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 It recycles an outdated feature phone wall adapter to create a lead-acid battery float charger.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: batteryuniversity.com)

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 bsselektronika.hu.

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: http://www.ti.com/tool/tlc555calc.

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.

<p>Hi Nobilis,</p><p>Thank you<br>for your project. </p><p>I have a<br>question to make. Do to the bad level of our economy, I couldn&rsquo;t find in the<br>market the materials as exactly as you say. So I bought BF246B instead of BF246A, 390mH<br>Inductor instead of 330mH, IRF640 instead IRF630 and 220 Ohms resistor instead<br>of 210 (in order to make the 190 Ohms in parallel connection).</p><p>So please<br>tell me if I must make changes in the schematics or other materials.</p><p>Thank you<br>in advance. </p><p>CaptainJ</p>
<p>It would be better to get a 15v wall brick and use a switchmode regulator adjusted to 13.8v straight.</p>
<p>As of Lead-acid batteries naturally decreases charging current to near zero towards reaching a full charge.</p>
<p>does this charger cause exposable fumes or explosive vapors?</p><p>you should find out. there is a very large difference between the two. goodluck</p>
<p>&quot;This is a float charger as it provides constant current to the battery.&quot; This directly contradicts your statement not two sentences previous that a float charger supplies constant voltage.</p>
<p>My fault... as this charger produces constant voltage _and_ constant current, both statements are true. A float charger provides fixed voltage, however this charger aviods overcharging by fixed low current, and this is the reason I emphasized the constant current.</p><p>Note that I refer to this charger as a float charger as it is definitely not a bulk charger nor a trickle charger.</p>
So, basically, this is really a trickle charger, and you are just confused. It is physically impossible for a power supply to be both constant voltage and constant current, and few supplies are either. The fact that this supply changes its voltage to supply a particular current makes this a constant-current supply, and therefore it physically cannot be a constant voltage supply. Therefore this is a trickle charger, not a float charger.
<p>The wall adaptor as the power source of this charger does provide constant voltage without a load connected to it. On the other hand under load it provides constant current.</p><p>As a trickle charger vary its current output to adapt to the actual charge level of the battery connected to it, this is not a trickle charger.</p>
<p>That is not what &quot;constant voltage&quot; means. ANY power supply will provide a constant no-load voltage. A Constant Voltage supply OTOH will provide the same voltage across a wide range of load currents. And a Constant Current supply provides the same current across a wide range of load voltages. Here is an image that shows load-line graphs for four common types of supplies:</p><p><a href="http://www.chromaate.com/chroma/webdrive/products/63600/2013/63600-05-en.jpg" rel="nofollow">http://www.chromaate.com/chroma/webdrive/products/...</a></p><p>As can be seen, constant voltage means constant WRT current, not just 'has some particular value at each current'. And constant current means constant WRT voltage.</p>
<p>Good graphs!</p>
<p>Could you let me know what changes would be required to modify the circuit for a 6V float charger for a lead acid battery. Required for old motorcycle 6V battery.</p>
<p>You can build such a charger with the same type of components but unfortunately I can not give you exact values for that. For sure you will need a smaller inductor and different values for R5 and R6, also probably different frequency and duty cycle set by C1, R1 and R2. Beyond the difference in values the circuit could be the same.</p>
<p>D1, D2, and D# are 1N4148 type?</p>
<p>Yes, I used 1N4148 signaling diodes.</p>
Thanks for answer me
<p>&quot;you need to give ~15A to the battery to get it fully recharged. With 150mAh it will take 100 hours&quot;<br>-Just to be technically correct, you mean 15Ah and 150mA right? </p>
<p>Well, I think that the original numbers are correct: the battery in the example requires 15A, but not in an hour; and the float charger can provide 150 mA per hour.</p>
<p>I'm just saying that when you are talking about charging a battery, you are talking about storing energy in it. </p><p>&quot;Ah&quot; is a unit for stored energy. &quot;A&quot; is a unit for currend. You can't say a battery requires 15A. </p><p>Even the calculations shows your use of the units are not proper:</p><p>&quot;You neet 15A what is 100h with 150mAh&quot;<br>That means 100h times 150mAh which is 15000mAh^2. <br><br>The correct calculations are:<br><br>You need to store 15Ah of energy, and you managed to do this in 100h with a 150mA charging current power supply. <br>100h*150mA = 15000mAh = 15Ah</p><p>Is this clear now?</p>
<p>I am really sorry, but still think that my use of A versus Ah is correct.</p><p>Ah is not just a unit for stored energy - it express how much amperage you can exchange with the battery in one hour. (https://en.wikipedia.org/wiki/Ampere-hour)</p><p>150mAh power of a charger express that the charger can provide 150 mA per hour. So it can provide 150x100=15 000 mA in 100 hours. You can multiple hour by hours, but you should not.</p><p>Anyhow, I understand your thread - just don't think its better or more correct than mine. So let me don't alter the instructable. ;-)</p><p>Thank you for your feedback, I recognised and appreciate the constructive criticism.</p>
<p>Ah = A * h</p><p>The Ah rating of a battery is in theory the number of hours(h) you can draw 1 ampere(A) from the battery before it is completely discharged.</p><p>Alternatively, how many ampere(A) you can draw for 1 hour(h).</p><p>In practice, you won't get the full capacity (as you already noted in your article).</p><p>You are definitely confusing A and Ah.</p><p>Let's concentrate on this quote:</p><blockquote>With 150mAh it will take 100 hours</blockquote><p>If you charge 150mAh over 100 hours, your charging current is </p><p>150 / 100 = 1.5mA.</p><p>What you wanted to say is that you are charging with 150mA for 100 hours, resulting in a charge of 15000mAh; 15Ah.</p><p>When it comes to</p><blockquote>Ah is not just a unit for stored energy - it express how much amperage you can exchange with the battery in one hour.</blockquote><p>That is exactly what Ah is - charge, the number of hours you can draw one ampere from the battery.</p><p>The Wikipedia article for Ampere is more complete than the Ah article, and probably explains this better. </p><p><a href="https://en.wikipedia.org/wiki/Ampere" rel="nofollow">https://en.wikipedia.org/wiki/Ampere</a></p><p>Other than the confusion of A and Ah, I really like the layout of your perf. board, it looks nice. Mine tend to look a lot messier.</p><p>Thanks for the article, it is a nice starting point for making a simple charger for the two small gel batteries I use when I am testing circuits made to run off of car batteries.</p>
<p>Thanks for taking your time to make it clear!</p>
<p>To be accurate, amp-hours are a unit of charge, not energy (charge x voltage = energy). But Oddstr13 is correct that you should be saying &quot;Well, if your battery miss 75% of its consumable 15Ah capacity, then it needs 11.25Ah to be pumped into it. As a charging process has no more than 75% efficiency, you need to give ~15Ah to the battery to get it fully recharged. With 150mA it will take 100 hours, or more than 4 days to fully charge your depleted 50Ah nominal capacity battery.&quot;</p>
<p>Lloyd b</p><p>Please go to Tesla chargers and go to a book called solar secrets by Peter Lindemann this is a book that explains the charge discharge of a 12 volt lead acid battery very well. It has exerpts from a book written in 1923 on the care and repair of lead acid batteries and the technology holds to this day. Basically if you charge a lead acid battery with too much current the sulfation burns off, falls to the bottom of the cell and shorts out the cell. The sulphation needs to be returned to a liquid and in order to do this you will need 15.2 to 15.4 volts at very well controlled current The best charger to do this is a solar panel, thus the book solar secrets. The people at Tesla Chargers are all Electronics engineers and are very knowledgable in this field. </p><p>Thanks for your time.</p>
<p>Happy to hear that solar panels are good chargers, as I originally made this charger to maintain my solar battery bank under dark winter conditions.</p>
<p>so interesting instructable, how can i modify the circuit to charge a tablet pc battery or a cellular phone battery without damaging it ? </p>
<p>SeanB10 said it very well. Don't try to make Li-ion charger without solid knowledge in that field. This charger is inadequate for that purpose.</p><p>From amazon or e-bay you can buy little Li-ion charger circuit boards for DIY projects if you want. They expect 5V DC input, so you can use a salvaged phone wall adaptor as the power source. However with 500 mA the charging process may take a long time. (For example my 10&quot; tablet's power charger provides 1500 mA.)</p>
<p>Laptops and cell phones do not use lead acid batteries (well, I could betray my age and say the very first cellular phones did, but that was a long time ago, and they looked more like suit cases than phones). Laptops and cell phones use Lithium based batteries, either Lithium Ion (LION) or Lithium Polymer (LIPO) batteries, and the chargers monitor every cell, and discharge also monitors every cell, so in short, do not build your own chargers for these devices. Lithium batteries are much, much more dangerous than lead acid batteries, and can catch fire and explode quite easily when improperly charged. There are Darwin Award contestants on YouTube that hit lithium batteries with HAMMERS to get them to explode. I do not recommend hacking lithium batteries, especially if you still own all your body parts.</p>
<p>Questions about your circuit operation, Step-6</p><p>555 Timer Astable operation. It looks like you have R1 bypassed with a diode, so the C1 charging is just &quot;0.693 * (R2) * C1&quot;. Discharging is &quot;0.693 * (R1) * C1&quot;. Your frequency therefore is 1 / [0.693*C1(R2+R1)]</p><p>Voltage Multiplier Operation: How did you decide on the value of the inductor? Also, your description/write-up says C1 when I believe that you meant C3.</p><p>Last question, could you describe the operation of the feedback circuit and how it controls the output voltage please?</p><p>Thanks</p>
<p>For R1/R2 and C1 calculations I used the Excel sheet calculator from Texas Instruments.</p><p>The only inductor I had at the time I was prototyping this circuit was a 330 &micro;H inductor. I am sure that lower values also could work.</p><p>Indeed, the capacitor for the output voltage is C3 and not C1. I have corrected this in the write up. Thank you for pointing out the typo!</p><p>NE555 control pin is there to influence the PWM pattern, which in this case determines the voltage produced by the inductor. You can read more about it here: http://www.electro-tech-online.com/threads/ne555-control-voltage.125619/#post-1041244</p>
<p>The schematic may be incomplete. The cathode of D3 is open, and I suppose it should show the battery under charge attached to it, so add that to the schematic.</p>
<p>Yes, you are right. As I mentioned in the description D3 is the output of the charger.</p>
<p>I was taught to call it current, not amperage (and resistance, not ohmage for that matter)...</p>
<p>Nice instructable. I didn't know what float charging meant and only after going through it did I realize it's the same as what I know as trickle charging(or is it different?). Also in some places, you mention capacitance in relation to Ah values when you probably mean capacity. Thanks for sharing your knowledge...</p>
<p>Thank you for your feedback! I made minor corrections in the article.</p><p>Also extended step 2 to explain the difference between float charging and trickle charging.</p>
<p>Your explanation is incorrect. Float charging is a very specific thing. It is not as you wrote &quot;A float charger provides a very small constant amount of electric current to the battery.&quot;</p><p>Float charging is not constant current. Float charging is constant voltage from the power supply, holding the battery to that specific voltage which the battery manufacturer specifies as the float voltage specification.</p><p>Now it's entirely possible that you might just happen to arrive at a current that equals the battery self discharge rate, but frankly it is unlikely because lead acid batteries self discharge quite slowly. What you are probably doing instead (if the battery is staying at peak charge level) is creating excess hydrogen gas.</p>
<p>Yes, float charging in general is about a fixed voltage. But in this case the charger also has a constant current, so what I wrote is not incorrect.</p><p>One criteria of gas production is the voltage above 14V. However the charging current being large enough relative to the nominal capacity of the battery is also a necessary criteria of making a battery bubbling. And this charger provides very small charging current (in terms of lead-acid batteries).</p><p>This charger is not specific to any lead-acid battery, so you are right: on low nominal capacity batteries it definitely can produce gas exhaust. But this is a very simple charger intended to reuse a died mobil phone's adapter, and does not trying to compete with professional float/trickle chargers.</p>
<p>There are a number of &quot;smart&quot; chargers available commercially - CTEK and Ring do what you are doing at reasonable prices. CTEK is from Sweden so they should know something about keeping batteries charged.</p><p>Modern batteries do not like the crude old chargers of transformer/rectifier build with a large ripple voltage. The CTEK units are switch mode, very small and generate very little waste heat ( I don't work for them!).</p><p>My own vehicle gets very little use and in the cold weather needed charging every couple of weeks. I have a CTEK unit permanently wired in under the bonnet, plus a panel heater under the battery to keep the volts up to peak.</p><p>You could do much the same thing with an Arduino to do the charging stages, plus add in things like temperature sensors to make sure you don't fry the battery.</p><p>Always fascinating to see the charger going through the different charge states.</p><p>Nice project.</p>
<p>HI friend , I would like to know how can build the inductor coil?</p><p>Thanks</p>
<p>The simplest ones of DIY coils are air core inductor coils. You can make them easily: <a href="https://www.instructables.com/id/Making-a-Simple-Induction-Coil/">https://www.instructables.com/id/Making-a-Simple-In...</a></p><p>However with a home-made inductor you will hardly know how much inductance you can get out of it. Calculations are complex and a common multimeter is unable to measure it. See more here: <a href="http://www.wikihow.com/Measure-Inductance">http://www.wikihow.com/Measure-Inductance</a></p><p>I would advise you to buy ready-to-use ferrit core inductors, like the one I used in this instructable. If this is not an option to you then walk the trial and error way with handmade inductors until you get some useful amount of induction. Good luck!</p>
Thanks Nobilis .<br>I think is better get the ferrit core coil, I know a professional transformers builder Im shure he can make it.<br>
<p>Fantastic intructable. Great explanation of all the various considerations. I think that just a little bit of math would improve some parts, especially your explanation of why this float charger won't cause overcharge. I would love to see you make other instructables for charging other types of batteries (NiCd / NiMH).</p>
<p>Thanks for the kind words and the positive criticism!</p><p>I am going to give some math background.</p><p>My main focus at the time is on lead-acid solar batteries, so Li-ion and other battery type chargers will probably not be on my repertoire in the next few months.</p><p>However using old mobile phone wall adapters as AA, AAA, CR123 or 18650 cell chargers are even more obvious as to use as a lead-acid battery charger. So thanks for the tip! ;-)</p><p>If you are interested you may try this DIY charger: <a href="http://amzn.com/B00YZV8TRG">http://amzn.com/B00YZV8TRG</a><br>Proper charging of NiCd/NiMH batteries are not that difficult as Li-ion bats.</p>
<p>&quot;Proper charging of NiCd/NiMH batteries are not that difficult as Li-ion bats&quot;</p><p>People all ways confuse possibility of catastrophic failure with simple charging circuitry.</p><p>The &quot;simple&quot; NiCd/Nimh Chargers all ways abuse the cells, fast charging without abuse is very complex(even was kind of a religion) with expensive chargers(5-10 times more then the simple ones)</p><p>I all ways charge my 18650 with a lab power suppy, which I can set to stable 4.20V, since it regulates down at 2.5Amps it reaches 1C.</p><p>With current technology it is easy to get stable 4.20 V.</p><p>The only thing you can (but must not)do better is to charge it first to 3.3 Volts with a resistor in series to be below 1710C Current. And not charging a to hot /cold cell? You handle it or not, so you will feel the temperature.</p>
<p>For people with different chargers with varying output current, it will be nice to add a few lines on maths about values for trickle charging with relation to current, such as explaining about keeping it between C/40 -C/100.</p>
<p>Thanks young man. I will definitely build one for myself and then perhaps for family members. Happy New Year</p>
<p>You're welcome. Let me know if you need further assistance.</p>
<p>Great float charger! :)</p>
<p>Great float charger! :)</p>
<p>i have a battery from my previous car that i wanted to make a project with. i just might use this idea thanks.</p>
Nice write up! Explained it all very well, and a very handy device. Thanks for taking the time to share this

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