Need help building a 24V Li-ion pack

[protocol217]

Hello, i am building a 24v Li-ion battery pack for my electric skateboard.
I am using 18650 cells from laptop battery's and i am testing the capacity of each cell wit ImaxB6, and will be using cells with 2000 mAh +-200 mAh.
The plan is to build 7 cells in series, with 6 or 8 (haven't decided yet) in parallel.

i have ordered this BMS board: **broken link removed**

I know how to connect all the balancing wires, but what input power for charging should i use? What voltage what amperage ? Can i use a 120W laptop charger 20V ?

Here is the English board drawing:

 

[audioguru]

You need a Lithium battery charger circuit that matches the spec's of your battery. The charger circuit will have a voltage drop specified for you to select its input voltage and the battery will have a recommended charging current for you to program the charger current. A 20V laptop charger is not a charger circuit, instead it is a power supply for the charger circuit inside the laptop. Your battery with 7 cells will be 29.4V when fully charged and the charger circuit might need an input of 31.4V or more so how can you use a 20V power supply?

The spec's for the board says 18A but also says 40A which will probably explode many 18650 battery cells so ebay does not know what this board was made for and neither do you. Not enough details for that cheap Chinese junk.

 

[protocol217]

Hello, i am now finishing the assembly of my pack now i need to resolve only the charging method so,
i found this tutorial of building Li-ion packs: https://www.ebikeschool.com/how-to-build-a-diy-electric-bicycle-lithium-battery-from-18650-cells/

Here he is using the same BMS board only for 10 cells 36V.

True there is not much specs on this board. And i found a better one: 24V Lithium ion Li-ion Li-Po LiPo Polymer Battery BMS PCB System
I intend to use the BMS board only for charging, so my idea is to make a 7S8P pack, and for BMS charging input can i use a laptop charger with a steap up module with CV CC, and set voltage and current limit that i need ?
If yes what input voltage and current limit must i set on the steap up module if i want to charge each cell with 300 ma of current? I think 2,4 A is that correct?

 

[tcmtech]

That charge control board should work just fine for your battery bank but it may have limitations on keeping the cells balanced at discharge rates over 18 amps if one set of cells drops off faster than the rest.

As for the proper power supply to run it that's up to you to decide what you want to use. As long as it can supply the minimum voltage require to properly charge the battery bank and enough amps to meet your needs it should work.

My math says that to charge seven 18650 cells in series you will need ~30 volts. What amperage you want to charge things at that voltage is up to you.

 

[audioguru]

True there is not much specs on this board.

Because it is cheap Chinese junk and ebay knows nothing about electronics.
The first one does not say it is not a charger circuit. The second one shows how to connect it to an external charger circuit.

I think ebay sells this cheap junk thinking that people will not bother to return it.

Why is every e-bike I see, creeping along slowly like it has a dead battery? I am glad that ebikeschool revealed that *fire battery cells are cheap factory rejects.

 

[protocol217]

OK, i have a few more questions:
1. The Voltage for charging all the cells is 4,2V x 7 = 29,4V Do i have to limit the input Voltage for charging on exactly 29,4V or can limit it to a little more like 30-32V and then the BMS board will regulate the rest?
2. I want each cell to be charged with 300 mA, so i will need to set the steap up module to 2,4 Amps right?
3. Is a 120W laptop charger with a steap up module with current and voltage control a good idea for this or should i search for another power supply?

 

[tcmtech]

I charge my cordless tool battery packs at around 4.25 volts per cell so by my standards anywhere around 30 volts will be fine.

Just monitor the cell temperature by hand the first time you run the charging system. If none get overly warm to the point of concern your voltage is close enough.

 

[audioguru]

Because the battery monitoring system is cheap and the manufacturer's name and reliability are unknown then I would not trust it to prevent an explosion or fire when it fails. It is supposed to limit the voltage on each cell to 4.20V and limit the current but for how long? One week, one month or one year? Therefore I think you need a reliable charging circuit, not a cheap Chinese one from ebay or Hobby King.

Does anytbody make a "steap up" module (do you mean stepup?) that steps up a laptop's power supply of 20V to 29.40V at the current you need? I doubt it. I think you need a trustworthy Lithium battery charger instead.

 

[spec]

Just for reference here is an English language version of the eBay advertisement for the seven cell LiIon Battery balancing protection circuit.

**broken link removed**

spec

 

[spec]

Hy Protocol217,

Unlike audioguru, I can find nothing wrong or contradictionary about the battery pack protection board that you have ordered, and while some eBay products are unreliable and badly specified, in my experience most are OK, and excellent value for money.

audioguru is quite right about needing a separate battery charger as the circuit you have ordered does not do that.

Before you go any further though you need to do a few basic calculations.

(1) What maximum current will your skateboard take? LiIon batteries have a maximum continuous current rating, typically, C/10 or C/1, where C is the Amp/hour capacity of the battery, in your case 2A/H.
There are two consequences of taking too much current from a battery.
(1.1) You will damage the battery and possibly cause an explosion.
(1.2) You will radically reduce the battery duration so that a 2A/H battery may only provide 0.2/AH
There are high current LiIon batteries that are designed to provide a high current, typically 20A and 30A safely and without reducing capacity too much.

(2) You need to decide what your priorities are:
(2.1) Long battery life
(2.2) High battery duration.

For (2.1) if you charge at a current of no more than C/10 and charge to no more than 4V and discharge to no less than 3V you will have a good battery life.
With this approach you can use a simple constant voltage charger with a current limited of your chosen charge current, 300mA.

For (2.2) you can charge to 4.2V and discharge to 2.6V to get maximum capacity but reduced battery life. In practice, unless you have a constant voltage inverter between the battery pack and skateboard motor, there would be little point in discharging to 2.6V.

If you are going to charge your batteries for maximum duration, you need a smart charger with -dv/dt detection to complete the topping charge without wrecking the batteries.

This will be anathema to tcmtech, but my strong recommendation would be to build your battery packs so that the individual batteries can be removed for charging. To charge seven batteries you would probably need two four-way chargers, but I think you can get eight-way chargers. This will give you maximum flexibility and give you the best battery life and endurance.

Just the routine word of caution, only buy main-line manufacturer's batteries: Sanyo, Panasonic, Sony, Varta, LG, Samsung, from a reliable supplier, because batteries are the biggest rip-off items on the net and generally. Don't be tempted by fantastic paper specifications and low prices.

spec

 

[audioguru]

I have radio controlled model airplanes. An American brand of Li-Po battery I use is rated at a discharge current of 70C and peak currents double that. They say they can be charged at 5C and the high currents do not affect the life.
There is a cheap Chinese battery brand with a range of maximum discharge current because I guess most of them are garbage but occasionally you get a good one.
In another thread or another website there is a thread about an electric skateboard or scooter and a link to a manufacturer who says that usually a battery brand ending with "fire" (Ultrafire, etc) is a fake one or are factory rejects.

 

[protocol217]

Hello spec and thanks for replying.

My skateboard draws max 16-17 amps when going up hill. so each cell will be providing 2 amps on max load.
I decided to use only cells which have 2000 mAh or more of capacity, all the cells are from old laptop battery's, they are not all the same, but most of them are Samsung.
I know that those ultra fire or other Chinese battery's that are marked with 4000 or more mAh are all fake and some don't even have 600 mAh of capacity, so i am never using them, i get all my battery's from laptop battery's, and i test each cell for capacity with imaxB6.

I mean to use this charger with the BMS board: **broken link removed**

This charger has 2 amps output so each cell will get 250ma of charge (which is OK). what do you think about this charger? Or can somebody recommend a better one for <50$

 

[tcmtech]

This will be anathema to tcmtech, but my strong recommendation would be to build your battery packs so that the individual batteries can be removed for charging. To charge seven batteries you would probably need two four-way chargers, but I think you can get eight-way chargers. This will give you maximum flexibility and give you the best battery life and endurance.

I have no problem with that when using a homemade battery bank made from misc lipo cells.

LA batteries generally don't blow up and burn your rig down if you give them a bit too much voltage and current for too long. Lipos do.

 

[spec]

Hy protocol217

Hello spec and thanks for replying.

No problem.

My skateboard draws max 16-17 amps when going up hill. so each cell will be providing 2 amps on max load.

Just to clarify. I take it from your figures that you will be using banks of 7 cells and 8 banks in parallel to make one battery pack comprising 7 x 8 = 56 cells in total.

On that basis your figures are correct, but the batteries will not necessarily share current and voltage equally either on charge or discharge.

On the other hand, my rercomendation of charging each cell individually, which would be optimum, is not really practical with 56 batteries.

I translated the eBay listing that you linked to but could not find much detail about the charger so I am not able to comment on it.

Your approach will work, but it would be wise to check the voltage across each cell, especially for the first charge discharge cycles to see how equal the cell voltages are.

You may have to replace any cell that has a voltage significantly higher or lower than the other cells.

I would recommend that you still design your battery pack of 56 cells so that you can measure the voltage of each cell and can easily replace any individual cell.

Just a bit of general advice, try and keep the battery pack temperature down by providing vents for air circulation and cooling by convection and heatsinking by conduction.

You do know that 56 LiIon batteries alone will weigh around 2.8Kg.

spec

 

[SimonTHK]

This project will most likely not work, unless you use some kind of intelligent charger for each cell´s, not even then. BMS is just monitoring your charging voltage and discharge voltage, and cut off any current either way. For these cell´s that means 4.2v charge and 2,75 discharge.
You are gonna use what..21 cells? with no intelligent charging.

One cell reaching 4,2v faster than another cell, will shut down the whole charging process.
And you say you are gonna use old cell´s, different cell´s, differens ages, different usages with a BMS.... That just aint gonna work for the time put into it at all, unless ofcourse, it is learning by doing.
I will give a shot at 35% capacity on your battery pack. And with no proper test equipment, your are probably gonna change some cell´s quite some times.

You are way better off by forgetting the BMS, charge the pack with a cheap 29,4v charger and let em die when time is. If you pick em out of laptop´s, old ones. Then time has allready happened.

If you are gonna use a BMS anyway, then be sure that the voltage´s of the cell´s are equal. And also be sure to know, that any slightly difference in voltage under load and under charging, can ruin your day of fun.
Do make a setup with lamp´s or something that can draw a load, to measure the voltage on each cell under load. You can not get a clear reading of the cell´s condition by measuring the surface voltage. Also measure the voltage of all cell´s under charge. Make a setup without BMS first, and see and write down on the behaviour (voltage) of each cell under charge. If any cell reach 4.2v you will know that this is the point, where the BMS will shut down your circuit your charging of the whole pack. Throw out them bad cell´s that go up quickly or fall quickly. Keep on doing this till you lose your breath and finally in the end... realize...that changing cell´s is your new day job.

Also slightly bad cell´s tend to draw the cell´s around them up quicker aswell.

 

[audioguru]

no intelligent charging.
One cell reaching 4,2v faster than another cell, will shut down the whole charging process.

Maybe in a cheap Chinese BMS (like the ebay one?). A good balanced charger bypasses most of the charging current past a cell that has reached its 4.2V maximum voltage so that the remaining series cells continue charging.

 

[SimonTHK]

Maybe in a cheap Chinese BMS (like the ebay one?). A good balanced charger bypasses most of the charging current past a cell that has reached its 4.2V maximum voltage so that the remaining series cells continue charging.

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.

 

[audioguru]

Why do you like CHEAP stuff? Cheap usually means many problems and poor performance.

 

[SimonTHK]

Why do you like CHEAP stuff? Cheap usually means many problems and poor performance.

First if I guesd this project with old batteries fit more a cheap bms
I dont like cheap stuff allways. But many times it can be ok. And used coreectly under right circumstances with more precaution, **** stuff can be ok. A 20 amp bms china, i would not put 20 amp through more like 15. Also never drop stuff from china on floor

 

[tcmtech]

One cell reaching 4,2v faster than another cell, will shut down the whole charging process.

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?