[Ziggythewiz]

Since there's no winning this debate, I figure we can at least keep score.

Let us know how you manage your pack, as well as your reasons for doing so. Let's try to stick to non-repetitive arguments (at least for the first few pages) so those trying to learn something aren't eternally lost to the black hole.

Quick intro for noobs (potential converts):
To prevent damage to your expensive lithium battery pack it needs to be managed. This can be automated by a trustworthy and capable Battery Management System (BMS) or manually using top or bottom balancing and some basic monitoring.

The ideal pack will have very similar cells to facilitate balancing (same size, manufacturer, batch, quality). No matter how alike they are, no two cells are identical, much less 30-100 cells in series as found in typical EVs.

Types of BMS Poll and info here.

Top balancing seeks to match all cells near 100% SOC so they finish charging together, providing better protection while charging at high SOC but greater variance while discharging at low SOC.

Bottom balancing seeks to match all cells near 0% SOC so they finish discharging together, providing better protection while discharging at low SOC but greater variance while charging at high SOC.

Basic monitoring should include the use of a Digital Volt Meter (DVM), Amp Hour (AH) Counter, and ideally some form of simple split pack voltage comparator to detect a cell anomaly.


Some historical perspective:
Note: Lots of learning has occurred since then.

This debate has recurred at several times in the last 3 or 4 years. The first "discussions" were in late 2009. A later one was started by Electriccar I think. Only the earliest "pioneers" like Jukka Jarvin and Cedric Lynch were using lithium cells in vehicles much before 2009, but they and others were using them in 2004. I recall them dividing them into pre-2005 and post 2005, with quality much worse in the pre group. There was also much discussion on the old Yahoo Groups Thundersky group (now removed) which Cedric participated in a bit. It also had Jack's early efforts at a crude BMS using a Darlington pair to shunt 2-3A, which got very hot resulting in him railing against the danger of fire with a BMS. Here are some links for late 2009/early 2010 in this site:

Lots of discussion of top v bottom balance by JRP3, Jack, and Dimitri in this thread of 11/22/09 which also references another one of same date, and has Jack’s “ragged top” graph of cell V:
http://www.diyelectriccar.com/forums/showthread.php/bottom-balancing-thread-saferi-more-usefuli-38662.html

Here is another thread on BMSs from 10/20/09 before Dimitri had developed the minibms:
http://www.diyelectriccar.com/forums/showthread.php/minimal-full-function-bms-li-37512.html

Here is Dimitri’s 2/16/10 thread with a poll asking what people want in a low cost BMS, which led to the minibms development:
http://www.diyelectriccar.com/forums/showthread.php/new-cheap-bms-poll-38549.html

Most of those involved in the earlier debates have long since grown tired of it so no longer participate.

 

[Ziggythewiz]

My pack is top balanced with no BMS.

• I see a BMS as a large expense that is unnecessary with proper care and feeding.
• 70 or 80% DOD is recomended to prolong cell life, while charging is nearly always to 90-100% SOC. It's easier to stay away from the bottom than it is to stay away from the top, so better protection on the top end is more useful.
• Potential overdischarge would happen when you are in the car, able to notice and respond to any warnings, limp triggers, or disable features you've installed. Potential overcharge would happen when the car is unattended, usually when you are having dinner, watching the game, or sleeping. That makes it more important to protect from overcharging.
• Top balancing is easier
  • Cells (at least CALBs) usually arrive at ~60% SOC. It's faster to charge them to nearly 100% than to drain them to nearly 0% just to charge them again.
  • There are more purpose built battery chargers available than dischargers.

 

[vmrod]

BMS (Lithiumate Lite)
Top Balance Cells
Valet Mode input to Zilla controller during BMS warning
HV Contactor open for BMS fault (not main contactor, but an additional contactor)
Disable charger for HCV or BMS indicates complete charge

Note:
Batteries not purchased yet, but coming very, very soon. (Test pack wired in place now) All other equipment either purchased and/or somewhat wired up.

 

[Yabert]

I use Mini bms (top balance) since more than a years now with great result. It saved my cells at least two times during deep discharge (rare event in my case).

But if I have to restart a simple conversion (48v-72v) I will probably try bottom balancing without BMS because I know now that can work properly.

 

[Siwastaja]

Poll = a great idea.

And a... balanced starting post!

Of course a BMS. It's "active security", and while passive is typically better and more reliable, li-ion does not have passive security (such as a physical mechanism to naturally change overcharge into heat without damaging the cells) -- and pretending it does doesn't change the fact. So, either use an automated system, or really know what you are doing. For very tight budget, I can imagine going without a BMS.

 

[frodus]

While I don't own a DIY build anymore, the build I was working on had a Lithiumate Pro integrated to work with a Curtis AC controller and DeltaQ charger.

My current vehicle, Brammo Empulse R, has a full BMS designed and built by Brammo. It throttles back if a cell is too cold, too low, etc. and displays faults on the screen.

Interestingly though, I'm using 40 of my cells leftover from my DIY project to build a 12V 100Ah pack + inverter for portable power/emergency charging @120V L1 rates. That will not have a BMS due to the voltage being so low. The differential isn't so extreme, so a pack monitor like the JLD404 will work. To charge, I'm using 4 3.7V DC-DC converters to charge each group of cells individually... so they're automatically balanced each charge.

IMHO basic monitoring with throttle cutback/charger cutoff is bare minimum.

 

[Elithion]

Quick intro for noobs ....

I commend you for your ability to describe each option so concisely and accurately. Thanks.
(I added to your "reputation"

for this post.)

 

[dladd]

I top balanced, then installed MiniBMS to hopefully keep them top balanced (there's a little balancing going on at the top of charge with the shunts). Mostly the MiniBMS is there to cut off the charger in the event of high voltage, and to warn me of low voltage cells when driving. My setup is really more of a Battery Monitoring System as opposed to Management.

On the charge side, my charger is set to charge to an average of 3.5vpc. The charger runs through a SSR that is controlled by the MiniBMS. If any single cell reaches 3.6v, AC power is immediately cut from the charger. In over a year and 10k miles, this has happened exactly zero times.

On the discharge side, my controller is set to limit pack voltage drop, then the MiniBMS will sound an audible alarm if any single cell drops below 2.7 (or so) volts. No further action is taken by the BMS, it's up to the driver to back off or stop driving if a cell is alarming. I get this alarm a lot, especially when it's cold out. I have 3-4 cells that routinely sag to 2.3v when the rest of the pack stays up at 2.8v. So I'm not really protected on the low side, but I am warned.

Mostly I use an amp counter for range. I have the EVDisplay, and it is set up so that zero ah remaining is actually 100ah used from my pack. I have 130ah cells that range in capacity from 119 - 140ah in actual testing, so at 0% I'm really at ~15% SOC on my weakest cells. I've never once brought them past zero. In fact in the year plus I've been driving, the lowest I've seen is 4ah remaining (which is 96ah used from my 130ah pack).

I am much more concerned about the charge side, since that happens unattended in my garage while I sleep. That is why I decided to use a BMS with a high voltage cutoff that will shut down the charger if any single cell runs away.

 

[Tyn245GL]

I use the Elithion Lithiumate Pro BMS on my 90 cell CALB SE130AHA series pack. The system has 9 battery banks.
In addition to Lithiumate Pro's core function: managing and monitoring the pack, I also use most of the charge and discharge control and safety functions that are built-in in the Lithiumate Pro:
- Plus, Minus and Precharge contactors switched via vehicle key signal via BMS
- CAN Bus integration with my Brusa NLG513 charger: current monitoring, variable CCL and cut-off at High Limit all via CAN Bus
- CAN Bus integration with the MES TIM600 inverter: DCL variable current limiting, native torque reduction at low SOC and cut-off at bottom SOC, and CCL variable current limiting and torque limiting during regen at top SOC
- regen switched off at High Limit and low temperature
- CAN Bus integration with Lithiumate Pro's HVFE - High Voltage Front End which provides the additional safety test functions described below:
- Plus, Minus and Precharge contactors switched via vehicle key signal via BMS, only after safety tests have been performed, such as:
* battery voltage present
* contactor health checks
* isolation test
* successful precharge

For daily driver info I use MetricMind's Evision2 digital instrument which shows:
- Pack Voltage, Amps, Temperature, Split Pack Balance, SOC
- Actual Kw, Wh/km, Remaining Range (DTE), speed, Ah used (coulomb count)
- Charge indicator, Time Till Full (TTF)
- aux battery voltage (12V)
At the moment Evision2 and LithiumatePro work independently. Both have their own current, voltage and temp sensors.
I wish some time there will be an integration between the two systems. Both are very valuable but the combination would be state of the art for me.

 

[Yukon_Shane]

I top balance and use a mini-BMS. I allow the BMS to top balance my cells but truthfully I'm not terrible concerned about them going out of balance, my guess is they'll stay pretty much in balance. The reason I use a BMS is because I like the idea of activiley monitoring my cells. I want to know if any given cell falls below or above the threshhold I've set for them.

It seems like a very simple and straighforward way to ensure that nothing strange is going on during charging or discharging.

 

[ndplume]

52S1P GBS 100AH
Lithiumate Pro, 4 banks
Top Balanced EVERY time I fully charge.

Over Charge protected by BMS control of charger, secondary by Voltage cut off of charger.

Low Cell signal sounds tone (primary) and flashes strobe under passenger dash board. Not connected to my controller. I depend on driver to not kill cells at low end based on warning signals.

Cell boards are used to keep cells warm for charging in winter (About 15F capable) in my un-insulated boxes. (Dallas = 3 weeks of winter)

Torque via android bluetooth to see low/high cells, monitor temperature.

Properly installed and configured, I consider my BMS as my best chance to get entitlement life from my pack. I sleep easy with it watching my pack. Lithium Cells are considered more cost effective, However Only if you get 2,000+ charge cycles.

( The improved ride performance of Lithium over lead because of the diet is lagniappe ! )

 

[PStechPaul]

Although I don't (yet) have a lithium pack or a roadworthy EV, I voted for BMS, because that is what I plan to use, even if I go for a 300V SLA pack for my tractor project. I believe in the concept of the BMS and I think every pack should have one, although the exact type and capabilities are open to discussion and personal choice. To me, the challenge of designing and building and testing a BMS (and an EV) is my main motivation, and the actual end product, not so much.

My ideas are expressed in detail in the BMS Design Guidelines thread, but essentially I like the idea of a full time one per cell system which can at least partially prevent overcharge or depletion and cell reversal, and ideally remove (isolate) a compromised cell from the pack, and also provide real time data to a main controller and/or charger for warning, display, and shut-down.

 

[Ampster]

Cell monitoring and top balancing by Emus BMS.
In the first phase of my VW build I tried monitoring my pack with cell logs and had no active cell balancing. I really liked the data about how each cell was doing. Despite the low voltage audible warnings my controller went into limp mode once because I miscalculated the energy needed for one trip. No cell damage was done because the Curtiss LVC was set high for my pack of 32 cells.
On phase two of my build I wanted a Android app to view my individual cell health and the ability to set the BMS parameters. Right now I use a programable volt meter (JLD 404) to control my charger but plan on wiring up the bells and whistles from the Emus to give me redundency. Currently I charge my Winston's to 3.4 volts and begin shunting at 3.35 volts. I will experiment with turning off shunting by setting the shunt voltage higher than the HVC of the charger. I like that flexibility and in that mode I guess you could say I will have a top balanced pack with an expensive monitoring system.

 

[azdeltawye]

• Battery pack: 45 cell series string of TS 180Ah prismatic LiFePO4
  
• Balance methodology: Top balanced
• BMS: first generation mini-BMS (similar to Volt Blocher) with 10W shunt resistor and optically coupled LVC & HVC 4-wire bus to control unit.
• Cell Undervoltage condition: Dash warning indication & throttle reduced 50% (valet mode) after time delay.
• Pack Undervoltage condition: Controller reduces battery current until undervoltage condition clears.
• Cell Overvoltage condition: Battery charger output reduced to 2.0 A & activate shunt resistor on cell BMS board.
• Pack Overvoltage condition: Independent Overvoltage relay will trip lockout relay & cut AC power to battery charger.
• Total E-miles driven as of 3/15/13: 30,102
• Vehicle in service date: 5/2/10

 

[dougingraham]

My pack is 52 cells of the GBS 100AH cells.

Bottom balanced which makes me the BMS.

I chose to do this because when it goes flat at the bottom there will be no damage to the cells. My Soliton will stop me from running the pack to death with its low voltage settings. Cutoff at charge is something everyone worries about but it becomes completely unimportant once you realize that it is better to not completely charge the cells anyway. On the first charge you watch for the cell that gets to 3.5 volts first and then measure the pack voltage at that point and set the charger to CV at that pack voltage.

I plan to put in a half pack comparator which will indicate a single cell failure.

 

[onegreenev]

Below in red is my addition to the previous quotes.

Top balancing seeks to match all cells near 100% SOC and low currents so they finish charging together, providing better protection while charging at high SOC but greater variance while discharging at low SOC and high current levels.

Bottom balancing seeks to match all cells near 0% SOC so they finish discharging together, providing better protection while discharging at low SOC and high current levels, but greater variance while charging at high SOC and low current levels.

Basic monitoring should include the use of a Digital Volt Meter (DVM), Amp Hour (AH) Counter, and ideally some form of simple split pack voltage comparator to detect a cell anomaly.

Bottom balance. Charger set to cut off current at 3.65 volts nominal and no cell reaches over 3.8 volts during the charge at low current levels. Elcon 3KW 120 volt charger with 10 different voltage levels. Mine was set up like that from Elcon in Sacramento. I can set my charger to 3.5 volts per cell nominal if needed.
Controller: Synkromotive is set to 2.4 volts cut back and 1.8 volts sag shut off under current load. Will be set higher with higher voltage pack.

Volt meter, Amp meter and will add in a JLD AH meter and considering the split pack comparator. I believe in monitoring the pack but not needed on a single cell daily basis.

Pete

 

[EVfun]

I'd say top balance and learn when to call a tow truck.

Top balancing seeks to match all cells near 100% SOC so they finish charging together when your not there to watch them, providing better protection while charging at high SOC but greater variance while discharging at low SOC when your right foot is in control.

 

[onegreenev]

I say bottom balance and know your batteries will be fine when you need to call a tow truck. It will happen.

 

[EVfun]

I say top balance and know your batteries will be fine if the charger timer fails (you know -- when you are not there to notice.)

 

[onegreenev]

Well poop, it could happen. Less likely to happen vs driving your car that last bit. You know, when you let your teenager loose with your car. It will happen.

I trust my Elcon. I trust my Controller.