My idea for a long-lasting, fast accelerating ebike battery

[CUDAcores89]

So far I have been enbarking on a quest to build an ebike battery that has all of the following parameters. My system is a 36v 12.5A ebike rear hub motor I purchased from ebay 4 months ago and the system has been working really great. Although the problem is my battery is built from salvaged laptop cells and while it does work, the laptop pack does not have nearly as much "punch" during acceleration as my small lipo pack I use for emergencies. Therefore I have been exploring other options and other battery technologies and chemistries to develop a seemingly unicorn battery that will somehow be able to meet all of the following parameters:

1. Low cost
2. Long range (when I need it)
3. Plenty of "punch" during acceleration
4. long lifespan (thousands of cycles)
5. Safer than HK lipo
6. Ability to use an RC hobby charger (sorry, no battery murdering systems for me)


I think I may have just found a way to do it.

First, let's talk about how I am going to solve each problem:

1. Low cost: What my plan is for now is to purchase new laptop batteries from ebay in lots of 10. I recently purchased a single laptop battery on ebay for $10, and they contained 6 Genuine Samsung 28A cells inside. I tested the cells by charging and discharging all of them with my opus BTC-3100 and sure enough, they came out to 2800mah. The same guy is selling said laptop batteries in lots of 10 for $50, so This is how I am going to keep costs low.

2. Long range: This problem is going to be solved by simply using a ton of cells. If I place 10 cells in parallel, I will achieve a capacity of 28AH maximum capacity. This will keep discharge rates under 0.5C (my hub motor draws around 12.5A most of the time) therefore extending the cycle life of the battery itself.

3. Plenty of "punch" during acceleration: How I am solving this one is actually kind of interesting. Instead of building a typical 10S battery, I am actually going to build a 12S battery. Two days ago I crached open my ebike controller and after checking the MOSFETS and input capacitors inside, it can take up to a 50v input. Therefore if I build a 12S battery pack instead of a 10S battery pack and purposefully undercharge each cell to 4.0v per cell, this will give me a fully-charged voltage of 48v fresh off the charger, just under what my controller can take. This will mean that my new battery will have 48v fresh off the charger, 43.2v nominal (vs 36v) and 36v completely flat. Therefore because the voltage is higher, I will be able to get much more acceleration while still staying under the parameters of my 36v system. This will also do another thing which I will talk about later in the next point.

4. Long Lifespan: This is an important one. I not only want the battery to be cheap, but also last thousands of cycles. As noted in point four, I am only going to charge the cells to a maximum of 4.0v. Doing this I will take a 20% hit in capacity, but that will be made up by the fact that the battery will still be 22.4AH, and I will still be able to get a good 33 miles range out of the pack.

5. Safer than HK lipo: This should be an obvious one. I currently ride my bike to and from work and it would be very cool if I could take out my battery and bring it inside every day to charge. I am much more willing to bring in an 18650 pack into into my work than a lipo, since I have personally experienced two lipo fires first hand, while I have yet to see an 18650 go up in flames. Yes, I know 18650s can still explode, but at least in my experience they are safer than lipo.

6. Ability to use an RC hobby charger: This one is very important to me. I know that there a lot of folks out there that like to use a BMS, and that's perfectly fine. But in my experience, the cheap BMSs that yo can buy from ebay or aliexpress are absolutely garbage. I spent my summer of 2016 this year fixing a few of peoples hoverboards, and the most common problem I saw was a cooked BMS. And many of the BMSs in the hoverboard packs were the exact same BMS I have seem many ebike batteries use. In order to integrate many of the safety features a BMS provides, I am going to be doing a couple of things:

over-voltage protection: I will be charging with an RC lipo hobby charger

Under-voltage protection: I will be using 2-8S cell loggers that will beep when any cell hits 3.0v. This is much better than a BMS, since some of the cheap ones either cut off at a really low voltage per cell or only cut off at a low voltage for the entire pack.

Over-current and thermal protection - This one is going to be solved by using PTC fuses. PTC fuses are basically resettable fuses that when they heat up, they go open circuit and cut the power. If my battery pack get's very hot, the PTC fuses will get hot too and cut off the pack. If I try discharging over the hold current of the PTC fuses, the PTC fuses will heat up and cut the power again.

Now let's look at the bill of materials:

20 laptop batteries (+ an extra): $110
Solid core wire: $7
200G solder: $5
PVC heat-shrink: $5
PTC fuses: $11
Electrical tape and heatshrink: $3

Total: $141

However, there is only a single disadvantage I can think of from doing this: The pack will he quite heavy. It will likely weigh around 15lbs fully constructed. Although this really isn't a huge deal to me, since The pack will be installed in a large triangle bag most of the time, so weight won't matter to me a ton.

What do you guys think. I think this could actually work. I would be able to build at the very least a much nicer pack than I have now, at a low cost, that would be user-manageable, and last thousands of cycles.

Anything I haven't thought of?

[moderator edit to fix title spelling]

 

[dogman dan]

FWIW,, build it big enough, and you won't need to use those lv beepers till you are actually at a pretty low voltage. Big enough, even laptop cells will provide plenty of punch. You might consider 30 ah for that reason, so .75c discharge rate is still a solid 20 amps.

Once it's big enough, you will know, sag under load will be less than 2v. It does not have to be one big pack. It can be a 20 ah for most rides, but add another 10 in parallel as needed. personally, I like to make 10 ah packs. a 10 ah carries nearly anywhere easy. rear rack, handle bar bag, small bag in the triangle, etc. Connect as many as you need for a particular ride.

Just run a wattmeter, CA, or even just a simple pack voltage display, and you will know enough to ride the first 25 miles or so without putting the beepers on. If you leave beepers on the pack all the time, they run on the voltage from one or two cells, and will tend to unbalance your pack.

Use it conservatively enough, and you won't need to bother with balancing or worry about lvc for months at a time. Make it big and charge to 4.1v, then leave plenty at the bottom, and good cells will tend to stay balanced enough for safety.

Then occasionally, run your balance charge to 4.2v.

Put the beepers on the pack when you know it will be run to 100% empty. For a 14s setup, put em on when you get below 48v resting for example. Otherwise, not needed. Once you are using them,,, if you know which cell group in which pack has the lowest capacity, you only need to watch that pack. the other additional packs will be fine.

 

[redilast]

The 28A's are 4.30v charge voltage cells. So charging to 4.0v will yield likely 70-75% capacity.

Also I always recommend avoiding soldering cells together to make a pack. But if you must soldier, if you leave some of the tab on the cell and do the soldiering quickly then it is less damaging.

No BMS, a fuse and careful monitoring is the way to go if you know what you are doing IMO.

 

[Hwy89]

Yes, what you are planning works. I started out five years ago trying to accumulate enough used laptop cells for a 12s 10p pack. After sorting and testing and resorting and retesting I put together a pack of a rainbow of different colors and makes of cells. The step up from 36 to 48v was definitely an improvement but it still sagged a lot on hills and I wasn't comfortable with mixing all those different cells. I reconfigured the pack to 12x12 and it helped but still sagged.
In my EBay search for more cells I stumbled across a seller with a pallet of new two cell packs from a discontinued product.
These were all Samsung cells in perfect condition every cell at 3.6 or3.7V after an unknown amount of time in the box. So suddenly I had more cells than I knew what to do with.
I built two 12s 16p packs in 6s halves that charged separately on my RC balance charger. Yes the 16p packs are heavy but there is very little sag and the range is incredible. Most of the time they would only need charging once a week.
My suggestions;
Put as many in parallel as you have room for or can afford.
When disassembling the packs leave as much of the nickel tab as you can so you can solder the tabs not the cans.
Get a roll of nickel strip and use that for parallel connections.
Solder output wires to nickel strip then nickel strip to cell.
On series connections use multiple strips to increase current carrying capacity.
Use extra insulation on the "shoulders" of the positive cell ends. There are paper washers for this but electrical tape works.
Use extra insulation between each separate cell group. I like craft foam sheets cut to size.
Work slow and carefull, with no distractions.
An easy way to design the layout is to get a roll of dimes and play around on a template of your frame space. Heads positive tails negative to figure out what will fit.
Good luck and post pictures

 

[eflyersteve]

Sub'd - doing something very similar and will be interested in seeing how things turn out.

 

[999zip999]

when using used cells be very careful not to short the nickel strip at the pos. end to the can neg.
I don't know where the fast accelerating comes in ?

 

[CUDAcores89]

OK guys, just pulled the trigger on 30 of those 6-cell laptop batteries from the ebay seller. Let's hope this works out.

 

[Green Machine]

OK guys, just pulled the trigger on 30 of those 6-cell laptop batteries from the ebay seller. Let's hope this works out.

gamble gamble

 

[CUDAcores89]

OK guys, just pulled the trigger on 30 of those 6-cell laptop batteries from the ebay seller. Let's hope this works out.

gamble gamble

To date I have only lost 100 dollars on this stuff and I have probably scrapped hunderds of laptop batteries. And even then I was able to resell the worthless cells for nearly it's origional purchase price. On top of that I ordered one and checked it out first and when I received the single laptop battery, it still had the OEM wrapping on it. I suspect these laptop batteries are new, but the seller is claiming they are used just to cover their ass if the battery doesn't have full capacity.

 

[dogman dan]

pardon me for assuming the cells full charge voltage was 4.2v. But the point I was making was that making it really big has it's benefits.

If you are looking for zoom, you are looking for less sag under load. Build it, and if it sags, build another and parallel them till it's big enough to not sag.