Can I power and charge my boombox simultaneously? + advanced wiring help needed

Thread Starter

MechaMatt

Joined Nov 19, 2018
5
My boombox project is currently designed to have x4 3S4P (11.1v) 18650 battery packs, each will have it's own BMS. x2 on one side (11.1v each and connected in parallel) ) and x2 on the other side (same arrangement). The power feeds a 12v regulator to power a monitor, then converted down for a set of 5v LED strips and RPi4 and then is combined (all four packs = 22.2v) and boosted to 24v to feed two amplifiers. I have calculated the load to be between 4-9 amps continuous.
  • Could I plug this into a suitable power supply and charge the batteries (through the BMS's) and use it at the same time? I have heard of some power banks that can do this so I thought I'd just scale it up a little. Perhaps if the battery is only below 50% charge to avoid cc/cv complications. Maybe a solid state relay to switch between using one 'bank' of x2 battery packs whilst charging the other two? This would mean disconnecting the x2 parallel packs (11.1v) + x2 parallel packs (11.1v) = 22.2v into x2 series packs (22.2v) = load and x2 series packs (22.2v) = charge.
  • Also, I'm aware that I have to stop the batteries trying to charge/rob/inrush power from each other when connected and have to isolate one bank from the other when charging but I'm not sure how to do it (3 way switch rated for the load?). Biggest concern is how to isolate/bypass the balance wires to stop them trying to supply the load or short circuit when one bank of packs is switched off to charge the other or when each pack is paralleled and each bank is series connected to make 22.2v.
Anyone got any ideas to help avert a small, expensive fire? I have just enough knowledge to be dangerous and I need to get my head around all this wiring!
Thanks.
 

KeithWalker

Joined Jul 10, 2017
909
Can you draw a block diagram showing how the power is routed from the batteries to everything else? Your description is a little difficult to follow.
Regards,
Keith
 

Tonyr1084

Joined Sep 24, 2015
4,543
You mentioned regulators. If you're talking about the LM78xx series - they waste power. A better option would be a buck converter. They don't waste nearly as much power. You can get them in various voltages and you can get them as adjustable types as well. So having all the voltages you want would be an easy task to accomplish.
 

Thread Starter

MechaMatt

Joined Nov 19, 2018
5
Oh my god, what a pain! After giving up on EasyEDA, Fritzing and Windows Paint - I came up with this from Digikey.
I also realized that I can't series connect the batteries to provide 22.2v as well as being connected in parallel to provide 11.1v.
schemeit-project.png
 

jpanhalt

Joined Jan 18, 2008
9,445
Do the BMS's state that the device can be run while charging? I suspect not, but some do -- at least ones for single cells do.

Can you charge two batteries (one of COMP2 and one of COMP4) while using two batteries, then exchange working and charging batteries? If you are good with that, a simple switching arrange can be designed, i.e., one each, other each, both.

Edit: I suspect you want a forth state, both charge, system off. Please confirm.
 
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Thread Starter

MechaMatt

Joined Nov 19, 2018
5
Regarding buck/boost converters, I have these 12v to 24v 15Amp (I was going to use 12v LEDs originally) and a 12/24v to 5v 30Amp.20200510_224344.jpg
 

Thread Starter

MechaMatt

Joined Nov 19, 2018
5
Do the BMS's state that the device can be run while charging? I suspect not, but some do -- at least ones for single cells do.

Can you charge two batteries (one of COMP2 and one of COMP4) while using two batteries, then exchange working and charging batteries? If you are good with that, a simple switching arrange can be designed, i.e., one each, other each, both.

Edit: I suspect you want a forth state, both charge, system off. Please confirm.
It is possible but depends on the BMS being able to use both charge and balance FETs simultaneously, I haven't found one yet.
As the inept diagram above demonstrates, COMP2 and COMP4 are both two groups of 11.1v battery packs connected in parallel to output 11.1v. I assume that you meant to say 'charge either COMP2 or COMP4'.
I originally suggested a three-way switch in my opening post but upon further thought I'd now go with two (one per 'bank') 2-pole-2-throw switches to avoid any big sparks (no halfway between a shared earth scenario). The states I would need are ON-OFF, OFF-ON, OFF-OFF (system off,charging) and a powered-from-the-wall battery bypass (could also charge both battery banks). These would trigger relays to disengage the balance wires connection from the 'loaded' bank and reconnect the ones for the 'charging' bank or both. I'm starting to see the light..
 

jpanhalt

Joined Jan 18, 2008
9,445
No, I didn't mean charge COMP2 or COMP4; although you could. Since you showed COMP2 and COMP4 as separate, rather than 4 packs in parallel, I assumed there was a reason for that. So, I was suggesting charging one of each pair, e.g., U1 and U3 while powering with U2 and U4; then charging U2 and U4 while powering with U1 and U3.

If you cannot charge and run simultaneously from the same battery, you are going to have to reduce available run time while charging one or more of the batteries while powering from the others and then repeat that process. Of course, lithium batteries are often discharged at a faster rate than charged in units of C. If that is not the case, then it becomes easier.

Once you decide what compromises you will make, then drawing a switching diagram should be relatively simple. 4 A to 9 A total is really not that much.
 

Thread Starter

MechaMatt

Joined Nov 19, 2018
5
I've had a re-design. I was kind of locked into using four of the smaller 3S4P packs (2 at each end of the boombox before),
batt picture1.png
now I can fit one 6S4P 22.2v pack either side to be parallel connected together, which means I can power it differently.
x2 6S4P gives me 28 Ah and 22.2v. I'm back to 3 hours operating time at 9 amps!
I will have the parallel connection between the packs wired through a switch so I can use/charge each pack separately.
A relay when the PSU plug is inserted will switch the load to bypass both batteries and power everything directly, and a switch can then divert that power to charging the batteries.
With the RPi4 on board I'll monitor the battery cell voltages and possibly switch banks automatically then use a hobby charger to balance the batteries overnight. Each bank will have be fused.
I think the power distribution will go: 22.2v -- 24v boost (5A max load) -- 5v buck (10A max load) -- 12v boost (2A max load), this is because I think it's more efficient that way to power the biggest loads first?
I'll work on the wiring diagram next now I've got the plan outlined.
 
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