Sunday, 29 November 2015

Going Green - Rechargable Battery Packs

In my first article about Going Green - Transportation I touched on Electropeds/Electro-Peds and the battery requirements and costs.  That was on November 22nd.  The thing that was burning my butt was the cost, a 48volt, 1500 Watt - 20Ah battery pack that uses Lithium-Manganese based chemistry was expensive ($850.00 USD) and I seriously got to wondering if I could construct one of these bad boys myself?  Could I make a better design?  Could I make it cheaper? Could I make a higher power density Battery Pack(BP)  The answers are yes, yes, yes and theoretically yes.

Cost of Battery Pack Technology

There are four major components to Battery Pack(BP) Technology you have to consider
  1. Cost of the cells.  Typically you are looking at 100s of individual cells in each pack.  This will vary according to the voltage of the motor, the power the motor draws (Watts) and amp-hours you require (how long the battery will last on full charge).  For the Electro-Ped I am designing from scratch, I will need 48volts at 1500 Watts, switchable to 3000 Watts and as much amp-hours as I can pack into the size.  It seems that the best cell tech right now is the 18650 cell configuration and they are costly. 
  2. The BCM, Battery Controller Module.  Now some people say you don't need a BCM other say you do.  I say it is mandatory!!!  The first reason is safety, the pricier 18650 Cells you can buy claim to have circuitry protection built into each cell.  This protection is limited at best being chiefly short circuit, open circuit and thermal protection.  For this level of protection you can end up spending $20.00 per cell, low end $10.00 per cells.  How good is this circuitry?  No idea, you would  have to buy a couple of these pricey cells and dissect one and then test several to failure to find out.  I have seen about a dozen youtube videos of one guy doing this and unless you are buying TOP OF THE LINE, you are wasting your money.  Also this type of cell protection only makes economic sense when you are using the cells in limited number, in expensive electronic equipment and using a crappy charger.  Since I want a cheap BP, a good BCM is mandatory.  More on BCMs in another article.
  3. The BP Housing.  Some people go with making their BPs as small and as light weight as possible.  In order to do this you sacrifice maintainability, serviceability and safety.  Basically, they connect the cells in various parallel and series connections and make the connections with nickel metal strips and spot weld them directly to the cells (say good bye to maintenance and serviceability).  They bundle the cells into what are called offset parallel cell configuration and hot glue them all together(say good bye to maintenance and serviceability again).  This makes for a smallest and lightest weight parallel cell configuration possible (don't worry, in yet another article I will go into great detail about connect the cells and you will not need a freaking spot welder, and by that I mean parallel-series design).  This small, light design has a serious safety design flaw.  If you have a single or multiple bad cells in your pack (this can happen with a faulty cell(s) or a cell(s) that has aged into failure) it can cause a run away discharge and cause your BP to catch fire or even explode.  Remember we are talking about a very high density power source here, it can go all explody-wody and going BOOM!  With this in mind, it seems to make sense to put in a few extra design features that will increase safety and allow for serviceability.  Again, this will be another entire article,... maybe several. Ain't Engineering Design grand?
  4. The Battery charger.  Ok, no sense beating around the bush here, buy a very good one, and one that will meet your BP Technology charging needs.  You want safety, testing and rapid charging capability, you may also want a good portable charger as well if you plan longer trips where you will need to charge on the road.  Sorry kids, cheap charges are just that CHEAP.  Even with a BCM on your BP you still don't want to risk it, cause things can go BOOM when charging as well.  Annnd once again, this will another article. 
So you kiddies have three (maybe more) boring articles on EV-Cell rechargeable technology to look forward to.  I know this might sound boring, but understanding this technology is key when you want to do solar, wind, alternate energy production and alternative transportation.  In the long term it makes you a smarter consumer, saves you money (I hope) and provide you with greater degree of safety.  There are some many variables at play here and juggling them all at once is a bit overwhelming if you don't have the math and sciences to understand them.  Hopefully I will assist you in this regard.

The first article I will be doing will by BP Housing and and BCM design/selection.  Since these two things are the core of a safe Battery Package.  "Wait, hold on a second Grumpy Hobbit, what about the cost of the battery cells?" you all cry.  Well problem solved, I have found a source for inexpensive 18650 Cells.  I plan to buy about 600 cells in January.  Before I reveal the cells in questions, I first want to test them, see how they fail, when they fail, under what conditions they fail and how Explody-Wody they are. Also I will need to test their safe discharge/charging and power performance (this is kind of important)  This will involve a video or two and maybe a trip to the hospital,... don't worry I will be using extreme care and safety measures when I do this, I is a Engineer after all.

A Word on Safety

This should be address, please DO NOT attempt this stuff yourself, some of the things I'll be doing require a high degree of Engineering Science, Education and Experience.  One of the reasons why I am doing this series is to help you understand the risks and if you choose to build your own BP, provide you with the safest and easiest construction design for the end product.  Many of the tests I will be doing will be done to demonstrate the problems, risk and dangers of cutting corners.  I have seen some pretty piss poor designs for BPs on the web and if you follow those designs without understanding the risk, you could easily kill yourself and I am not exaggerating here one bit.

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