My first question - which BMS to choose

[rcraats]

I'am new on this forum and very happy with al the information i did found here. I've allready read a lot about the working of a BMS, but I still have a question. I want to build a 280 Ah battery with LiFePO4 cells as a replacement of 2 105Ah lead acid batterys in my RV. Because i also want to travel in winter, it is necessary to look for a BMS with temp cut off in cold circumstances. So far so good. Then I have to make a choice for how many Amp's the BMS have to support. I want to connect a pure sine 1500 watt continuous (3000 watt peak) 220v Inverter. I will use this inverter for a 1400 watt coffee machine. So I think i have to look for a 125 Amp BMS in this case. Is this assumption correct or do I have to buy a 250 Amps BMS because of the 3000 watt peak load that can flow?

To be complete: The LiFePO4 battery will be charged by 2 100 watt solar panels and the Victron Smartsolar 75/15 and through a Schaudt EBL 101 Electroblock (220v charging).

Can someone give me advice or tips I should pay attention to?

Thanks in advance,

Greetings Rinus

 

[John Frum]

Assuming 12 volts system.

1500 ac watts / .85 conversion factor / 12 volts low cutoff = ~147 dc amps
~147 dc amps * 1.25 fuse headroom = ~183 fuse amps.

Plus you should consider de-rating if you get a commodity "Ebay" bms.
There is no consensus on how much to de-rate.
I would say the range is .5 to .8.
~147 * .5 = 294 amps.
~147 * .66 = 222 amps.

 

[rcraats]

@smoothJoey Yes, it's a 12v system. Thanks for your advice. I think i'll go for a 250 amps BMS. Do you have a suggestion for a brand and type that have a good price-quality ratio? The low temp cut off is for me very important.

 

[John Frum]

@smoothJoey Yes, it's a 12v system. Thanks for your advice. I think i'll go for a 250 amps BMS. Do you have a suggestion for a brand and type that have a good price-quality ratio? The low temp cut off is for me very important.

Sorry not really.
I know a 120 amp one that I feel comfortable to suggest.
For most installation the inverter is the lion's share of the load.
I like this solution and tested it myself.

 

[Factory400]

@smoothJoey Yes, it's a 12v system. Thanks for your advice. I think i'll go for a 250 amps BMS. Do you have a suggestion for a brand and type that have a good price-quality ratio? The low temp cut off is for me very important.

This is a tough one - the BMS market is FLOODED with cheap Chinese stuff that is very difficult to predict. I have an RV system that needs to operate in the cold, be very reliable, and not malfunction in a way that can damage the pack or any individual cell.

I examined the usual suspect cheap models and was unwilling to even try them on my cells that took 12 weeks to arrive. I have tried the Chargery unit, and it sorta works but not well enough.

I am now moving to Batrium https://www.batrium.com/collections
Hoping it is closer to 'set it and forget it' even when the going gets tough. The price may seem high until you figure the cost of trying 2-3 other cheap units, damaged cells, being without power in the middle of nowhere, etc, etc.

If I had more time - I would design and build my own from scratch but for now I just need to get a proven system. In BMS world - it is rather hard to save money long term. The cheap stuff seems to be the most expensive to me.

 

[Steve_S]

Have a look at this Poll for which BMS' people here are using in general.
https://diysolarforum.com/threads/2020-poll-what-bms-are-you-using-in-your-system-updated.8698/

One year ago, there was not as many BMS' with Low Temp cutoff but there are more available today "Thank Goodness !"
If you decide to go with a MOSFET based BMS then having one "over rated" to allow for room to prevent pushing it too hard is wisdom that has been thoroughly tested. A 250-300A BMS would work quite nicely. If you decide to go with a Relay/Contactor based BMS, the amperage is handled by the Relays/Contactors and can be for any amperage but again, it is also best to leave room so you never exceed the handling capacity even with a deep surge.

 

[rcraats]

Sorry not really.
I know a 120 amp one that I feel comfortable to suggest.
For most installation the inverter is the lion's share of the load.
I like this solution and tested it myself.

Thanx

 

[rcraats]

This is a tough one - the BMS market is FLOODED with cheap Chinese stuff that is very difficult to predict. I have an RV system that needs to operate in the cold, be very reliable, and not malfunction in a way that can damage the pack or any individual cell.

I examined the usual suspect cheap models and was unwilling to even try them on my cells that took 12 weeks to arrive. I have tried the Chargery unit, and it sorta works but not well enough.

I am now moving to Batrium https://www.batrium.com/collections
Hoping it is closer to 'set it and forget it' even when the going gets tough. The price may seem high until you figure the cost of trying 2-3 other cheap units, damaged cells, being without power in the middle of nowhere, etc, etc.

If I had more time - I would design and build my own from scratch but for now I just need to get a proven system. In BMS world - it is rather hard to save money long term. The cheap stuff seems to be the most expensive to me.

In the Netherlands we also know the saying "Cheap is expensive." ;-)

 

[rcraats]

Have a look at this Poll for which BMS' people here are using in general.
https://diysolarforum.com/threads/2020-poll-what-bms-are-you-using-in-your-system-updated.8698/

One year ago, there was not as many BMS' with Low Temp cutoff but there are more available today "Thank Goodness !"
If you decide to go with a MOSFET based BMS then having one "over rated" to allow for room to prevent pushing it too hard is wisdom that has been thoroughly tested. A 250-300A BMS would work quite nicely. If you decide to go with a Relay/Contactor based BMS, the amperage is handled by the Relays/Contactors and can be for any amperage but again, it is also best to leave room so you never exceed the handling capacity even with a deep surge.

I'll take a look at it, thanx

 

[squowse]

I'm planning on exactly the same thing. The 1400W draw from my coffee machine is only for 10-15 second pulses though? Maybe yours is the same?
It makes it a lot easier as the 100A JBD/Overkill BMS is what I'm hoping to use. It can deliver 120A peak (possibnly a bit more before it trips out, it;s adjustable). 120A times 12V min should cover it. Otherwise it's possibel to use 2 in parallel if you can't find a 250A BMS you like at a reasonable price.
Which inverter are you getting? I got a cheap pure sine wave inverter by Sunyima 1500W/3000W peak and I'll see if it can handle the coffee machine. Otherwise I might ask for my money back or just use it for computer etc worse case scenario. The coffee machine is a resistive load (heat) so not so many problems as with running motors.