BSM Sizing Question

[arborist.boi]

Hi Folks,

Hope I have found the right place to ask this question. I am looking at building my first battery but I am a little confused about sizing the BMS. I plan on using a 2000 watt inverter so my understanding is a 100 amp BMS on a 24 volt battery system could handle 2000 watts. But the inverter notes its peak wattage is 4000 watts. So do I instead need 2x 100 amp BMS on a 24volt battery to handle this peak?

Thanks,

Erik

 

[John Frum]

I am looking at building my first battery but I am a little confused about sizing the BMS. I plan on using a 2000 watt inverter so my understanding is a 100 amp BMS on a 24 volt battery system could handle 2000 watts.

2000 ac watts / .85 conversion factor / 24 volts low cutoff = 98.039215686 service amps
98.039215686 service amps / .8 fuse headroom = 122.549019608 fault amps

But the inverter notes its peak wattage is 4000 watts.

If its a high frequency inverter the surge rating will be ~20 milli-seconds or 1 cycle of alternating current and is only significant to the marketing team.
If its a low frequency inverter it might matter.

So do I instead need 2x 100 amp BMS on a 24volt battery to handle this peak?

Depending on how much the BMS manufacturer embellished the continuous rating you might squeak by with one 8s 100 amp bms.
Which BMS and what inverter are we talking about about?
Also what cells?
Devil is in the details.

 

[BentleyJ]

Without additional info on your inverter like a datasheet, a general answer is the best that can be done. Most likely the 4,000W peak you mentioned is a surge rating and is only valid for a few milliseconds. A 100A BMS should be OK in your situation. The BMS probably has its own specifications and may give some data regarding surge capability that you may compare to the inverter surge values.

 

[arborist.boi]

@BentleyJ & @smoothJoey. I was looking at the Overkill solar 24v 8s 100 Amp BMS. Seems to be the only trusted BMS with low temperature protection which I desperatly need up here in Canada. The inverter I was looking at is linked below. Says 2000watt continuous and 4000 watt surge. If you know any other trusted BMS that are have higher amperage than the overkill that could also fix my problem lol.

https://www.amazon.ca/2000Watt-Inverter-Outlets-Control-VOLTWORK/dp/B081YVNZX1?th=1

 

[John Frum]

@BentleyJ & @smoothJoey. I was looking at the Overkill solar 24v 8s 100 Amp BMS. Seems to be the only trusted BMS with low temperature protection which I desperatly need up here in Canada. The inverter I was looking at is linked below. Says 2000watt continuous and 4000 watt surge. If you know any other trusted BMS that are have higher amperage than the overkill that could also fix my problem lol.

https://www.amazon.ca/2000Watt-Inverter-Outlets-Control-VOLTWORK/dp/B081YVNZX1?th=1

We still don't know what cells you are using and that can matter.
Since the low voltage cutoff for that inverter is lower than the minimum voltage for a lfp battery the math changes a bit.
2000 ac watts / .85 conversion factor / 20 volts low cutoff = 117.647058824 service amps
117.647058824 service amps / .8 fuse headroom = 147.058823529 fault amps.

Maybe the lfp version of this https://www.aliexpress.com/item/1005003632587827.html

 

[upnorthandpersonal]

was looking at the Overkill solar 24v 8s 100 Amp BMS. Seems to be the only trusted BMS with low temperature protection which I desperatly need up here in Canada. The inverter I was looking at is linked below.

JK BMS could be an option.

 

[John Frum]

JK BMS could be an option.

Link please.

 

[upnorthandpersonal]

134.21US $ |Jikong 2a Active Balance Bms 150a Canbus Rs485 Bt 36v 48v 60v Li-ion Lto Battery 18650 Battery Lifepo4 Battery Storage - Battery Accessories & Charger Accessories - AliExpress

Smarter Shopping, Better Living! Aliexpress.com

www.aliexpress.com

You want for example the B2A8S20P - 8S configuration, 200A. There are others that would be suitable, check the chart on that page.

 

[arborist.boi]

134.21US $ |Jikong 2a Active Balance Bms 150a Canbus Rs485 Bt 36v 48v 60v Li-ion Lto Battery 18650 Battery Lifepo4 Battery Storage - Battery Accessories & Charger Accessories - AliExpress

Smarter Shopping, Better Living! Aliexpress.com

www.aliexpress.com

You want for example the B2A8S20P - 8S configuration, 200A. There are others that would be suitable, check the chart on that page.

@upnorthandpersonal am I miss reading the link you shared and this has low temperature protection?

 

[arborist.boi]

We still don't know what cells you are using and that can matter.
Since the low voltage cutoff for that inverter is lower than the minimum voltage for a lfp battery the math changes a bit.
2000 ac watts / .85 conversion factor / 20 volts low cutoff = 117.647058824 service amps
117.647058824 service amps / .8 fuse headroom = 147.058823529 fault amps.

Maybe the lfp version of this https://www.aliexpress.com/item/1005003632587827.html

@smoothJoey I haven't figure out the exact cells I am going to buy yet. Still trying to figure out what suppliers are good but I am leaning towards the cells linked below. Seems the forum is generally supportive of this one.

Luyuan 4pcs Eve 304ah Automotive Lifepo4 (lfp) 3.2v Cells Battery (fully Matched&batched)--genuine Grade A - Buy Eve lifepo4 rechargeable Battery Product on Alibaba.com

Luyuan 4pcs Eve 304ah Automotive Lifepo4 (lfp) 3.2v Cells Battery (fully Matched&batched)--genuine Grade A - Buy Eve lifepo4 rechargeable Battery Product on Alibaba.com

www.alibaba.com

 

[upnorthandpersonal]

am I miss reading the link you shared and this has low temperature protection?

Yes. Two temperature probes are included to mount on the batteries. Low temperature protection works perfectly. I've run these JK's for almost two years now (I'm also in a cold climate, this was one of the reasons I got them).

 

[John Frum]

@smoothJoey I haven't figure out the exact cells I am going to buy yet. Still trying to figure out what suppliers are good but I am leaning towards the cells linked below. Seems the forum is generally supportive of this one.

Luyuan 4pcs Eve 304ah Automotive Lifepo4 (lfp) 3.2v Cells Battery (fully Matched&batched)--genuine Grade A - Buy Eve lifepo4 rechargeable Battery Product on Alibaba.com

Luyuan 4pcs Eve 304ah Automotive Lifepo4 (lfp) 3.2v Cells Battery (fully Matched&batched)--genuine Grade A - Buy Eve lifepo4 rechargeable Battery Product on Alibaba.com

www.alibaba.com

Close enough for my purposes.
Most folks these days are going with one of the different flavors of big blue prismatic LFP cells.
Those will be able to power a 2000 watt 24 volt inverter just fine.

 

[rickst29]

When the Inverter is connected, first turned on (before turning on any 120v loads), it may create demand high 12v current while charging up its internal capacitors. This current can exceed the maximum allowed by your BMS. A "pre-charge" 12v with a moderate size (e.g. 10-25 ohms) but high current/high power resistor (e.g. 100 watts) can be used to control the maximum current which occurs during this in-rush process.

I would never plan on using a "2000 Watt continuous" Inverter with an actual 120-VAC load of 2000 Watts, and some 120v devices (with motors) will want to pull "peak" loads instead. You haven't yet described your downstream 120-VAC load devices, but I'll go ahead and offer a SWAG in agreement with SmoothJoey (rather than Bentley):

If you've already determined that an Inverter rated at "2000 Watts" will be appropriate (and a "1500 watt" Inverter would likely be inadequate), then your minimum BMS size must be 120A continuous - and I'd recommend BMS rated for 200A instead (maybe even 250A). I'm in total agreement with 'SmoothJoey' and 'UpNorthAndPersonal' on this size, which is significant;y LESS than 1C for the 304ah batteries linked above. (BMS on my own 2x12V parallel battery string is configured to allow almost 1C current, though I never approach that amount of load.)

BMS "over-current" should never be invoked by your configuration - your main fuse should be a smaller limiting factor. In desigining for an overcurrent situation:

1. The load really shouldn't blow anything. Then:
2. Blow the fuse first.
3. invoke BMS shutdown next.
4. overrun the battery cells or overheat the supply wires LAST. (Never reach either of these limits).

Your original question seems to ask how to size the BMS according to the load. The BMS limiter should be significantly BIGGER than the maximum load, and also bigger than the limiting fuse.

 

[Horsefly]

134.21US $ |Jikong 2a Active Balance Bms 150a Canbus Rs485 Bt 36v 48v 60v Li-ion Lto Battery 18650 Battery Lifepo4 Battery Storage - Battery Accessories & Charger Accessories - AliExpress

Smarter Shopping, Better Living! Aliexpress.com

www.aliexpress.com

You want for example the B2A8S20P - 8S configuration, 200A. There are others that would be suitable, check the chart on that page.

I think I've somewhat lost track of things on the JK BMS. For an 8S, would it still require an external boost converter to power the BMS from an 8S / 24V battery? I thought I had seen that once and I thought @Off-Grid-Garage had a video showing it.

 

[upnorthandpersonal]

I think I've somewhat lost track of things on the JK BMS. For an 8S, would it still require an external boost converter to power the BMS from an 8S / 24V battery? I thought I had seen that once and I thought @Off-Grid-Garage had a video showing it.

No, you only need that for a 4S battery.

 

[John Frum]

No, you only need that for a 4S battery.

Cool, I'm not keen on the wimpy high current leads, but its seems like a reasonable recommend.

 

[upnorthandpersonal]

They're high temperature silicone ones, so they're ok. Granted, for 12V systems you still want to mind the load a little...