Parallel and series pondering

[EraJomppa]

I´ll divide my question in 2 parts, but let me start by saying that I have some knowledge of the subject but making the logical conclusion is sometimes difficult so I want to get some suggestions and opinions from you guys.

1st.
I got two 6cell (6s) batteries connected in parallel under load. The other battery is older than the other, but are otherwise the same, 6s 4000mAh. When I charge them I notice that the older battery lost 1900mAh during load, and the new battery lost 2100mAh. Using only new batteries, both lose 2000mAh, so the consumption is same, but the capacity loss is distributed unevenly.

My logic is that the parallel batteries act as parallel resistors;
-Both batteries operate on the same voltage
-The current flowing through both batteries is I=U/R
-New battery loses more capacity thus more current flows thru it
-> This means that the new battery has lower internal resistance (doesn't it?)

Now this is actually disadvantageous to the new battery since it drains faster than the old one I guess. The system I'm using does have low voltage cut-off but still I'm worried that this strains the batteries unevenly. Is there a way to prevent this? I suppose calculating the difference in resistance inside the batteries and increasing the resistance of the new battery with a resistor?... I dont know if you can or if you should

2nd.
This is my configuration:


I got total of 9s1600mAh of batteries (~36V). Connected as seen in the picture: (2x 6s4000mAh in parallel) in series with (2x 3s4000mAh in parallel), those and in parallel with an identical set.

My question is that one of the 6s batteries is older than the other 6s batteries and causes the other 6s it is in parallel with to drain more rapidly in this current configuration; How can I improve my configuration in order to minimize this effect? would it be better if I had all the 6s batteries in parallel with each other and the in series with all the 3s batteries (that are also in parallel with each other)? Or rather make 4 pairs of 6s+3s batteries in series and parallel all of those (effectively 9s) sets with each other?

I feel that my current configuration is forcing all the strain into the one battery that gets paired with the old 6s. That is the question

__________
Just ranting here, since someone might wonder why all these batteries...

I am using these batteries in my electric bicycle with 500W motor. It does not create huge currents (compared to my 2kW plane that flies with just one 6s4000mAh) and I get away with smaller diameter wires in the connections.
I also use these same batteries with my 200% Dusty, but right now I use the bicycle daily and they see their primary use there.

Here is my charging case that fits neatly into my bicycle saddle-pack if I need to charge batteries on the road.


The legal limit of power in electric bikes in Finland is 250W so I dont really use the 500W setting, also the speed is limited to 25km/h (if you go faster the motor wont be helping you along). I got roughly 575Wh (=36V*16Ah) of batteries so if I use the allowed 250W of motor assistance and go 25km/h I get 2h 20min of riding, which at 25km/h is 57.5km. This is the theoretical minimum I should get. But when you ride you dont allways use all the 250W and in the uphill you might use more (I can if I want since it's 500W motor). Also I never drain my batteries fully. Thus my range is somewhere around 50-70km, or even more if I feel frisky and use smaller level of assistance. Thanks for reading

 

[pressalltheknobs]

The first problem I see with you charging station is that you do not have balance leads. You should have those anyway. If you are parallel charging, to safely parallel charge you need to combine the balance leads in parallel.

Here's an article

You could buying new batteries (expensive) that are guaranteed to have similar internal resistance. Probably not an option


Possibly if you put the poorer batteries in series with the good batteries you will get a more even discharge on that path. Maybe trying to balance each parallel path by switching the series batteries around to the try and get the total resistance of each parallel path to be similar might be an advantage

I'm not sure it really matters since if one path is favored it will discharge first and then the second path will kick in. I doubt you will notice any gain. If you have one known "bad" battery I would consider replacing it since it will make the others work harder and possibly reduce your potential run time. However, if you are not discharging the batteries at an excessive rate it's hard to see how it would affect the life of the other batteries. I'm just guessing.

 

[makattack]

I actually have the blue iMax charger shown in the pictures. The balance ports are flush with the charger on the right side. These photos will never show them. I assume it's the same on the other chargers. No separate balance board/extension.

As to the OP's original questions, yeah, with batteries in parallel, or serial, ideally, they should be equivalent batteries -- it's more important for serial batteries, but in parallel, they should actually balance out themselves... any batteries with less stored energy should essentially receive a charge from those that have more stored charge. So a battery with higher internal resistance connected in parallel should just lower the overall parallel capacity. If you're seeing a discrepancy, I would be worried about a flaw in your harness (a short or bad connection) that is causing one pack to discharge more than another...

 

[pressalltheknobs]

I know those chargers have balance ports. The (potential) issue is they are not connected.... May have been omitted for simplicity of the photo but there are 4 chargers and 8 batteries so there is a possibility that parallel charging is an intent and that requires a special balance harness

 

[EraJomppa]

Thanks for the insight.

Possibly if you put the poorer batteries in series with the good batteries you will get a more even discharge on that path. Maybe trying to balance each parallel path by switching the series batteries around to the try and get the total resistance of each parallel path to be similar might be an advantage

I'm not sure it really matters since if one path is favored it will discharge first and then the second path will kick in. I doubt you will notice any gain. If you have one known "bad" battery I would consider replacing it since it will make the others work harder and possibly reduce your potential run time. However, if you are not discharging the batteries at an excessive rate it's hard to see how it would affect the life of the other batteries. I'm just guessing.

I see what you are getting at. But I understand the best option would indeed be to get a new battery. My problem though is that all the batteries are bought 1 or 2 at a time so they are all from different "generation" so I would have to buy a whole new set to get the most balanced battery pack.

I am not concerned about the whole situation really, but if there is a way to improve the life of the batteries and overall capacity that would be nice. I think I will just keep changing the battery that I couple with the old one to discharge all the newer batteries evenly over time.

NOTE: the chargers have balancing ports, so no problems there. I don't do parallel charging, only 1 battery per charger at a time (and I always use balance charging). Takes only 2 hours to charge all 8 of them.

 

[razor02097]

Putting battery packs in series is no different than putting individual cells in series. Parallel is a little different since you have each pack in a "unit" The big difference is you have 2 balance leads to contend with. Each "unit" is going to drain a bit different just due to the nature of having that separation. If they were parallel in a single pack each cell would be paired to another then each pair would be series to another pair.

It is never a good idea to mix batteries of different age or type in a series or parallel fashion. It can be done but you're risking a lot. Ideally if you pair 2 packs together you want to find ones that match capacity and internal resistance as much as possible.

As long as you monitor all of your battery cells and cease operations once a battery gets low you will be okay. Just keep in mind if you have a poor performing battery in your array it will drag down the entire array's performance.

 

[pressalltheknobs]

As long as you monitor all of your battery cells and cease operations once a battery gets low you will be okay.

That is a very good point. I don't see any kind of battery voltage monitoring. An easy way would be to get a bunch of these alarms since you can find them quite cheap (in the US anyway). Ideally you would need 8 which adds up and be a bit of a pain to plug in unless you made a harness. However, I would have at least two on one of the parallel paths, one for the 6S and one for the 3S. It's not perfect but it should give you an idea. If you track the batteries you will find the weakest ones and choose those to monitor.

 

[EraJomppa]

Alarms WOULD be a good idea, if it wasn't for my routine.. I listen to audiobooks while I ride my bike. I gotta keep the volume a bit high, since I want to hear the voice of the reader over the passing cars. (mind you, I pay extra attention to the traffic. And even though I keep the volume up, I still HEAR the cars etc).

BUT anyway. I'v been keeping my eyes on the total voltage when riding and the amount of capacity being put into the batteries when charging them. They have been very steady. I usually charge them after about 50km of travel. Then the batteries take around 2900-3200mAh inside them, keeping the maximum battery discharge below 80% of the total capacity. I haven't witnessed any major inequalities between one battery or the other. If anything, I think the gap between the discharge amounts might have started to drop. But I have not kept any records of them (I would have to charge them all with the same charger [or at least the same type] and thats slow).

PS. I also rarely use the buzzer type alarm since under load the voltage will sag and the buzzer will go off, but when the load chages it might turn off the buzzer.. Which is kinda annoying to my taste, especially with quadcopters. But with my bike also, when I put the highest power setting on when going uphill the voltage sags up to 1,5V from the regular level.

 

[pressalltheknobs]

PS. I also rarely use the buzzer type alarm since under load the voltage will sag and the buzzer will go off, but when the load chages it might turn off the buzzer.. Which is kinda annoying to my taste, especially with quadcopters. But with my bike also, when I put the highest power setting on when going uphill the voltage sags up to 1,5V from the regular level.

That would be annoying. The nice thing about these kind of alarms...

https://store.flitetest.com/battery-voltage-alarm-1-8s/

is you can switch off the alarm function and just use it as a cheap monitor. Possibly you could route some cable from your battery box to the handle bars. Just an idea.