Smart Float Charger to protect your battery

The Problem

A lot of SX Owners these days only drive their cars on weekends, but all the modern electronics, alarm systems and so on, will flatten the battery in just a few days.

Yes, you can disconnect the battery, but this is not always practical and disables security etc. Also, most budget battery chargers will supply a very high voltage to the battery, which would damage the electrical systems in the car if you don't disconnect the battery first. So what to do?

The Solution

This circuit mimicks the way that your car's electrical system charges the battery in the first place. This design is not suitable for bringing a battery "back from the dead", but it should stop the battery from being fatally discharged in the first place. This design provides the following features:

The Circuit

Label Item
TRANSFORMER 15+15 80VA Toroid
(Altronics M5115 or similar)
BR1 GBPC3504 200V, 35A Rectifier
(Altronics Z0093 or similar) 
D1  1N4004
R1 3K9 0.5W
R2   290R 0.5W
R3 15R 0.5W
VR1   500R
VR2 1K0
  C1 - C2   4700uF 35VW

The transformer used here is a big unit. At first glance, 80VA seems overkill for this application, but remember that in many cases, this power supply is what will run the car.  For some applications, you could use a larger or smaller unit.

The theory of operation is very simple.  When the terminal voltage on the battery drops below a preset value (typically 11 - 11.5 Volts), the transistor switches the relay on, and current flows into the battery.  When the battery's terminal voltage reaches the cut-off point (13.8V), the relay is switched off again, and we're back on the battery.


If you have not tackled electronic construction before, you may wish to get an experienced person to tackle the wiring for you. It is not difficult, but mains, and high-current DC wiring should not be taken lightly. Mistakes can be fatal, so you have been warned.

Picking an Injector

Installation and testing

Going Further

If you want, you could add an ammeter .
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