Introduction: Battery Float Charger

About: The instructables I publishing are here because of Arnold Schwarzenegger's sixth rule of life: "Always give something back." Back to the community. Share the knowledge.…

Ensuring your vehicle batteries are always in good condition is important.

Preserving of our environment is/should be mandatory.

This project supports both purpose. It recycles an outdated feature phone wall adapter to create a lead-acid battery float charger. The wall adapter gets a new life and the lifetime of the battery extends.

You can build a reliable float charger unit using a few items, almost for free!


  • gently keeps charged 12V lead-acid batteries from 5Ah to 100 Ah nominal capacity
  • very low (<1 Watt) power consumption
  • power on LED
  • can be left on battery permanently (for weeks to months)

What do you need to build a float charger?

  • a salvaged wall charger of an old mobile phone with 5 to 6 Volts and 500 to 800 mA output
  • an inductor
  • NE555 timer chip with a socket
  • 2x N-channel transistors
  • resistors
  • signaling diodes
  • a LED
  • project box
  • wires
  • crocodile clips
  • wired fuse holder

Expected total cost: less than $5 (or free if you already have these items in your repository).

Required tools:

  • soldering iron
  • drill
  • pliers

Note: if you want to jump immediately to the circuit diagram and the build instructions, go to step 6. Otherwise stay with me and let's discuss a thing or two about battery charging.

Step 1: Why Do You Need a Float Charger?

Lead-acid batteries needs regular charging.

Even if they are not used their charge level drops in weeks due to self-discharge. Even worse a discharged battery may lose its ability to get recharged.

If you do not use your motorbike on the winter or your car stands for weeks -- you better have a float charger connected to the battery. Otherwise you probably encounter engine starting problems, and your battery's lifetime is getting shorter. In a depleted battery the emerging sulfation on the negative plates is avoid the battery to take adequate charge again, and a vicious circle happens: the battery is unable to get fully charged, which increase sulfation, which avoids proper charging, and so on, until your rather new battery is ready to be recycled.

(Good to know: even a sulfated battery can be restored by desulfation process, and high current impulse tools called desulfators are available to decrease or eliminate sulfation. However, your best shot is preventing sulfation at all, and the best way to do this is to never let you battery to get/stay depleted.)

What is the difference between float charging and trickle charging? The active current regulation is the difference.

A battery under continuous float voltage charging is said to be under float-charging. Trickle charging means charging a fully charged battery under no-load at a rate equal to its self-discharge rate. (These definitions are from Wikipedia.)

This is a float charger as it provides constant current to the battery. This way we can avoid overcharging without a much more complex feedback circuit (which would surely requires the utilisation of a programmed microcontroller). Overcharging is avoided here by the very low current relative to the nominal capacity of the battery.

(Image source:

Step 2: Bulk Charge Vs. Float Charge

Most older battery charger units designed to bulk charge, which is a reltively fast process to recharge a depleted battery. Though if you permanently leave a fast charger on your battery it will overcharge and destroy the battery, or in best case it simply switch itself off after an initial full charge and leave the battery alone. An old-fashioned bulk charger has a heavy transformator to provide lots of amperage.

To avoid self-depletion a battery needs float charging to maintain its voltage near to maximum and elongate the lifetime of the battery. A float charger provides a very small constant amount of electric current to the battery. This current is enough to compensate self-discharge and to give a gently charge to the cells, still underdosed to overcharge the battery. In fact such a charger can be connected onto the battery permanently, even for months without any drawback.

The power source of this float charger is a salvaged mobile phone wall adapter. The phone itself is retired by years ago and probably will never be used again. This is a very effective switch-mode charger with 5.9V and 800 mA output power. So let's transform it and give it a new life as a 12V lead-acid battery charger!

Step 3: How Does It Work?

Let's keep it simple. Get an expendable old mobile phone charger (wall adapter) or a USB charger. Build a circuit with an inductor to boost the voltage from about 5V DC to 13-15V DC, and connect it to your battery. That's all.

As a typical feature phone charger provides 450-800 mA current, this fortunately limits the max current of your float charger. Which is good! We do not need to bother with current limiting methods in our circuit.

How much amperage can you expect? While the input voltage will be increased (about three times) the power stays constant. So if you triple the voltage, a 600 mA input from the phone charger will produce 200 mA output current. In practice the actual output current will be even less due to internal resistance and ineffectiveness of the components of the circuit. So what you can expect is about 150 mA at 15 Volts.

Will be this adequate as a float charging? Absolutely! It's close to be perfect. I found that with these parameters the charger have good use on batteries from 4Ah to 150Ah capacity. My partly depleted 4Ah scooter battery get charged with this current in hours (but still never get overcharged), and the charger was still able to constantly maintain 12.7V of a 150Ah (actually 2x75Ah in parallel) battery.

Step 4: How to Avoid Overcharge?

Overcharge is dangerous, so we want to avoid overcharging. It "boils" the battery, forms exposable (and toxic) gas, and can destroy the battery by deforming the plates inside and causing a short circuit, which even can explode the accumulator. The last thing you ever want to experience is the explosion of a box filled with hot sulfuric acid and lead.

So, the question is: can our float charger cause overcharge? Well, until you use it on 12V lead acid batteries no, it can not cause overcharge.

The gassing voltage of a 12V nominal lead–acid battery is about 14.4 V. This means that when the voltage of the battery getting pushed above this point the battery starts "bubbling" and forms exposable gases, also getting warm. So what we want to avoid is letting the battery voltage to get higher than approximately 14 Volts.

But our float charger produces 15V. Does it push the battery above 14V? No. It can not. As you connect your battery to the charger, the voltage gets equal to the battery's voltage. As the battery getting charged, its voltage rises. But such a small current as 150 mA will never push the battery voltages as high as 14V, or above that. 15V on the charger's unloaded output is neccessary to form a voltage difference. This is the force which "pushes" the electron flow into the battery.

My 150Ah battery bank's top voltage stayed for days at 12.7V with this conditioner (float) charger. (Without the charger during this time it would drop 0.1 to 0.2 volts.) The smallest, 4.4Ah battery's voltage topped at 13.8V, and did not rised higher even after a couple of hours.

Step 5: Is This a Universal Charger?

So is this float charger can actually charge a depleted battery to its fully charged state?

Probably yes, it can! However it will take a loooong time to charge a depleted battery with just milliamps.

For example if you have a 50Ah car battery, and you can measure 12.1V on its terminals then your battery is depleted to about 50%, according to information widely available on the internet.

However, you really can not elicit 50Ah from a 50Ah battery. In reality you can extract approximately 30% of its nominal capatity, which in this case is 15 ampers. (Deep cycle "marine" batteries can be depleted more.)

Look at this table:

State of ChargeVoltage

(Source: Battery Basics: A Layman's Guide to Batteries)

So at 12.0V the real charge level of a car battery is about 25%. (At 20 °C -- lower and higher temperature degrades battery efficiency and lifetime dramatically.)

Returning to the original question, in how much time can you recharge your depleted 50Ah battery with float charging?

Well, if your battery miss 75% of its consumable 15A capacity, then it needs 11.25A to be pumped into it. As a charging process has no more than 75% efficiency, you need to give ~15A to the battery to get it fully recharged. With 150mAh it will take 100 hours, or more than 4 days to fully charge your depleted 50Ah nominal capacity battery.

So the answer is: yes, you can swap a fast charger with this low-current float charger if you have the time to charge a battery for days. You can even leave the battery on the float charger for weeks or even for months, and it will take good care of the battery, never damaging it while extending its shelf life.

Step 6: The Circuit: Schematic and Working Theory

The circuit divides to three logical parts:

  1. Modulator with NE555 chip
  2. Inductor
  3. Output voltage feedback

NE555 is configured as an astable multivibrator. It produces an alternating high/low level output on pin 3, which turns on/off transistor Q1. R1 and R2 determines the frequency of the modulation. The best visual explanation I found is from

Q1 transistor drive the inductor, which is constantly building up electromagnetic flux and collapse it, inducating voltage rise.

D2 avoids this higher voltage to feed back to the power source.

C3 stores the higher voltage and provides it while Q1 is in off state - without capacitance the rised voltage would not appear on the output of the charger.

D3 avoids current to flow from the battery to the charger. Also, this is the positive output terminal of the charger.

Q2 with its resistors forms the feedback circuit to NE555, and makes output voltage adjustable.

The other parameters which determine output voltage is R1, R2 and C1 of NE555. Equations are:

  • Positive Time Interval (T1) = 0.693 * (R1+R2) * C1
  • Negative Time Interval (T2) = 0.693 * R2 * C1
  • Frequency = 1.44 / ( (R1+R2+R2) * C1)

Look for online 555 calculators or see this Excel sheet calculator from Texas Instruments:

LED is fixed to Vcc and ground, indicating that the charger is powered.

Warning #1: Without the feedback this circuit can produce far over 100 volts (with very low amperage). Be aware of the high voltage could destroy your voltage meter if you are not careful enough.

Warning #2: Fuse is a must have if you work with high energy density power sources like lead-acid batteries. If anything goes wrong and for example your terminal connectors in your project box accidentally get connected by a broken soldering than it directly connects the + and - terminals of your battery. Your wires will glow and got fire and your battery may get ruined, even it can explode. So althrough the charger itself has low voltages and low amperage, a fuse is a must have. Always be on the safe side!

Note: the circuit illustrations made by a tool named Fritzing.

Step 7: Build Design

Before this project I used to cut the circuit board and then solder the parts into it. However handling a small and crowded plate is uncomfortable. So this time I made my circuit board finished with all parts, tested it, and as a final step cutted by a rotary tool. Waaay simpler, believe me! :)

I used a TO-220 house MOSFET because I didn't have smaller one at the moment. As the switching transistor does not get hot at all a TO-92 package would work well.

Also I ran out of LEDs with diffused lens, so I made one diffused. With a 400 or higher grade sandpaper any water-clean LED can made nicely diffused in about a minute.

I don't like using surface mounted parts as they are intended to be machine-assembled onto cirtuit boards. However they had their benefits: at this time I used two 1 kOhm SMD resistors as R1 and R2 of the NE555 chip. They are positioned below the DIP socket and using no place at all on the board.

R5 (190Ω) is actually formed on the circuit board from a 2kΩ resistor and a 210Ω resistor in parallel. Q2 is a simple N-channel switching transistor.

The float charger I built now serves as a gift to my brother, and works really well.