# Is this a float charger or trickle charger?

Below is a simple charging circuit using LM317 and BC547 and it is mostly classified as a float charger in many forums. This seems to reduce the current flow when the battery is fully charged, however does not turn off the charger completely. Hence can this be connected to SLA batteries indefinitely without affecting the battery life? or is this a trickle charger, which should be disconnected after charging.

By definition float charger will only turn on and charge your battery when it has self-discharged below a certain level, whereas a trickle charger continuously emits a small current of electricity regardless of the charge level or even if it's full. It is also mentioned that for SLA batteries, a float charger could be connected indefinitely where as the trickle chargers needs to be disconnected after charging.

simulate this circuit – Schematic created using CircuitLab

Updated components as per recommendation

simulate this circuit

When no battery is connected the circuit operates as a constant-voltage source having an output voltage of $$\\mathrm{V_o=11\cdot 1.25V = 13.75VDC}\$$ with zero current flow.

When a battery is connected, if the battery voltage is less than, say, 12V then the circuit will work as a constant-current source having a constant output current of $$\\mathrm{I_o[A]=V_{BE-BC547}[mV]/470\approx1.5[A]}\$$.

During the battery is charging the battery voltage will rise up to a point that the D1 gets reverse biased (Note: Charge current may drop as the battery voltage increases, but I'm not sure about that). So the circuit will switch its operating mode from constant current to constant voltage with zero current flow even if the battery is not removed.

So, this circuit fits in your "float charger" definition:

float charger will only turn on and charge your battery when it has self-discharged below a certain level

PS: The circuit may not work with 15V source since the LM317 requires a minimum difference of 3V between the input and output. For a source voltage of 18VDC and an output current of 1.5A, the LM317 will dissipate at least 7.5W. I'm sure you know what that means.

• Will it really reach zero current flow? Can it be connected to the battery indefinitely? If so, this seems to be a better solution than those with relay cutoff, provided we use a voltage regulator with least drop out (eg. MIC29302) and a schottky diode.
– Zac
Mar 22, 2020 at 20:10
• Will it really reach zero current flow? The diode guarantees this. Can it be connected to the battery indefinitely? It seems so. Mar 23, 2020 at 6:58
• The diode is actually added to stop current from battery flowing back to the circuit when the power supply is turned off. Wouldn't the diode fail or heats up when current is applied from both sides?
– Zac
Mar 23, 2020 at 7:29
• @Zacson The diode is actually added to stop current from battery flowing back to the circuit when the power supply is turned off. True but not that only. What if the battery voltage gets high enough (e.g. 13.7VDC)? Can the current flow? Wouldn't the diode fail or heats up when current is applied from both sides? How can you apply current to a diode from both sides? Side note (I forgot to mention in my answer): 1N4007 is a terrible choice for this application since the current can be as high as 1.5A when a fully drained battery is connected. A Schottky diode with low Vf should be used. Mar 23, 2020 at 7:36
• Sorry, it is voltage applied from both sides. I shall update the diagram with better components. Though this circuit is simple, cheap and effective, why do we find many or most SLA chargers in the market using a relay cutoff? Apart from the heat generated, does it have other drawbacks?
– Zac
Mar 23, 2020 at 18:07