# BMS vs. bigger battery for electric motorcycle



## Duncan (Dec 8, 2008)

Hi
My tuppence worth
There are BMS's that top balance your cells
- I would avoid
Then there are BMS's that simply tell you when a problem has appeared
They can be as cheap as
http://www.evdl.org/pages/battbridge.html

If you are going to be the only driver you can be the BMS
If you want other people to be able to zoom off and use your bike you need a more robust system

The main things I have learned
(1) cells can just die - no BMS can fix that
(2) you can kill your cells
Most BMS's won't stop you from doing that

I killed some cells because they were sold as 16Ah - so I used 80% 
Unfortunately they were only 12Ah - so I killed them

Buy one of the nice single cell charge/discharge units and test all of your cells


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## polodiy (Oct 25, 2015)

I am in similar situation and considering doing bottom balancing the first time,
and after control Low Voltage Cut off of the pack with controller
and High Voltage Cut off with programmable charger.
When you operate in optimal working voltage range the cells should not become disbalanced.


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## Raphael (Dec 8, 2015)

Thank you very much!
That does make a lot of sense to me!

I'll be pretty much the only driver except to my wife, and she'd take good care the rare times she'd drive. So I believe it will be possible to be the BMS or/and limit via controller charging which seems to be an excelent idea.

How do I do the "bottom balancing the first time"?
Is this just testing the cells by charching, discharging (how?) and so "getting to know" the individual cells and therefore learning the weakest link in the chain, or is there more to it?


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## dcb (Dec 5, 2009)

this is what I'm using to good effect on my tractor and bike:
http://www.diyelectriccar.com/forum...-pack-balance-monitor-batt-bridge-165953.html

super cheap, super easy to install, tells me when I have a balance problem, gives me pack voltage (so I don't kill it), no phantom loads, or imbalanced loads of its own. Less than $5 in parts (for < 60v pack) less than $10 for 200v pack


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## Raphael (Dec 8, 2015)

Interesting!
You have an electric tractor???


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## polodiy (Oct 25, 2015)

Raphael said:


> Thank you very much!
> That does make a lot of sense to me!
> 
> I'll be pretty much the only driver except to my wife, and she'd take good care the rare times she'd drive. So I believe it will be possible to be the BMS or/and limit via controller charging which seems to be an excelent idea.
> ...


This is why you *bottom* balance: https://www.youtube.com/watch?v=84oseQR1jpY
Here is instructional video how to: https://www.youtube.com/watch?v=J2WvQre8sAQ
In general advice you to check the videos on the chanel - very helpful to newbie.


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## madderscience (Jun 28, 2008)

I know that to some degree this is a religious war, but In my opinion, a BMS of some sort is an absolute requirement. Size of battery is irrelevant. Without a BMS you have no idea what is going on at the individual cell level when you are driving.

I have an eliithon BMS system in my xB, which recently passed 20,000 EV miles. It is top balanced. I recently went through and manually verified 100% SOC for every cell in the pack using a single cell LiFEPO4 charger. The charger (which uses a CC/CV algorithm to charge up to 3.65V) took no more than a couple minutes for each cell to reach cutoff voltage. This means the cells were all well balanced. I attribute this to the presence of the BMS.

A good BMS should:

1) monitor for low and high voltage
2) monitor for excess temperature
3) monitor charge/discharge current
3) Cut off charging and discharging loads under conditions that could damage cells (LV, HV, current, temperature, etc)
4) Allow you to know what the hell is going on (ideally real time, but after the fact is OK too)

My BMS has let me identify individual bus bars that needed cleaning/tightening and allowed me to identify the weakest cells in the pack WITHOUT over discharging them. I know #63 is my weakest cell right now. When they hit the steep part of the voltage discharge curve (when they pass 80% DOD) it is clearly apparent on the BMS display (the display is DIY in my case) and I can stop driving before even hitting LVC.

It is possible to get by with a simpler BMS or one that only does monitoring, (as opposed to management and at its simplest you can just monitor blocks of cells (like every 12V worth, or 4 LiFePO4 cells) for voltage. That's enough resolution you will likely be able to spot a low or high block of cells and infer a low or high cell within. However a low and high cell in a given block might cancel each other out. Still its way better than nothing.

BTW the stupid-simplest balancing load for a LiFEPO4 cell: Five 1n4001 diodes in series. They don't conduct more than a couple mA below about 3.5V, but by the time a cell is nearing 4V, they are conducting about 250mA. I am using such balancers on a 24V, 60AH LiFePO4 pack being used in an off grid solar array. the charger is a 24V MPPT solar charge controller (so it handles maximum pack voltage, the individual balancers help pull down high cells). So far (3 years) working great. I wouldn't use this approach unless the cells were regularly charged though.

Good luck.


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## Raphael (Dec 8, 2015)

Thank you very much!
This is all very helpfull. Especially since, no matter pro or contra BMS, I can see now what the reasons are!


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## dcb (Dec 5, 2009)

fyi, it is generally understood that not pushing a battery to full charge or full discharge is good for cycle life, which is facilitated by a "larger" battery than what you might otherwise need.

http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries

i.e.


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## Sunking (Aug 10, 2009)

Raphael what you have stumbled onto is there is no Definition of a BMS. Ask 10 people what a BMS is and you get 10 different answers. 

Long battery cycle life is super easy to do. Never charge any cell to more than 90% SOC, and never discharge a cell more than 90% DOD. Never charge faster than C/2. Do those three things, and you will get good cycle life. None of the commercial EV manufactures allow the user to fully charge or discharge any cell. It is the onlyu way they can off the warranties they do. 

That should set off Red Flags in your mind. Most BMS incorporate what I call Vampire Boards, aka Bleeder Shunt Bypass circuit boards. Guess how they work? Yep most of them requires you to charge the cells to 100% SOC. Vampire Boards IMO do not solve a problem, they are the problem like governments. 

So you are on the right track that eludes most DIY builders. Most are tightwads and do not understand the consequences of their actions. Example they calculate it takes say 20 Kwh battery pack to go X amount of miles. So they go right out and buy a 20 to 22 Kwh battery. Huge mistake.

First Chi-Com batteries are pretty low quality. Capacity tolerance is roughly -0% + 12%. Secondly operating temps, Peukert effect, and not using all the capacity is completely ignored and not taken into account. So what they end up doing is end up racing to both Over Charge and Over Discharge cliffs and hope the BMS slams on the brakes before they go over the cliff. So instead of that 20 to 22 Kwh battery they should have been looking at a 30 Kwh battery. 

My advice is duplicate best as you can what the EV manufactures do. Run Between the Sheets. Never allow your battery to ever be fully charged or discharged. Stay between 10 and 90% SOC. To do that takes some sort of BMS, but never use Bypass Vampire Boards to do that unless you have some way to limit current to match what can be bypassed.


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## Hollie Maea (Dec 9, 2009)

Sunking said:


> That should set off Red Flags in your mind. Most BMS incorporate what I call Vampire Boards, aka Bleeder Shunt Bypass circuit boards. Guess how they work? Yep most of them requires you to charge the cells to 100% SOC. Vampire Boards IMO do not solve a problem, they are the problem like governments.


Why can this balancing only occur at 100% SOC?


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## madderscience (Jun 28, 2008)

It doesn't have to occur at 100%.

If the shunt boards can be programmed, they can balance at any voltage. Any decent, configurable BMS will allow this.

100% voltage for my GBS cells is 3.65v. I balance to 3.50v (which in reality is still about a 98-99% charge) Much lower than that and it gets way harder to detect the actual SOC of the cell as the voltage/SOC discharge curve flattens out.

Some other lithium chemistries do have a more linear discharge curve than LiFePO4, and with those its easier to balance and limit to 90% maximum charge.


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## dcb (Dec 5, 2009)

there is something to be said about bottom balancing that is often overlooked.

charging is done under relatively controlled conditions, i.e. constant currents/voltages, at comparatively low power usually.

discharging is random, and often high power.

over-discharge kills cells quickly.

so bottom balancing aids the signal to noise ratio on the bottom end, and adds a bit of self-protection to the pack. 

but bottom isn't visited as often as top, so there is a bit more uncertainty (and a tiny bit more energy in a top balanced pack)

But you should have a half-pack monitor at a minimum, it is just too trivial to do, ideally with authority over charge and discharge, and know what the spread *should* be at end charge and end discharge regardless where you balance. If you "automatically" top balance at every charge (any data on how often that happens in practice?) and ignore the low end, that isn't good either, worse in some ways.


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