Using USB power banks in parallel

[raresserban]

Hello, I am new to this whole battery world
I am building a 5S2P battery pack for TPA3116D2 amp. I have a CC CV Step-Up board to charge it and a BMS. The amp will be directly connected to the BMS.

I want to increase the capacity (and run time) of the amp. I was thinking of connected a USB power bank to the CC CV board to charge the battery through the BMS.

The problem is that a standard USB power bank outputs 5V2A. So that is 10W. If my amp draws ~50w, my battery will still deplete quite fast.

I was thinking of using 2 USB power banks in parallel, so if they each provide 5V2A, that would be 20W, but from what I read it wouldn't work quite like that and I want to understand why.

I was thinking of connected the USB power banks to the CC CV board through diodes so they don't send power to the other bank (in case they don't have internal protection) and maybe a capacitor to smooth the output (but I believe this should be handled by the CC CV board).

Why wouldn't this work like I expect, to get more than 2A/10W charging?If both banks are full and output 5V, shouldn't it work like this?

Thank you and I hope this is the right section for this

 

[john61ct]

Forget USB except maybe for slow charging.

Use an output voltage in the top 80% range of what the amp allows.

Enough Ah capacity for 2-3hrs at high volume, obviously sensitive efficient 4ohm speakers will help.

 

[amberwolf]

How many Wh do the USB packs hold? My guess is they only have one cell each in them, which is probably a couple of Ah, at most, depending on the pack physical size. You can find out the actual cell capacity and redo the calculations below for better guesstimates. (and no, it won't be 30,000mAh or whatever these things often claim; if it was, it would have to be as big as or bigger than a typical large ebike battery).

So call that 2Ah * 3.7v = 7.4wh.

It also has a converter inside that steps up the cell voltage to the 5v, so that wastes some of the power (10-30% wasted, depending on efficiency).

Let's use the middle case, about 20% wasted, so you only get about 80% of that 7.4wh actually out of the pack: 0.8 * 7.4 = 5.92wh.

Then you're going to run it thru another stepup board that's also going to waste power as heat, so we'll be generous and call it 80% too, so 0.8 * 5.92wh = 4.736wh. Meaning, now you're only getting a little more than half the cell capacity out and into your pack--and I would guess there will be even less than this in reality.


The diodes will create a voltage drop based on the current flow thru them, but probably around 0.7v, so that * 2A would be another percentage of power wasted, since that would be 0.7 * 2 = 1.4w of heat on each diode, or 2.8w of heat total (variable, depending on actual flow and voltage drop)).

I think you would be MUCH better off making a bigger battery pack to start with. Or make a second pack you can simply swap out. At least that way you actually get all the power into the amp, and not waste *at least* half of it as heat before it even gets to the amp's charging plug.


Another issue is the charging rate of your cells in the amp's pack, and what it's BMS charge port can handle. Most 18650 cells have low charging rates, and most small BMS's do too. So if you actually do pull more than that thru the BMS cahrge fets, they'll get hot and may fail, or worse may desolder themselves and slide around on the board, leaving solder trails taht short out your pack and destroy it or even start a fire.

If the BMS doesn't fail, and the cells are passing the charging current thru to the amp rather than the BMS (though it should be doing that), or they are accepting the charge current rather than powering the amp, then if the charge current is higher than they are made for, they will heat up. The more it exceeds the rating, the more heat there will be.

 

[raresserban]

Thanks for you suggestions, unfortunately I already order 10 Samsung cells, which should arrive in 1-2 weeks.
I wanted to hold an outside party using this pack, calculated I would get like 5 hours of play time at 25% volume, and I wanted to extend that, that's why I was thinking of charging using power banks.
I have one that is 37Wh, and a friend has one that is 111Wh (it is quite bulky, but maybe it is not really the correct capacity), and I wanted to use those for charging, and maybe some others.
I guess for the time being, I will just use one power bank to charge the pack, and offload some of the load off my battery.

My BMS has the same port for charging/discharging so I think this should work.

 

[john61ct]

Charging from stored power is **very** wasteful.

You really want to charge all your various packs at once, when a real charge source is active.

e.g. mains power, genset, solar, alternator while driving etc

You are also much better off with just one bigger storage bank, maybe two for redundancy

USB powerpacks are just for tiny loads devices like phones or tablets.

Driving audio gear uses a lot of power, especially for outdoor parties, even a vehicle starter battery at 1200+Wh is easy to run down flat.

40-100W output is **a lot** for USB even PD spec not yet fully implemented out in the marketplace

but it is very little compared to charging up the size bank that use case requires.

 

[raresserban]

Hello again, just realized that even though you guys gave me good ideas, I didn't actually get an answer regarding the main question, what happens when using USB power banks in parallel?

If using 2 powerbanks, each with a USB 2A port at 5V, will I get 4A?Or how those it exactly work?Is it like the same case if I want to use 2 USB wall sockets, each 2A?I won't get 4A at the output since the one with the higher voltage will "win"?

 

[amberwolf]

Depends on how the output of the DC-DC in the powerbanks is designed. If they aren't meant to be paralleled, you may have problems (which problem depends on the design). One possible problem is that if one is slightly higher output voltage than the other, it may flow current back into that one; what happens then depends on the design. It wont' be much current, as the difference would likely be very small (hundredths of a volt).


Most likely it is safe to parallel them...but you would have to test for the actual behavior under the conditions you'll use them in.


Diodes would prevent the backfeeding...but they will also drop voltage and waste some power as heat, so you get even less out of the packs than you already will (whcih won't be much).