What is the difference between single-rate battery chargers and two-rate chargers

Technical notes of interest to Marine Engineers

Battery Chargers

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What is the difference between single-rate battery chargers and two-rate chargers? Is there a benefit to one over the other?

Generator set battery systems are very important for many reasons. Their reliability is of much importance when critical loads are a factor. While the battery and charger work together to insure uninterrupted power, the battery is ultimately responsible for reliable engine starts. If the battery fails your generator might not start resulting in lack of power to equipment and facilities.

It is the battery charger's duty to maintain fully charged batteries at all times to assure positive engine starts under routine or emergency conditions. The battery charger is designed to provide the batteries with a charging current anytime the charge level falls below acceptable limits. There are many designs of battery chargers, but most operate under one of the following two concepts:

  1. Single-Rate Charger - Also known as "trickle charger", this unit will produce its maximum current for only a very short time, then begins to taper off as the batteries' state of charge increases. The only way to maintain a higher current output into the battery would be to increase the charger's output voltage. This, however, would cause serious overcharging problems as the battery becomes fully charged. Thus, the voltage chosen for any single-rate charger is a compromise between fast charging and optimum battery maintenance voltage. The most common problem with the single-rate charger is the loss of electrolyte in batteries caused by a "boiling" effect as they are overcharged. This leads to extensive labor hours replacing or servicing batteries. It should also be noted that due to their nature, most single-rate chargers sold as "5 amp" never reach full current output and therefore may offer the performance of a 2 amp charger!
  2. Two-Rate Charger - Also known as "float chargers", the two-rate charger automatically operates at a "boost" charging voltage that allows the battery to draw the charger's maximum output until is almost fully charged. When the battery reaches a high rate of charge, the charger shifts to the optimum float voltage to minimize battery electrolyte consumption. Since the charger monitors the battery 100% of the time, the optimum charge is provided without adjustments. The correct two-rate battery charger will offer faster charging performance and will reduce the requirement for maintenance. No compromise is made.
The benefits of the two-rate battery charger make for a much more reliable battery system. However, attention must be given to the various designs out in the marketplace. The minimum requirements should include: (a) temperature compensation which insures correct charging in most conditions, (b) output voltage regulation to maintain rated output regardless of input volts and frequency variations, (c) current limiting and (d) overload protection.

Calculation of battery charger size with automatic two-rate charger - Engine Starting Application

The following formula can be used to determine the required charger ampere rating to recharge a battery used in an engine-starting application. This formula assumes that there is little or no continuous current drain on the charger and it is useful only in calulating ampere rating of two-rate chargers. It is also assumed that the charger will replenish only the ampere-hours withdrawn by the engine-cranking event. The voltage of the battery system is immaterial to this calculation.

Calculation Steps

Determine the current that the starter draws for the entire starting cycle. Assume, for example, that:

- The starter draws 900 amps rolling current, worst case
- The maximum cranking time per start attempt is 15 seconds, which equals 0.0042 of an hour
- The maximum number of start attempts will be 5

Ampere-hours (AH) drawn by the starter for this example is (900) (0.0042) (5), or 18.9 AH

Decide how quickly you wish the battery to be recharged. Assume, for example, that you wish to recharge this battery in 5 hours

Plug variables into this formula to find the charger ampere rating (use 1.4 for lead acid and 1.8 for nickel-cadmium when asked for inefficiency constan):

Total AH drawn by starter X recharge inefficiency constant / desired recharge hours


Answer (for above example): 18.9 x 1.4 / 5 = 5.29 amp charger