Noob batteries in series question

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

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.

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

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.

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

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

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.

Thanks, I've been looking in to that seems like a good way to go. What are the main daw backs?

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

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

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.

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

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

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.

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

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