Disconnection of load when charging battery pack with 5V USB

Thread Starter

503

Joined Aug 15, 2020
7
Sorry for laymen question, however I am trying to build a device that is to be chargeable from USB charger.

It's an audio circuit consisting of 2x 30w 8ohm speakers, amplifier, bluetooth module, step down voltage converter and power isolator.

Battery pack will be made of Li-Ion cells equipped with BMS. Question 1:

1) Assuming voltage of each parallel connected cell bank has max charging voltage of 4.2V is it possible to charge 11.1 V bank with 5V USB input through BMS? Does BMS charge each cell individually with fixed voltage or does it need full 11.1V or above to do this?

Assuming answer to above is 'yes, can be charged with 5v USB', then next question is:

2) What is the simplest/compact method of disconnecting the loads when charger is plugged in? Solid state relay or is there any better way?

I never dealt with BMS but remember model battery pack chargers plugged to car 12V socket being able to fast charge various voltages battery packs so I assume as long as input voltage is above cell's voltage BMS would charge.
 

KeithWalker

Joined Jul 10, 2017
1,386
1) Assuming voltage of each parallel connected cell bank has max charging voltage of 4.2V is it possible to charge 11.1 V bank with 5V USB input through BMS? Does BMS charge each cell individually with fixed voltage or does it need full 11.1V or above to do this?

Answer 1) There are multi-cell Li-Ion chargers available for different input voltages. You need to select one that will match your battery and availble power source.

2) What is the simplest/compact method of disconnecting the loads when charger is plugged in? Solid state relay or is there any better way?

Answer 2) Use a three way switch with "ON, "OFF"(center) and "CHARGE settings.

I never dealt with BMS but remember model battery pack chargers plugged to car 12V socket being able to fast charge various voltages battery packs so I assume as long as input voltage is above cell's voltage BMS would charge.
The voltage of power source does not have to be higher than the voltage across the battery - see answer 1.

Regards,
Keith
 

MisterBill2

Joined Jan 23, 2018
7,679
The one thing to understand is that an arrangement that charges each cell separately will take much longer to charge the whole system because the cells are charged in sequence.So it may be inconvenient to wait three times as long for charging to be complete.
 

Thread Starter

503

Joined Aug 15, 2020
7
Thanks for your answers, further questions:

Keith, I meant simple but less crude way than having to put a switch. Rather than automatic action when charger cable gets plugged in loads are instantly disconnected. I was thinking of solid state relay through which loads are connected from battery pack via NC contacts and battery from power source via NO, same as coil. Therefore after plugging in relay coil energises and disconnects loads whilst bypassing power to charge battery. Though this seems to be still on 'crude' side of ways.

Bill, I anticipate charging time will be long. The idea is to size battery pack for required usage time say 4hr at 70% volume to minimise recharging time. I will be using 2A USB charger most of the time and if 1A is used recharge will simply be longer. Is it better then to step up voltage to take advantage of 2A 5V supply or doesn't it matter too much?

My idea of working out required cells size and pack configuration is to obtain discharge curve for given current draw and integrate over required time. However I appreciate that it may not be as simple as it sounds as those curves may look very differently for different current load. I do not even know whether manufacturers publish such data or it will be required to make homework of own test by logging voltage drop over time at applied required load. As mentioned I do not know much about batteries at this stage. I have designed nice speaker but now need to make it work somehow. Can anyone with batteries experience point out to some literature that would help to resolve this?

Many thanks!
 

Audioguru again

Joined Oct 21, 2019
2,353
Your "11.1V" lithium battery will be (3 x 4.2V) 12.6V when fully charged and its balancing BMS needs a higher input voltage of about 14V. So you need a voltage boost module to boost the 5V USB to about 14V to power the BMS. Then the total charging current will be (5V/14V) x 2A= 0.71A.

When the battery is fully charged at 12.6V then a bridged stereo amplifier such as a TDA7492 will produce 8.2W per channel into 8 ohm speakers. The maximum power will drop to 5W per channel when the battery had dropped to 10V and it should be disconnected to avoid battery damage.

Which amplifier will you use? If it is not bridged then its output power will be only a couple of Watts per channel.
 

MisterBill2

Joined Jan 23, 2018
7,679
Why not just have a charger that will supply the required voltage to charge the batteries? And have it built into the package so that it will be very convenient to use? Or is there some other reason to hang on to using a 5 volt charger? AND, if the the bluetooth device also needs charging then the charge sequence will be even longer and more complex. Is this by any chance a sound package for use in a plant where folks are allowed to bring in their music boxes but not allowed to use company power? I have seen that in at least one auto plant.
 

Thread Starter

503

Joined Aug 15, 2020
7
Your "11.1V" lithium battery will be (3 x 4.2V) 12.6V when fully charged and its balancing BMS needs a higher input voltage of about 14V. So you need a voltage boost module to boost the 5V USB to about 14V to power the BMS. Then the total charging current will be (5V/14V) x 2A= 0.71A.

When the battery is fully charged at 12.6V then a bridged stereo amplifier such as a TDA7492 will produce 8.2W per channel into 8 ohm speakers. The maximum power will drop to 5W per channel when the battery had dropped to 10V and it should be disconnected to avoid battery damage.

Which amplifier will you use? If it is not bridged then its output power will be only a couple of Watts per channel.
Thanks for your input, the amplifier I am using is OEP30WX2 Audio Module Class D Digital Power Amplifier Board (Replace TDA8932). At what voltage form the pack speakers need for for full power and what for ~70%? Is linear relationship from ohm's law or loses (if any) from amplifier differ in non-linear fashion?

The idea of 5V charger was simply because those ones are always available everywhere (phones, tablets, other gadgets etc.). Though now I am getting to conclusion that using higher voltage PSU for recharging will be inevitable.

Yet I need to understand the BMS itself, is it an addition to charger or does it have a function of a charger itself. I mean do I need to use actual charger that will cut when voltage is no longer rising or simple continuous power supply is enough with BMS. I guess rather the former, do if guess is correct could anyone suggest the world's most compact 230V charger module for this duty?

What voltage from the pack would be optimal with this amp?
 

Thread Starter

503

Joined Aug 15, 2020
7
Why not just have a charger that will supply the required voltage to charge the batteries? And have it built into the package so that it will be very convenient to use? Or is there some other reason to hang on to using a 5 volt charger? AND, if the the bluetooth device also needs charging then the charge sequence will be even longer and more complex. Is this by any chance a sound package for use in a plant where folks are allowed to bring in their music boxes but not allowed to use company power? I have seen that in at least one auto plant.
As per my previous post your concept of built in higher power charging module is more appealing now and I'll rather go this route. It's a table speaker and idea of battery backup is to avoid trailing cables when in use. When put away could be being charged even for long hours, however prospect of the juice running out in the middle of enjoyment is discouraging. Therefore I think I try to squeeze larger pack in and some compact charging module.
 

Audioguru again

Joined Oct 21, 2019
2,353
The Chinese amplifier can produce a maximum output power of 15W per channel into your 8 ohm speakers when it is powered from 18V or 20V. The amplifier is cheap with no heatsink so it will overheat if the power output is higher.
When powered from your 3 cells battery the output power will be about 5W to 8W per channel.

A BMS "might" have a balanced charger circuit in it but the Chinese online selling places (ebay, AliExpress, Banggood and Amazon) and their sellers do not know anything or do not translate about it.

The voltage if a charging lithium battery rises quickly to the limit of 4.2V per cell that each cell keeps charging for a fairly long time. The charger stops when the battery drops its charging current to a low amount.

Why do you talk about 70% of speaker power when the difference in loudness between 70% and 100% of speaker power is tiny? 10 times the power sounds twice as loud.
 

Thread Starter

503

Joined Aug 15, 2020
7
The Chinese amplifier can produce a maximum output power of 15W per channel into your 8 ohm speakers when it is powered from 18V or 20V. The amplifier is cheap with no heatsink so it will overheat if the power output is higher.
When powered from your 3 cells battery the output power will be about 5W to 8W per channel.

A BMS "might" have a balanced charger circuit in it but the Chinese online selling places (ebay, AliExpress, Banggood and Amazon) and their sellers do not know anything or do not translate about it.

The voltage if a charging lithium battery rises quickly to the limit of 4.2V per cell that each cell keeps charging for a fairly long time. The charger stops when the battery drops its charging current to a low amount.

Why do you talk about 70% of speaker power when the difference in loudness between 70% and 100% of speaker power is tiny? 10 times the power sounds twice as loud.
I considered this amp after reading very positive reviews about it and I was going to add heat sink to it. Can you recommend better source for this sort of components. This speaker must be safe and I will use better quality parts if needed.

I meant 70% of maximum sound pressure roughly (dbA). It is not going to be banging loud all the time. I understand now why logarithmic pots are used. I am not sure whether forum rules permits but could point out better source of components?

Speakers I selected are DaytonAudio ND65-8, made in China but claimed to be designed/engineered in USA. Purchased from importer in Netherlands, they came with decent documentation and look well made. Also reviews were really good.

It is very difficult to know for a newbie where to buy decent components as all seem to come from one place.
 

Attachments

Audioguru again

Joined Oct 21, 2019
2,353
For your small 3-cells battery you need 4 ohm speakers to get near 15W each from the bridged amplifier.
The 8 ohm speakers you selected are tiny (2.5") then they produce no bass sounds. Many high quality 4" speakers are available that sound fairly good. I would buy Dayton speakers from Parts Express in America.

The Chinese amplifier you selected is sold by Amazon, AliExpress, ebay and maybe Banggood. It is no-name-brand and they do not know who makes it. It might be poor quality and not last long. They sell amplifiers that use American ICs that are high quality. Look for a Chinese amplifier that uses the Texas Instruments TPA3116 IC and has a little heatsink. Many of them have a Bluetooth wireless audio input.
 

Thread Starter

503

Joined Aug 15, 2020
7
For your small 3-cells battery you need 4 ohm speakers to get near 15W each from the bridged amplifier.
The 8 ohm speakers you selected are tiny (2.5") then they produce no bass sounds. Many high quality 4" speakers are available that sound fairly good. I would buy Dayton speakers from Parts Express in America.

The Chinese amplifier you selected is sold by Amazon, AliExpress, ebay and maybe Banggood. It is no-name-brand and they do not know who makes it. It might be poor quality and not last long. They sell amplifiers that use American ICs that are high quality. Look for a Chinese amplifier that uses the Texas Instruments TPA3116 IC and has a little heatsink. Many of them have a Bluetooth wireless audio input.
Thanks for the advise, I actually have separate BT module so if this could all be on one board would be great. This is small garden table top speaker that I am making as a present for someone so the safety and performance is paramount. Yet I want to keep it compact. I may want to scale it up to 4" tough it may be too bulky, perhaps concept for MK2. The low freq sound production was my concern from the beginning, though anything is always better than smartphone speaker i thought those tiny 8 ohm speakers would produce acceptable output for reasonably quiet evening tune playing in the background without upsetting the neighbors too much. I am based in UK so sourcing parts from EU suppliers, Dayton seemed to be good choice. Can I get decent bass producing speaker in size less than 4"? In our tiny homes compactness is important.
 

Audioguru again

Joined Oct 21, 2019
2,353
A cheap tinny-sounding clock radio speaker is 3".
Another problem with a tiny speaker is that it takes more power to make a certain loudness.
I heard JBL and Bose little speaker systems that have tiny speakers in them and they produce good bass at fairly high loudness. But they are extremely expensive.
If the speakers are 4 ohms then the battery can be smaller with 2-cells (6V to 8.4V).

Many years ago I used a pair of 4" speakers in my car then later used one as a sub-woofer with two 3" satellite speakers for the beach.
 

Thread Starter

503

Joined Aug 15, 2020
7
I heard JBL and Bose little speaker systems that have tiny speakers in them and they produce good bass at fairly high loudness. But they are extremely expensive.
Interesting enough, I heard one of the Bose tiny bluetooth speaker sounding surprisingly well and this was an inspiration to build this small stereo set that could nicely decorate garden table whilst used. When I look at the price of the speaker i immediately decided that will be wiser to actually build one. Now not sure whether it was wiser decision, nonetheless I am learning a lot of the audio electronics which is quite interesting as I never ventured into those areas of electronics.
 
Top