Hi!

I'm an absolute newbie in electronics (I'm industrial designer, working as motion and graphic designer )

I'm trying to make a bluetooth speaker... My problem is here:

I have 6 (and I ordered 6 more) lithium batteries (3.7V - 3500mA) and they have to power a bluetooth audio amplifier that needs between 5 and 24v (I would like to have between 12v and 20v) and 3A to work properly.

How can I do that?
What's the best way to connect/link my batteries? .. To get the best output..
What module do I need between my batteries and the bluetooth/ampli module?

Thank you in advance for your help!!

 

Put the six in series. It'll run for a bit more than an hour on a charge.

 

I'm an absolute newbie in electronics

You probably know that when you stack up batteries the voltages add. 6 batteries = 3.7X6=22.2V

Do you more information on the speakers. It is a little strange that they need 3A at 5V through 24V. I find it more likely that they need 3A at 5V and 1.5A at 10V and 0.75A to 20V. But we really don't know. If I am right they might run for a good long time.

 

If I am right they might run for a good long time.

Yeah. And volume counts.

 

Thank you for your help!

Ok for the batteries in series, that gives me 22,2V but not 3A.... no?

Here are the specs of the bluetooth amp:

 

Thank you for your help!

Ok for the batteries in series, that gives me 22,2V but not 3A.... no?

Here are the specs of the bluetooth amp:

Worry about the voltage. The amps will take care of themselves.

 

Ok, and do I just regulate the voltage by the way I connect my batteries together? Doesn't I need to put a "regulator" module or something like that between the batteries and the amp ?(I saw this on other projects)

 

Welcome to AAC. Thanks for joining, I hope you find it worth hanging around.

It is critically important that you add cell protection and a BMS (Battery Management System) to your pack, no matter what configuration you end up with.

These devices protect the battery from overcharge and overdischarge, as well as balancing the charge among the individual cells as needed. Making a battery pack properly is not a trivial undertaking. The danger of Lithium chemistry cells improperly charged is quite extreme. A very difficult to extguish and extremely hit fire is a real possibility.

Given the potential for serious problems, I strongly suggest you purchase a prebuilt battery. I think a LiPo, pouch type pack is probably the most practical. A LiFePO4 pack would be even better because of its inherently safer chemistry but might be hard to source.

A prebuilt battery will include the necessary protection circuitry but you will still need to be sure to provide an appropriate charger. This can be an external one, or one built into the device. Whatever the choice it needs to be something designed to charge a battery of the configuration and chemistry you choose.

Batteries of there types—be they made of cylindrical cells like the common 18650 Li-Ion or prismatic cells like the common LiPo pouch cell—are specified by the number of cells that are wired in series and parallel. The notation is:

nSnP

Where S is the number of cells in series and P the number of those series strings in parallel. So a battery constructed of 6 cells in series would be “6s” while one with three sets of two cells in series would be “2s3p”.

Cells in series determine voltage while cells in parallel determine capacity (usually measured in Amp Hours (AH) and rendered in mAH (milli Amp Hours). So if you need a 12V, high capacity battery you might choose to build or buy a 4s2p battery out of Lithium cells rated at 3000mAH. This would would provide (~3.7 x 4)= 14.8V at (2 x 3000mAH)= 6000mAH or 6AH.

In order to provide the 12V, if that is what is needed, you’d have to include a buck converter which is. a small circuit that converts the 14.8VDC to AC and uses that AC to reduce the voltage—then converts it back to DC with the new voltage. It is the most efficient way to reduce the voltage.

Because batteries will change voltage over their usable run time, the buck converter also ensures the output will remain constant. A boost converter does the opposite, increasing the voltage using the DC-DC conversion method at the cost of amperage (nothing is free). A buck-boost converter can do both ensuring the output voltage remains in the acceptable range.

If your device can be powered from ~14V to 16.8V, which represents the approximate range a 4s2p Lithium battery would vary over its runtime, you don’t need the DC-DC converter. However, since Lithium batteries can.produce enormous currents (even up to hundreds of amps) it is important to include over-current protection, such as a fuse, to prevent very unpleasant side effects from a short circuit across the battery.

There are many batteries designed for remote controlled vehicles readily available, I would suggest you explore the LiPo packs for that application and choose one that meets your electrical needs and will physically fit into your project.

They will come with standard connectors and chargers are cheap and everywhere. To calculate your capacity requirements you have to choose a target run time, then use the average current the device will use over that time to decide on the capacity of the battery. The instantaneous current requirements are much less important because the LiPo pack will provide more current than you can use.

So, for example if you want a runtime of 3 hours, and have an average current of 1A multiply 3 x 1 (or 1000 for mAH) and get 3AH, or 3000mAH. Taking this number and the physical dimensions will allow you to select a battery. Amazon has a huge selection, and there are specialty dealers as well.

 

You probably know that when you stack up batteries the voltages add. 6 batteries = 3.7X6=22.2V

Do you more information on the speakers. It is a little strange that they need 3A at 5V through 24V. I find it more likely that they need 3A at 5V and 1.5A at 10V and 0.75A to 20V. But we really don't know. If I am right they might run for a good long time.

Class D amps can work over a wide range of voltages, but at 5V it can provide only about 1.5W of power into an 8 Ohms, whereas at 24V it can provide 35W.

When run at max power, it will draw more current at higher voltage, but at the same output, it will draw less current at a higher voltage.

 

A Lithium battery cell is not 3.7V. Its half-charged storage and selling voltage is 3.7V and the fully charged voltage is 4.2V. Each cell must never be allowed to drop below 3.0V which will destroy the battery and make it dangerous to attempt charging.

The amplifier probably uses a TPA3116 power amplifier IC. Its datasheet says the output power at low distortion into 8 ohms is 30W for each stereo channel when powered from 24VDC. Then the maximum battery current will be 66W/24V= 2.75A.

You are not playing a continuous tone at 30W per channel, Instead only the loudest peaks are 30W and the average power of music and speech is only 3W per channel then the average current is only 6.6W/24V= 0.275A (plus what current the Bluetooth uses) then the 3500mAh battery cells will last for at least 5 hours.

Note that some 18650 batteries sold on the internet are fakes and have a tiny little 66mAh cell inside.

 

Hi!
Thank you so much for all tose informations! It's taking place in my mind
- Ok, I will have to buy a BMS, but before I need to know how I will arrange my batteries together (nSnP) to buy the good one (3S2P or 6S,...)

- I have a charger (see attachment) ... But if I understant it well, the output is only between 3,7 and 4,2V... (?!) If that's the case, my choice of nSnP doesn't matter? or just for the amperage? .. I don't understand.

Does that charger have a protection system? ... Like a fuse? or do I still need to place a fuse somewhere to protect against over-current problems?

- Next, after my batteries, I have a boost converter (see attachment too) that seems good to me (certainly if my charger output is only around 4V)... is this ok? ...

Thanks in advance for you answers!

 

You are better off using the batteries in series up to to the the voltage you want. A buck converter will cut your capacity by about 20%.

Then you have two options to charge. Get a charger designed to charge them in series, or charge them in parallel (or singly) with your current charger.

 

Each Lithium battery cell is 4.2V when fully charged, 3.7V when half discharged and 3.0V when the load should be disconnected by something that detects 3.0V for each cell. A BMS disconnects at 2.5V per cell which is too low and reduces the battery life.

Your load is an audio amplifier that works fine when its total voltage is 4.5V (low maximum output power) to 26V (high maximum output power). 6 fully charged cells in series produce 25.2V and maximum undistorted power of 32W per channel into 8 ohms then the battery slowly discharges down to 18V producing maximum undistorted power of 16W into 8 ohms which sounds only a little less loud than when the battery is fully charged. The volume control controls the amount of distortion, when you hear distortion then turn it down a little.

A boost converter will not boost one 4.2V/3.5A cell (14.7W) to 25.6V at 2.6A (66W). Use 6 cells in series instead.
Then you do not need a voltage regulator for the amplifier but maybe it is needed for a Bluetooth player.

 

The first question you need to answer is how loud you want the sound and what's the minimum voltage to get that loud. This will depend a lot on the speakers and the listening environment. Then you can decide on the battery arrangement.

 

In case it was missed in my long post:

If your device can be powered from ~14V to 16.8V, which represents the approximate range a 4s2p Lithium battery would vary over its runtime, you don’t need the DC-DC converter. However, since Lithium batteries can produce enormous currents (even up to hundreds of amps) it is important to include over-current protection, such as a fuse, to prevent very unpleasant side effects from a short circuit across the battery

 

You have a charger board for a single 18650 battery cell. I asked Google for "a balanced charger and protection board for six 18650 battery cells" and they were all sold out and protected from 15A to 50A which would not protect, instead it would allow a spectacular explosion. Therefore a fuse is important.

 

I'd try 3S or 4S (12 to 16 volts nominal); if that's not loud enough, 35 watts won't be either. It'll be easy to find a wall-wart charger for 3S lithium (package must say 12.6V lithium charger, not power supply).

 

The TPA3116 stereo amplifier class-D audio amplifier IC produces 50W at low distortion into 4 ohms per channel when the supply is 24VDC.

 

Regarding the fuse, there is a "over current protection" in the charger module, is that enough or should I had a fuse in addition? And if yes, where should I place it?
I don't have time to work on this now, but it is processing in my mind and I will make a little schema before I buy the missing components and assemble it and I will submit it to your expertise to feel safer... Thank you very much for your help and very valuable informations.

 

Your charger board is for only one 4.2V battery cell. Instead you need a "balanced charger for 5 or 6 cells" and a power Supply that can feed it (not USB). The balancing prevents the weakest cell from overcharging and exploding.