Need help building a 24V Li-ion pack

[spec]

Bit of thinking out loud here. Assume that there are a total of 56 cells in the battery pack and the skateboard motor is 24V. 7 batteries in series will give 7 * 3.6V = 25.2V which is suitable for the motor (corrected 2016_07_26).
(1) Instead of making battery packs of 7 cells in series, make battery packs of 8 batteries in parallel, giving a total of 7 packs.
(2) Charge each battery pack independently with 7 chargers, each with a capacity of around 1.6A (this can be done with isolated chargers without dismantling the battery pack)
(3) Charge to 4.1 Volts to avoid the need for a smart charger with topping algorithm and to give the batteries an easier time.
What do you think?

In any instance, I would recommend converting each cell to a protected type. Small protection circuits are available quite cheaply from eBay (sorry AG) for example. **broken link removed**

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[spec]

How exactly will that happen if multiple cells are in parallel?

The cells with the lowest capacity will have their charging current drop off faster than the ones with the most capacity but all of them will still reach 4.2 volts at the same time. Or is there some law of electrical physics related to battery cells that bypasses Kirchoff's law that states that any number of parallel loads will all see the same voltage potential across them?

The cells are not in parallel; they are in series and that is the problem.

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[spec]

I didn't know this, but some advanced battery management systems use active balancing on discharge whereby a weak cell in a serial chain of cells is charged from the other stronger cells. This extends the duration of the serial bank as a whole.

Of course, this problem is automatically taken care of, to a degree, with parallel connected batteries.

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[audioguru]

A Lithium cell does not have the voltage "hiccup" that occurs with Ni-Cad and Ni-MH cells when it becomes fully charged. Instead its voltage slowly rises until at about 70% fully charged it reaches its highest allowed voltage of 4.20V. Then the charger is supposed to limit the voltage across each cell with a "balancing" circuit until the current in the series string of cells drops to a low amount, then disconnect.

 

[tcmtech]

Then whats this mean? First post fourth line.

The plan is to build 7 cells in series, with 6 or 8 (haven't decided yet) in parallel.

The cells are not in parallel; they are in series and that is the problem.

Isn't that the whole purpose of the charge control system in lipo packs?

To make sure each cell only gets charged to its proper voltage then limits the charging voltage to that cell by bypassing the unneeded current around it while the rest of the cells in the series keep charging?

 

[spec]

A Lithium cell does not have the voltage "hiccup" that occurs with Ni-Cad and Ni-MH cells when it becomes fully charged. Instead its voltage slowly rises until at about 70% fully charged it reaches its highest allowed voltage of 4.20V. Then the charger is supposed to limit the voltage across each cell with a "balancing" circuit until the current in the series string of cells drops to a low amount, then disconnect.

Quite right AG. **broken link removed**

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[protocol217]

My skateboard originally had 2 12V 7Ah lead acid battery's connected in series making 24V, and the motor is 24V 400W rated.
My new battery pack is total 56 cells 7 in series and 8 in parallel.

I want to charge the complete pack at once simply by connecting 2 leads from one charger, like e-bike charging, so i will try the charger that i linked earlier, with the ebay BMS board and see how it goes, i think that this BMS board will do just fine with limiting the voltage to 4,2 V for each of the 7 groups, like balance charging.

Normally for the first time and the second time i will be there to monitor the voltage of the pack and temperature.
I have watched a lot of YouTube videos of people making this packs and charging them this way and are very happy about it.

Now i am waiting for the BMS board to arrive so that i can test this.

 

[spec]

Then whats this mean? First post fourth line.

Isn't that the whole purpose of the charge control system in lipo packs?

To make sure each cell only gets charged to its proper voltage then limits the charging voltage to that cell by bypassing the unneeded current around it while the rest of the cells in the series keep charging?

tcmtech, this is my understanding of how the battery pack is wired up. Notice that none of the batteries are in parallel. But in view of what you say perhaps this diagram is not complete and there are connections running horizontally so that the individual batteries are in fact wired in parallel.

(I have corrected the typo in my post #21 above)

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[protocol217]

That schematic from spec is exactly how i made the battery pack.

 

[tcmtech]

That schematic from spec is exactly how i made the battery pack.

That info would have been useful a few days ago.

If that's how you plan to wire your battery bank you will need a charge controller board for each string of cells whereas if it was wired with each group of cells parallel you would only need one charge controller.

Each set of 6 - 8 2000 mAh cells works as a single 12,000 - 16,000 mAh cell.

This is why I get cranky in battery related threads. Too little critical info gets presented up front where it needs to be so proper suggestions can be made around it.

 

[audioguru]

The battery in post #28 has 56 cells connected with 7 cells in series in each string (vertically) and it has 8 strings. The balancing connections are not shown. All the cells should be connected in parallel (horizontally) so that they all will have balanced charging.

 

[spec]

The battery in post #28 has 56 cells connected with 7 cells in series in each string (vertically) and it has 8 strings. The balancing connections are not shown. All the cells should be connected in parallel (horizontally) so that they all will have balanced charging.

Unless there is a BMS nodule for each stack of 7 cells which would be normal and what I had envisaged. The battery management system specified by the OP only has a balance current of 55mA maximum, so placing 8 cells in parallel and using a single BMS is not really a practical proposition.

(It is not necessary to describe the circuit of post #28. The schematic is quite clear and, as they say, one drawing is worth a thousand words.)

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[spec]

That is what I meant by intelligent charging, by discharging through a resistor or similar, over each cell.
I dont believe this happens in many ebay BMS modules. And I would like to know, where to get a cheap BMS that does this for you.

Hy SimonTHK,

The OP's BMS module has a maximum balancing current of 55mA: **broken link removed**

but this BMS module has a maximum balancing current of 80mA. **broken link removed**

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[tcmtech]

The battery in post #28 has 56 cells connected with 7 cells in series in each string (vertically) and it has 8 strings. The balancing connections are not shown. All the cells should be connected in parallel (horizontally) so that they all will have balanced charging.

Same thoughts here as well.

With this proposed layout every single string of cells has to have a dedicated charge management system which adds a lot of cost and physical bulk to the whole design which is totally counter productive to being integrated into a powered skateboard.

To me for a battery pack this large made of that many cells something like this is what I would be looking to design things around.

**broken link removed**

 

[audioguru]

Since it is an ebay ad, the SKYRC charger has no detailed spec's but it might work fine. It has switches on top that might accidently be bumped into the wrong position (LiFe or too high a charging current) but they can be fixed with a cover glued over them.
Since it charges only a single string of cells then the cells in the battery of 56 must all be connected together horizontally.

 

[tcmtech]

Since it is an ebay ad, the SKYRC charger has no detailed spec's

They are there but you have to scroll down through several pages of pictures to get them.

--------
Specifications:
Item brand: SKYRC
Item model: e8
Charge battery type: LiPo/LiFe
Battery cell count: 2-8 cells
Input voltage: AC 100-240V
Circuit power: 100W
Charge current: 1A/3A/5A/6A ± 10%
Current drain for balancing: 200mA
Weight: 448g / 15.8oz
Size: 150 * 93 * 51mm / 5.9 * 3.7 * 2.0in

----------

Given the mix and match of cell ages and mAh capacities, I don't see any other practical way to make charging that many cells together work from one charge control unit.

 

[protocol217]

That charger is perfect, and i can use it without the BMS board.
For these laptop 18650 cells what charging option should i use Li-po or Li-fe ? And what is the difference?

 

[Tony Stewart]

Keep in mind if load balancing starts when current is maximum, load dumping is VxI per board which won't work well with the size of components shown. e.g. 4.1Vx2A >8W per cell of diverted power that must be heat sunk.

Thus the smart charger must reduce current before any of the cells go into balance mode to shunt the current such as 200mA or <1W which may be doable.

Normally this regulation is done inside the laptop, not in the charger.

If you don't know how to do this, then get a universal laptop charger with selectable voltages and a DMM and monitor the temp rise of each balancer board with your finger, then reduce Vdc out. Universal types vary from 14 to 24V typ. 65W-150W but may be hard to find.

 

[tcmtech]

That charger is perfect, and i can use it without the BMS board.

Just ordered one myself!

 

[spec]

Nice find TCM

spec