BMS 120Amp to Inverter question

[BigGun]

Then you go back to where you were before earlier today -- The cells still should stay around .5C continuous ( 140 amps )
140 amps/24 volts / .85 conversion = 2856 watts ----- 3000 watt inverter

 

[time2roll]

200 amps x 24v is 4800 watts. So good on 4000 watts. Best to have a bit of headroom so please stop at 4000

 

[V84x4]

Sorry, spoke to quickly.
The next bottleneck will be the cells themselves.
The limit will be ~140 amps(.5c).
140 amps * 24 volts low cutoff * .85 conversion factor = 2,856.00 ac watts.

This would allow a 3000 watt inverter.

OK Great now I understand
What 200A or 150a BMS do you recommend for a 8S 24V LiPo ?

Thank you so much


best regards, Rick

 

[V84x4]

Sorry, spoke to quickly.
The next bottleneck will be the cells themselves.
The limit will be ~140 amps(.5c).
140 amps * 24 volts low cutoff * .85 conversion factor = 2,856.00 ac watts.

This would allow a 3000 watt inverter.

Sorry .....
one last question just so I understand your math
Where is the .85 conversion factor coming from?
Is it a percentage?
Or is it the 150/85 controller?

Thanks for everything
regards, Rick

 

[John Frum]

What 200A BMS do you recommend for a 8S 24V LiPo ?

I don't have a super good recommend for a 200 amp 8s BMS
I'm not a big fan of the Daly but this should be "ok" https://www.currentconnected.com/product/daly-sbms/
They have the 250 amp in stock so I that is what I suggest.
The Current Connected owner is a respected member of this forum.

 

[John Frum]

Sorry .....
one last question just so I understand your math
Where is the .85 conversion factor coming from?

Its a general estimate of the conversion efficiency of a generic inverter.
Quality ones are more efficient, ghetto ones are less efficient.
Warning: most inverters advertise their maximum efficiency not their typical efficiency.

 

[John Frum]

@V84x4 its good that you ask these questions.

 

[V84x4]

@V84x4 its good that you ask these questions.

Thank you for saying that

Do you have a recommendation for an Inverter/charger ?
3000w Victron?
3000w Aims?
or something else

regards, Rick

 

[John Frum]

Thank you for saying that

Do you have a recommendation for an Inverter/charger ?
3000w Victron?
3000w Aims?
or something else

regards, Rick

This one is my fave https://www.donrowe.com/samlex-evo-2224-pure-sine-inverter-charger-p/evo-2224.htm

Victron Energy PMP243021102 Multi Plus 24/3000/70-50 120V VE Bus Inverter/Charger

shop.pkys.com

The victron is rated in VA=volt amps
If you dig into the specs its a 2400 watt device.
The Samlex is conservatively rated at 2200 watts.
These are both top shelf products.

 

[RedTechNeck]

You need to be aware that inverters are rated in different ways. Some are rated to draw 150% surge, and they will do that. For some the advertised wattage is the surge or peak rating. Consider the rating of your BMS to be the absolute peak current rating. I would not run at that continuous depending on how good the heatsink is on the BMS and what generation of MOSFETS it uses. Also, 200 amps is .714c for a 280 amp cell. Depending on the quality of the cell and its internal resistance, this could cause overheating in continuous use. IR squared U know, may need cell cooling.

So, a 3000 watt inverter can draw as much as 4500 watts for some brands and models. 200 amps at 12v nominal is 2400 watts. You would need two parallel stacks rated at 200 amps each to be safe.

Most folks go to 24 volt nominal stacks, and 24 volt inverters for this range of power. Now you take the same 8 cells you need anyway, and just use one 200 amp 24 volt BMS. These can be be future expanded by adding additional 8s stacks in parallel.

You can also get 36, 48, and even 96 volt inverters. Each increase in feed voltage increases inverter efficiency as the first boost stage of the inverter has a lower conversion ratio at higher DC feed voltages. Increased efficiency translates into your energy system lasting longer at night and on cloudy days without having to run the gen.. This means less or no fuel bills when the sun is not out.

Also, be aware that modified sine wave inverters, while bragging about being so much more efficient "up front" causes most of your equipment and downstream devices to become severely less efficient. So much to the extent that this effect more than makes up the difference that a pure sine wave inverter has in lower "up front" efficiency. Now days, modern technology, such as silicon carbide mosfets, has made pure sine wave inverters as efficient as yesterdays modified wave inverters, and there is no question as to what devices you have that will run on it.

Anything with an electric motor will cause an inverter to draw a surge current too, or destroy the inverter with excessive surge current. Most motors, such as an air conditioning compressor will draw up to 5x running current when starting. You can find this by looking at the plate or sticker on the side of the motor or compressor, and looking for "locked rotor" current, or LRC abbreviated. Your inverter needs to be rated over this amount, and that is with any other stuff u have running added to that.

And you also need to consider transient and surge protection in certain parts of the system.

A lot goes into to figuring a system, but you will never regret it if done right. It will make your life will be easy and trouble free, and best of all....... electric bill free!

 

[Shep]

Those 280ah cells should not be discharged at a continuous rate of >.5c(140 amps) as they get hotter inside.
So the bms is only really a small bottleneck at 120 amps.

Is this statement saying that- it's not actually the amperage rating of the installed bms that matters (for the inverter size or otherwise) if the settings for the battery (programed into the bms) need to be at a lower amperage than the bms itself?

I have the same question as the author here, except my system is 304ah 12v battery (4 cells). I have an overkill solar 120a 4s bms and a 2200 watt inverter from Giandel. I started to scratch my head when I saw that Will said a 1500 watt inverter would work for that size of bms. But maybe there is more to it than meets the eye? I didn't quite follow this thread well enough since it was 24v and not 12v. But maybe the info is here for me somewhere? Thanks yallz

 

[HRTKD]

The battery rating in Ah indicates how long it will discharge. The BMS rating in amps indicates how long it should continuously operate at the number of amps. The two numbers don't correlate. Buy the BMS to match the load amps. Buy the battery to match the total discharge in amp hours.

I planned to run a microwave from my LiFePO4 battery bank. I put in a 4s2p configuration. Two batteries, each with its own 120 amp BMS. I have ~240 amps of continuous discharge through the BMS. Each 280 Ah battery would be discharging at less than .5C which is the recommended threshold.

 

[John Frum]

Is this statement saying that- it's not actually the amperage rating of the installed bms that matters (for the inverter size or otherwise) if the settings for the battery (programed into the bms) need to be at a lower amperage than the bms itself?

I have the same question as the author here, except my system is 304ah 12v battery (4 cells). I have an overkill solar 120a 4s bms and a 2200 watt inverter from Giandel. I started to scratch my head when I saw that Will said a 1500 watt inverter would work for that size of bms. But maybe there is more to it than meets the eye? I didn't quite follow this thread well enough since it was 24v and not 12v. But maybe the info is here for me somewhere? Thanks yallz

The BMS will be the gating factor.
.5c for 304ah cells is 152 amps but the BMS is only rated 120 amps continuous.

2200 ac watts / .85 conversion factor / 10 volts low cutoff = 258.823529412 service amps
258.823529412 service amps / .8 fuse headroom = 323.529411765 fault amps

That means pure copper 3/0 awg wire with 105c insulation and a 350 amp fuse with breaking capacity of at least 10000 amps.
It also means you need 2 sets of 304 ah hour batteries in parallel each with a BMS rated for 150 amps or better.

If any of this is not clear please ask specific questions and I will be glad to answer.

 

[Shep]

The BMS will be the gating factor.
.5c for 304ah cells is 152 amps but the BMS is only rated 120 amps continuous.

2200 ac watts / .85 conversion factor / 10 volts low cutoff = 258.823529412 service amps
258.823529412 service amps / .8 fuse headroom = 323.529411765 fault amps

That means 3/0 awg wire with a 350 amp fuse with breaking capacity of at least 10000 amps.
It also means you need 2 sets of 304 ah hour batteries in parralel each with a bms rated for 150 amps or better.

If any of this is not clear please ask specific questions and I will be glad to answer.

Alright ya, I have a question or two. The needing another set of a 304ah battery.. You mean all 4 cells of the 304ah type right? Or are you saying that the 4 cells I have would be split up and put in parallel?

This may be teaching me a thing or two, but to be clear- I only plan on having the 4 cells 304ah pack used in my van, which is sounding a fairly overdone for my purposes already (a 12v fridge, a little PS4 gaming here and there, a small projector, maybe a diesel heater in the future).
Would I need to aim for a smaller inverter anyway for my simple setup? What I'm saying is, I have more than enough capacity with this pack as it is.

My heads already starting to turn towards a Daly however, I know you aren't a big fan of them. Or... What size inverter would be more kosher with my setup you think? If any more info is needed, I can provide that. Thanks

That wire sizing was helpful to see too btw. I've been piecing my system together for almost a drawn out year now, and it can be helpful to see info about specific wire sizes and fuses.

 

[John Frum]

Alright ya, I have a question or two. The needing another set of a 304ah battery.. You mean all 4 cells of the 304ah type right? Or are you saying that the 4 cells I have would be split up and put in parallel?

4 cells makes 1 12 volt battery.
You need 4 more to make another battery.
If you go with another 120 amp bms your aggregate continous amperage would be 240 amps which is slightly less than the service amps I calculated.

This may be teaching me a thing or two, but to be clear- I only plan on having the 4 cells 304ah pack used in my van, which is sounding a fairly overdone for my purposes already (a 12v fridge, a little PS4 gaming here and there, a small projector, maybe a diesel heater in the future).
Would I need to aim for a smaller inverter anyway for my simple setup?

120 continous amps * 10 volts low cutoff *.85 conversion factor = 1020 ac watts
You bms can handle a 1000 watt inverter

What I'm saying is, I have more than enough capacity with this pack as it is.

Capacity is measure in amps or time or watts over time.
The continuous rating is measure in c rate or watts or amps.

My heads already starting to turn towards a Daly however, I know you aren't a big fan of them.

Daly BMSs have very load capacity to balance your batteries.
Most of the cells that folks purchase here would not stay balanced with a Daly BMS.
That means adding an active balancer which is a whole other kettle of fish.

Or... What size inverter would be more kosher with my setup you think?

See above ^

If any more info is needed, I can provide that. Thanks

What vendor did you get your cells from?

That wire sizing was helpful to see too btw. I've been piecing my system together for almost a drawn out year now, and it can be helpful to see info about specific wire sizes and fuses.

I'm glad to help.
Is your setup mobile or stationary?

 

[HRTKD]

This may be teaching me a thing or two, but to be clear- I only plan on having the 4 cells 304ah pack used in my van, which is sounding a fairly overdone for my purposes already (a 12v fridge, a little PS4 gaming here and there, a small projector, maybe a diesel heater in the future).
Would I need to aim for a smaller inverter anyway for my simple setup? What I'm saying is, I have more than enough capacity with this pack as it is.

Your inverter may be rated for more than you need. If your needs never exceed say 1000 watts (on the AC side) then the inverter, battery and BMS are sized about right and no further changes are necessary.

You can figure out the maximum number of watts that you'll pull at any one time and plug that number into John's formulas.

 

[John Frum]

Its my policy to always size the system to the max continuous rating of the inverter.
Others may have different policies.

 

[Shep]

4 cells makes 1 12 volt battery.
You need 4 more to make another battery.
If you go with another 120 amp bms your aggregate continous amperage would be 240 amps which is slightly less than the service amps I calculated.

120 continous amps * 10 volts low cutoff *.85 conversion factor = 1020 ac watts
You bms can handle a 1000 watt inverter

Capacity is measure in amps or time or watts over time.
The continuous rating is measure in c rate or watts or amps.

Daly BMSs have very load capacity to balance your batteries.
Most of the cells that folks purchase here would not stay balanced with a Daly BMS.
That means adding an active balancer which is a whole other kettle of fish.

See above ^

What vendor did you get your cells from?

I'm glad to help.
Is your setup mobile or stationary?

So mine are from Amy with Docan Power. They are eve cells, grade A. Right now everything is sitting at my sister's place while I plan to tweak things into shape but the system will go into a cargo van under my bed. And I'm planning to build a custom box for it that controls the temp with a fan and some heating pads too. In this video (a short segment)

At 11:14 he states that this exact bms can handle 1500 watts from a 12v inverter, like the one he is testing with.

I've always heard that you want to go for the largest inverter that you can afford/will work for your system. I don't really have a problem emailing these companies though to swap this inverter out for a smaller one and the bms out for something more suitable too. But it sounds to me like the bms may just be fine. He also multiples by a different number to get that result.

 

[John Frum]

So mine are from Amy with Docan Power. They are eve cells, grade A.

Docan supplies good cells but they are very lilkely not grade A.
Grade A means automotive grade and they come with test reports from the manufacturer.

Right now everything is sitting at my sister's place while I plan to tweak things into shape but the system will go into a cargo van under my bed. And I'm planning to build a custom box for it that controls the temp with a fan and some heating pads too. In this video (a short segment)

At 11:14 he states that this exact bms can handle 1500 watts from a 12v inverter, like the one he is testing with.

I showed my math.

I've always heard that you want to go for the largest inverter that you can afford/will work for your system.

I gave you my advice in good faith.
Do with it as you will.

I don't really have a problem emailing these companies though to swap this inverter out for a smaller one and the bms out for something more suitable too. But it sounds to me like the bms may just be fine. He also multiples by a different number to get that result.

Please share the math.

 

[Shep]

Docan supplies good cells but they are very lilkely not grade A.
Grade A means automotive grade and they come with test reports from the manufacturer.

I showed my math.

I gave you my advice in good faith.
Do with it as you will.

Please share the math.

In that video I shared, he's multiplying the 120a from the bms to "12.8 nominal". I suppose that # is just the voltage of the pack?

I don't mean to offend you or sound like I'm not listening to your advice John. I'm all ears! This is just the exact video and segment that got me to start questioning my inverter size/bms size.