Charging LiPo Batteries in Series / Parallel

#1

I’m looking to build a circuit that powers both a portable LCD display that requires 7+ volts and a pi zero w along with a small USB hub. This requires that I have two 3.7v LiPo batteries run in series in order to obtain the 7.4v current needed to run the display.

The issue I’m having is figuring out how I’m going to run them in series and also be able to balance charge them without having to remove them.

Does anyone know if it’s safe to use something like the Zero LiPo Shim and run these batteries in series off the side terminals to something like an Adafruit Power Boost 1000 or something similar? I’d rather not burn out my LiPo Shim or my Pi Zero W if I can at all help it.

This has been perplexing, to be sure. I don’t see a lot of information floating out there on the web that has been of much use. I’d rather not use a mechanical switch for this if I don’t have to, as I’d like for it to be as automated as possible.

Ultimately I’d like to be able to run the unit, and when it registers a low battery condition, I could simply plug it in and it would charge the batteries while providing enough power to keep the whole show running.

Any ideas?

#2

I don’t think two lipo’s in series will reliably give you your 7+ volts. Fully charged they will be somewhere around 4.2 volts, but discharged they will be closer to 3 Volts.
And just so you know, the current output doesn’t double when they are in series. You won’t get any more current than a single cell can supply, you’ll just get it for a longer time.

#3

I think you’re thinking “parallel” which would be correct in that you wind up with the same voltage but with roughly double the mAh.

“Series” adds the voltages of each battery, so in this case with two fully charged 3.7v batteries (4.2v each), you’d wind up with 8.4v plus or minus with the same mAh rating as a single battery.

The problem I’m having is that I want the load to be the product of a series configuration, but charging wants to be parallel.

#4

I found the above statement a bit confusing? Namely the “7.v current” part, voltage and current are two different things.

I wish I could offer more help but this is all I have at the moment. I don’t know what I would do in your situation? Get a different monitor maybe? Adafruit and pimoroni have displays that run on +5V.
Anyway, I wouldn’t attempt what your thinking of doing, mostly because of the charging issue.

I have a Pi A+ with Sense Hat attached that I run from a powerboost 1000c and 6600 MAH battery pack. Its 3 2200 MAH cells in parallel. Everything runs on 5V so its and easy setup. Not like what your trying to do. My 3 batteries came prepackaged as a set / pack. And are fine to charge from the powerboost. Adafruit cautions against making up you own set of parallel battery packs. Bad things can happen etc. I want to have a nice long run time on battery power, so I went with the high capacity cell.

#5

Most lithium chargers are based on a single, or paralleled identical, cell(s) that are isolated from a load so the charger can measure only the charging current going into the cell and not get confused by loading currents.

There are two phases - constant current typically 0.5C until 4.2v cell voltage then switch to constant voltage at 4.2v and monitor the charging current stopping the charge when the current drops to a few tens of mA. That all goes to pot if a load is still connected to the cell.

Quite a few battery banks work the same way - charge or use but not both.

#6

Hrm… I’m guessing then that I might want to run in parallel and maybe use a step up converter. I was hoping to avoid adding another layer of conversion to save on power draw, but it may be that I have little choice in the matter.

Actually, I’d have to use two step up converters. One for 5v and one for either 9 or 12v. The display unit in question is technically rated for either 9v or 12v but runs happily at just about anything over 6.5v.

This proves to be something of a technical challenge. I wonder of I could create a logical triple pole - triple throw switch using transistors… or possibly even relays? That might be an interesting avenue to explore?

#7

How much current does the display draw?
You “might” be able to run two powerboosts in series to get 10 V
So display + to + (powerboost 1 output) - connected to + (powerboost 2 output) - to display -.
You’d be limited to 1 A max. The two batteries would be isolated from the load though and should charge Ok. And the power boost can power the load and charge the battery at the same time. Assuming the power supply can supply enough current for both.
I’m just not sure what kind of sensing if any there is on the powerboost output, which is why I said might work. You’d also need two power supplies as I’d be leary of having any kind of common ground between the two power boosts.

#8

If you look here: https://www.amazon.com/Sylvania-SDVD8732-Portable-Lithium-Battery/dp/B0044FCQIG

You’ll see that I’m using the non-dvd screen in this set. It claims that it will use anything from 9-12v at 1A draw, so technically I suppose a couple of Power Boost 1000s would do that trick. I’ve tested the unit and I know that it’ll run all the way down to pretty close to 6v. I would have used a different controller board for the screen, but the display unit is some whacky LCD-RGB format and not LVDS, as I had hoped. I does, however, take composite in.

I also have a Smakn SBT4312 DC-DC step up converter that I can use. I’m pretty sure it’ll supply something on the order of 4a. Nobody really sells these anymore except for some Chinese companies, but there are similar (Newer) units out there that are comparable. I don’t know how efficient this model is, so I guess I’ll just have to test it and find out.

More news as it happens.

#9

The powerboost might be iffy, your right at its 1A out max.

#10

So I think I’ve found a more feasible solution. I have two identical “Lipo charger basic” (Tiny little red lipo 500mA chargers that have battery out terminals. This eliminates the balance charging problem. I’ll run power in parallel out of the Bat terminals using rectifier diodes to ensure there’s no accidental push-back voltage. I’ll run the combined 3.7v (4.2v) out to the Lipo Shim to power the Pi ZW, the Powerboost 1000 to power the Zero4U USB hub AND to the Smakn SBT4312 to power the display board at whatever minimum voltage I can get that to run without it bugging out on me. I’ll throw in a power switch right after the diodes, as there’s no way I’ll be able to run AND charge at the same time.

I have no idea how long 4600 mAh is going to last in this configuration, but it’s worth testing to see. Now I just need to wire it all up and see what happens!