How to Build a Safer Battery - Looking to Collect Knowledge

[Braddudya]

Been reading all that I can here. I want to build a 52V battery for BBSHD emtb conversion. I have assembled packs before using 60 LG cells for a mobile power station. This was before I could find affordable spot welding equipment so it was the "work as fast as possible with a very hot soldering iron" technique. This time I want to do it right and make it as safe as possible. I live in Reno and my biggest concern is an accident while out on a trail. We have some significant fire risk around here.

What I have decided so far:
1. 18650 lithium ion cells
2. Will carry a fire extinguisher on bike (not so much for putting out the battery, but more for putting out anything around the bike in case of thermal runaway)
3. Will charge outside or at my warehouse.

What I am considering:
1. Single rows of cells in a triangular shape within my frame
A. This should help reduce a chain reaction or at the very least slow it down by reducing density without sacrificing space efficiency.
2. 3D printed custom battery case. (I have 6 printers which I run daily)
A. Allows for a perfect fit within my frame and allows for the minimal density design.
3. Salvaged lightly used name brand cells
A. Looking at options from Jag37 and batteryhookup
B. I will do whatever is needed to test for good condition cells.

What I need is advice on:

1. How is my plan so far?
2. What are the best ways to up the safety factor of such a battery?
3. Are there any must watch or must read resources I should be focusing on?

Once I get the safety features planned out I will be looking into the optimum BMS so any advice there would be hugely appreciated as well.

 

[SafeDiscDancing]

Pouch type cells don't have the bottleneck problems that the cylindrical cells do so they are (in my opinion) better just by the design itself.

You know Tesla has a new battery design now where they correct the bottleneck problem by having a serrated lip along one side that spreads the current around making it more like the pouch cells while retaining the light weight of cylindrical.

Pouch also needs an external way of applying pressure to prevent them from "puffing".

But if I were to "guess" the cause of most battery failures I'd say an excessive trust in an automated BMS.

The joke is BMS means "Battery Murdering System".

What I do is not typical but I design my battery so I can actually access each cell group in the series.

Then I do a "bottom balance" on the pack.

Next I identify the weakest cell in the pack.

Finally I manually (fast) charge while watching the weakest cell and stop when it hits maximum voltage.

So in my opinion "maximum safety" is when you have "maximum awareness".

Few people are as hardcore about it as I am I guess.

--------------------

And let me add I used to have a "charge and pray" attitude and that caused me to wreck a lot of cells because I would "top balance" them and not be too sure when the weakest cell was hitting bottom during discharge.

The biggest advantage with "bottom balancing" is when you hit bottom the voltage radically drops.

There is no possible way to not notice the end of charge when you "bottom balance".

 

[Braddudya]

That is quite interesting and makes sense to me. Basically it is focusing on the stress applied to the weakest link in a chain versus the strongest or the average. Correct?

If you have any more specific information on how this is done I would love to see if. Again I am not new to batteries or electronics but I would say I am new to powerful pack design. My first attempt I would never do again today and it was never completed to the actual use stage.

 

[SafeDiscDancing]

That is quite interesting and makes sense to me. Basically it is focusing on the stress applied to the weakest link in a chain versus the strongest or the average. Correct?

Yeah that's the idea. Be aware of how your pack is behaving in every way. My pouch cells are the SPIM08hp variety and they need to get warmed up before they work to full potential.

But let others offer their advice.

I like to do this as a hobby and I like to get into all this stuff because I enjoy it.

Others will offer their ways.

Then think about what people say and decide what best suits your habits.

 

[calab]

You make some good points you are right you have to be at one with your ebike and your battery, be at one with the vibrations and the sounds, its the ying and the yang

Yeah that's the idea. Be aware of how your pack is behaving in every way. My pouch cells are the SPIM08hp variety and they need to get warmed up before they work to full potential.

But let others offer their advice.

I like to do this as a hobby and I like to get into all this stuff because I enjoy it.

Others will offer their ways.

Then think about what people say and decide what best suits your habits.

 

[SafeDiscDancing]

While we wait for others to offer their ideas I want to bring up the whole philosophy of the BMS.

Assume your pack holds this sequence of cells:

10.01 ah
10.03 ah
---> 9.98 ah the weak cell.
10.02 ah
10.04 ah

What can a BMS do for you?

Well... it can shut down if the weak cell either goes too high or too low. That makes sense.

But then you get to balancing... this is where it gets goofy.

If you charge up the pack and the BMS balances as a "top balancer" then as the pack drains it MUST correctly identify the weak cell hitting bottom or the BMS has failed it's job.

However, some BMS try to balance any time there is a difference between cells.

Well wouldn't that condition occur EVERY TIME?

Every time the BMS would try to "save" that weak cell and heroically shuffle energy to it.

Okay now what happens? Eventually you charge again.

Now the weak cell is already OVERCHARGED relative to the "top balance" targets so now the BMS heroically tries to shuffle energy back out of the weak cell to prevent overcharge.

See how that's all nonsense?

In objective reality your overall pack (above) has a 9.98 ah capacity and you might as well just live with that through "bottom balancing".

And if you are like me and do short 20 minute rides with high C rates over and over you just want repeatable performance each time.

The BMS tends to get confused and that's why they often have a bad reputation.

 

[Braddudya]

Very interesting. So are there advocates against using a BMS at all? Or are you suggesting some other type of battery management board to achieve the the safest balancing? Manual balancing? Just trying to wrap my head around the suggestion.

A bit more about myself and my use case: I am incredibly anal about not draining anything lithium to cut off. I buy phones with the biggest battery I can get and charge them every night. I would have a similar philosophy with the ebike. Most of my rides would be incredibly short (commuting to work) about 3.5miles round trip (I do live on the side of a mountain though so a killer incline coming home). For fun I would like to do some 5-15 mile trails around here. I am assuming with non aggressive riding a 12-15ah (52v) should do the trick without risking running it too low. Not sure how relevant this information is but it might help for understanding my use case and design needs.

Edit: just read some other forum posts about bottom balancing and I was definitely misunderstanding the strategy. I have the concept but the actual technique is way different than I was imagining. I like the idea and will keep reading into. Probably need to watch a video of someone doing it before I will fully grasp it. How do you drain the cells to a specific voltage? Would imagine it's incredibly time consuming and would require a discharger tool. I do have an RC charger that does discharging I believe. I'll have to look into that more as well.

 

[E-HP]

I am assuming with non aggressive riding a 12-15ah (52v) should do the trick without risking running it too low.

I think the Luna Wolf Pack would be the perfect choice. https://lunacycle.com/luna-wolf-v2-52v-battery-pack/

 

[SafeDiscDancing]

I buy phones with the biggest battery I can get and charge them every night. I would have a similar philosophy with the ebike.

That's another area.

Lithium based batteries HATE both the high and the low voltage.

What? You mean I'm hurting the battery keeping it fully charged? Yes.

Wow... Right?

So my little "routine" on my ebike is to watch my "Watt Hour" counter which tells me how much energy I have used. I always park the bike about half way down from full since I know my full capacity.

To start my day I usually take a quick warm up ride to get near empty then I charge to full.

This warms up the pouch cells I use.

Then I start my full battery runs which are at (50 amp) 5C+ discharge rates and (38 amp) 4C charge rates.

...when I'm done for the day I again leave a half empty battery.

-----------------------

In my experience all major wear occurs at the extremes of high and low. Avoid that and you can repeat forever some seriously impressive performance. But a single drop into the low voltage area below 2.5 volts and you permanently begin doing damage. Above the 4.2 volt level is bad, but it seems more tolerant of that.

 

[SafeDiscDancing]

...some other forum posts about bottom balancing and I was definitely misunderstanding the strategy. I have the concept but the actual technique is way different than I was imagining. I like the idea and will keep reading into. Probably need to watch a video of someone doing it before I will fully grasp it. How do you drain the cells to a specific voltage?

I have XT-60 plugs for each cell since mine is a 12S with pouch cells.

Discharge:

Four 0.1 ohm 100 watt resistors in Series.

Charge:

A USB charger at 5 volts and 2 amps which I "hacked" together from an old electric device.

--------------------

The first time you balance can take some time, but eventually you find the right place and it's surprising how consistent the cells behave over time.

Once you get it right is stays there after many runs.

You might need a checkup every month or two.

--------------------

https://endless-sphere.com/forums/viewtopic.php?f=14&t=114594

That's another direction I went into to reduce the number of connectors needed.

Rather than 12 of the XT-60 connectors I use a Rotary Switch.

But that's limited to 11S battery packs unless you find a Rotary Switch that can do 12S.

 

[JackFlorey]

Very interesting. So are there advocates against using a BMS at all?

Under any normal use case with li-ion batteries you HAVE to have a BMS. And it has to do several things:
1) Monitor cell voltage and cut off charging when any cell hits its upper voltage limit (usually 4.2 volts)
2) Monitor cell voltage and cut off discharging when any cell hits its lower voltage limit (usually 2.7 volts)
3) Monitor cell current and cut off discharging when the pack sees too much discharge current (depends on the cells used, usually >5C.)

A BMS may also do some other things:
1) Monitor cell current and cut off charging when the pack sees too much charging current. Less important since this is an unusual failure.
2) Report on cell condition. Many BMSes allow you to measure voltages on a per-cell basis. A few will even give you things like a charge accumulator (so you know how much is in the battery) and ESR (calculated by looking at voltage drop vs current.)
3) Balance. There are a lot of ways to balance:
-a) Top balance. This is the most common. When the cell exceeds (say) 4.15 volts a load is placed on it. Generally the load is between C/100 and C/20. This tends to bring that cell's voltage down and allows the other cells to "catch up."
-b) Full time flying capacitor balancing, where a "flying cap" moves from segment to segment, charging and discharging as needed to maintain an average voltage.
-c) Full time active boost converter. This is a boost converter (one per parallel cell bank) that is constantly moving charge from the highest voltage cell back to the pack.

Both b) and c) above have the advantage of being relatively lossless (i.e. does not heat up the pack and retains more charge for you to use.)

 

[SafeDiscDancing]

Under any normal use case with li-ion batteries you HAVE to have a BMS.

It's good you offer the "other" viewpoint of buying the top end BMS (like the ANT from what I read) and turning over control to the robots.

Artificial Intelligence can equal everything a human can do.

Heck, they might even own the planet eventually.

------------------

I think we can agree a low priced junk BMS is worse than doing nothing.

BatteryHookup seems to exist as a supplier of batteries with failed BMS projects. (there have been so many)

 

[Braddudya]

Thanks for the further information.

I have to say I am fairly convinced about the bottom balancing and no traditional BMS route. Some of those features I will implement in another way (pack low voltage cutoff, high voltage cut off, and monitoring). To me the main downfall of the BMS seems to be that it is an attempt to replace the human for battery maintenance. That's great as long as it works. If it doesn't the human won't know and things will go badly. The BMS tries to squeeze every joule out of a pack by compensating for battery variance and in doing so introduces a lot of additional complexity and fail points. With bottom balancing you just compensate for battery variation by building in an actual margin of safety. It seems like more effort but also seems easier on your pack. I am of course NO EXPERT but this is my thinking after reading up more on the topic.

My current plan is to use bottom balancing as suggested. I also am considering upping the battery capacity so that I can set my charger to 57.4v max (4.1v per cell) and still get great range. Then for my daily commute I can turn it down even further since it's such a short round trip.

This seems ideal for me because it isn't excessive amount of effort, should give great cycle life, and should have a great margin of safety.

Now I am looking for the best way to monitor the balance. I would love to get real time information on the balance just to feel confident in it staying even. It would be amazing if this could be done through the ebike display. Is this a thing? I don't see much that supports 14s packs or really even more than 8s.

I am also trying to figure out the charger. It seems all that I need is CCCV capabilities and the correct final voltage. I have a CCCV Variable power supply controller (just the controller but not a high enough voltage supply) but not sure I trust it haha and I'd hate to have to manually set it each time I go to charge.

So far everyone's advice has been immensely helpful so please keep it coming! If anyone really thinks I need a BMS I would greatly appreciate those arguments as well. This is a wonderful community.

 

[SafeDiscDancing]

I am also trying to figure out the charger. It seems all that I need is CCCV capabilities and the correct final voltage. I have a CCCV Variable power supply controller (just the controller but not a high enough voltage supply) but not sure I trust it haha and I'd hate to have to manually set it each time I go to charge.

My case is unusual.

I'm going after all out performance along every dimension so I'm doing hard charging with a fast high C rate and hard discharging as well.

And for me the charging occurs so fast that in 20 minutes I'm up and running again.

My routine is 20 minutes ride, then 20 minutes of charge over and over until exhausted.

The biggest exercise day was I think seven rides in a day. Often the charging times allow a rest break.

People doing slow charging on the 18650 cells are NOT going to sit around for an hour or two or three.

----------------

Charging is a whole other thing because you need to watch four parameters at once:

Voltage
Current
Watts
Watt Hours

My 120v AC line in the garage maxes out at 1500 watts so I have to be careful not to pop the breaker switch.

Generally the Watt Hours give me a predictable knowledge of progress.

Current is limited to 38 amps based on the power supply design not me.

You watch the Voltage as it rises towards it's desired maximum, but I also watch the weak cell with a manual digital voltage meter that reads from 2.5 volts to 30 volts. They cost about a buck or two on eBay. It has a XT-60 plug that I added.

https://www.ebay.com/itm/2-Wires-Mini-DC-2-5-30V-Voltmeter-LED-Panel-3-Digital-Display-Voltage-Meter-hm/303979822563?hash=item46c69bfde3:g:OLsAAOSwe9VcJbgq&var=603143572554&pageci=8f46899a-4cdf-4d92-98b1-c10a66005aeb&redirect=mobile

(note... there is a way to "tune" these on the back. Use a real voltmeter to set them right first)

My overall scheme is like LEGO blocks... many things can plug into different places.

 

[JackFlorey]

I have to say I am fairly convinced about the bottom balancing and no traditional BMS route. Some of those features I will implement in another way (pack low voltage cutoff, high voltage cut off, and monitoring).

If you have all that done automatically you have a BMS.

To me the main downfall of the BMS seems to be that it is an attempt to replace the human for battery maintenance.

Most don't do that. At best they will top balance, but 99% of their job is to shut down the battery when it's being mistreated. This is not only important to protect the battery, but to protect you from fire.

The BMS tries to squeeze every joule out of a pack by compensating for battery variance and in doing so introduces a lot of additional complexity and fail points.

Most ebike BMSes do not do this.

With bottom balancing you just compensate for battery variation by building in an actual margin of safety.

You can top balance or bottom balance; doesn't matter too much. You are still limited by the weakest cell. And of course you MUST monitor cells to protect them at both ends.

Now I am looking for the best way to monitor the balance. I would love to get real time information on the balance just to feel confident in it staying even. It would be amazing if this could be done through the ebike display. Is this a thing? I don't see much that supports 14s packs or really even more than 8s.

There are several BMSes that will show you the voltage at every cell. If you go this route be VERY VERY sure that the BMS draws the same current from each cell. If it does not (as in some cheap BMSes) it will gradually unbalance the pack.

I am also trying to figure out the charger. It seems all that I need is CCCV capabilities and the correct final voltage. I have a CCCV Variable power supply controller (just the controller but not a high enough voltage supply) but not sure I trust it haha and I'd hate to have to manually set it each time I go to charge.

You can use a CCCV supply (like a Meanwell) to do quick charges - but for long term life and safety I strongly recommend an actual charger that will terminate charge. Your batteries will last longer and be safer overall.

 

[SafeDiscDancing]

THE BOTTOM LINE

If you want things to go well with a battery you either:

------------------------

1) Know what you are doing and get involved with watching everything when you use it.

2) Trust a BMS that has a high reputation.

------------------------

Like in life itself.... some prefer to not know, but trust while others like knowing.

Ask yourself what type of person you are and what type of performance you anticipate having.

Think of a gasoline powered racing car and think "would I check the spark plugs?"

If your answer is "yes" then choose 1) but if your answer is "no" then pick 2).

Many race drivers defer most all technical issues to their mechanics and simply drive the car.

----------------------

One last thing to remember is a battery tends to repeat nearly the identical behavior for at least a thousand cycles these days.

So if you do things on your own it's mostly getting the setup right and knowing your start and stop points.

Very little wandering around happens with a battery used intelligently.

 

[Braddudya]

If you have all that done automatically you have a BMS.

I was talking mostly about pack overall voltage vs the parallel cells voltage. Maybe BMS is a more broad term than I am realizing.

Most don't do that. At best they will top balance, but 99% of their job is to shut down the battery when it's being mistreated. This is not only important to protect the battery, but to protect you from fire.

There are several BMSes that will show you the voltage at every cell. If you go this route be VERY VERY sure that the BMS draws the same current from each cell. If it does not (as in some cheap BMSes) it will gradually unbalance the pack.

If there are BMS options that dont do any balancing then I am interested in that! I assumed that the balance function was built into all of them. Cell level monitoring and shutoff would be great to have. If it can display the balance levels that is even better! Do you have any recommendations for a BMS like this?

You can use a CCCV supply (like a Meanwell) to do quick charges - but for long term life and safety I strongly recommend an actual charger that will terminate charge. Your batteries will last longer and be safer overall.

Do you have any recommendations for a real charger? I am just looking for somewhere to start the search. I google ebike charger and it seems like everything is either cheap chinese stuff or ill stumble upon a $700 charger that seems quite overkill.

Advice much appreciated!

 

[SafeDiscDancing]

Advice much appreciated!

Just a random search on eBay found a $15 BMS that has no balance feature listed:

-------------------------

10S LI-ION BATTERIES. The protection board is for 10 series cell Li-ion batteries, it can be used for ternary batteries, manganese acid batteries and cobalt acid batteries
35A DISCHARGE CURRENT. Charging connector and discharging connector are separated, discharging current is 35A
STABLE PROTECTIVE FUNCTIONS. Multiple protective functions including overcharge protection, overdischarge protection and short circuit protection
Color: Silver + Green
Material: metal + PCB
PROLONG THE SERVICE TIME. Help prolong the service time of your battery pack
Discharge Continuous Discharge Current 35A
Momentary Discharge Current 100A
Charge Charge Voltage 42V
Charge Current 8A(Max.)
Overcharge Protection Overcharge Detection Voltage 4.28±0.05V
Overcharge Protection Delay 100mS
Overcharge Cancel Voltage 4.08±0.05V
Over-discharge Protection Over-discharge Detection Voltage 2.55±0.08V
Over-discharge Detection Delay 50mS
Over-discharge Cancel Voltage 3.0±0.1V
Overcurrent Protection Overcurrent Detection Voltage 150mV
Overcurrent Detection Delay 15mS
Overcurrent Protection Current 120±5A
Protection Cancel Condition Disconnect Load
Short Circuit Protection Short Circuit Protection Condition Outside Load Short Circuit
Short Circuit Detection Delay 200uS
Protection Cancel Condition Disconnect Load Internal Resistance
Main Circuit Conduction Internal Resistance ≤15mΩ
Self Power Consumption Working Current ≤50uA
Sleeping Current(Battery Over-discharge) ≤10uA
Operating Temperature -30℃~+80℃
Size 69*51.5*10mm/2.71*2.02*0.39
Package Contents: 1pc x 10S 35A Lithium Battery BMS PCB

---------------------

But this represents a "partial solution" because you need to start off with cells already balanced.

See how this is being set up?

If you add the balancing into the BMS you can save labor costs necessary to balance a pack. Just add the balancing feature and "in theory" it fixes any big disparities.

Everything is driven by profit margins.

Having to carefully balance a pack is effort which is profits lost.

And the key phrase you are seeking is "protection board".

 

[JackFlorey]

If there are BMS options that dont do any balancing then I am interested in that! I assumed that the balance function was built into all of them. Cell level monitoring and shutoff would be great to have. If it can display the balance levels that is even better! Do you have any recommendations for a BMS like this?

Is this for a 14S li-ion battery? (I assume so since you said 52V)

None will have cell-level shutoff. That doesn't really make any sense. Almost all of them have pack level shutoff.

Do you have any recommendations for a real charger? I am just looking for somewhere to start the search. I google ebike charger and it seems like everything is either cheap chinese stuff or ill stumble upon a $700 charger that seems quite overkill.

Sure, a Satiator. It's by Justin of Grin Technologies and it costs about $340. It will do everything you could ever want a charger to do.

 

[SafeDiscDancing]

None will have cell-level shutoff. That doesn't really make any sense. Almost all of them have pack level shutoff.

A good time to bring up a point which many new to electrical theory do not know.

The battery has cells in Series and the most basic truth of an electric circuit is that "Current is the Same" all the way around.

This means you cannot simply "turn off" a cell unless you had some way to physically disconnect it from the path of the Series string.

Long ago there were experiments into systems that actually did regulate the cells on an individual basis but they disappeared because it was not cost effective.

With the original brushed controller it only must regulate One pulse width modulation (PWM) in a brushed motor and this was why the early systems were brushed and really simple.

Later the brushless controllers and motors arrived and now you have Three phases to use PWM.

Then it got worse (or better) in that the latest controllers carefully bend the PWM to simulate a sinusoidal curve best matching the motor electrical requirements.

So what happened is that once "crazy" idea of single cell PWM is now seen as ten times crazier.

The technology went in a direction where that potential is long gone.

What we have is a simple decision point during discharge and if you "Top Balance" that decision point arrives without your knowledge because the weak cell starts dying while all the others are raging ahead without it.

That's why the BMS must "save" that hidden weak cell and shut down the pack.

"Bottom Balance" and the pack voltage drop as it nears the bottom is so strong a signal you "just know" by feel it's time to get home and recharge.

Trust must be absolute in the BMS because if it fails to warn you and stop the pack it will kill the weak cell and possibly cause a fire.

 

[calab]

Some try to put a fuse at every cell.

 

[SafeDiscDancing]

Some try to put a fuse at every cell.

Tesla did that in their packs of 18650's. (or was that the larger cells?)

Yes... the cell that starts to fail will begin filling and draining at faster and faster rates until it starts to threaten a fire.

What the Tesla engineers carefully figured out was the point where the wire would burn up and cause it to disconnect before the cell went into thermal runaway.

It's done with machine level precision and seemed to have been okay.

That was "state of the art" a few years ago.

 

[Braddudya]

To clarify when I was mentioning cell level shutdown I meant shutting down the pack via monitoring parallel cell set voltages. I do know that cells are not individually shut down by the BMS. Still tons of useful information though! Now I'm unsure of what to do but I know more! Haha

 

[docw009]

Questions for the DIY battery builder

-Is it worth running LiFePO4?
-Can a DIY battery have individual cell fusing?
-Is spot welding unsafe?
-Is it worth the weight penalty to size a battery so it can be used in the 80%-60% SOC range for longevity? How about 90%-60%?
-Where I can get 'lightly used" cells that are trustworthy. How do know they weren't drained to zero and recovered?
-How about salvage cells that had their spot welds stripped and the ends repolished. Is the metal thinner and susceptible to a weld punching thru? Will they rust?
-Speaking of rust, what about condensation?

 

[Chalo]

Questions for the DIY battery builder

-Is it worth running LiFePO4?

You're weighing different values against each other. Longevity? Performance? Weight? These things trade off differently for different applications and different desires.