Editor’s note: Battery charging in a float or maintenance mode has been around for a while, especially for standby power, emergency lighting, telecom, uninterruptible power, and other critical power needs. The float method of charging is a voltage of 0.2 to 0.6 V above the battery rest state voltage that is continuously applied so that the battery will be ready to deliver a full charge when called upon in an emergency situation. The author adds reliability to this function by taking temperature compensation into account.
Steve Taranovich

Many of the circuits for maintenance or float charging a lead acid battery omit the need to compensate the float voltage for temperature effects. This can lead to loss of water and production of hydrogen gas, which can cause explosions or fires.

This Design Idea incorporates two important functions necessary for a float charger: 1) constant charge current up to the float voltage, and 2) temperature compensated float voltage.

Figure 1 The design provides constant charge current and temperature compensated float voltage with a small number of common components.

The charging current may be adjusted by changing the value of R1. The current limit in amperes is found by this formula: I(limit) = 0.65/R1. The base-emitter voltage of Q1 has a −2.2mV temperature coefficient, which will cause the limit value to decrease with rising temperature. For the values shown, this amounts to −8mA over 10 to 50ºF temperature range.

The float voltage is determined by the voltage at the U1-GND terminal, which is controlled by the ratio of R2 to NTC1 and D1, D2. R3 adjusts the range of temperature compensation to more closely fit the change of battery float voltage with respect to temperature that will not cause gassing or water loss in the battery.

M1 may be replaced by a relay which is driven by the input voltage, V1. M1 may have a small leakage current in the 10s of µA, which could discharge the battery in the event of a very long power outage.

There is no provision in this design for protection against connecting the battery in reverse polarity. I’ll leave that detail up to the needs of your application.

Figure 2 The battery voltage will float at a current somewhere between the knee and 0A. This would be the current needed to maintain the float voltage at a particular battery temperature. Red to green traces depict operation at temperatures of 50 to 10ºC in 10ºC intervals.

M1 should be an N-FET with low RDS(on) and Vg(th).

Dave Conrad is a retired electronics engineer with experience in power, video, analog, digital, mixed signal, and software design.


  1. Visit the PowerStream website for more details about float charging a lead acid battery at various temperatures.
  2. Vishay thermistor NTCLE100E3339JB0, Digikey Electronics
  3. LT1086-12, Linear Technology