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High current BMS 300A plus

GSXR1000

GSXR1000

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One of the forum power users has said numerous times that the cheap Chinese BMS need to derated 50% off of the marketing maximum amps.
 
get whatever BMS you like the features of, and run a contactor.
FETs are gonna get hot AKA waste energy at those kinds of currents. Personally anything that you intend on frequently using over 100A continuous i would go for a contactor
 
Those BMSs are probably ok but there is no way to monitor what is happening. No bluetooth, no connector for external display.
I really like to see what is going on with each cell so I bought a smart BMS with bluetooth

edit: I mean they are ok at 50% per the other posts
 
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HRTKD said:
One of the forum power users has said numerous times that the cheap Chinese BMS need to derated 50% off of the marketing maximum amps.
Click to expand...
LMAO, "One Of" includes MANY including Will.... Ratings provided by the BMS companies are optimistic and edge case values.
Then FET based BMS makes Heat and inside a Battery Box can also result in assorted issues.

For 4S packs you will be hard pressed to find a High Amp FET Based BMS. Heltec only has TWO Smart 4S BMS'. They have 8S+ which can also do 4S which are higher Amp capacity but they have to be checked to verify which is suitable.
 
Daly make a 500a smart BMS. Heat sink looks huge with a fan. I was considering purchasing one .

Contactor seems like a good solution also, perhaps better.

I’d be keen to understand the pros/cons of a conactor vs a 100% overrated FET with a good heat sink from some of the more experienced diy crew on the forum .
 
sid the seagull said:
I’d be keen to understand the pros/cons of a conactor vs a 100% overrated FET with a good heat sink from some of the more experienced diy crew on the forum .
Click to expand...

In principle you can switch pretty much any currents with FETs (there's plenty of FETs switching kA currents in the industrial world...). The main problem comes from the fact they use cheap FETs and so they have a high power dissipation at high current. They also use a very poor thermal design, hence the high derating needed.

To give you an idea of the price a real 300 A continuous (400 A peak) FET based bidirectional switch will cost around 100-150 USD for the full BoM (losses in my example would be: 3.8 W @ 100 A, 15 W @ 200 A, 34 W @ 300 A).

Con of the contactor is the high power consumption of the coil (unless you use a latching one), basically about 5 to 15 W constantly.

Con of the FETs is higher cost at higher current while a contactor will be only a bit more expensive for higher currents (at least to make, not talking about the MSRP...).
 
BiduleOhm said:
In principle you can switch pretty much any currents with FETs (there's plenty of FETs switching kA currents in the industrial world...). The main problem comes from the fact they use cheap FETs and so they have a high power dissipation at high current. They also use a very poor thermal design, hence the high derating needed.

To give you an idea of the price a real 300 A continuous (400 A peak) FET based bidirectional switch will cost around 100-150 USD for the full BoM (losses in my example would be: 3.8 W @ 100 A, 15 W @ 200 A, 34 W @ 300 A).

Con of the contactor is the high power consumption of the coil (unless you use a latching one), basically about 5 to 15 W constantly.

Con of the FETs is higher cost at higher current while a contactor will be only a bit more expensive for higher currents (at least to make, not talking about the MSRP...).
Click to expand...
Thank you, very insightful.

OP was planning 300a , similar to myself so. The 500a Daly is probably on target for this.

300a would be an upper limit for me, however current draw may sit at level for a while as I’m using 12v high power inverter.

Con or downside to the Daly seems that it has a very primitive and App running that is a bit of a lucky dip for certain critical features.

personally I like the JBD/overfkill however it’s not available beyond 120a from what I can tell. Such a shame. internally it’s probable similar or better than the Daly , just without the massive heatsink of the Daly.
 
BiduleOhm said:
In principle you can switch pretty much any currents with FETs (there's plenty of FETs switching kA currents in the industrial world...). The main problem comes from the fact they use cheap FETs and so they have a high power dissipation at high current. They also use a very poor thermal design, hence the high derating needed.

To give you an idea of the price a real 300 A continuous (400 A peak) FET based bidirectional switch will cost around 100-150 USD for the full BoM (losses in my example would be: 3.8 W @ 100 A, 15 W @ 200 A, 34 W @ 300 A).

Con of the contactor is the high power consumption of the coil (unless you use a latching one), basically about 5 to 15 W constantly.

Con of the FETs is higher cost at higher current while a contactor will be only a bit more expensive for higher currents (at least to make, not talking about the MSRP...).
Click to expand...
In addition to this, FET based the losses not only increase with current, they also increase with temperature. so if it has a crummy cooling solution, it will lose even more.

A contactors power use is constant. A quality contactor with an economiser (or you could DIY it too) will use less than 5w
 
Aaron_85 said:
In addition to this, FET based the losses not only increase with current, they also increase with temperature. so if it has a crummy cooling solution, it will lose even more.
Click to expand...

Yep, the figures I quoted are at Tj = 120 °C so already taking into account that fact ;)
 
Aaron_85 said:
In addition to this, FET based the losses not only increase with current, they also increase with temperature. so if it has a crummy cooling solution, it will lose even more.

A contactors power use is constant. A quality contactor with an economiser (or you could DIY it too) will use less than 5w
Click to expand...
Aaron_85 said:
In addition to this, FET based the losses not only increase with current, they also increase with temperature. so if it has a crummy cooling solution, it will lose even more.

A contactors power use is constant. A quality contactor with an economiser (or you could DIY it too) will use less than 5w
Click to expand...

I guess a contactor such as this would do the job.

Dc Contactor,Power relays,shunts,circuit breaker - China Dongya Electronic404 Not Found

Dongya DC contactor manufacturer specializes in the production, research and development, and sales of dc rated contactors, High Voltage DC Contactor,shunt and hydraulic magnetic circuit breaker.
en.cndongya.com en.cndongya.com

they seem to be normally open and use 8w. When closed , which is most of the time . I guess it would be possible to add an isolator in the circuit so that the battery is cut out when not in use. Perhaps a bit of a PITA in a first world kinda way. I guess this is not a bad solution and I gather the contractors would make a pretty nice clunk sound every time they operate.

I like the idea of the contactor , the 8w drain is a pretty major downside from what I can see. If the battery’s are left unattended for any length of time that 8w continuous drain can’t be good.
 
Well not ALL contactor are equal either.
You have the standard ones, some of which can be Real Power Hogs.
You have Every Savers which are considerably more energy efficient.
Then you cross into Solids State territory, even more energy efficient but also with some tradeoffs depending on type and quality of components within.

TE Connectivity Kilovac EV200AAANA Contactor 500A 12~24VDC contactors
SPECS DOC LINK: https://www.te.com/commerce/Documen...200_Ser_Contactors&DocType=CS&DocLang=English

Dongya DH (Unidirectional) & DHV series (Bi-Directional)
High Voltage DC Contactor,High Voltage Contactor,DHV dc contactor - China Dongya Electronic (cndongya.com)
* You have to request DVH Bi-Directional.
DATASHEET PDF: https://diysolarforum.com/attachments/dhv200-engery-saving-datasheet-2233-pdf.13061/

These Biig SSR's came out as a result of Craig & I's Relay, Contactor SSR testing a while back. At that time no one had Large Lugs SSRS to actually handle real battery cables. These guys took on the challenge and created this Shown below is the 500A versions I received. After these were made up, and Jason from Chargery had communicated with that Company, he realized it would be prudent to develop the DCC inhouse which is how they came to be in existence. NB: These are Unidirectional Versions suitable ONLY for Split Port config. Bi-Directionals are available but +$$$ and are special order, which would have to goto Manufacturing to be made on req. Both of these versions were made up specifically at my request and my additional expenses. They are now available for everyone

The Real Deal 500A (1/2" Lugs)
500a-ssr-face-jpg.12566


500a-ssr-profile-jpg.12567



IF you are Curious:
Big Lug 500A SSR : https://www.alibaba.com/product-detail/DC-control-DC-solid-state-relay_62549649837.html?spm=a2747.manage.0.0.3d1571d2AmThhr&bypass=true

Big Lug 1000A SSR : https://www.alibaba.com/product-detail/DC-control-DC-solid-state-relay_62549285366.html?spm=a2700.icbuShop.41413.11.72317242imd258&fullFirstScreen=true&bypass=true
 
On MOSFET switches, I like to try and get the part numbers used for the MOSFET's used for the series cutout switch and look up their Rds_ON resistance spec. 90+% of time the ad picture is either not high enough resolution to read part number or they purposely blacked out part number but sometimes you can search around various sites for that model BMS and find a picture you can enlarge and read their part number. Sometimes the BMS PCB picture is not even the model you have selected.

There will be multiple parallel, end to end pairs of MOSFET's. Bi-directional switch requires two end to end devices in series so each end to end pair has twice the series resistance of individual MOSFET. Higher amperage BMS often have parallel pairs on top and bottom side of PCB. I have seen as high as 14 pairs in parallel with 7 on top side and 7 on bottom side of PCB (total 28 power MOSFET's). There may be a single extra MOSFET on the end of the pairs string. It is for a charging path when BMS switch cuts out for low cell voltage.

Heat sinks and fans can be good and bad. It is good to keep things cooler but because the MOSFET's are significant component expense, having a heat sink and fan allows the manufacturer to skimp more on their net series resistance. I like to keep series MOSFET net resistance heating to less then 5 to 8 watts at maximum sustained current I want to be capable of. Most of the BMS's sold will have over 20 watts heating at their stated maximum current rating. This is why everyone says derate stated max current by half.

For example, if the MOSFET part number spec for typical Rds_ON is 2 milliohms, each back to back series connected pair has 4 milliohms. If there are a total of 10 parallel pairs then the net cutout switch resistance is 0.4 milliohms. At 100 amps they would have I^2 * R watts of heating = 100A squared x 0.0004 ohms = 4 watts of heating., There will likely be another 10-15% heating for PCB/busbar resistance within BMS. This example would likely have a BMS max current stated as 250 amps which would be 25 watts of heating and get very hot if that current was sustained very long.

Another thing to check is particular MOSFET Vds breakdown voltage spec. A 48v system should have breakdown voltage greater then 75v. A 24v system greater then 40v breakdown voltage. For same Rds_ON, a higher breakdown voltage MOSFET costs more then lower breakdown voltage part. I have seen 8s BMS's built with 25v breakdown MOSFET's. Breakdown voltage is only important when cutout switch opens so you may never know it is marginal until you need it. This is also why you need to be careful buying 12v self contained BMS LFP batteries Their internal BMS may be built with 20v breakdown MOSFET which is fine if only using for 12v system but if stacking batteries you need to check to see they state what maximum number of batteries are allowed for series stacking. BattleBorn states maximum series batteries stacking of four so they are capable of supporting a 48v system.
 
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Aaron_85 said:
Personally anything that you intend on frequently using over 100A continuous i would go for a contactor
Click to expand...
In addition, FETs and SSRs tend to fail closed. That is another reason to run them at 50 %. I agree with @Aaron_85 and also use a contactor on my large pack. My 12v packs rarely see more than 15 or 20 Amps so a simple BMS with Bluetooth works great there.
 
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I personally would never use a contactor. Besides the continuous coil power they can become very erratic for contact resistance after a few high current opening and closings. For starter motor they are okay but for a BMS you need a solid contact every time. You can mitigate the coil current by backing down the voltage after relay closure but this also reduces holding force which for high DC current with its resultant strong magnetic proximity fields can cause contact to be jared around when DC current surge happens from inverter loading surges.

Similar reason you should not charge your inverter DC input caps with breaker switch closing.
 
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RCinFLA said:
I personally would never use a contactor. Besides the continuous coil power they can become very erratic for contact resistance after a few high current opening and closings.

Similar reason you should not charge your inverter DC input caps with breaker switch closing.
Click to expand...
Each of us have slightly different use cases. My use of Lithium batteries started with an EV conversion where contactors are commonly used. It should also be noted that all the production EV manufacturers use contactors for safety. High quality contactors can survive many thousands of cycles. It all depends on where you are standing. I use a Kilovac which has an energy saver feature. I am on grid so a few Watts per hour is nothing to me to protect a 30 kWh pack. Also the contactor in a storage pack would only be required to open as a last resort not several times a day like in an EV. The expensive contactors like Kilovac do have arc suppression.
I am not disagreeing with the sentiment of @RCinFLA as it applies to less expensive power hogging contactors used often without arc suppression or precharge circuits.
 
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Steve_S said:
Well not ALL contactor are equal either.
You have the standard ones, some of which can be Real Power Hogs.
You have Every Savers which are considerably more energy efficient.
Then you cross into Solids State territory, even more energy efficient but also with some tradeoffs depending on type and quality of components within.

TE Connectivity Kilovac EV200AAANA Contactor 500A 12~24VDC contactors
SPECS DOC LINK: https://www.te.com/commerce/Documen...200_Ser_Contactors&DocType=CS&DocLang=English

Dongya DH (Unidirectional) & DHV series (Bi-Directional)
High Voltage DC Contactor,High Voltage Contactor,DHV dc contactor - China Dongya Electronic (cndongya.com)
* You have to request DVH Bi-Directional.
DATASHEET PDF: https://diysolarforum.com/attachments/dhv200-engery-saving-datasheet-2233-pdf.13061/

These Biig SSR's came out as a result of Craig & I's Relay, Contactor SSR testing a while back. At that time no one had Large Lugs SSRS to actually handle real battery cables. These guys took on the challenge and created this Shown below is the 500A versions I received. After these were made up, and Jason from Chargery had communicated with that Company, he realized it would be prudent to develop the DCC inhouse which is how they came to be in existence. NB: These are Unidirectional Versions suitable ONLY for Split Port config. Bi-Directionals are available but +$$$ and are special order, which would have to goto Manufacturing to be made on req. Both of these versions were made up specifically at my request and my additional expenses. They are now available for everyone

The Real Deal 500A (1/2" Lugs)
500a-ssr-face-jpg.12566


500a-ssr-profile-jpg.12567



IF you are Curious:
Big Lug 500A SSR : https://www.alibaba.com/product-detail/DC-control-DC-solid-state-relay_62549649837.html?spm=a2747.manage.0.0.3d1571d2AmThhr&bypass=true

Big Lug 1000A SSR : https://www.alibaba.com/product-detail/DC-control-DC-solid-state-relay_62549285366.html?spm=a2700.icbuShop.41413.11.72317242imd258&fullFirstScreen=true&bypass=true
Click to expand...

Steve , that’s great info, I think I stumbled across some of that info on this forum previously. I have to admit some of it went over head and I’d need to examine closely to understand. I think what I’m reading is that the SSR’s are probably a better design for for high current 12v compared to an array of FETs.

do theses SSRs need a separate port to operate rather than a common port BMS ?

is split port the same as separate port , some discussion in another post on that I see.

From memory other forum member had concerns regarding that the SSR failing in a closed mode. Definitely a risk . However with 1000A SSR operating at 300-500a peak and normal operation only 10% of that I’d personally say the risk is low. However I have no experience here and by my own admission NFI.

I love the half inch bolts.
 
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The Big Lug SSR's I posted above are Unidirectional, only good for a Separate Port configuration.
Common Port uses the same lines for charge & discharge.
Separate Port has a separate line for Charging Devices and a line for Discharge devices.

EV's like the Chevy Bolt & Volt, Tesla's etc all use Relays & Contactors, most commonly the Panasonic ones (they are dear too). The TE/Kilovacs & Gigavacs can also be found in some EV models. The Panasonic ones can be found used & tested and they are still somewhat dear even then.
 
Steve_S said:
The Big Lug SSR's I posted above are Unidirectional, only good for a Separate Port configuration.
Common Port uses the same lines for charge & discharge.
Separate Port has a separate line for Charging Devices and a line for Discharge devices.

EV's like the Chevy Bolt & Volt, Tesla's etc all use Relays & Contactors, most commonly the Panasonic ones (they are dear too). The TE/Kilovacs & Gigavacs can also be found in some EV models. The Panasonic ones can be found used & tested and they are still somewhat dear even then.
Click to expand...
Steve , I think you’ve solve my challenge and saved me considerable expense.

I was about to purchase 2 x the Daly 500a 12v smart Bms units. However after reading this thread a better outcome for my application will be to purchase two separate port BMS’s and two of those 1000a ssr units .

that way I can use my preferred jbd/overkill style Bms rather than the Daly which by my reading has a very alpha style app.



I’ll be using 70mm2 cable as much as possible and those big 1/2 terminals are perfect. Most other connects I’ve seen are just too small for the currents needed in a 12v setup.

I have 8 eve batteries currently in transport from Amy so the plan is to set up two separate 12v packs that are ergonomically manageable
 
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