperes, respectively. Even though the marketing description may still be applicable, again, in a limited context, we can say now that the BTJR has a larger brain that enables it to create maximum charge effectiveness with minimal output current amplitude.
After connecting the BTJR to a battery and then applying AC power, it first conducts a number of checks during Initialization Mode to ensure that the battery functioning normally. Then it will deliver its full charge at a constant rate of 0.75 amperes. This is called the Bulk Charge Mode. The battery voltage will rise and when it reaches a predetermined level the BTJR will hold the battery charge voltage constant at that level, allowing the charge current amplitude to drop. This is the Absorption Charge Mode. The Absorption Charge Mode is complete when the battery charge current drops below a very low value, usually below 1/8 ampere. Some BTJR models have timers to limit the duration of the Absorption Charge Mode.
After the current drops or the allotted time expires (typically several hours), the BTJR automatically switches to a Float / Maintenance Charge Mode. The purpose of the Float / Maintenance Charge Mode is to maintain the battery voltage just slightly (typically between 1/10 and ˝ volt) above where it would be if it were fully charged and sitting at rest. This keeps the battery topped off at voltages well below the gassing voltage of a lead acid battery.
Based on price alone, trickle chargers often appear to be a better economic choice for the typical consumer, but trickle chargers do not have the advantage of sophisticated electronic control. Therefore, as they allow the value of charge current to trickle down to what appears to be safe levels, the output voltage of the charger may very well rise to an unacceptably high level, sometimes even going higher than 16 VDC depending on the charger type and the battery that is connected to it. This magnitude of voltage is far above the gassing voltage of a lead acid battery. If the battery remains connected to this high level of voltage for an extended period of time, extreme damage may be done to the battery. Without Battery Tender® type electronic safety controls, what appears to be an initial cost savings for the charger may actually cost several times the charger price in replacement batteries.
1. How are the BatteryTender® Plus and Junior battery chargers different from other automatic battery chargers?
Many automatic battery chargers simply turn off when the battery voltage rises to a preset level or when the charge current falls below a certain level. With the battery sitting idle, its internal losses will consume much of its stored charge. Depending upon the age and condition of the battery, it may only take a couple of months before the battery loses more than 90% of its charge. The amount of charge lost tracks pretty well with the reduction in battery terminal voltage.
Some automatic chargers will restart when they sense that the battery voltage is too low. As a battery goes through these types of cycles of repeated charging and idle self-discharge to low capacity levels, the useful battery life may be dramatically reduced.
Both the BatteryTender® Plus & Junior battery chargers do not turn off after they charge the battery. They automatically switch to a safe float voltage level that keeps the battery charged and yet does not do any harm to the battery. In fact, in most cases, this type of charge maintenance will extend the battery’s useful life by at least 50%.
Some customers have reported battery life increases of more than double what they had before using the Battery Tender® Plus or Battery Tender® Junior battery chargers.
From this URL:
There is more information there. You can save yourself untold levels of battery grief by using the best storage methods.