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BMS sizing for LifePO4

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WillAK

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Nov 17, 2021
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I've just bought 8x 280Ah LifePO4 cells and i'm trying to figure out the best size BMS for them.

At the moment I only need to pull 500w at any one time.

If i wire the batteries 12v and buy a 60a BMS then I can use upto 720w (12v * 60a) right?

So if I wire the batteries 24v would the same apply? 24v * 60a = 1440w ?
 
Yes :)

P=VI, so higher voltage will deliver more power for same current (or same power for less current).
 
280AH cells (battery pack) can output 1C or 280Amps for 1 hour from 100% SOC to 0% SOC.
They can accept up to 0.5C or 140 Amps for charging.
The most commonly used SmartBMS' used on such packs is a 250A Capable FET Based BMS. Above 250A we generally recommend a Relay/Contactor based BMS due to the high amperage.
A Good BMS is the JKBMS with 200A / 250A Capability, 2A Active Balancing to keep cells properly balanced, and these are configurable via Apps.
A BMS does not use / waste power and to maximize your capabilities do not restrict it's potential.
Generally we recommend that a limit of 250A Pull from the bank. 24V @ 250A = 6000W - or 120V/60A or 240V/25A (uncorrected for efficiency losses or derration)
24V/280AH Battery will provide 7168 Watt Hours (7.1kwh).

JKBMS listing from Hankzor (AliExpress)

HankzorBMS Store - Amazing products with exclusive discounts on AliExpress

Discover the wide range of from AliExpress Top Seller HankzorBMS Store.Enjoy ✓Free Shipping Worldwide! ✓Limited Time Sale ✓Easy Return.
hankzor.aliexpress.com hankzor.aliexpress.com

See the links in my signature for Basic Assembly Guides, Voltage Charts and more.
Hope it helps, Good Luck
 
Thanks for the replies.

My basic set up will be 8x 3.2v 280ah cells wired in 8S to 24v. I live south of Spain so the solar puts in about 5kw per day off 1800w panels this time of year and I use exactly 4500w of power every day. (My solar powers my outside kitchen which are fridges and freezers mainly).
I have a 3000w 24v inverter. The biggest energy draw is a 2400w kettle, for about 5 minutes, and the remaining 23.55 hours never goes over 300w.

So I was contemplating just buying a small 60a BMS and a 500w inverter and just leaving it to run the 300w i use for outside equipment.

I was worried that if i purchased a 250a BMS it would draw a lot of power on standby, as does my huge 3000w inverter.
 
I will have to correct you here a little bit.
A BMS does not use much power, we are talking milliwatts.
Active Balancing "moves" power from a High Voltage Cell to a Low Voltage cell, there is very little loss and is balanced out by incoming power so it is nothing.
! Passive Balancing is very slow and only wastes (burns off) high voltage cell power and does not transfer it.

Do use a 200 to 250A BMS because that will provide lots of elbow room for surges etc without creating any stress on it. This will also allow you to charge at full 0.5C if you have the capability.

Inverter/Chargers. There are many types from really cheap Chinese imports to really high-quality Tier-1 products like Samlex, Victron and so on.
The Value Grade (cheapo's) have a considerable power use even just idling (not power-save mode). Many cheapo's do not have a power save feature but some do, albeit they are not really very good and you will still likely be using 12W in eco mode.

Step up to a Samlex EVO (Pure Sine Low Frequency *1)
*2 Low Frequency is nearly mandatory for starting Fridges/Freezers & Heavy AC system because thy have 3X Surge handling capability and use Toroidial Coils to provide clean consistent power. The EVO family uses 11W in Idle & 6W in Power-Save and in power save the sense pulse is mild and the moment a load demand is sensed, the Inverter powers up transparently.

NB, High Frequency inverters are Much cheaper & Lighter BUT they are MOSFET Based without coils and can only handle 2X surge and they are NOT GOOD with High Draw motors (fridge/freezer etc).

REFERENCE HERE:

Inversion Methods Explained: High Frequency vs Low Frequency | Magnum Dimensions

Understand the difference between high frequency and low frequency inverters with this quick article.
www.magnum-dimensions.com www.magnum-dimensions.com

Your Kettle is NOT your biggest draw ! Your Freezers & Fridges will likely be a minimum of 230V/10A and if that is the case the start surge will more than likely be 20-25A albeit short, that's what it takes to spin the compressors up under load.

230V X 10A = 2300W

Hope it helps.
 
I have two chest freezers, and to run both of them takes 100w per hour. Even when I turn them off/on the watt meter I have installed never shows any higher output than 100w. They then sit comfortably turning on and off either pulling 0, or 50w (1 freezer) or 100w (both). Although saying that, I never run the freezers from defrosted. They are always fully frozen inside.

What I meant by the kettle is, it's the highest pull of power. When i switch it on it shows 2200w immediately until it finishes the boil. So if I had a 500w inverter the freezers would run perfectly fine, but I wouldn't be able to use the kettle. And I want to eventually run a 2500w table top oven from the solar also.

I have a 3000/6000w 24v pure sine inverter. Chinese brand (Lvyuan). How much do you think this pulls per hour on top? I feel like it's wasting so much energy so I wanted to go for a smaller one.
 
WillAK said:
So if I had a 500w inverter the freezers would run perfectly fine
Click to expand...
Almost certainly not...

What @Steve_S meant is that a kettle is a resistive load, so it will just draw the 2200W, no more.

An inductive load, such as motor / compressor in a fridge or freezer will draw potentially up to 10x its rated power on start-up.

As an example, I have a 3.6kW inverter. During a power cut, I tried to power start it to power the house load of just 2 lights, 2 fridges and 2 freezers, it tripped immediately. Why did that happen when the fridge / freezers only draw about 40W-100W each when on? It was because the start up surge of those 4 appliances was greater than 3600W and the inverter had a wobbly. The only way I could get things going was to switch off 3 of the fridge-freezers, get the inverter going and start up the others one-by-one.

Lesson learnt:) - don't underestimate the start-up draw of an inductive load.
 
Ah i see. Thank you for the explanation.

Although, I'm sure I have had a 500w inverter running my freezers perfectly fine. But they never switch off unless I unplug them.

I guess I'll stick with my 3000w inverter then

:giggle:


So just to clarify one more time, with the 3000w inverter and 24v I should buy a 200w BMS? This way I can add more cells in the future.
 
WillAK said:
So just to clarify one more time, with the 3000w inverter and 24v I should buy a 200w BMS? This way I can add more cells in the future.
Click to expand...
Sounds like a good plan, for the reasons @Steve_S said in post #5, assuming you meant 200A BMS (y)
 
Steve_S said:
280AH cells (battery pack) can output 1C or 280Amps for 1 hour from 100% SOC to 0% SOC.
They can accept up to 0.5C or 140 Amps for charging.
The most commonly used SmartBMS' used on such packs is a 250A Capable FET Based BMS. Above 250A we generally recommend a Relay/Contactor based BMS due to the high amperage.
A Good BMS is the JKBMS with 200A / 250A Capability, 2A Active Balancing to keep cells properly balanced, and these are configurable via Apps.
A BMS does not use / waste power and to maximize your capabilities do not restrict it's potential.
Generally we recommend that a limit of 250A Pull from the bank. 24V @ 250A = 6000W - or 120V/60A or 240V/25A (uncorrected for efficiency losses or derration)
24V/280AH Battery will provide 7168 Watt Hours (7.1kwh).

JKBMS listing from Hankzor (AliExpress)

HankzorBMS Store - Amazing products with exclusive discounts on AliExpress

Discover the wide range of from AliExpress Top Seller HankzorBMS Store.Enjoy ✓Free Shipping Worldwide! ✓Limited Time Sale ✓Easy Return.
hankzor.aliexpress.com hankzor.aliexpress.com

See the links in my signature for Basic Assembly Guides, Voltage Charts and more.
Hope it helps, Good Luck
Click to expand...
They list working with Deye inverters, wonder if that would include Sol-Arks?

Didn’t see any place to ask seller questions.

 
WillAK said:
I have a 3000/6000w 24v pure sine inverter. Chinese brand (Lvyuan). How much do you think this pulls per hour on top? I feel like it's wasting so much energy so I wanted to go for a smaller one.
Click to expand...
The manual for it OR the products webpage should tell you what that Inverter/Charger uses in stand-by, eco mode (if it has one) and so on.
I do very strongly suggest you install a Shunt Monitor between your battery pack(s) and the inverter. That will allow you to monitor the voltage & amperage in real time and will let you see the surges from motor starts etc, additionally it's an extra tool that helps to see the state of charge etc.

There are many varieties of Shunt available, some use Apps to view, some have digital displays, some even have wifi & more...
The Victron Shunts are really good high quality products and they have different models and you can likely get those locally in Spain. Or amazon & EBay can drop you one in no time.

My own system, as well as those I've helped build & setup, all have a Battery Shunt as well as an AC Monitoring shunt between the inverter & AC Panel and that's made monitoring & troubleshooting a heck of a lot easier... small things can make a huge difference.
 
Danke said:
They are made by Deye, but wondering if software/firmware is different. Will communications be the same?
Click to expand...
Good question and obviously I have no idea. I understood that Sunsynk at least had a different (=? better) user interface.

With my software engineering hat on, I'd be surprised if they tinkered with the low-level communication protocols as that would burden the OEM manufacturers with a whole load of extra tech support issues and create issues when Deye update something.
 
WillAK said:
Thanks for the replies.

My basic set up will be 8x 3.2v 280ah cells wired in 8S to 24v. I live south of Spain so the solar puts in about 5kw per day off 1800w panels this time of year and I use exactly 4500w of power every day. (My solar powers my outside kitchen which are fridges and freezers mainly).
I have a 3000w 24v inverter. The biggest energy draw is a 2400w kettle, for about 5 minutes, and the remaining 23.55 hours never goes over 300w.

So I was contemplating just buying a small 60a BMS and a 500w inverter and just leaving it to run the 300w i use for outside equipment.

I was worried that if i purchased a 250a BMS it would draw a lot of power on standby, as does my huge 3000w inverter.
Click to expand...
I also built a 280Ah 24v battery earlier this year. I used a JK 200A BMS with 2A active balance. It is excellent. We have a 3000w inverter that we regularly pull 2000w from for electric cooking via an induction hob. We boil the kettle on the induction hob. We have 920w of solar with 3.6kw average daily solar harvest.
The power consumption of the BMS is negligible. Its tiny. The power consumption of your inverter on standby, will likely be massively more. Don't worry about BMS running load. It is irrelevant in the scale of things. It is more important to get a good BMS. The JK will do this for you.
Over sizing the BMS amp capability is a good thing not a bad thing. The mosfets will run cooler for a given load. The cost of the JK 200A BMS is tiny compared to the rest of your investment
 
Steve_S said:
The manual for it OR the products webpage should tell you what that Inverter/Charger uses in stand-by, eco mode (if it has one) and so on.
I do very strongly suggest you install a Shunt Monitor between your battery pack(s) and the inverter. That will allow you to monitor the voltage & amperage in real time and will let you see the surges from motor starts etc, additionally it's an extra tool that helps to see the state of charge etc.

There are many varieties of Shunt available, some use Apps to view, some have digital displays, some even have wifi & more...
The Victron Shunts are really good high quality products and they have different models and you can likely get those locally in Spain. Or amazon & EBay can drop you one in no time.

My own system, as well as those I've helped build & setup, all have a Battery Shunt as well as an AC Monitoring shunt between the inverter & AC Panel and that's made monitoring & troubleshooting a heck of a lot easier... small things can make a huge difference.
Click to expand...
Thanks very much, that sounds like a great idea. I'll search amazon for one.

I'm very interested in exactly how much my panels produce (to the extent I even keep a spreadsheet of the daily output depending on the time of year) so I would like to install monitors onto every connection so I know exactly what its putting in, and taking out and when.

So if you have any suggestions for monitors I can connect from the panels to the charge controller, CC to batteries ETC...
 
TimE said:
I also built a 280Ah 24v battery earlier this year. I used a JK 200A BMS with 2A active balance. It is excellent. We have a 3000w inverter that we regularly pull 2000w from for electric cooking via an induction hob. We boil the kettle on the induction hob. We have 920w of solar with 3.6kw average daily solar harvest.
The power consumption of the BMS is negligible. Its tiny. The power consumption of your inverter on standby, will likely be massively more. Don't worry about BMS running load. It is irrelevant in the scale of things. It is more important to get a good BMS. The JK will do this for you.
Over sizing the BMS amp capability is a good thing not a bad thing. The mosfets will run cooler for a given load. The cost of the JK 200A BMS is tiny compared to the rest of your investment
Click to expand...
Sounds exactly like my setup and what I want to use it for.

Did you buy your JK BMS from Alibaba? I have a guy who ive bought all my batteries from (Alibaba, straight from the manufacturer), who also makes the JK BMS but I cant buy them on Amazon where I am.

Amazon.es have a good selection of Daly BMS. 200a 24v BMS are 201EUR at the moment. I was thinking of buying one of those.
 
WillAK said:
Sounds exactly like my setup and what I want to use it for.

Did you buy your JK BMS from Alibaba? I have a guy who ive bought all my batteries from (Alibaba, straight from the manufacturer), who also makes the JK BMS but I cant buy them on Amazon where I am.

Amazon.es have a good selection of Daly BMS. 200a 24v BMS are 201EUR at the moment. I was thinking of buying one of those.
Click to expand...
I purchased my JK BMS off Ebay. £125 in the UK with display screen
 
WillAK said:
So if you have any suggestions for monitors I can connect from the panels to the charge controller, CC to batteries ETC...
Click to expand...
I cannot suggest a device that can be used between solar array & solar Controller. Most solar controllers have that capability built-in, some have PC software, some have Phone Apps. Depending on exactly WHAT equipment you have there are some 3rd party software applications which can interface with Battery Packs, Inverters, Solar Controllers (although AIO (All in ones) are more supported).

This one is quite popular, it is opensource so it is quite dynamic with updates & maintenance.

Real-time solar monitoring from a Raspberry Pi | SolarAssistant Software

Real-time charts, analytics and power management from via a Raspberry pi - the most powerful, cost effective device on the planet.
solar-assistant.io solar-assistant.io

I will make s suggestion that will help us here to help you better.
Please give us a full description of your equipement. Make Model of Solar Controller, Inverter/Charger, solar panel specs & configuration etc. The more complete info you provide & Links to products used is most helpful....
 
Steve_S said:
Please give us a full description of your equipement. Make Model of Solar Controller, Inverter/Charger, solar panel specs & configuration etc. The more complete info you provide & Links to products used is most helpful....
Click to expand...

Sure, I'll write down what I have and how i've set it up. I would appreciate improvements which I'm sure there will be lots of!

I have 2x 450w 40v panels which are wired in parallel. (On the sunniest days, the highest I've ever seen on the charge controller is 630w 15/16a).
I do want to buy 2 more panels so I have 4 in parallel.

Epever Triron 3210N 30A Charge Controller (https://www.amazon.com/EPEVER-Controller-Intelligent-Modular-Designed-Regulator/dp/B081Q8NQ17?th=1)

LVYUAN 24v 3000w Inverter. (https://lvyuan.co/products/voltage-...0v-pure-sine-wave-inverter-remote-control-led)

8x 3.2v REPT 280Ah cells wired 8S to 24v and 200A Daly BMS.

It's all connected using 10AWG wires. There's only 1 metre from the panels to the CC and about 6 metres from the CC to the batteries.

It's a very simple setup and In the summer months I get about 5000w of input power each day, currently putting in about 3000w on a clear day.
 
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