Hi,

I am trying to build a DIY battery pack with 4x 18650 Li-Ion batteries 3.6V 3200mAh each. This is for a camera rig by the way.

My schematic might be wrong (I apologize in advance) but my goal is to have a 3.6V, a 7.2V and a 12V regulated outputs, to add some sort of circuit protection or BMS and a charging port, perhaps USB if that is easier to source, but I am not sure if a USB charging system would be efficient or quick enough for this setup.



I do not intend to build all the circuits from scratch since my level is not there unless there is something that might be easy enough, rather I would prefer to source parts that already exist and assemble them all together in the most compact and efficient way. I have been looking at batteryspace.com which seems to have most of the components I need.

My questions are:

-Voltage regulators
I have found this one to regulate 14.4V to 12V. Although it might be a bit too big. Is there anything smaller?

For the 7.2V and 3.6V outputs, do I need regulators? I am trying to keep those voltages as consistent as possible, so my initial thought is that I would need something to regulate them but perhaps this might be overkill?

If I use them, then I still yet need to find them. Any recommendations?

-BMS
Do you guys recommend a BMS for this setup? And if so, is there one in particular that you would recommend? There are so many options at Battery Space. And how would that be wired? Splitting from the main 14.4v output?

-Charging.
I am not sure how that is implemented in the circuit. Splitting the two main 14.4v leads + and - as well? Something like this?



And also, is a 5V USB charging circuit ok, or should I be looking for something more suited for a 14.4v 3200mAh?

-Battery Level Indicator
I am trying to find a very small and subtle battery level indicator. Battery Space has this one, but I want to eventually integrate it in the enclosure that I will build for this, so I wonder if there is a more DIY simple circuit that I can build myself for this? Or perhaps I could also buy that one from Battery Space and take it apart repurposing it for my enclosure. I am also not sure how that would look in the circuit, something like this?



I need to draw the correct schematic once all this is clarified.

Thank you so much!

 

Your concept has a major problem: The cells will be discharged at different rates to each other. That will make it very difficult to recharge them using a balanced charger.
The best way is to connect the pack in whatever configuration is convenient to you for recharging, then use buck or boost supply modules connected to the output from the pack to provide each of the required voltages.

 

Your concept has a major problem: The cells will be discharged at different rates to each other. That will make it very difficult to recharge them using a balanced charger.
The best way is to connect the pack in whatever configuration is convenient to you for recharging, then use buck or boost supply modules connected to the output from the pack to provide each of the required voltages.

Thank you so much for the response. Is this because of the way I have the 3.6v output? What if my circuit looks something like this? Perhaps this way all the batteries are being discharged at the same rate?

 

What the each of the load currents and duration?
That will help determine how to generate the various voltages.

 

What the each of the load currents and duration?
That will help determine how to generate the various voltages.

The 12v has a 1000mAh load, the 7.2v varies between 200mAh and 500 mAh and the 3.6v is super low current, it is basically for a photo cell, but unfortunately, I do not know the current value for that one.

 

The 12v has a 1000mAh load, the 7.2v varies between 200mAh and 500 mAh

A mAh is normally used as a battery capacity rating, not a load value.
A 1000mAh load could be 1mA for 1000 hours, or 1A for 1 hour, or 10A for 1/10 hour, none of which tells me anything about what type of regulator is needed.

I'm asked about the maximum current for each voltage output, and the duration of that current.

 

A mAh is a battery capacity rating, not a load value.
A 1000mAh load could be 1mA for 1000 hours or 1A for 1 hour, which tells me nothing about what type of regulator is needed.

I'm asking about the maximum current for each voltage output, and the duration of that current.

I am not that savvy when it comes to all this, I apologize. This is why I am here

I am not sure how to answer this. All I know is that I would be using 4 batteries that are rated at 3.6v 3200mAh for this pack and that the devices that will be plugged into this battery pack are as follows:

-An external monitor that is rated at DC12V 1A.
-The camera operating motor at 7.2v which has a power consumption dependent on filming speed, and lies between 200 and 500 milliamps.
-And lastly, that photocell which needs a voltage of 3.6v, but I have no other information.

As I understand it, the only device that would be draining at a higher rate would be the external monitor, so with an output of 12v 3200mAh, I would get about 3 hours of operation for that one, right?

Does this answer anything better?

 

Thank you so much for the response. Is this because of the way I have the 3.6v output? What if my circuit looks something like this? Perhaps this way all the batteries are being discharged at the same rate?

View attachment 291522

Take a closer look at your diagram. You are shorting out each cell. That would just generate a lot of heat rather quickly.

 

Perhaps it has to do with the way I am drawing it, but I currently have this pack, with no regulators or anything in it works perfectly:



All I am trying to do is implement this to have also a 12v output.

 

I might have drawn that wrong. I think this is what I currently have:

 

so with an output of 12v 3200mAh, I would get about 3 hours of operation for that one, right?

Approximately.
The battery data sheet for a 1A load would give you a more exact answer, as the actual mAh value depends goes down as the load current increases.

It likely would be the easiest and most efficient to use buck switching regulators for all the larger current loads (12V and 7.2V) directly from the series battery 14.4V.
If the 3.6V current is low enough, then just a linear regulator (e.g. LM307) form the 7.2V switching regulator could be used.
That way all batteries are equally discharged, giving the maximum operate time for the combined loads, and also making it somewhat easier to charge al the batteries at once.

For example, a couple of these should work, with an output adjustable from 1.25V to 30V, and are only 1.69"L x 0.83"W x 0.51"H.

 

This is what you should be considering. Using inverters will be much more efficient than using regulators:
:

 

This is what you should be considering. Using inverters will be much more efficient than using regulators:
View attachment 291528:

Thank you so much! That is super helpful, I so appreciate it. I have never used inverters before, aren't those to convert DC to AC tho? The devices that will be plugged to this battery pack are DC. Did you mean converters?

 

You are attempting to connect the batteries in series for 12V and in parallel for the 3.6V, which shorts all of the cells. Look at your original diagram. You can trace wires from the + to the - terminal of each cell.

You cannot do that. Look at @KeithWalker’s post.

(but change the word “inverter” to “DC to DC converter”)

 

Did you mean converters?

Look at the last sentence in my post #11 for an example module.

 

You are attempting to connect the batteries in series for 12V and in parallel for the 3.6V, which shorts all of the cells. Look at your original diagram. You can trace wires from the + to the - terminal of each cell.

You cannot do that. Look at @KeithWalker’s post.

(but change the word “inverter” to “DC to DC converter”)

Makes sense now. Thank you!

 

Approximately.
The battery data sheet for a 1A load would give you a more exact answer, as the actual mAh value depends goes down as the load current increases.

It likely would be the easiest and most efficient to use buck switching regulators for all the larger current loads (12V and 7.2V) directly from the series battery 14.4V.
If the 3.6V current is low enough, then just a linear regulator (e.g. LM307) form the 7.2V switching regulator could be used.
That way all batteries are equally discharged, giving the maximum operate time for the combined loads, and also making it somewhat easier to charge al the batteries at once.

For example, a couple of these should work, with an output adjustable from 1.25V to 30V, and are only 1.69"L x 0.83"W x 0.51"H.

This is great, thank you @crutschow ! So a schematic for something like this would be like the first drawing that @KeithWalker posted? Except that you are saying to step down 7.2v from the 14.4V?

 

Except that you are saying to step down 7.2v from the 14.4V?

It can be done either way.
But based upon the efficiency curves below from the LM2596 data sheet (used in the module I referenced), connecting them all to the 14.4V battery would be slightly more efficient.

 

Thank you so much for all the replies. So If I am understanding this correctly, the most efficient way to do this is with step-down converters. I implemented the drawing that @KeithWalker initially made, is this correct?



If so, what happens with the output negative leads from each converter?

I will be ordering the buck converters that @crutschow recommended.

What PCM or BMS do you recommend? Considering I do not have much space.

Thanks again!

 

I implemented the drawing that @KeithWalker initially made, is this correct?

Looks good.

If so, what happens with the output negative leads from each converter?

The negative output is connected to the input common, so can be connected to the output load common.

What PCM or BMS do you recommend?

Sorry, I'm not that familiar with them.
Perhaps someone else can help with that.