1000Ah Huge capacity 100V+ BMS?

Batteries, Chargers, and Battery Management Systems.
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rg12   10 kW

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1000Ah Huge capacity 100V+ BMS?

Post by rg12 » Feb 04 2018 6:15am

Hey Guys,

There is a company that needs some kind of a super battery pack.
I was wondering what would be different than a regular pack.
First of all no BMS that I have ever heard of can balance such a capacity.
also, what about cell chemistry? will LiIon be the right chemistry? maybe LifePo4 since they got those big blocks with screw terminals that are used many times in electric car batteries?
What if a cell is dead? should the pack be in a single huge piece? doesn't seem right, but if split to many modules, how will it work with the BMS? many different BMS's for each pack module?
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jonescg   1 GW

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Re: 1000Ah Huge capacity 100V+ BMS?

Post by jonescg » Feb 04 2018 7:06am

1000 Ah at what? 12 volts? 48?

No BMS should ever need to balance the battery pack, because the battery pack shouldn't drift out of balance. But if it does, it addresses it a little each day until it's back to normal.

For capacities this big, you would probably be looking at large format cells to minimise the part count. You could use ~400 x 18650 cells in parallel if you wanted to, but that's a lot of work.

Any chemistry will work - just make sure it's the right tool for the job. If it's a hot or cold climate, pick a chemistry which performs better in those conditions. An investment this big calls for a thermal management system - adds cost and complexity, but ensures the pack lasts as long as possible.

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Re: 1000Ah Huge capacity 100V+ BMS?

Post by liveforphysics » Feb 04 2018 7:38am

If you have good cells, you will need less than 10-50mA balance currents to manage 1,000Ah. If you have bad cells, no amount of balance current can fix them from making gasses and failing.
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rg12   10 kW

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Re: 1000Ah Huge capacity 100V+ BMS?

Post by rg12 » Feb 04 2018 8:17am

How can a 50mA balance current ever balance such a pack? it can take 3 days to balance a 20Ah that went out of balance...

How do Tesla packs work? I know they use 18650 cells but what if a cell fails? because many times when cells start to fail they take out all the cells connected in parallel and if 400 cells in parallel then thats a disaster...

Is there a smart way to turn it into modules in order to be able to service the pack more easily?
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jonescg   1 GW

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Re: 1000Ah Huge capacity 100V+ BMS?

Post by jonescg » Feb 04 2018 8:57am

By not having cells which drift out of balance to begin with; so select your cells carefully. Plus if you have active balancing coming on at, or near the top of charge, it does a little bit of balancing every day, day after day, charge after charge. It can't fix a big imbalance, but it will help. If you suspect imbalance might be an issue, go for a BMS with a high bleed current, like 500 mA.

Tesla use 18650 cells, and they have up to 84 cells in parallel. If any cell should self-discharge due to a manufacturing intolerance, it will take the rest with it. So they just have really strict QC to ensure every cell is matched before assembling into modules.

They use 16 modules to a battery pack, and each module is 6s, 84p. If you really needed to you can replace a module, but it's a big job and most of the time they simply replace the whole battery and take the dud back to the lab for testing. It's a pretty rare occurrence.

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rg12   10 kW

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Re: 1000Ah Huge capacity 100V+ BMS?

Post by rg12 » Feb 04 2018 11:58am

Do you have any idea how the balancing works in tesla's?
and is there a way to buy 144v bms that balance's at a high current or is it all custom?
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Re: 1000Ah Huge capacity 100V+ BMS?

Post by dustNbone » Feb 04 2018 2:27pm

Does this battery need to be mobile or is it stationary? Is it going to be deep cycled on a regular basis or is it a backup system? How much current needs to go in and out of it?

Large batteries aren't uncommon, there are management systems available for basically every cell chemistry and usage scenario imaginable, but you'll want to tailor it to your specific needs as much as possible, because these things aren't cheap.

Large format cells for sure though, going that big in 18650 sounds like a nightmare, unless maybe you could find a way to do it with complete Tesla modules or something like that that's gone through some kind of serious testing/QC to ensure it will stay in relative balance.

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Re: 1000Ah Huge capacity 100V+ BMS?

Post by fechter » Feb 04 2018 2:45pm

I've seen some big BMS units made for car packs but they are quite expensive. The DIY electric car forum would be one place to search for those.

http://www.diyelectriccar.com/

My experience is LiFePO4 tend to need much higher balancing current than Li-ion.
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Re: 1000Ah Huge capacity 100V+ BMS?

Post by jonyjoe303 » Feb 04 2018 2:50pm

I been researching how to balance my 110ah 4s20p lifepo4, one of the cells always drifts off balance. These were all new cells when I bought them.
So far the bms 60ma balancing wont do the job even when I'm charging the pack at less then 2 amps. I found some balance modules that bleed at 1.4 amps but that would limit your max charge rate to about 5 amps.
These are like 7 dollars each from us suppliers, you need one for each cell. From china they cost 3 dollars each but the shipping can be expensive, for 12 units shipping alone would be 35 dollars. They have these available for lifepo4 and li-ion cells. If you charge at a low rate and the battery isnt too far out of balance these might help.
a a balance module.jpg
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Another balance module that I think I will be getting can balance at 6 amps (peak 10 amps), these are available from electriccarpartscompany, they ain't cheap a 4s module will cost 80 dollars plus 10 dollars shipping. this module instead of bleeding and wasting the energy, actually takes from the high cell and transfers it to the low cells. They are always balancing while charging and discharging, they suppose to keep the voltage difference between cells to less then 10mv. According to the website you can add as many modules as you need for you battery pack. This would work good on a very out of balance pack and higher charge rates.
a balance modules 10 amp.jpg
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all the other balancers , bleed the amps at too low of rate to make any difference.

What if one cell goes bad in your massive battery pack? if you put fuses on every cell, if one goes bad that fuse will blow and take that 1x cell off the pack. If each individual cell was 5 amps, your battery pack will lose 5 amps capacity. The pack will still be fully operational just minus 5 amps. As long as you have good balancing modules it should keep the pack balanced.

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rg12   10 kW

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Re: 1000Ah Huge capacity 100V+ BMS?

Post by rg12 » Feb 04 2018 3:55pm

If I use those modules for balancing, how do I control the rest of the functions that a BMS does like HVC/LVC etc?
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Re: 1000Ah Huge capacity 100V+ BMS?

Post by dustNbone » Feb 04 2018 4:49pm

You can use separate high/low voltage cutoff boards for those functions, and just have them switch relays or contactors on your charge/discharge lines. For something that size I think I'd prefer separating those functions anyway, as you could tailor them to your needs. You're not dealing with high currents on the actual modules, just the contactor/relay has to be big (and expensive)

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Re: 1000Ah Huge capacity 100V+ BMS?

Post by jonyjoe303 » Feb 04 2018 5:07pm

you control with a regular bms, a regular bms is good for that but not good for balancing. The balance modules only aid when the pack is very out of balance or you charging at high rate. It wont interfere with the bms functions.

Also you can turn off charging discharging with a voltage relay. Like this one, just make sure you get one that handles your voltage range. They cost around 6 dollars. If the voltage goes below or above a certain range it will pull the relay. I have this small relay connected to a larger automotive 40 amp relay that can handle higher amps. I use this relay as a deadman switch on my lifepo4 in case the bms fails.

There is a post here in the forum where a bms fail and caused a fire, this relay would have prevented something like that from happening.
a overvoltage relay.jpg
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Re: 1000Ah Huge capacity 100V+ BMS?

Post by Hillhater » Feb 04 2018 6:49pm

1000 Ah, 100 v, is 100kWh. Which is the same capacity as a Tesla car pack and half the size of a tesla commercial Power Pack.
Im not suggesting you use the same approach as Tesla., 1000s of small cells ,..but there are good lessons to learn from them...such as
Lots of cells in parallel will balance themselves.
Good quality, matched capacity, cells need little balancing
Higher voltage cells mean less series groups to manage with a BMS
So a 100v pack of 3.8 - 4 volt lithium needs only a 26 chanel BMS
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rg12   10 kW

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Re: 1000Ah Huge capacity 100V+ BMS?

Post by rg12 » Feb 05 2018 6:31am

I think that if this project will get real I will look into larger capacity cells and make them into a few modules rather than spot weld like 13k cells.
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Re: 1000Ah Huge capacity 100V+ BMS?

Post by flippy » Feb 05 2018 7:41am

tesla uses active power distribution to balance cells. not bleed off type balancing liek discusses above. tesla uses a very expensive setup from texas instruments that allows balancing chips on each set to pump power to any other set. stupendusly expensive to make your own setup that does this.

you can use basically any bms with balancing that is rated for that voltage. you only need to modify the bms to trigger a mosfet instead of a bleed off resistor and direct the bleed off power to a power resistor bank that can bleed off at ~5 amps. the bms is none the wiser and it is a fairlly cheap way to still use a modest bms in order to keep the pack in check. some bms sytems allow for manually enableing (enforcing) balancing. that basically keeps the pack in balcing mode all the time until you turn it off.

i would recommend also tapping the gate signal of the bms mosfet pack and run it to a seriously heavy duty contactor and bypass the mosfets completly and use a contactor to break the pack if the bms senses something out of voltage. you dont want it to trigger on current. just use a regular fuse for that.
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