35 Replies Latest reply on Mar 17, 2019 1:48 AM by moderategamer

    Noob batteries in series question

    moderategamer

      I was wondering, if I wire for example 2 2700ma batteries in series to get roughly  8.4 v cell fully charged but use a step down convertor to bring the voltage down to 5v do I gain back any of the capacity between the two cells? Ideally I would want to wire in parallel for the project but unfortunately I need to draw up to 3.7ish V which would become impossible as the batteries discharge. I'm new here so please go easy on me I'm trying to do as much reading as I can but would appreciate the help.

        • Re: Noob batteries in series question
          gecoz

          Hi Steven,

           

          The short answer to your question is no, with the batteries in series the max capacity you can aim for is the original capacity of the single battery, if they are identical, otherwise the capacity of the resulting battery would be equal to the lowest capacity.

           

          Generally speaking, if you use higher voltage to power up your load you end up drawing more current from the battery, hence making it discharge faster. If you imagine the device you are powering up with the battery as a simple resistor (approximation), and think about Ohm's law (V = R * I), you can understand why this happens: your load would stay constant, so if you increase the voltage (V), the current draw increases too, so that R remain constant.

           

          If you are planning to use the battery in parallel instead, to increase the capacity, just make sure the 2 batteries are identical (or as identical as they can be), otherwise they won't be balanced, which will lead to one of them eventually failing.

           

          I hope it helps,

           

          Fabio

          4 of 4 people found this helpful
            • Re: Noob batteries in series question
              moderategamer

              Thanks for the reply Fabio, my project uses a Mosfet to control the resulting voltage I'm just a bit confused as to where all the extra voltage is going. if the resistance of the coil I'm using remains constant and the voltage I'm effectively supplying is around 3-3.7v am I conserving power at all. sorry I'm very new to electronics programming is more my deal.

                • Re: Noob batteries in series question
                  gecoz

                  Hi Steven,

                   

                  If I understand your question correctly, you are asking why, if the nominal voltage of the battery if 4.2V, I read a voltage of 3-3.7V once the battery is connected to the circuit.

                   

                  The battery nominal voltage is defined as the voltage read at the terminals when the battery is disconnected, i.e. with no load attached to the battery. Ideally, the perfect battery could provide any amount of current, keeping its nominal voltage the same, i.e. the battery would have no internal resistance.

                   

                  The reality is quite different: batteries do have some internal resistance, and you can see its effects only when you connect the battery to a circuit. Once attached to a load, the current drawn from the battery causes a voltage drop on the internal resistance, and the result is a lower voltage read at the battery terminals, as the sum of the voltage drop on the internal resistance and the voltage drop on the load must still be equal to the nominal voltage of the battery.

                   

                  I hope I'm not making things more confusing with my explanation.

                   

                  Fabio

                  3 of 3 people found this helpful
                    • Re: Noob batteries in series question
                      moderategamer

                      Nope that all makes perfect sense thanks again. I guess I'm just unsure about if I have two batteries in series that under load are lets say 6.2 v and I for example use the mosfet to modulate the voltage down to say 3v surely that would mean I had gained capacity or run time. Which I know you're saying is not possible just struggling to understand what reducing the voltage does because I would still be drawing the same amperes or current as the resistance of the coil hasn't changed. So I'm using a lower effective voltage due to the mosfet and the draw is lower than it would be at full voltage before the mosfet, so what happens to the extra voltage I'm stripping away cause in my mind that should equate to effective more capacity? Sorry if that makes no sense.

                        • Re: Noob batteries in series question
                          michaelkellett

                          If I understand your circuit correctly (a schematic would help !) you have a battery, a mosfet (controlled we don't know how) and a load.

                           

                          if the mosfet is operating in a linear way it drops the voltage between the battery and the load, and the wasted power (current x drop across mosfet) comes out in heat in the mosfet.

                           

                          If you use a switching controller (Google 'buck regulator') it acts differently and the current in the load will be greater than the current drawn from the battery but the voltage across the load will be less than that across the battery.

                           

                          The power into the load will always be less than the power drawn from the battery because no controller can ever be 100% efficient.

                           

                          MK

                          3 of 3 people found this helpful
                            • Re: Noob batteries in series question
                              Jan Cumps

                              yes. at least with the buck converter way less energy will be wasted as heat compared to the linear FET.

                              With a buck-boost converter, Steven would also be able to get some more energy out of the cell when the combined battery voltage drops below the required voltage.

                              2 of 2 people found this helpful
                              • Re: Noob batteries in series question
                                moderategamer

                                Sorry I haven't drawn up a diagram but atm it's a very simple circuit it's just a mosfet controlled by one of the pwm pins of an Arduino the draw is simply a roughly 0.24ohm coil and ideally 2 to 3 18650 in parallel but more likely either 2 in series or 4 in 2s2p

                                  • Re: Noob batteries in series question
                                    michaelkellett

                                    We really need to see a schematic - I think what you have described is is PWM drive of a mainly inductive load, which will work rather like a buck regulator and can be efficient.

                                     

                                    There should be a diode somewhere (to help with efficiency and keep the MOSFET safe) - without a schematic I can't say where, and I need more information about the coil - part number, data sheet whatever.

                                    If you coil is really a 0.24 ohm load the current with 3.7V across it would be 15.4A - seems a lot for batteries.

                                     

                                    What does the coil do - in many cases it isn't necessary to put anything like as much power into a solenoid to keep it energised as there is to get it to pull in at first.

                                     

                                    MK

                                    2 of 2 people found this helpful
                                      • Re: Noob batteries in series question
                                        moderategamer

                                        I don't have a datasheet for the coil as I buid them, 15A is within the batteries regulation and completely normal for the application. I'm not sure if you mean using a mosfet is similar to a buck regulator or if I were to make a buck boost convertor but I can't increase the voltage with just a mosfet only reduce it. I'll draw up a simple diagram but I'm not sure what use it will be.

                                        • Re: Noob batteries in series question
                                          moderategamer

                                          Here's a rudimentary diagram it's missing a few items but nothing that is important for demonstration purposes

                                          2 of 2 people found this helpful
                                            • Re: Noob batteries in series question
                                              michaelkellett

                                              Now we're are getting somewhere - but to get anywhere sensible you will need to tell me what it's for.

                                               

                                              What PWM signal are you using (frequency and duty cycle).

                                               

                                              There should be a diode across the coil (probably - if it was a relay coil almost certainly although there are applications where a zener diode or transorb would be better).

                                               

                                              If it's a door lock solenoid you would want a diode and in many cases a capacitor across the battery.

                                               

                                              You've said that you wind the coils yourself, how many turns, what diameter is the coil, how thick is the wire (from that we can work out the inductance).

                                               

                                              Why does all this matter,

                                               

                                              If the component values and PWM are suitable you can have any average current you like in the coil for pretty much any battery voltage, if you add the diode across the coil (anode to positive battery connection), the efficiency can be good.

                                               

                                              MK

                                                • Re: Noob batteries in series question
                                                  moderategamer

                                                  I'm hesitant to tell you what it's for as you will most likely tell me to not do it but it's for a regulated mod for a vape. I'm in the early stages right now and plan on making 100% sure it's all safe. For example I will be measuring battery voltage and cut use when it dips below recommended battery voltage, I'll have a 5 second fire limit, I'll also be adding a fuse between the drain and coil to avoid over current so the coil doesn't melt, I'll be capping the wattage at 75W to avoid drawing too much current from the battery which is well below safe draw, only allowing certain resistance coils  basically I'm going all out on safety and I'll be doing my homework.

                                                   

                                                  As far as the coils go I use 26 guage stainless steel wire between 6 and 8 wraps which usually gives around 0.24 ohm coils and the pwm frequency will be variable depending on wattage selected but it's pin 3 on the arduino.

                                                   

                                                  I'd appreciate it if you could explain the buck convertor to me as it seems like the way to go.

                                                   

                                                  also should I add a resistor between the gate of mosfet and ground as from what I understand the gate can store power between cycles causing it to not switch off.

                                                   

                                                  I am new to electronic design as I'm a lowly programmer but I'm a quick study.

                                                    • Re: Noob batteries in series question
                                                      Jan Cumps

                                                      moderategamer  wrote:

                                                       

                                                      I'm hesitant to tell you what it's for as you will most likely tell me to not do it but it's for a regulated mod for a vape.

                                                       

                                                      .....

                                                      Don't vape

                                                        • Re: Noob batteries in series question
                                                          moderategamer

                                                          Hey if it stops me smoking, haven't smoked in 6 years. xD

                                                            • Re: Noob batteries in series question
                                                              michaelkellett

                                                              OK - I understand now:

                                                               

                                                              The "coil" is a heater so it will have minimal inductance. It will have a little so the diode or possibly a resistor in series with a capacitor in // with the heater would be a good idea - bu may not be required.

                                                               

                                                              Is this a one off or do you plan to sell it ?

                                                               

                                                              The problem is that to make it safe you will need a much more complex circuit  - and if it is to be sold there is no way it can be controlled by an Arduino.

                                                              If I were designing it I would do it so that it could not become unsafe with 1 (or possibly 2) hardware faults, and any software fault including complete replacement of the code with malicious code.

                                                              (Because these things are sold in large numbers, get used with poor quality batteries and have already killed a few people (by fire) - so there are several high risk factors)

                                                               

                                                              So, unless you want to discuss a commercial relationship , I'm going to have to bow out here.

                                                               

                                                              Still happy to answer specific questions but very worried that pulling 75W from 18650 cells is not exactly a beginner's project.

                                                               

                                                              MK

                                                                • Re: Noob batteries in series question
                                                                  moderategamer

                                                                  No this is a one off making it as a present for my dad who is most likely never going to use it. I get the best batteries on the market all individually tested. I think it's entirely possible to do it with an Arduino safely and 75 watts is just the limit I'm setting actuall use will be 45 - 50 watts at short draw of around 2 seconds I have many high quality commercial mods that can draw 200 watts safely so I think 45 -50 is safe. Also 18650's can give out high amp loads well. I think there's a point where you can be overly safe and think mine will be 100x safer than the unregulated mods which are the ones that cause accidents. The actuall power delivery is on a closed circuit independant of the Arduino due to the mosfet so there's no real chance of any hardware fault on the Arduino side and with the correct resistors and fuse (which btw no commercial mods use) the circuit can be very safe. I understand if you don't feel comfortable helping though that said I would appreciate the help, I mean I'm going to do it anyway may as well get some sound advice xD

                                                                   

                                                                  If you do feel like advising me would love to hear your thoughts and knowledge about the buck convertor.

                                                                  2 of 2 people found this helpful
                                                                    • Re: Noob batteries in series question
                                                                      michaelkellett

                                                                      For a heater load a buck converter has limited advantage, one needs to know a lot more about the load to decide if its worthwhile.

                                                                       

                                                                      Do you know the resistance of the heater cold and at working temperature (there is usually quite a difference).

                                                                       

                                                                      MK

                                                                      2 of 2 people found this helpful
                                                                        • Re: Noob batteries in series question
                                                                          moderategamer

                                                                          I'm not able to measure the resistance at temperature but the maths say at 500 degrees Celsius the resistance would be roughly 0.58344 ohms which would bring the amp draw down to about 6-7 amperes. Cold temp is the first number I quoted at 0.24 ohms.

                                                                           

                                                                          Take these numbers with a grain of salt though as they're calculated with googled coefficients for stainless steel.

                                                                           

                                                                          sorry I made a mistake there got my Fahrenheit and Celsius mixed up at 500 degrees Fahrenheit resitance would be 0.38904 ohms and draw 9.95 (2dp) amperes.

                                                                          1 of 1 people found this helpful
                                                                            • Re: Noob batteries in series question
                                                                              gecoz

                                                                              So, what you are trying to build is basically a PWM-controlled MOSFET driven heater, whose heating element is powered by a 2.7Ah battery.

                                                                               

                                                                              Have you already worked out the duty cycle for heating up the coil? This will determine how long one charge will last for but, more importantly, will determine if the battery is working within safe operating margins.

                                                                               

                                                                              This leads to my second question: do you know the maximum continuous discharge current for your battery? For example, a Sony 2.2Ah High Discharge Rate battery is only safe up to 10A max continuous current. Drawing 15A, if done continuously, will definitely put your battery under huge stress, with risks of over-heating.

                                                                               

                                                                              Assuming your coil resistance will vary between 0.24 ohm and 0.39 ohm, you definitely will also have to take into account the battery internal resistance in your calculations. For a good quality brand new battery, the internal resistance will be around 0.03 ohm (will increase as battery ages).

                                                                               

                                                                              Fabio

                                                                              1 of 1 people found this helpful
                                                      • Re: Noob batteries in series question
                                                        gecoz

                                                        To try and grab this concept, lets think the mosfet just as a "black-box" converter, placed between the battery and the load (the battery is connected the converter input terminals, and the load is connected to the converter's output terminals). In your case, your converter should transform your voltage from let's say 6V down to 3V. If your transformer was ideal, this conversion would happen with no losses, i.e. the power (P = V * I) transferred from the converter to the load would be equal to the power transferred from the battery to the converter (no losses).

                                                         

                                                        Again, in the real world, there is no such thing as no losses, and the converters have a defined efficiency (i.e. the fraction/percentage of power effectively transferred from the input to the output of the converter), as Michael pointed out. Therefore, using higher voltages leads to more power wasted because of the losses.

                                                         

                                                        But I think what you are finding more confusing is the fact that battery capacity and run time are not the same thing. The capacity of a battery is a property of the battery, due to the physics of how it is built (the chemical reactions involved, etc...), and it is independent from the way the battery is used. The run rime of a battery depends entirely on the way it is used: if you have 2 identical batteries, and you connect them to 2 circuits that draw one double the amount of the current than the other, the run time of the battery attached to the higher-drawing circuit will last half the time compared to the other.

                                                         

                                                        Again, I'm trying to simplify things hoping that this doesn't confuse you more, as I know some concepts are hard to grasp when you are moving your first steps in the world of electronics.

                                                         

                                                        Fabio

                                                        1 of 1 people found this helpful
                                                          • Re: Noob batteries in series question
                                                            moderategamer

                                                            Thanks again Fabio, I get that no circuit is lossless just seems like a huge waste of battery when you're essentially just throwing away all the remaining voltage after the conversion. I just thought it was maybe different with a mosfet because of the way it drops the voltage, as far as I'm aware the mosfet which i'll be controlling with one of the pwm pins of an Arduino drops the voltage by opening and closing the circuit by the desired frequency to do such. I thought even though there would be a loss it wouldn't be the entire difference.