Some suggestion needed before starting a battery pack

[desmond007]

Hi there,

I'm fairly new here, also new to this subject, so some suggestions about creating a battery pack will be highly appreciated.

My goal is to create a 48v 26ah battery pack. For this, I'm using Samsung 26c 18650 cells. The specs are below.


This is because it's a 2600mah battery I'm doing 13s10p pack. I'm going to use a BMS for this and thinking about using this one. ( http://www.ebay.com/itm/13S-Li-ion-Lithium-Cell-48V-20A-18650-Battery-Protection-BMS-PCB-Board-Balance-/272291298325 )

So my questions are,

1. For the pack first I need to put 10 battery in parallel and 13 of them, right? I mean, 10 battery with parallel connection and then connecting them with series for 48v.

2. To charge the battery I'm going to use a 54.6, 4A/5A charger. Is that okay? What is the maximum ampere charger I can use to charge it? 26A?

3. In above image, I can see that each cell takes around 3 hours to fully charge. So if I connect a 5A charger, each battery will get 5000ma/10 = 500ma, so the charging time will be around 7.8 hours for the pack, right?

4. What the BMS will do for balancing? Will it divide the 54.6v to 13 packs so that each pack can get 4.2 volts? The ampere will be same for each pack, right?

5. I can see that there is also an option to discharge through BMS. If I use that what gonna happen? Will it take 3.7 volts from each 13 battery pack?

6. Can I charge and discharge the battery pack at the same time?

I know all these questions are very basic. But it will be helpful if someone answers it.

Thanks.

 

[spinningmagnets]

The first half of a battery can be charged at a slightly higher rate, but if you want to keep it simple with a continuous-charge setting, a lower rate will keep the battery cooler, and that will make it last longer. Heat degrades the chemistry, whether it is on charging or discharge. A higher C-rate cell can be charged faster, a lower C-rate cell will get hot at high charging amps. How high can the 26C cell be charged at? 100F / 38C would be fairly normal, but I wouldn't charge it at a higher rate than the amps that make it 100F.

Packs are almost always paralleled into sub-packs first (strings?), because that allows the builder to use a common BMS with P-string balancing capabilities, or an RC charger with P-string balancing capabilities. Then combine P-strings in series to attain pack voltage. It is possible to make 13S strings and then assemble them in parallel, but...I don't know anyone who does that, there doesn't seem to be any benefit.

I'm looking forward to the results. What continuous and temporary peak amps to you want the final pack to provide?

edit: 10P of the listed 5.2A is 52A...

 

[desmond007]

Howdy,

Thanks for your reply.

I'm little confused about the half of the battery higher charge rate. It's better I attach an image for it.


I will use a CC-CV charger to charge it, maybe something like https://www.aliexpress.com/item/Rohs-54-6V-5A-Smart-Lithium-Battery-Charger-For-48V-LifePO4-Li-ion-Electric-Car-Power/32742009327.html

my question is if I charge it with this 54.6v, 5A charger, the individual pack of 10 cells will get 4.2v and 5000mA. So each 18650 cell will get around 4.2v & (5000/10)=500mA charge rate, is this correct?

As I can see on the table, charging @ 520mA rate is idle and it gives around 100% relative capacity. So, will it be good for the battery health if I charge it with 500mA? Will it keep the battery pack cool?

Well, right now my requirement is fairly not much (Later I will try taking more amps from it). I will use it with an e-bike (which has VRLA battery[48v,24ah] and wanted to convert it to li-ion). It's controller and motor need is 250watts. So 48v with 250 watts means the motor will take around 5.2A from the pack. Also, I can see that discharge rate at 520mA gives 100% capacity. So 5.2A for the total battery pack means it will take 520mA from each 18650 cells, right?

Last question, Can I charge and discharge the pack through the BMS at the same time?

Best Regards,
Desmond

 

[wesnewell]

1. As long as they're all around the same voltage, it doesn't matter one bit if parallel them or put them in series first. That goes for any battery cell.
2. Standard charge rate for the cells is .5C, so for a 26ah pack that's 13A, with a rated max charge rate of 26A. I didn't see a max charge rate listed for the BMS, but 5A should be ok. I wouldn't go any higher without finding out the max charge rate for it. Heck it may be 3A. A lower charge rate is better for longevity of the cells.
3. 26ah divided by 5A is roughly a 5.something charge time. Most chargers drop to a lower charge rate at the top, plus time to balance, etc, so actual times will vary depending on condition of the pack, starting pack voltage , etc.
4. The bms will cut power to each cell group when it reaches it set limit and then slowly bleed that group down to whatever level it's set for. It does this for all cell groups until they are equal.
5. It will take the juice from the pack until one of the cell groups hits the bottom limit (2.8V if the specs are right) and then shut discharge power off. I wouldn't run it taht low if i were you. The lower you run the pack , the more out of balance it will get and take longer to recharge. It's also not good to run batteries down to rated specs imo.
6. If you have a way to connect the charger to it, I don't see why not.

 

[desmond007]

1. But for this BMS, I need to first take 10 cells in parallel and then connect them in series, right,?

2. Yes I need to first ask the seller what is the max input voltage and ampere for that BMS. If it supports, will 5A differ the longevity of the battery?

3. Clear one of my confusion please, if I use a 5A charger, what current an individual 18650 cell (not the 10 cells parallel pack) will get? 500mA?

4. Generally 18650 cells charges at 4.2v,then why the output discharge voltage is only 3.7 max?

5. So what is the safe capacity to stop discharging? 20%

6. That BMS has that option to discharge through it, so it will support simultaneous charge/discharge, right?
- also the connected charger will be 54.6v and the attached controller and motor is 48v, so if I charge/discharge simultaneously, will it affect the controller and motor?

Regards.

 

[wesnewell]

1. No, it doesn't matter how you do it, Just have the whole pack built to 13s10p before you connect the bms to it. Every time you connect another cell to a cell group, each cell in that group will equalize.
2. Yes, but if the bms supports 13A, you could go that high if you need to charge faster. I would not go higher unless you really need a faster charge for some reason. If you need a faster charge, then consider a cell that has a higher charge rate. A lot of rc lipo is rated for a 5C or higher charge rate with 1C being the standard rate.
3. Yes, 5A divided by 10 is 500ma.
4. Don't know where you're getting that, but any time you load a cell, the voltage will drop. If you're putting a high enough load on it to drop from 4.2V to 3.7V, you're probably at its max discharge rate or above it.
5. Hard to say, I'd say 3.0V resting for the 26C, which is a pretty weak battery btw.
6. Assuming this is for a brushless motor, the motor won't matter, only the controller, as it's what draws power from the battery, not the motor. If this is for a stationary application instead of an ev, I don't know why you just wouldn't use a ups and redundant power supply.

 

[desmond007]

1. But there in BMS I can see that voltage is increasing step by step, like 24v, 36v, 48v. If I create a 48v stack first and then connect 10 of them in parallel then how the voltage will be step by step like any generic BMS?

2 & 3. As I can see that 26C standard charge rate is 0.5C(1300mA), and @ 0.2C(520mA) its relative capacity is 100%. So, this is because 5A charger delivers around 500mA current to each cell, so it will be idle for both battery life and temperature. Is that correct I'm assuming?

4. As you can see in the first image, charging voltage should be 4.2+-.05v. That's why I'm going to use a 54.6V charger. So that each pack can get 4.2v. Is that not okay?
i - also, there can be seen that nominal voltage is 3.7v. That's why 3.7v x 13 = 48.1v. I saw everyone doing 13s to achieve 48v voltage. Am I doing any wrong?
ii - tell me one thing. When the battery will be fully charged, what will be its total voltage? will it be 3.7vx13=48.1v or 4.2x13=54.6v?
iii - This is because the fact sheet is telling me to charge it by 54.6v charger and other professional makers are also using same voltage charger (not 48v) to charge this kind of pack, what will happen if I use 48v charger instead of 54.6v?

5. Can my current battery meter(which is regulated by the controller, and the current battery is VRLA) show the battery level? Or, it won't work this is because it's li-ion?

6. No. It's for an EV (electric scooter actually), of course, I don't need to charge and discharge at the same time, but via the controller, I think it steps down some current to 12v which operates lamps, horn etc. Currently, there is a switch from the manufacturer, which needs to turn on when charging the battery. It's called 'Running Stage' & 'Charging stage' switch. At charging stage, the lamps and horns can operate but not the motor. In running everything can operate but no charging will be done.
i - So this is because if I charge the vehicle and turn on the light, will everything blow up? This is because the controller is 48v 250watts and connected charger will be 54.6v?
ii - My current charger is 48v 4A. On the charger, it's written, 'Only for VRLA Battery', so if I charge the li-ion pack with this charger, things won't work, right?
iii - Generally what maximum voltage a 48v 250watts controller can sustain? Is it exactly 48v or little higher than this?

7. It's off track question maybe but can't resist asking. The vehicle is around 50kg, I'm 65kg. So around how much mileage I can expect from a single charge from this setup?

Thanks!

 

[wesnewell]

1. Don't understand the question. Each cell in series increases voltage. Each cell in parallel increases capacity. That's all there is to it. A 13s pack fully charged will be 54.6V using 4.2V charged cells. The charger should be 54.6V CC/CV. They will not charge higher than 54.6V.
2&3. Not sure what you're asking.
4. See 1. Chargers designed for any lead acid batteries will continue to charge until the batteries won't take the charge or it reaches its max output voltage, which is usually ~60V for a 48V charger. Lithium batteries will always except more voltage until they puff and possibly vent and catch fire. That's why it's not a good idea to use chargers designed for lead batteries on lithium packs. You can use them, I have, but if you happen to forget them and over charge the pack, well, bad things can happen.
5. Because of the different voltage curve between lead and lithium batteries, lead battery type indicators don't work well with lipo. If it displays actual voltage, then it should be accurate. A lead battery cells range is ~2.25V full down to ~1.75V at 0% soc. A lipo cell is ~4.2V down to ~3.2V.

 

[dogman dan]

Wes does not run a bms to my knowledge. To run naked,, it does not matter if you have 13 groups of paralleled cells in series. or 10 13s strings.

But to wire a bms,, the convention is to make parallel groups, then series connect the groups. Each group has a small wire to the bms that reports that cell groups voltage.

Yes, you could series first, but it would mean a large and complicated wire assembly to connect the 10 individual cells to the bms, 13 times.

13 wires, or 130? Which would you choose. Battery engineers chose 13.

Charge it with an affordable 48v lithium charger that puts out 5 amps,, a 250w charger. This 48v charger will be set to 13s,, which is 54.6v fully charged. Your current charger is for lead, is likely not the right voltage for lithium, and if used for a long time, might actually damage your battery. If at some point you need to,, you can use that lead charger in a pinch,, like for a week while you wait for a new charger to arrive. You could also use it anytime, for charges that will not fill the battery,, such as grabbing a few wh while you eat lunch somewhere. It should not be outputting 60v,, but check with a voltmeter. If it's putting out above 56.4v,, DO NOT use it till the battery is full. 60v would kill your new battery.

You will also need a 13s bms,, if you choose to use a bms. Again,, if you want to run naked, then you could wire the pack any way you choose.

Max voltage for your controller is likely to be 60v, or 63v, typically 63. So if you want to, you could run a 14s battery, that charges to 58v . with a 14s bms,, charger, yadda yadda.

Mileage varies with weight and speed. But if you calculate for 35 watt hours per mile,, you won't likely get less than that.

Battery voltage times amp hours equals watt hours,, then divide by 35.

In this case,, your pack should deliver 25.5 ah. Lets round down to 23 and do the math. Most of your ride, the battery will be at least 52v. 52 x 23= 1196 watt hours. You should have about 34 miles range, at about 20-25 mph.

 

[desmond007]

Hello all,

Thanks for all the information.

Yes I was thinking about the same. First I need to create parallel group and then connect in series.

So the best thing is I need to use a 54.6v charger cc/cv charger and after full charge the battery pack will be 54.6v. Now the question is this is because I'm using a 48v bms, will it regulate the total output voltage to 48v? I mean stepping down to 48v from 54.6v.?

About the charger all I know it's 250watt. There's nothing written on it so it's hard for me to know what is the max input voltage. Do you know how to figure out?

Can a 250watt motor deliver 25mph.? Right now I get hardly 15-18mph.

regards