# How to build a simple lithium ion battery pack with or without bms



## typci (Feb 16, 2012)

Ok, I am considering building a Li-ion battery pack. 

I tried reading the forums about how to set up a pack and all I got is a headache out of it. I understand circuits and batteries, and I took chemistry and physics in college. But there are so many ideas and arguing on this forum and others that I can't make head or tails of it.

I have an electric motorcycle that has Lead Acid batteries. There is a red and black wire coming from the battery pack which splits going to the charger port and the control unit. I ride my bike, plug it in to charge, and repeat. If the battery meter starts pointing towards the Low symbol, I know I'm close to being out of electricity and should charge up. When the batteries don't give me the distance I need, I replace the pack. Very very very simple.

If I build a Li-pack it needs to just bolt/plug right into the existing system, because if it is a pain in the a$$ to use daily I don't want to deal with it.

I can find good batteries and a good charger. I know how to wire the batteries together. What else do I need to get/do this project working? I want something that is simple to use on a daily basis because I don't have time to mess around with it.


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## batterytang (Nov 17, 2011)

Hey Typci, You can ask Dimitri,

About 2009 he started a thread about How to DIY BMS circuit.

And his mini BMS is not expensive too.

Just take a look at this,

http://www.diyelectriccar.com/forum...ms-affordable-and-easy-install-bms-41020.html

and this,

http://www.cleanpowerauto.com/MiniBMS.html 

Lithium batteries are weak so they need protection, BMS is necessary


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## EVfun (Mar 14, 2010)

There is a lot of disagreement about the need for a BMS with LiFePO4 battery packs. Many EVs are running around with Lithium batteries and no BMS. 

Most EVs with a simple battery fuel gauge simply use voltage to determine the state of charge. These types of meters will not work with lithium batteries because the voltage is nearly constant until very nearly dead. You can just use range (X number of miles per full charge) if you undercut a bit. For instance, my EV can go up to 37 miles on a charge -- so I just don't go over 30 miles.


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## typci (Feb 16, 2012)

Ok, that makes sense since the remaining capacity on lead acid is measured by voltage.

So I'll either need to measure by distance traveled or if I could find an something to count the amps spent, that might be better. On the measuring the distance note, What about driving style and weight changing the amount of energy used? With a car, if you add a person or two, it doesn't affect the total weight as much as on a motorcycle. If I have me and two girls on my bike, I'm not going to be able to go as far as if it is just me. Also, driving in China's traffic requires a lot of braking and acceleration, and this is going to be a factor to. I'm going to need a more accurate way to measure reserve capacity.

On the bms note, I'd rather buy one than build one as I don't trust my skills building something that important. I see lots of centralized BMSes for sale in china. Here's and example of one that comes built into a battery pack: http://item.taobao.com/item.htm?id=13805031856. The seem to cover all the basics: voltage, amperage, shorts, etc. Is there anything else that the BMS should have? 

What I really want is a BMS that will display some information on my control panel/dashboard with an LED that lights up when there is a problem.

Thanks, I'm going to go read the articles now and see if I can make some more sense out of the BMSes available in China. I think the BMSes I've been looking at have been battery management (protection) systems, when I need to both to manage and monitor the batteries. Once I get this figured out I can hopefully upgrade my bike.


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## ElectriCar (Jun 15, 2008)

As EVFun said, lots of people run them without BMS systems. That said you would be foolish to use lithium with no means whatsoever to monitor your pack. Too many things can go wrong that could destroy the entire pack quickly if you're not monitoring it in some manner. 

Click on the link to my conversion thread and you can read about the things I use and how I did them. Some of them I've yet to see anyone else use, ie voltage monitor relays. 

Though my system may cost as much as the least expensive BMS systems, I like it because it doesn't have wires running to every single cell. A large pack like my 50 cell pack would have a large bundle of wiring going to it that I prefer not to have nor install.


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## tomofreno (Mar 3, 2009)

ElectriCar said:


> Though my system may cost as much as the least expensive BMS systems, I like it because it doesn't have wires running to every single cell. A large pack like my 50 cell pack would have a large bundle of wiring going to it that I prefer not to have nor install.


 Uh, not so with the minibms ElectiCar. There is only one wire from one on-cell board to another, and only two low voltage optically coupled wires from the pack to the main board - though with the centralized option, there would be wires from each cell out to the centralized boards, which I don't like.


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## ElectriCar (Jun 15, 2008)

I did not know that. So they use that for serial data xfer I guess. Much better than a harness. I also didn't like the idea of a board failure of some sort on a cell shorting and draining the cell to death either.


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## EVfun (Mar 14, 2010)

typci said:


> Ok, that makes sense since the remaining capacity on lead acid is measured by voltage.
> 
> So I'll either need to measure by distance traveled or if I could find an something to count the amps spent, that might be better. On the measuring the distance note, What about driving style and weight changing the amount of energy used? With a car, if you add a person or two, it doesn't affect the total weight as much as on a motorcycle. If I have me and two girls on my bike, I'm not going to be able to go as far as if it is just me. Also, driving in China's traffic requires a lot of braking and acceleration, and this is going to be a factor to. I'm going to need a more accurate way to measure reserve capacity.


With a cycle you would have to learn more about what diminishes range and likely use a more conservative range limit too. I have to figure in my head that each freeway mile is about 1.5 city miles when estimating range remaining because it is a light car with bad aerodynamics. 



> On the bms note, I'd rather buy one than build one as I don't trust my skills building something that important. I see lots of centralized BMSes for sale in china. Here's and example of one that comes built into a battery pack: http://item.taobao.com/item.htm?id=13805031856. The seem to cover all the basics: voltage, amperage, shorts, etc. Is there anything else that the BMS should have?
> 
> What I really want is a BMS that will display some information on my control panel/dashboard with an LED that lights up when there is a problem.
> 
> Thanks, I'm going to go read the articles now and see if I can make some more sense out of the BMSes available in China. I think the BMSes I've been looking at have been battery management (protection) systems, when I need to both to manage and monitor the batteries. Once I get this figured out I can hopefully upgrade my bike.


I think the miniBMS from Clean Power Auto or the BMS boards from EV Works in Australia would be the easiest way to go. These units don't tell you much, they provide a normally closed signal loop that will open if any cell in the string goes too high or too low in voltage. What you do with that signal is up to you. Finding which cell went over or under is up to you. The resulting cell boards are inexpensive and don't draw a lot of power from the cells.

Clean Power Auto and EV Works both sell amp hour counters that function as a fuel gauge. This is likely the best way to go for estimating range available, especially if you want to regularly use more than 70% of the available capacity.


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## tomofreno (Mar 3, 2009)

ElectriCar said:


> I did not know that. So they use that for serial data xfer I guess. Much better than a harness. I also didn't like the idea of a board failure of some sort on a cell shorting and draining the cell to death either.


 There is no serial transfer on the minibms since it is an analog device with no digital signal processing. Each on cell board has optocouplers that complete the N.C. circuit. If voltage goes too high or too low on a cell, an optocoupler on that cell board opens the loop, triggering an HVC or LVC alarm. 

Regarding shorts, there is a fuse on the input of each cell level board which would blow in event of a significantly higher than normal shunt current flow between cell terminals. This guards against a short between terminals occurring on the board as well as a cell failing with a high internal resistance which results in large voltage across terminals of the cell level board. The minibms is well-designed and quite safe I think.


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## typci (Feb 16, 2012)

Just looked at miniBMS master/slave board from Clean Power Auto. I can't buy that. The master board costs more than my pack of 16 cells, and the slave boards costs the same price as a pack of cells, and that's before shipping. That's insane, it would double the cost of my conversion.

I'd really rather not import anything. The prices are just going to be too expensive. The only reason I'm really able to do the conversion is because I'm in China, which means I can get the batteries for half of what they cost in America.


So, I've got my base setup:

a) 2 or 3 battery packs: 48V, 20Ah, probably a123 20Ah batteries. 2 20Ah packs is what I have now, a 3rd would be a nice bonus for extended range.
b) An amp counter to monitor remaining capacity. Still exploring options on this one as I haven't started looking at what China has to offer.
c) A Chinese BMS: overcharge, over discharge, over current, short, and high temp protection. I'm going to continue looking at Chinese BMSes to get the best one I can. I figure replace the BMS every year or two when I do my maintenance, since the BMSes are pretty cheap, about $25.
d) Individual voltage LED displays for each pack on the console, with low voltage alarm.

Things I'm still looking at:
a) Saw the ammeter at Clean Power Auto. Going to see if I can find something like that in China.
b) Still reading Electricar's conversion thread to see what else he did.
c) Still got to choose a charger, but I know there are plenty to choose from here in China.
d) Should I install circuit breakers for each pack? My bike currently just has one.

What else am I missing that is essential?


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## GizmoEV (Nov 28, 2009)

You could just use a 1/2 pack voltage monitor like I'm using. Check my blog for details on it. In my case I'm using a top balanced pack. Also, you might want to look at ebikes.ca and the CycleAnalyst. I don't know what it would cost to import the CA from Canada but it is designed to be mounted on an eBike.


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## typci (Feb 16, 2012)

GizmoEV said:


> You could just use a 1/2 pack voltage monitor like I'm using. Check my blog for details on it. In my case I'm using a top balanced pack. Also, you might want to look at ebikes.ca and the CycleAnalyst. I don't know what it would cost to import the CA from Canada but it is designed to be mounted on an eBike.


Is the 1/2 pack voltage monitor the same as the battery balance volt meter?
The battery balance volt meter, if I understand it correctly, is exactly what I was looking for to supplement the Chinese BMS. The Chinese BMS can protect the cells and the battery balance volt meter can tell me when something is wrong with a cell.

Also I checked out the cycle analyst. It's great, but it's $120, that's half the cost of a battery pack, or about 20% of expected cost for the battery upgrade. It just doesn't make sense to spend that much money on a single accessory component. Now when I get back to the states and want to convert my prelude, that's a different story. But right now my income is in Chinese RMB, and right now they don't stack up too well against US$. For now I need just the bare minimum to protect the investment in the batteries. The bike used with 6 month old lead acid batteries only cost me $675, new it's about $1000. When I do the upgrade, the batteries will be worth more than the bike, at the Chinese price for the batteries.


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## GizmoEV (Nov 28, 2009)

typci said:


> Is the 1/2 pack voltage monitor the same as the battery balance volt meter?


Yes, that is the same thing with different ways to implement it. The way I implemented it, with resistors at the end terminals and a somewhat smaller one on the center tap, I eliminate the need for high voltage low current fuses. In my case the current draw across the pack is really low. There is 201kohms of resistance across the pack so the drain is at most 0.33mA.



> The battery balance volt meter, if I understand it correctly, is exactly what I was looking for to supplement the Chinese BMS. The Chinese BMS can protect the cells and the battery balance volt meter can tell me when something is wrong with a cell.


There is a low chance that a cell in each half will "die" at the same time in which case the balance meter won't notice it. I figure that probability is quite low so am willing to go with it.



> Also I checked out the cycle analyst. It's great, but it's $120, that's half the cost of a battery pack, or about 20% of expected cost for the battery upgrade. It just doesn't make sense to spend that much money on a single accessory component.


I understand the issue with cost and exchange rate. Remember, however, that the CA or similar device can be used for a long time and it is there to make sure you don't over discharge your pack and risk loosing the whole thing. Naturally with my Gizmo's $5000 pack it was worth the $120. There may be other Ah counters available over there you could use.


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