18650 Li-Ion Battery Cells ma/h Confusion

Life Technology
1

I don’t know how to name the problem, but here it goes.

I intended to build my own Li-Ion Battery Pack for my electric bike. The problem was that my engine is laid out to consume 480W at 29V.
Most battery packs however come at 10 Amps witch should deliver around 290 W.
Since they easily break during heavy duty, I decided to build my own at 29V 50-60 AH.
On Bade Road I found a store selling 18650 Li-Ion Battery Cells with 4600 ma/h. So I thought with 7 units containing 12 18650 Li-Ion Battery Cells each, this should give me about 55AH at 29V.

Ok break down
12 18650 Li-Ion Battery Cells 4.2V 4600 ma/h
+12 18650 Li-Ion Battery Cells 4.2V 4600 ma/h

  • 12 18650 Li-Ion Battery Cells 4.2V 4600 ma/h
  • 12 18650 Li-Ion Battery Cells 4.2V 4600 ma/h
  • 12 18650 Li-Ion Battery Cells 4.2V 4600 ma/h
  • 12 18650 Li-Ion Battery Cells 4.2V 4600 ma/h
  • 12 18650 Li-Ion Battery Cells 4.2V 4600 ma/h

7 x 4.2V= 29.4V and 12 X 4600=55200

So, I build my battery with a special holder for each individual cell leaving a good deal of space in between for ventilation.
I used all the necessary electronics for balancing the 7 units at max 4.2V and minimum 3.67 or so.

However, today I had my first test drive to Tamshui about 32 KM round trip and the battery pack was depleted (empty) :cry:

I measured 29.2V at full charge and 25.6V when the pack was empty.
The pack did not get hot or produce any electricity in the outer case. I was very careful with that.
I opened the case after returning home right away and checked all 84 cells. Each of them was hand warm and had a charge of about 3.65.
The 18650 Li-Ion Battery Cells are resting on 2x7 1mm thick copper plates, each 4 cm by 14 cm. One plate is for the 12 positive sides and the other for the negative.

Is there anyhow a way that I have overlooked something? I went back to the store and told them that I think the cells only hold about 1500ma/h.
They agreed to take them back and order me some 3100 ma/h cells Panasonic in Japan. This should give me around 37.200 AH but for an additional 8000NT$.
Should I go for it or am I missing something here?

Help will be appreciated.

2

Easiest thing to do is to check one of the cells individually and confirm it’s capacity,if it’s significantly less than stated then bring them back for a refund.

3

I may be misunderstanding, but…

  1. By “ma/h”, do you mean “mAh”?
  2. Where are you getting this from: “12 X 4600=55200”? Never mind, just re-read it.
4

Where and how do I confirm that?
I am a little lost here.

5

[quote=“irishstu”]I may be misunderstanding, but…

  1. By “ma/h”, do you mean “mAh”?
  2. Where are you getting this from: “12 X 4600=55200”? You can’t do that unless they’re in parallel, can you? You either put them in series to get more volts (as you did) or in parallel to get more mAh, right?[/quote]

I mean mAH.
12 are in parallel and the 7 units of 12 are in line to add up the voltage.

6

Where and how do I confirm that?
I am a little lost here.[/quote]

All depends on how much equipment you have, at it’s most basic you just need to load the battery and check how long it takes to discharge. Check with google and find a method that matches the equipment you have or are willing to invest in.

7

This is what the pack looks like.

8

These copper sheets are put in between to make sure every battery makes a connection even if it slipped to the side.
Luckily, they also protected the head which will make returning them easier.

However, could they be at fault? But there are only two ways I can think of electricity being lost, one is heat and the other is electricity flowing through the case.
30AH 29V missing should make quite some fire and I did not measure any electricity in the case, even during operation, I measured it.

Last, did the copper-sheets somehow create radio magnetic waves going into space? No, that’s absurd.

9

Why do you think you are not getting the stated capacity? I see that you did 32km but at what speed, what was the motor consuming in terms of watts? assuming you previously had 10Ah pack, how far did you get with under similar load conditions? Also are you 100% sure that all the batteries are identical and all had proper conduct as well as all being fully charged before the trip?

10

A 10 AH pack would get me to Tamshui and half way back, about 25 Km. Two of those packs brand new would get me on the same bike forth and back with a lot left to go.

I took the bike for a short test ride after charging the pack. When I came back home I found the voltage to have dropped a bit too much so I opened the case and checked every cell. Each of them was at 4.08V then. I charged the pack up again and measured 29.2V.
29.2 ÷ 7 = 4.17 and that’s where I found the cells to be.
This morning, before taking the long ride, I measured again and it was still at 29.2V, therefor no lost over night.

When I came back from the trip, I opened the case and measured every single cell. Each of them was slightly warm and had about the same charge 3.65 or so.
If they hadn’t made a proper connection or non at all, they should have been at 4.17 V still. Actually that was my hope, to find one of the 7 units with only 3 of the cells connected, but NO!

11

I’d tried and do a test on the cells to see what the capacity in use actually is then. Is there any chance that they need several charging cycles before giving full capacity?

12

For a test, I need a reliable comparison. To be honest, I don’t know how to test those cells in order to determine their capacities.
If I had a Panasonic cell with roughly 3 to 4 AH and two identical power consuming devices, I could charge them up to 4.2V and see after how much time they reach 3.65V.
That should give me an idea but not any evidence for arguing.
Better would be, finding someone professional whose testimony can hold up in cord. I would then go to the consumer association and file a complaint there.

13

So basically, they can claim whatever they feel like.

14

OK an update!
I went back to the store and gave them all the 84 cells back.
They ordered the Panasonic 18650 3100 mah from Japan for me and I will have to pay an additional 100 NT$ per cell.
All together 240 NT$ per cell 20160 NT$.

Calculating every imaginable thing forth back and crosswise, I came to the conclusion that these cells would get me anywhere between 30 and 35 AH and 29V. or 70 Km to 80 Km.
Maximum discharge at burst should be around 30 amps. I require 16 amps of bursts.

While researching on the net, I found something looking very familiar to my blue cells which made me laugh.

At the bottom of that page. http://www.endless-sphere.com/forums/viewtopic.php?f=14&t=36802

So twice the cells as mine and the math adds up again.
I should be fine, or did you see any error I made in my design that could make 30 AH at 29V disappear without any heat and fire?

15

Although I’m not an expert in this…but you never get the full capacity…
First of all it depends on the type of cells (For example Polymer are most probably the best deal for capacity/reliability/price. But they usually come as flat for better performance).
Then even so, you will not get the capacity rated…I recently saw an article of a guy testing around 40+ Mobile Power Bank and tested the real capacity. The best of them was still only at 70% of rated capacity…some much lower (40%).
The manufacturing process is difficult to control but some brands are usually better. It’s apparently the case of Samsung cells.

Hope it helps a bit.

16

[quote=“dudumomo”]Although I’m not an expert in this…but you never get the full capacity…
First of all it depends on the type of cells (For example Polymer are most probably the best deal for capacity/reliability/price. But they usually come as flat for better performance).
Then even so, you will not get the capacity rated…I recently saw an article of a guy testing around 40+ Mobile Power Bank and tested the real capacity. The best of them was still only at 70% of rated capacity…some much lower (40%).
The manufacturing process is difficult to control but some brands are usually better. It’s apparently the case of Samsung cells.

Hope it helps a bit.[/quote]

thanks!
I kind of knew that and thought that with a 4.6 AH cell I should get 55 AH on a 84 cell 29V setup.
I thought 140 NT$ per cell is so cheap that even with a 40% cheating, I should get roughly 30 AH.

I only got about 12 AH.

There are discharge curves under various conditions for each type of battery cell. Since I use 84 cells, laid out to deliver 55 AH, instead of only 10 AH as before, I should be well within the better performance range of any battery type.

For the Panasonic, which I get tomorrow I expect to be within 2.5 to 3 AH per cell. 2.5 AH is rather pessimistic.
Still this should leave me with about 30 AH and 75 Km of range.
We will see tomorrow.

Wish me luck.

17

Everything looks very good so far.

First, I loaded the batteries into the pack and started charging them.
At 28.5 V, the controller and charger turned off, which was bad.
So, I opened the box and found 7 cells that did not charge at all. 5 cells that weren’t charging belonged to one compartment of twelve.

It took me some time re-balancing all the cells to the same voltage and to improve the connectors. The previous blue cells had top nipples, but the Panasonic cells have flat tops.
After doing all that, I started charging them again, and everything went as supposed to.
The charger cut off at 29.2 V leaving every cell with 4.17 V.

Next, real life testing.
I went to work 16.5 Km. The voltage measured 27.9V on arrival. Each cell should be at 3.98V which indicates a discharge of 0.19V per cell.
Then, back home. On arrival, the pack measured 26.9V. Each cell should be at 3.84 V and have discharged about 0.14V. You notice the drop of discharging for the second 16.5 vs the first 16.5 Km? That’s what the discharge curve should be. It’s right on.

In the evening, I went to my privates about 8.5 Km for the round trip.
Arriving at home, the pack was at 26.4 V. Each cell should be at 3.77 V and have discharged another 0.07V. Notice that 0.07+0.07=0.14 and 8.5 km+8.5 km=17 km. Again, it’s right on target.
So I opened the pack and measured each cell individually which only takes about 5 minutes. Each cell measured exactly 3.77 V. I was very pleased to find the pack so perfectly balanced.

Today, I overslept with a headache. So I got a wake-up call :frowning: .
Therefor, no trip to work. Instead I drove around the city. Altogether, I rode 18.5 km. The pack is at 25.3 V, each cell at about 3.61 v with a discharge of 0.16V.
I went[color=#FF0000] 59.5 km[/color] on a single charge so far.

If the final cut off is at 24V and 3.42V per cell, I should get an additional 0.19 V of discharging leaving me with another 10 to 18 km.

59.5+18=[color=#FF0000]77.5[/color]

Yeah! I am very pleased.

BTW. I used google map for the exact number of km.

18

Did you consider going with Lipo for your setup?

For instance this battery:http://hobbyking.com/hobbyking/store/__38502__Turnigy_7200mAh_4S_14_8V_40C_Hardcase_Pack.html

It’s 4s (4* 3.7V), so you could use 2 in parallel to get 8S for your system, and it’s already in a hard case. You could get 14400mah @ 29V for $130 and 840g. String a couple of those in parallel and you can quickly get up to your required output, at the expense of power density compared to Lion. Also, the packs here are very true to their rated capacity.

Any info about your motor/controller/setup?

19

Good that you got the result you wanted! Just as a matter of interest, how does your homebrew solutions stack up against the original equipment? Would it have been better/cheaper to simply buy 2 of the OEM batteries for the same range?

20

To answer both of the above questions.

Here is one of the packs I bought 2 years ago.

The packs are originally sold at 7500 to 10 000 NT$ and have 29 V at 10AH.
I bought 6 of those packs and paid only 3500 NT$ each since they went out of business and those were the remaining batteries they had.
Before that, we had two of those for about 2 years. Those packs were down already.

These new 6 packs lasted about a year before going down. Right now I need the remaining 5 to get me to work and back.

When we build this bike we used 4 different packs that had these 18650 cells inside and those packs lasted for about 3 to 4 years. We took them all the way up the highest passages Taiwan has to offer.
However, those packs weren’t at the right Amps and didn’t have a proper way to keep the cells cool.

3 packs with 10AH each cost about 24 000 NT$ and my pack is about the same including building costs.
However, I can, as I already did, check and replace all or the broken cells within 5 minutes now.
All cells stay cool at all times.
They are all within the optimal range of charging and discharging.
They are supposed to have 500 life cycles before going down to 90 %, but this is in lab conditions which I hopefully have translated into real life.
The 18650 are established as a new standard which means that I can buy cheaper and better cells in the future.

I thought of going lipo before. I even visited a battery pack company in the Taipei area. I did not like the way they arranged the packs.

Now look at those pro packs. I can trow away the whole thing. There is no way of easy replacement for the broken cells.
That’s what it looks like trying to separate them.

In this box< I now have three runaway cells giving up some ugly sweet smell and I don’t know where to go with them.