# Simple Full Charge Cutoff for a 6 V SLA Battery

#### zmint

##### Member
One of my hobby units comprise of 6V 4aH SLA Battery. The problem is that the charging section of the unit does not contain a automatic full charge cutoff (it is bad for the battery for not having one). I have to depend on my electronic instincts to estimate the fully charged state. So, I decide to assemble one myself. A simple one.
I came across this circuit

The charger power supply section is built around 3-terminal adjustable regulator IC LM317 (IC1). In the charger power supply section, input AC mains is stepped down by transformer X1 to deliver 9V, 500
mA to the bridge rectifier, which comprises diodes D1 through D4. Filter capacitor C1 eliminates ripples. Un-regulated DC voltage is fed to input pin 3 of IC1 and provides charging current through diode D5 and limiting resistor R16. By adjusting preset VR1, the output voltage can be adjusted to deliver the required charging current. When the battery gets charged to 6.8V, zener diode ZD1 conducts and charging current from regulator IC1 finds a path through transistor T1 to ground and it stops charging of the battery.

Should I go with it. Or do I have better options ?

Half of the components needed are with me and the rest are easily available. Please scrutinize and point out any flaws.

If it is Good enough to proceed then, “the output voltage can be adjusted to deliver the required charging current.” What should be the optimal output voltage ? Do I adjust it after connecting the battery ?

Thanks
Zmint

#### audioguru

##### Well-Known Member
The 6.8V zener diode might be 6.46V or it might be 7.14V. If it is exactly 6.8V the the transistor conducts when the battery voltage is (6.8V + 0.7V=)
7.5V which is too high. If the zener diode is actually 7.14V then the battery voltage will try to be 7.84V which will destroy the battery.

The circuit (from India?) is too old. Use a newer and better circuit.

#### MikeMl

##### Well-Known Member
That circuit is poorly and naively conceived. Why screw around with an imprecise Zener when the LM317 can simply be adjusted (by changing the feedback resistor voltage divider) to hold the battery terminal voltage at 6.90V. The voltage reference inside the '317 is an order of magnitude better than the Zener... Even with the Zener eliminated, this will create a simplistic "float" charger. The '317 will regulate the float voltage, and will prevent destruction of the battery due to chronic overcharge, but it is a poor "recharger".

The correct recharge (or rapid charge) algorithm for SLA batteries requires a three-state charger capable of
1) current-limited charging while the batter voltage slowly increases to prevent battery heating,
2) holding a constant "charge" voltage at 7.40V while the battery accumulates charge during which charging current slowly drops toward a low value, and
3) detecting the decrease in charging current and switching to the correct constant "float" voltage of 6.90V and to hold that voltage indefinitely.

This is best done by a ucontroller like a PIC with an A/D...