They used an off the shelf lithium charger board. It charges the battery till it hits 4.2 volts and then goes into a trickle charge mode. There is also a protection chip which prevents the battery voltage going over 4.2 volts or under 2.4 volts by disconnecting the battery.
However when playing loud music, there are both periods of brief extreme load on the battery (ie. Hundreds of amps), but also periods of brief extreme currents into the battery. For example imagine after a drum beat, the speaker cone which has a lot of momentum and coil with a lot of inductance and flowing current needs to be rapidly stopped, and that energy ends up recharging the battery (for a few milliseconds).
As the batteries age, they get higher internal resistance, and the current dumped into the battery causes the terminal voltage to briefly rise above 4.2 volts. The recharge circuit goes into trickle mode, and the protection circuit disconnects the battery.
Now the next drum beat of your music happens, and the charger in trickle mode is providing nowhere near enough current, and the whole speaker turns off.
I'm not aware of any audio architectures which are regenerative like this? Normally what happens to the energy is it ends up dumped in the MOSFET of the amplifier output stage.
A class D amplifier (ie. one like this[1]) is regenerative. Normally there would be big capacitors on the power rails to absorb the energy (and there won't be much - just the momentum of the speaker cone and inductance of the speaker coils). If those capacitors are a little small though...
I'm assuming that they need to have some type of power supply between the battery and the voltage rails of the amplifier to convert the 3.7V (or whatever) battery voltage to something higher. I would be surprised if this power supply worked the other way around (i.e., can sink current).
I always assumed that the damping action of an amplifier would ultimately result in the extra energy being dissipated as heat.
Class D amplifiers typically run direct off battery voltage in portable systems - although sometimes you go for a 2S or 3S or 4S design for more volts and more power output for a fixed impedance speaker. The controller chip compensates for any sag in the supply voltage.
Hey @londons_explore; author of the blog post here. I've added three new photos to help folks like yourself figure out what's going on. One of the battery specs, one of the charger specs and one of the T barrel connector.
Charger 14.5V x 3.3A = 47.85 Watts maximum rated power output under ideal conditions.
It's the law.
Ohms law.
Barrel connector looks like a laptop one, this could be even more of a bottleneck compared to the limited power of the charger. The physical point-contacts are so limited it can only pass so much amperage. Higher amounts of power usually require higher voltages than 14.4V unless beefier connectors are used, which allows more power (at the same maximum amperage) to be conducted through thinner copper wiring and lower-cost connectors.
It would be good to know the specs of the audio output amplifier and speaker assembly but chances are it takes a lot more than 47.85 watts of audio power output to get 126 dB from the speakers. More like 100W RMS delivered to the speakers as an estimate.
You can't get more out than you put in, plus significant energy efficiency is lost in places.
And that power would have to come from the battery if the recharging connector was not a strong enough source. Ideally there would at least be adequately beefy fully-soldered wiring & connections between the battery and audio output circuit, to supply it with the fully rated input power unless a lower engineering target had been designed.
So it seems to be full performance as issued was only ever within reach under battery power so it is basically a battery-powered speaker and the charger as it says on the device is merely a "battery charger" not an A/C line source for highest-performance operation.
Imagine if you will, you are running quite loud and drawing the maximum rated power from the battery pack, if this just happens to be 99.84W of power, the batteries will last only one hour like it says on the battery pack, 99.84Wh. So the most you can get is one hour if you're drawing close to 100W, and the audio output will still be measurably less than that.
Once the battery is discharged, the charging circuit including the external charger may or may not be intended to continue powering the speaker, but it could not realistically be expected to get very near full output dB performance.
This is absolutely the case, except you don’t need an empty battery to cause a high pitched tweeter sound and shutdown. Just having the charger connected to battery and charging while playing at any level above 5 will cause a shutdown. The charger connected by itself will cause speaker to shutdown with a high pitch sound always after 5. Also causing the battery to fail afterwards.
They used an off the shelf lithium charger board. It charges the battery till it hits 4.2 volts and then goes into a trickle charge mode. There is also a protection chip which prevents the battery voltage going over 4.2 volts or under 2.4 volts by disconnecting the battery.
However when playing loud music, there are both periods of brief extreme load on the battery (ie. Hundreds of amps), but also periods of brief extreme currents into the battery. For example imagine after a drum beat, the speaker cone which has a lot of momentum and coil with a lot of inductance and flowing current needs to be rapidly stopped, and that energy ends up recharging the battery (for a few milliseconds).
As the batteries age, they get higher internal resistance, and the current dumped into the battery causes the terminal voltage to briefly rise above 4.2 volts. The recharge circuit goes into trickle mode, and the protection circuit disconnects the battery.
Now the next drum beat of your music happens, and the charger in trickle mode is providing nowhere near enough current, and the whole speaker turns off.