Making my own battery pack

[Marsbar]

Had some troubles with range on my batteries. Before I used 4x12V 12Ah SLA (Heavy!)

After that I bought a 48V 10Ah lithium battery on e-bay, built in BMS. Charger is only 1.6A, so it takes approx. 6.5hrs to recharge it.

Problems: Battery seems to be fading even after 50-100 charges. Also, when cold (say 0 degrees Celsius) shuts down and have to be re-started (Power on and off motor)

Now problem is, if going to town (10km hills/wind) I seem to use up the entire battery. Have adjusted the motor to 48V 10A.

So I need to charge it in town. But shopping stop is often only 1hrs, not 6.5hrs it need to charge.
Also, I suspect the battery drains itself if left for a week  unused.

Well, seller has evaporated from e-bay so I thought of making one or two extra battery packs to extend the range.

So I found this:

http://www.ebay.com/itm/15x-5000mAh-3-7V-18650-Li-ion-LED-Flash-Lighting-Torch-Rechargeable-Battery-Pack-/300919589718?pt=US_Rechargeable_Batteries&hash=item4610348b56

Only USD 27!

Also I need this:

http://www.ebay.com/itm/BMS-for-48V-13-cells-Li-Ion-Battery-Pack-50A-10A-limit-With-Balance-Function-/151034601971?pt=US_Rechargeable_Batteries&hash=item232a5d21f3

Thinking of having two cells in parallel, this making a 48V 10Ah pack. Using a dremel to scratch surface and a 100W high temperature soldering iron to solder on wires by being quick.

Price will then be:

2x27+56=USD110. Shipping: 20USD.

Now there appear to be a problem shipping 48B batteries, but I don't think this applies to single cells.

I plan to use my existing charger 56V 1.6A og my SLA charger that also gives out 56V. I am not sure if I can use an SLA charger instead of lithium charger, but voltage out from both chargers are exactly 56V, and I assume the BMS cut's off charing current at the end.

Are these cells to good to be true? Instead of buying a pre-made battery pack I am determined to make my own instead, and when the cells are used I could re-use the charger and BMS.

I plan on having two or 3 battery packages, each with it's own BMS and charger. Reson for this is to split things up, if something fails I have backup!

[Andrew]

you may want to find out the discharge rate of those ebay batteries the 'C' rating.  As they may not give out the power you will need.

I build battery packs with 18650 li-ion cells and I do not use a balancing board for them.  I just charged them individually with a charger that takes 6 cells until they are full.  I leave them for a week and then measure the voltage of each one to see if they have dropped voltage. ones which have I do  not use, with the others I build up in lines of series (13cells  + to - ) to get 54v fully charged. and then I parallel them together at each end of the line + to +   and - to - .
Once done I add a fuse holder with a 40 amp fuse in it. hen use about 3 rolls of insulating tape to wrap around it all to keep it tight and to insulate.  I put wire connecting plastic screw block to put all the positive wires in one side of the block and the negative to the other side. then have the fuse holder coming out of the other side of the block and put an andersons 50amp connector on the end.

I use thick house hold grey cable and strip it to get the earth copper wire out of it and cut it into one inch pieces which I use for wiring the the cells pos to neg when putting them in series.

I wish I had some pictures at hand but I have not got around to it, to make a tutorial.  All you need is a good soldering iron.

[Andrew]

I found this......  The cells are already in pairs when I start.

[Marsbar]

In order to use more battery packs, and combine lithium and my old SLA
(I filled water in them, maybe they work better now?) I am thinking of using a diode
bridge as shown.

These diodes (two in one package) will allow me to connect two battery packs at once. The diodes ensures I can use two different packs, and they will not see each other, only deliver current.

(The diode shown need a good heat sink, since it will get hot!)

So I can use SLA and lithium together. The SLA will give good acceleration, and the lithium will give a good run...

But I want to get rid of the SLA all together. So back to lithium!

If I skip the BMS, how do I treat my battery pack? My suggestion is below:

1)Max discharge: I assume you simply add a glass fuse 10A in series with the batteries, to ensure   
   short circuit protection and fire

2)Under voltage: I assume you know your ride length or simply look at the voltage. I could add a   
   voltmeter and stop using the pack when the voltage is too low.
 
3)Charging: Now this is difficult. Could I connect a 56V charger and an Ampere meter.

So, when I start charging, say it draw 1.5A. After the current drops to say 300mA, it means the battery are full and I have to manually take the charger off. Could this work?

Now, with the above, do you think I can go ahead without a BMS?

It would greatly simplify my design and cost not using a BMS... I could simply add one later 

[Andrew]

i really have no idea at all about using differing battery chemistries together.

I use a Li-ion charger that came with a golden motor kit for a 48v pack, rated at 2amp out put.  I put a watt meter between the battery and charger and switch it off when the amp input drops to 0.50.  The charger will turn off on its own, but leaving the 0.50 space is probably good for not overly charging. I do a 10 mile round trip with an 8 to 9 AH pack.

As you say you could add a watt meter between the battery and controller to watch out for under charging.  I use cheap watt meters from China as off ebay.
i will try and get a photo up when the app on my phone decides  finishing loading up.

[Andrew]

I use anderson 50 amp connectors on the watt meter as seen on the charger adaptor that is also pictured.

[Marsbar]

The cells I found on e-bay seem to be FAKE, some report capacity is often 1/10! How do I know the cells really have 5000mAh capacity? If it's too good to be true it often is... 

I need a verified source, no use buying heaps of cells of poor quality.

Anybody that has bought cells from e-bay that matched the capacity? Would love to hear from you.

I am also willing to cut open PC batteries, since these probably are of good quality as an owner of an laptop would reveal the capacity instantly after some charge and discharge uses.

I bought a new battery for my computer, so that is verified. Contains 6 cells each 2200 real mAH.

Each cell would then cost me $6, but at least I know what I get  I have tested the cells in my laptop 100 times so I know they are the real deal 

[Andrew]

yes, lap top batteries are what I use. that's a good plan.  you would need 54 cells to make a 48v 8.8ah pack which would be easy to make. another 13 in parallel would get you up to 11ah.  They are usually soldered together in pairs.  I find that charging them like this is fine to get them up to full charge on a 18650 cell battery charger. I bought one off ebay that charges 6 cells at a time or 3 pairs as  I do. It takes a little while to get them all charged. Then I wait a week to see if any have dropped voltage before wiring them together.   it takes about 2.5 hours to charge the pack in bulk after I have done 8 - 9 miles.

[Marsbar]

Meanwhile, worked on my diode bridge. I now have 10Ah lithium in, and 12Ah SLA in.

Worked sweet. Total capacity is now 48V 22Ah. Should get me to town and back without recharging 

Had a small test drive, seemed to work well.

Now! Problem is that I am back where I started, SLA is simply too heavy for a bicycle wheel and a small bump could break my wheel or spokes... 

Also, today I read the following by a professor in battery chemistry:

http://www.tu.no/it/2014/01/19/slik-far-du-maksimalt-ut-av-mobil-batteriet

"If you keep a lithium battery between 40 and 60% charged, the life span on the battery will increase from 300 cycles to 20 and up to 25 times. As long as small changes in lithium going back and forth it will not stress the grid structure the same way.
Many laptop producers offers 100% or 80% discharge due to this".

Wish I had known this before, I should have two lithium packs at 2x10Ah and in a diode bridge, and never use more than 50% in each battery pack.

The battery would then last 6000 (!) charges according to that professor. So the most economical thing to do is to ensure you have enough capacity and only use half of it before charging, also keeping discharge current as low as possible.

[Andrew]

well I am impressed with you solution

[Bikemad]

"If you keep a lithium battery between 40 and 60% charged, the life span on the battery will increase from 300 cycles to 20 and up to 25 times."

If you only charge it to 60% of its capacity and never discharge it below 40% of its capacity you will only have a usable capacity of just 20% of the battery's stated capacity.
So, if your trip requires 10Ah of energy, you would need to have a 50Ah pack for this to work! 

You may have to sacrifice your battery's theoretical extended life expectancy for more practical reasons (size and weight).

Alan
 

[Marsbar]

Well what the article says is to never drain batteries more than 80 percent, preferably 60 to 40 percent to make them last really long. Would be interesting to see if a battery could get 2000 cycles instead of 500 by this way. That would half the battery cost! A bit more expensive at first, also extra weight, but maybee you get the money back in the long run if you use the bike every day.

[Andrew]

when using li-ion packs I try not to let the voltage when rested for a couple of minutes to go below 48v, as 54v is full charge. while using the battery I have a cut off of 42v to be safe. A programmable controller or a cycle analyst helps to do this.  As said above, practicality and weight are most important.  Keep the li-ion pack topped up after every trip.

If the pack pack does go down to 47v rested, and 41v when at full pull, then that is up to you, but you will be stressing the cells more.

48 / 13 = 3.7
42 / 13 = 3.2

47 / 13 = 3.6
41 / 13 = 3.15

[DirtyGinge]

Nice looking batteries Andrew lol

Marsbar, are you in the south of england ?...i got a few 18650 cells kicking around that I don't use at the mo.....

[Andrew]

yup, I am sure you recognise the cells. I have not mastered the artistic triangle packs as good as yours, but I am now fully educated (by you) to build my own packs. I tear down your packs and charge the cells that are still good to build my own. I owe you everything

You need to see my lipo set-up...    hard case/shell back pack with springy coiled cable going from back pack to under the seat to andersons connector.  A bit William Heath Robinson but with room in the coiled cable to stretch for a fall off road.   I must admit Paul, you are missing out

But then again, I had a really bad slam off one of my bikes going to work two weeks ago on black ice right outside my school. I had a dead leg for a week where I fell on my thigh with keys in my pocket doing 20mph, rippped trousers and jacket and a bruised ego, knees,elbow and thigh.  I have since developed what I think is a cracked rib which is getting worse as the days roll on. I was picked up off the floor by two young school girls that treated me like an old man and helped me onto my feet and even picked my bike up for me. As you can imagine I was the talk of the school and I got absolutely no sympathy from the kids I teach who all thought it was hilarious