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DIY bluetooth speaker

audioguru

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Most Helpful Member
I am glad that you bought a "proper bms" that is made to protect 18650 cells.
But its spec's say that its over-charge and over-discharge protect currents are 60A! which would make a very loud BANG if true.
Maybe the severely overloaded power supply will also make a loud BANG.
It shows that when you buy cheap you get cheap.
 
Well that is why i also ordered a 6A fuse and temperature fuse. Granted it would be easier to just buy a better BMS but the point of this project is to do it as cheap as possible (otherwise i would just buy 300 dollar unholy overpriced JBL thing). I will also use some thermal fuses to install an alarm on my lito 500 charger which doesnt have any temperature protection.

ps: when i said i bought a proper bms, i just meant that its diagram shows how to properly connect all 5 cells. It was still very cheap. Belive it or not, i am looking forward to testing at what voltage it stops the charging. This is 90% of the project for me, learning new things, testing, documenting.
 

audioguru

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Most Helpful Member
I think trying random things is "trial and error" and is a poor way to learn. An error with a lithium battery might result in an explosion and very hot fire.
Go to www.batteryuniversity.com to learn properly.

Cells in parallel will have strong cells charging weak cells and ending up all at the average charge.
 
I didn't ask this to avoid BMS, i will install BMS not to worry. But like Nigel said, parallel cells charge eachother which is good to know. And i am trying to understand if having 3 cells in parallel vs 1 cell, incase BMS fails, is there more, less or equal chance that 1 cell will go over 4.2. Gut feeling tells me that having 3 cells in parallel will equal to less chance of 1 cell going over 4.2 as oposed to having only 1 cell.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
If one cell dies in a series/parallel pack then you've still got problems, you were talking 4s3p - if one cell fails (say it goes O/C), then the two cells in parallel with it will receive 50% higher charge current, this means they will charge faster than the rest and are VERY likely to go too high in voltage - this is where protection boards are vitally important.

You can't go worrying about protection boards failing, it's an added level of protection, not a lifetime guarantee. Pretty well everything you have that uses a Li-Ion battery pack will have a protection board.

Work wise we change a LOT of batteries, both Li-Ion and NiMh - as such we have a spot welder for connecting them (really cool machine, lot easier and neater than soldering - and dead cheap from China), and every single Li-Ion battery pack we change the cells in already has a protection board, and normally a temperature sensor as well (as they commonly fast charge them). The majority of Li-Ion packs we replace cells in are 2s2p, and it's often 4 packs for each repair - we buy 18650's in boxes of 100! :D.
 
Btw i have a question about high voltage of liion batteries, for instance 3.7V being overcharged to 4.6V. Why does the battery explode ? Is it heat or something else ? Because if BMS fails, if heat is the only problem, heat fuse will solve that. If its not heat, then there is nothing i can do but trust my faith in BMS.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
A heat fuse will NOT prevent fires - why do you have this problem trusting the specially designed protection boards?.

I would imagine that the cause of battery fires is most likely heat (as most semiconductor failures are), but most likely it's internal to the battery - and heat fuses are extremely slow devices.
 
I will do that and try to educate myself further. While i am waiting for parts for the bluetooth speaker, i found an interesting video of a guy converting his old nicd into lithium from salvaged cells from laptops. And whlie that video is supposed to be video saying why it is not good to do that, it doesnt end up like that. His conversion works, since his tool only draws about 10 amps (mine draws something like this also). I planed to do 4s3p, which would mean about 3.3A per cell at most (but mostly much lower). Since i get cells for free and i still plan to keep 1 original nicd battery and experiment with another, i think it would be a fun project. I would use liion charger for 4S and use protection board. Thoughts ? Here is the video link, not the one i wanted to show but similar:


ps: its 14.4 drill i have, i planned to use 4S3P. But the option is also to use 3S4P. Thoughts ?
 
Would anyone dare to say anything about this ? I would love to do this project for the sake of having fun so any input would be great :))
 

audioguru

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Most Helpful Member
You did not say which amplifier so I found the TDA7492 in Google. Banggood and a few other Chinese Bluetooth modules use it and are rated at 2 x 25W.
3 series Lithium cells produce 11.1V when half charged then the TDA7492 amplifier produces such low power into an 8 ohm speaker that it is not shown on the datasheet graph that shows a minimum supply of 15V.
When the 3 cells are fully charged and produce 12.6V then the power is also so low that it is not shown on the graph.

The TDA7492 amplifier produces 25W per channel into 8 ohms when its supply is 20V and its distortion is horrible at 10%. It produces 25W into 8 ohms with low distortion when its supply voltage is about 28V.

When the 3 series cells battery is fully charged at 12.6V then the output swing into an 8 ohm speaker is probably 11V p-p which is only about 4W at fairly high distortion.

A 4 series cells battery averages 14.8V at half charge then the power into an 8 ohm speaker at low distortion is about 8W, a 5 cells battery= 15W, a 6 cells battery= 21W, a 7 cells battery= 26W and an 8 series cells battery produces a voltage too high when fully charged.
 
Actualy my last question was about a nicd to lithium conversion of my power drill :))
But speaking about a speaker, i plan to do 5S2P there and hope that it will play well. I could go for 7S1P or 7S2P but i will have a problem getting BMS for that, also using 7 cells gives me 29.4V at full charge, while for that amp it says input should be 10-25V. Can i go over 25V ?
 

audioguru

Well-Known Member
Most Helpful Member
The 14.4V drill will work fine with 4 series fairly new lithium cells.
The amplifier will also work fine with 5 series lithium cells.
The datasheet for the amplifier says a max supply of 30V on one page, 26V on another page and 25V on the graphs.
I assume the TDA7492 is a real ST one, not a cheap Chinese copy.
 
Its a cheap chinese copy for sure, for 7 dollars. I already ordered BMS for 5S and charger for that so for starters i will have to do with that. I am looking for 7S BMS but cant find anything cheap with good design :S
 
Btw, when i get all the items and asemble them together. And if i start up the speakers, put them to max and measure voltage and amperage of output on the batteries, will that give me a wattage ? voltage * amperage ? Or should i measure some other way ? It would be cool to just know how much watts am i getting out of the system
 

audioguru

Well-Known Member
Most Helpful Member
Your battery might not survive being charged at 15A. Also the BMS will probably shut off too soon when its charging voltage gets higher than 4.28V per cell even though the battery is nowhere near a full charge (when the charging current drops to a low level while still charging at 4.20V).
The cheap amplifier or speakers might not survive a test at full output continuously since music and speech has only momentary full power that most ordinary meters cannot measure. Make or buy an LED VU meter calibrated in power.
 

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