Quick question......lipo batteries

Thread Starter

Stillers48

Joined Jul 12, 2020
21
You have 4 lipo batteries. They are wires 2 pairs of two each are wired in series and the resulting two batteries are wired in parallel. Isn't his considered a 2S or 4S battery?
 

Thread Starter

Stillers48

Joined Jul 12, 2020
21
That is called 2S2P, which should give about 7.4V at twice the capacity of a single cell. That is, two, 2S in parallel.
Thank you for the reply. That raises another question however: How do I wire it up for balance charging? It seems to me that would be effected. Every cell is supposed to be charged in parallel but you can't do that now since each cell is actually 2 cells. But without putting it on paper, in my head it seems like it creates a cross-wiring issue if you try to wire all 4 cells in series-parallel and in parallel for discharge and balance charging. Can you split the two series stacks and wire them to charge as 2 separate 2S batteries?
 

jpanhalt

Joined Jan 18, 2008
10,087
Balancing parallel cells is a problem. Being parallel means that the voltage for each cell is the same. Check here: https://batteryuniversity.com/learn/article/serial_and_parallel_battery_configurations

Scan down to Parallel Connection. Note the comment that balancing is less important in parallel-connected cells.

Balancers are made for series-connected cells. If you want to balance individual cells in parallel, you must disconnect them from each other at one end. I would just treat the pack as a 2S pack. That is, the cells in parallel would be treated as a single cell.

As for splitting cell packs, sure that can be done. But is it worth the effort? You will eventually be putting them back parallel, presumably without using any protective device between them, like a resistor.
 

Thread Starter

Stillers48

Joined Jul 12, 2020
21
Balancing parallel cells is a problem. Being parallel means that the voltage for each cell is the same. Check here: https://batteryuniversity.com/learn/article/serial_and_parallel_battery_configurations

Scan down to Parallel Connection. Note the comment that balancing is less important in parallel-connected cells.

Balancers are made for series-connected cells. If you want to balance individual cells in parallel, you must disconnect them from each other at one end. I would just treat the pack as a 2S pack. That is, the cells in parallel would be treated as a single cell.

As for splitting cell packs, sure that can be done. But is it worth the effort? You will eventually be putting them back parallel, presumably without using any protective device between them, like a resistor.
Admittedly, my tech depth on balance charging needs polishing. But I seen a diagram of how Lipo batteries were wired for balance charging. The best way I can describe the the diagram is thet each battery shared positive lead but each had their own ground lead. And I know for fact that other 2S and above lipo batteries, specifically used for RC devices, do have a discharge lead and a balance lead. The balance lead for a 2S is 3 wires, a 3S is 4, etc etc. Which fits the idea that each one shares one lead with an individual ground. So I believe it is not only possible but is necessary for these type of lipo batteries in order to ensure that each battery charges equally to prevent one cell reaching full charge before another and becoming damaged from overcharging as it will still receive a charge current waiting on the other cell or cells to reach full charge.

Admittedly, I do not have a resistor between these batteries. I have 4 extra tablet batteries, 5000mAh each, that I wanted to throw together with a extra UBEC and a cree light bulb to make a generic emergency powerbank/flashlight.
 

jpanhalt

Joined Jan 18, 2008
10,087
Just as a thought experiment. Let's assume you have 2 cells in parallel, but you disconnect at one end (doesn't matter which end) and charge balance them to +/- 20 mV. They could be 40 mV different. You then connect them in parallel using connections, say 2 mohm (which may be high). Now, you connect them in parallel. E=IR (Ohm's law). 40 mV/2 mohm = 20 A! Of course, a better connection would give even more current. They would quickly reach some state of charge that gives the same voltage for each cell.

That is why I suggested that balancing "good" cells that are to be connected in parallel is worthless. Now, if one cell is shorted, of course, you will detect that while charging individually, but I suspect you will also detect it when charged in parallel too. Battery University discusses that scenario (I think).
 

Thread Starter

Stillers48

Joined Jul 12, 2020
21
Just as a thought experiment. Let's assume you have 2 cells in parallel, but you disconnect at one end (doesn't matter which end) and charge balance them to +/- 20 mV. They could be 40 mV different. You then connect them in parallel using connections, say 2 mohm (which may be high). Now, you connect them in parallel. E=IR (Ohm's law). 40 mV/2 mohm = 20 A! Of course, a better connection would give even more current. They would quickly reach some state of charge that gives the same voltage for each cell.

That is why I suggested that balancing "good" cells that are to be connected in parallel is worthless. Now, if one cell is shorted, of course, you will detect that while charging individually, but I suspect you will also detect it when charged in parallel too. Battery University discusses that scenario (I think).
Hm. Thank you for that information, sir. Appreciate it!
 

KeithWalker

Joined Jul 10, 2017
1,144
To keep it very simple, just regard two cells in parallel as a single cell with twice the capacity. The parallel cells don't need balancing but they need twice the charge current as a single cell.
A 2S2P battery only needs to balance voltages of the two parallel packs with each other.
Keith
 

Thread Starter

Stillers48

Joined Jul 12, 2020
21
No, even simpler and use a 2S balanced charger
View attachment 213986:
Whoop. Pump the brakes. Something has to be in series to get the voltage right. If everything is in parallel I end up with a 3.7 volts and 20,000mAh. I was targeting 7.4 voltd 10,000 mAh so I didn't have to constantly be boosting the voltage when using it. I'm not even sure my module can boost from 3.7 to 12V now. That T I think about it.
 

Tonyr1084

Joined Sep 24, 2015
4,910
That is why I suggested that balancing "good" cells that are to be connected in parallel • • • if one cell is shorted, of course, you will detect that while charging individually, but I suspect you will also detect it when charged in parallel too.
This is exactly what happened to one of the battery packs in my MIL's oxygen generator. A 4S4P setup was allowed to rest at a low voltage resulting in a few failed cells. Those failed cells dragged other cells down. In the end I disassembled the pack and removed all the batteries. Charging them individually I found four out of the 16 that were charged to 4.2 volts and in four months have not dropped below 4.12 volts. Those are the ONLY four batteries I'd consider reusing - but ONLY in a single cell configuration such as an LED flash light. JP suggests balancing "good" cells for a good reason. One bad cell in parallel with one good cell can harm the good one by dropping its voltage below 2 volts. At which point the battery starts undergoing chemical changes that lead to internal shorts. And those shorts can cause fires.

I'm new to Li-Ion batteries myself, so I'm learning a lot too. The biggest lesson I've taken to heart is to recycle cells that show any signs of failure. Even then I might still find it risky to reuse old cells.
 

Tonyr1084

Joined Sep 24, 2015
4,910
Awe heck! Your question inspired me to do a check of the batteries I've been saving. What I labeled "8L", after being charged has fallen to 4.03V. It was charged to 4.20V on June 14. Upon removing it from the charger it held 4.19V for 4 days. 4.18 for 3 days; 4.17 for 4 days; 4.15 for 4 days (skipped past 4.16); 4.14 x3d(ays) 4.13 x3d; 4.12 measured twice by 7/3/20. Today it's sitting at 4.03V. I haven't been measuring the drops in some batteries since 7/25. Of the 14 batteries I've been tracking, six of them have fallen into a range of 3.76 to 4.06 volts. And they were charged recently. Looks like I have more bad cells than I thought. 4.10 is the lowest of the batteries I'd consider good. 4.06, I don't like and 3.96 & 3.76 I have no faith in other than in a single cell flashlight.

Li-Ion's can bite.
 

KeithWalker

Joined Jul 10, 2017
1,144
I'm new to Li-Ion batteries myself, so I'm learning a lot too. The biggest lesson I've taken to heart is to recycle cells that show any signs of failure. Even then I might still find it risky to reuse old cells.
I usually use recycled cells in my projects. With a little care they can be a very inexpensive source of power.
Li-Ion cells lose capacity with time and use. When I acquire used batteries, it break down the packs into individual cells. I then check the capacity of each cell. I made an automatic charger - discharger that monitors the voltage and capacity of the cell. First it charges it to 4.2V. Then it discharges it through a known resistance, measuring the voltage every 5 seconds. When the voltage reaches 3V, the load is disconnected and the capacity is calculated for every 5 second period. The sum of those is the total capacity of the cell.
I discard any cells that have very low capacity. I mark the date and capacity on each cell tested. Then I recharge each cell to 3.8V and store it.
When I make up a pack, I select cells with similar capacities. First I charge them all to 4.2V, then I can connect any parallel cells together without problems, and connect the parallel banks in series. Some of the cells I use are more than 10 years old but I have never yet had a cell fail in one of my packs. For packs with multiple series cells, I always use a balanced charger.
Keith
 

Thread Starter

Stillers48

Joined Jul 12, 2020
21
As I'm extremely new to the subject, forgive me if I'm in error, but the batteries I have are Lithium Polymer. Is that the same as lithium ion? These are made for mobile devices and measure 101x92x3mm 5000mAh and I have 3 more thst are 136x59x4mm 5000mAh as well as 3 50x50x11mm 4000 mah. Perhaps I should have specified the exact batteries from the beginning.
 

Tonyr1084

Joined Sep 24, 2015
4,910
Well, until I'm more confident with 18650's I'll continue to take in more information. Eventually I may do as you have done with regards capacity. But let's not let this thread get hijacked. What do you call four LiPo's in 2 series 2 parallel arrangement? The answer is 2S2P.

I may have gotten off track with Li-Ion batteries. Not sure if I'm discussing the right kind of battery.
 

jpanhalt

Joined Jan 18, 2008
10,087
Here's Battery University's position: https://batteryuniversity.com/learn/article/the_li_polymer_battery_substance_or_hype
The difference is in the electrolyte which facilitates the prismatic shapes of polymer vs. cylindrical of ion.

As for behavior (Source: given above),
Charge and discharge characteristics of Li-polymer are identical to other Li-ion systems and do not require a dedicated charger. Safety issues are also similar in that protection circuits are needed. Gas buildup during charge can cause some prismatic and pouch cells to swell, and equipment manufacturers must make allowances for expansion. Li-polymer in a foil package may be less durable than Li-ion in the cylindrical package.
 
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