How to choose the correct BMS for my RV battery

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xtjoeywx

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May 17, 2020
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I have been searching online and nothing is making sense.I am making a 12v battery out of 4x 3.2v 135AH LifePO4 battery packs. I will have five of these 12v batteries in my battery bank.It is for my RV (recreational vehicle fifth wheel). I have 1240 watts of solar on the roofand the battery bank I'm addingwill be used to power 12v D/C and 30a/50aA/C loads (5000w inverter).

How do I know what amperageBMS to buy? I was told I should buy a 150ABMS by the seller of thebattery packs,but that doesn't make sense to me as that is more than the 135AH. I was thinking I should buy 120A BMS to be under the 135AH since it would use less of my battery overall.

Am I thinking about things all wrong?

Thanks in advance for any help provided.

Joey
 

OffGridInTheCity

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xtjoeywx said:
I have been searching online and nothing is making sense.I am making a 12v battery out of 4x 3.2v 135AH LifePO4 battery packs. I will have five of these 12v batteries in my battery bank.It is for my RV (recreational vehicle fifth wheel). I have 1240 watts of solar on the roofand the battery bank I'm addingwill be used to power 12v D/C and 30a/50aA/C loads (5000w inverter).

How do I know what amperageBMS to buy? I was told I should buy a 150ABMS by the seller of thebattery packs,but that doesn't make sense to me as that is more than the 135AH. I was thinking I should buy 120A BMS to be under the 135AH since it would use less of my battery overall.

Am I thinking about things all wrong?

Thanks in advance for any help provided.

Joey

>and 30a/50aA/C loads (5000w inverter).
If you have a 12v battery bank and you plan to run a 5000w inverter, that's 5000w/12v = 417amp draw from the battery bank. That's quite a lot of amps.
Your BMS 'amps' is all about allowing amps from the battery to the inverter when the battery is OK and cutting off the flow when the battery bank is not OK. A battery has a max amp draw as part of it's specifications... your LifePO4(s) will have this number. If you put 4 of them in series - then the number from 1 battery (of the 4 in series) will be the max amps you should draw and you should purchase a BMS with that as a key factor.

If you share the specs of your LifePO4 we can give you a more detailed answer - but I bet is 150amps range. 150a*12v = 1800w inverter - e.g. I suspect your 5000w inverter is 'too big' for a 4 x LifePO4 battery bank... you would more in the range of 8 LifePO4 arranged in 4s2p or even 12 of them in a 4s3p to get to 5000w.

The AH's of a battery is not its ability to delivery amps - but rather how long it will deliver power at a given load. AH is a measure of capacity rather that max power.
To get capacity you can do ah * v = wh - e.g. If you have 4 x 135ah batteries in series at 12v, then its 136ah * 12v = 1620wh. 1600wh is a 1600w load for 1 hour - like a 1500w heater would run for an hour. Of course you don't typically discharge a battery 100%, so in practical terms its less than 1600wh - maybe 60% - so 1600w * .6 = 960wh or just round it to 1000wh of power as a working number.
 

xtjoeywx

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OffGridInTheCity said:
If you share the specs of your LifePO4 we can give you a more detailed answer - but I bet is 150amps range. 150a*12v = 1800w inverter - e.g. I suspect your 5000w inverter is 'too big' for a 4 x LifePO4 battery bank... you would more in the range of 8 LifePO4 arranged in 4s2p or even 12 of them in a 4s3p to get to 5000w.

The specs are:
- Charge voltage: 3.65V
- Nominal voltage: 3.2V
- Nominal capacity: 135AH
- Charge current: Standard
- Charging/Discharging: 1C (135AH)
- Standard Charging method: CCCV
- Charging time: Standard Charge: 1C 60minutes(Ref.) Rapid Charge: 2C-30minutes (Ref)
- Max.charge current: 1C (135AH)
- Max.discharge current: 3C (405AH)
- Discharge cut-off voltage: 2.5V
- Operating temperature: Charging: -0C-55C Discharging: -20C-60C
- Storage temperature: -0C?+60C
- Battery Weight: About 5.25KG

I plan on buying 20 of these to make 5 batteries. Will that work?
 

OffGridInTheCity

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xtjoeywx said:
OffGridInTheCity said:
If you share the specs of your LifePO4 we can give you a more detailed answer - but I bet is 150amps range. 150a*12v = 1800w inverter - e.g. I suspect your 5000w inverter is 'too big' for a 4 x LifePO4 battery bank... you would more in the range of 8 LifePO4 arranged in 4s2p or even 12 of them in a 4s3p to get to 5000w.

The specs are:
- Charge voltage: 3.65V
- Nominal voltage: 3.2V
- Nominal capacity: 135AH
- Charge current: Standard
- Charging/Discharging: 1C (135AH)
- Standard Charging method: CCCV
- Charging time: Standard Charge: 1C 60minutes(Ref.) Rapid Charge: 2C-30minutes (Ref)
- Max.charge current: 1C (135AH)
- Max.discharge current: 3C (405AH)
- Discharge cut-off voltage: 2.5V
- Operating temperature: Charging: -0C-55C Discharging: -20C-60C
- Storage temperature: -0C?+60C
- Battery Weight: About 5.25KG

I plan on buying 20 of these to make 5 batteries. Will that work?
Excellent -specs really help and I think you'll get several comments. OK, to start htings off...

>I plan on buying 20 of theseto make 5 batteries. Will that work?
Yes sir. That would be 4s (4 in series), and 5p (5 batteries in parallel). That will give you 12v@675ah which is ~8100wh * 60% DOD = daily useable of at least5000wh.

>Charging/Discharging: 1C (135AH)

>Max.discharge current: 3C (405AH)
Great - then yes indeed you can do continuous ranges such as 135a * 5-batteries-in-parallel= 675amps of draw continous. This will indeed support a 5000w inverter. Very thick wire!! - like dual 4/0.

To your original question. A 5000w inverter @ 12v = 417a.

If you do 4s5p (hook all 5 set's of 4 in parallel to each other) then you need a 4s BMS that can cut-off a 417a (say 500a) load. This can be done by a singleBMS with low amps (cheaper) and have itturn on/off a contactor rated for 500a - maybe something like a Gigavac GV200 -https://www.waytekwire.com/item/770...MI4OGahe676QIVYSCtBh0nIw2tEAYYBSABEgJsbvD_BwE. and the power to the inverter would flow thru the contactor (rather than the BMS). Here's a youtube illustrating this idea - "4000a BMS?"

If you do 5 separate4s batteries, then you need 5 separate4s BMSs that can do 100-150a each - which is more standard to buy. You would then hook the output of all 5 (separate) batteries together to get the 500a for the inverter.
s
There are some pros and cons each way but either way is OK. I'll stop here for the moment and allow other comments to flow / give you a chance to come up to speed. I can chime in again if you don't getanswers to your questions.


 

gpn

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jonyjoe505

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You need to buy a bms that can handle at least a 4s battery pack. Thats the more important thing, as far as amps, if you only plan on using 40 amps or less, you need a bms that can handle 40 amps, if you plan on using 300 amps, you need a bms that can handle 300 amps.

Example I have a small 4s 10ah battery pack, since I use it to jump start cars, I use a bms that can handle 450 amps. Another example I had a 4s 220ah battery pack, and I only used a 30 amp bms since I never use more then 15 amps. You choose the bms by how many amps you plan to use not by the size of the battery amp hours.

Since you will be using solar I recommend the chargery bms8 (cost about 95 dollars). It uses contactors/relays to stop the charge/discharge, this bms can handle as much amps as your contactor/relay can handle.

The reason to use contactors is when the bms decides to stop the charge, a regular bms with mosfets, will cause voltage surges which damage equipment. With contactors, you place the contactor between the solar panel and the controller, when the bms activates it just disconnect the solar panel, and you get no voltage surges.

With the chargery you have to buy the contactors seperate, I use a 4 dollar 30 amp automotive relay, you can find large contactors/relays/solenoids for less then 50 dollars that can handle 300 or more amps. You need 2 one for charge and one for discharge, you can use a small one for charging since you charge at less amps then discharging. The chargery is a good solid bms, I been using it for the past year and proven very reliable. I used many bms on my solar system over the years but this is the best bms so far.
 

daromer

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Jonny thats not needed Still
You dont disconnect the battery unless its critical.

Its alot better to use a proper breaker with shunt trip than a contactor.
Mosfets is fine to and Will not cause a surge that Harms the electronics. If this would have been true 99% of all electronics out There would be harmfull and they aint. Pleasy supply picturews from you scoping this or scientific pappers showing in what scenario it happens.

Disconnecting the solar panels isnt needed with a proper mppt since its handled by the mppt controller. Charging is handled by the Charger in first hand and not the bms.
 

xtjoeywx

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gpn said:
https://www.solar-electric.com/learning-center/digital-nomads-work-remotely-using-solar-power/

This little write up and video shows what they did for this trailer. They went with a 48v system and then stepped down the dc voltage for the dc part of the trailer and used two inverters to share the load. They used really expensive parts but it is a cool setup. It can charge off solar, generator and shore power. Just some ideas.

This was pretty cool. I don't think I'm ready to spend that much money, but 48v sounds like the ideal way to go if you are going to be full-time in your RV. Thanks for sharing.



OffGridInTheCity said:
Great - then yes indeed you can do continuous ranges such as 135a * 5-batteries-in-parallel= 675amps of draw continous. This will indeed support a 5000w inverter. Very thick wire!! - like dual 4/0.

Are you saying dual 4/0 as in two positive wires on the positive terminal and two negative wire on the negative terminal? If so, that seems pretty large! lol I hope it's not that. I do have 4/0 wire on my lead acid batteries right now so I can reuse that.

OffGridInTheCity said:
If you do 4s5p (hook all 5 set's of 4 in parallel to each other) then you need a 4s BMS that can cut-off a 417a (say 500a) load. This can be done by a singleBMS with low amps (cheaper) and have itturn on/off a contactor rated for 500a - maybe something like a Gigavac GV200 -https://www.waytekwire.com/item/770...MI4OGahe676QIVYSCtBh0nIw2tEAYYBSABEgJsbvD_BwE. and the power to the inverter would flow thru the contactor (rather than the BMS). Here's a youtube illustrating this idea - "4000a BMS?"

If you do 5 separate4s batteries, then you need 5 separate4s BMSs that can do 100-150a each - which is more standard to buy. You would then hook the output of all 5 (separate) batteries together to get the 500a for the inverter.
There are some pros and cons each way but either way is OK. I'll stop here for the moment and allow other comments to flow / give you a chance to come up to speed. I can chime in again if you don't getanswers to your questions.

I'm thinking I'm going to use 5 separate 4S BMSs. The question is why would I use the 150a version over the 100a version BMS? Would I lose any power at all somewhere if I got the 100a version?


daromer said:
Jonny thats not needed Still
You dont disconnect the battery unless its critical.

Its alot better to use a proper breaker with shunt trip than a contactor.
Mosfets is fine to and Will not cause a surge that Harms the electronics. If this would have been true 99% of all electronics out There would be harmfull and they aint. Pleasy supply picturews from you scoping this or scientific pappers showing in what scenario it happens.

Disconnecting the solar panels isnt needed with a proper mppt since its handled by the mppt controller. Charging is handled by the Charger in first hand and not the bms.

Do you have any examples of this? A breaker with shunt trip? A contactor makes sense to me, but what you said doesn't because I don't know what that is.

Would I need a breaker with a shunt trip ora contactor if I have 5 BMSs (one for each battery)?
 

1nolaguy

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>and 30a/50aA/C loads (5000w inverter).
If you have a 12v battery bank and you plan to run a 5000w inverter, that's 5000w/12v = 417amp draw from the battery bank. That's quite a lot of amps.
Your BMS 'amps' is all about allowing amps from the battery to the inverter when the battery is OK and cutting off the flow when the battery bank is not OK. A battery has a max amp draw as part of it's specifications... your LifePO4(s) will have this number. If you put 4 of them in series - then the number from 1 battery (of the 4 in series) will be the max amps you should draw and you should purchase a BMS with that as a key factor.

If you share the specs of your LifePO4 we can give you a more detailed answer - but I bet is 150amps range. 150a*12v = 1800w inverter - e.g. I suspect your 5000w inverter is 'too big' for a 4 x LifePO4 battery bank... you would more in the range of 8 LifePO4 arranged in 4s2p or even 12 of them in a 4s3p to get to 5000w.

The AH's of a battery is not its ability to delivery amps - but rather how long it will deliver power at a given load. AH is a measure of capacity rather that max power.
To get capacity you can do ah * v = wh - e.g. If you have 4 x 135ah batteries in series at 12v, then its 136ah * 12v = 1620wh. 1600wh is a 1600w load for 1 hour - like a 1500w heater would run for an hour. Of course you don't typically discharge a battery 100%, so in practical terms its less than 1600wh - maybe 60% - so 1600w * .6 = 960wh or just round it to 1000wh of power as a working number.
I just found this post string and although it dates back to 2018 the information for me is timely. I am struggling with proper sizing of a system for my RV. I am replacing a Xantrex Freedom 458-25 (2500 watt 12vdc to 120vac I/C) and 4 - 150 AHr 6v wet cell GC batteries (2S2P) with a Xantrex SW 3012 (3000 watt 12vac to 120 vac I/C) and 2 - 280 AHr 12v LiFePO4 batteries (1S2P). The LiFePO4 batteries are to be DIY and I am working on siing the BMS for each 4 cell unit to get what is best for maximizing the new I/C and while I think it will be eldom that I will use 3k watt continuous often I do want the ability if needed. If I follow you math correctly 280ah*12v = 3360 watts, so 2 such batteries would operate at 3k watts for about 2.24 hours or say a 1500 watt heater for about 5 hours or a 1000 watt heater for say 7.5 hours. That assumes the battery BMSs will allow me to pull 3000 watts. If two batters at 280 AHr 12v each are in parallel they are the equivalent of 560 AHr at 12v. If the I/C is pulling equally from each battery it would pull 1500 watts each would be 125 amps each or 250 amps combined. If I used an S4 130 amp BMS for each battery that would give me 260 a combined. Do I have this correct or will I need a 250a BMS for each battery? Am I missing anything else?
 

daromer

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In theory here on the forum but in theory and in practical in the real world the current doesnt go evenly. It can easy go 20% off or more.

If you need a certain current and you add up bms systems oversize them alot if you ask me. You rather have the fuse blow than the bms and most cheap BMS system doesnt do over current very well.
 

OffGridInTheCity

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Do I have this correct or will I need a 250a BMS for each battery? Am I missing anything else?
Yes, agree with your logic/raw-numbers. And I'd head @daromer info and oversize the BMS as he suggests - which might lead you to go with 3 or 4 smaller batteries instead of 2 larger depending on what you find/prices.
 
Last edited:

1nolaguy

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I have given your recommendations some thought. Forgive me if I am a bit slow to follow but I am new to this. If I plan to build a battery bank with say about 600 amps I have the option of say 2 @ 280 or 3 @ 200 or 5 @120 or 6 @ 100, you get the point. The larger the cell in AHr the larger the required BMS; so while a 100 AHr might be fine with a BMS rated energy will be the same because there is only so many amps available total. If all this is correct and based on the limited space I have available, 3 @200 ahr might be the sweet spot for me. Is there any problem with having an odd number of batteries in parallel? I spoke to Overkill Solar tech support and that advised the 120 Amp 12v BMS will handle 150 continuous so I could go with 3 of those or 4 Daly 150 BMS. 2 @ 280 or 3 @ 200 cells cost about the same +/-. Smaller batteries require more BMSs abut also there is extra wiring etc and space is an issue. Am I missing something?
 

daromer

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No its not correct. Please dont mix Ah with A. Capacity with current.

If you have a system that max can draw 600A you either need 1 bms that easily can do 600!!. Not chinese Amps now...
If you plan to divide this battery into 2 or more you cant just split the current on the BMS. Thats because the batteries potentially can drift and if its 2nd hand cells they will and have drifted. Then i would say they should be 20% larger atleast. If you then take into account that potentially 1 bms could disconnect and leave the other one alone ;)

So if you have 600A max draw and have 2 BMS running and one disconnects that alone need to cope with 600. Thats also why i even recommend to have 1 system work alone. Ie n+1.

So please dont undersize this. Get a proper BMS directly. BMS is there to protect and protection need to be top priority.
 

chuckp

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Please heed Daromer advice.

If you have a 600A load and you split it across say two batteries 300A each i personal would over size the BMS by 50% so your looking at a BMS that can handle 450A. Now hypothetically if for some reason one of the BMS shuts down and cuts one of the batteries you still have a 600A load that now has to be supplied by one battery only. Your 450A BMS on the remaining battery isn’t going to handle a 600A load so that will also shut down and cut that battery too. However the cheaper 450A BMS’s probably wouldn’t handle a 600A surge that well and would most likely damage the BMS. Then you have the situation that when you fix the issues that caused the first BMS to shut down. You still only have one battery trying to supply a 600A load thought a 450A BMS. Hence why needing to use a BMS that can handle the full 600A load + on each battery. Next problem you have now is if your individual BMS’s can a handle 600A + then your separate batteries also as the be sized to be able to deliver a 600A + load without over stressing the individual cells.

I would personally build a 48V battery this way you will draw less amperage per cell from the battery and use only one BMS.
 
Last edited:

Riaz Ahmed

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I have a 48V 5kW inverter. I have 16 cells each of 3.2V and 100Ah. I have a bms 48V 40A but it trips the moment battery is connected to the invertor. Will a 100A bms work or is there another solution.
 

daromer

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@Riaz Ahmed You could have created your own thread ;) But i answer here.
5kw inverter at 48v will pull easily:

5000/51 = 98A....
And bursts 200A

1. Disconnect your system
2. Read the specs of your inverter
3. Make sure your battery can handle 200A. Thats 2C. Will they do that in bursts and 1C in continues? (They should with that size)
4. Install a BMS that easily can handle 200A. NOT a Ebike version BMS but a proper one
5. Put the system back in use.
 
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