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.

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?

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.

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

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

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

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.

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

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.

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

Hi Fabio, I have not worked out the duty cycle for the mosfet yet I've not started the code yet just trying to get a decent plan for the circuitry first.

as for your second question I will be using at least two batteries in parallel so the discharge current will be at least effectively halved, although the batteries I'm looking to use have a very high discharge current and have been tested at this rate.

https://www.fogstar.co.uk/products/molicel-p26a

Also the batteries will only have to endure short bursts of current draw over a short period.

Out of curiosity I looked for the datasheet, but it doesn't exist at that website I think.

The website link has a very brief specification only (it barely deserves the name 'specification'):

At a minimum, this is what specifications should look like:

http://www.farnell.com/datasheets/2608759.pdf

Last night I had a quick browse of Vapers "Mods" - astoundingly bad engineering practice seems common.

Many of the home brew things are built by people who obviously have almost no understanding of how the parts work.

Some of the commercial offerings look quite slick from outside but there are no proper specs.

I suspect that a lot of the 200W style claims are a bit like those made for matchbox sized loudspeakers.

Some of the designs just put 2 biggish cylindrical Li batteries in series with a button operated switch to connect them across a heater coil (of uncertain resistance). Often a MOSFET is used to buffer the button switch.

IRLB3034 seems popular.

The better commercial designs have temperature control based on the heater resistance.

MK

Yes there are certainly a lot of very unsafe diy mods but I can assure you legitimate commercial mods are very safe and have to follow the strictest guidelines and be approved. I use joytech and relaux mods which are both fantastic mods that are very safe.

I aim to do this as safely as possible something I think is very achievable given I take my time and do it right.