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How to choose the correct BMS for my RV battery
#1
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 roof and the battery bank I'm adding will be used to power 12v D/C and 30a/50a A/C loads (5000w inverter).

How do I know what amperage BMS to buy? I was told I should buy a 150A BMS by the seller of the battery 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
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#2
(05-17-2020, 05:22 PM)xtjoeywx Wrote: 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 roof and the battery bank I'm adding will be used to power 12v D/C and 30a/50a A/C loads (5000w inverter).

How do I know what amperage BMS to buy? I was told I should buy a 150A BMS by the seller of the battery 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/50a A/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.
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#3
(05-17-2020, 07:40 PM)OffGridInTheCity Wrote: 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: -0°C-55°C Discharging: -20°C-60°C
- Storage temperature: -0°C∼+60°C
- Battery Weight: About 5.25KG

I plan on buying 20 of these to make 5 batteries. Will that work?
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#4
(05-17-2020, 07:59 PM)xtjoeywx Wrote:
(05-17-2020, 07:40 PM)OffGridInTheCity Wrote: 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: -0°C-55°C Discharging: -20°C-60°C
- Storage temperature: -0°C∼+60°C
- 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 these to 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 least 5000wh.    

>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 single BMS with low amps (cheaper) and have it turn on/off a contactor rated for 500a - maybe something like a Gigavac GV200 - https://www.waytekwire.com/item/77078/50...gJsbvD_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?"  https://youtu.be/J2KH21Fwi9o

If you do 5 separate 4s batteries, then you need 5 separate 4s 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 get answers to your questions.  


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#5
https://www.solar-electric.com/learning-...lar-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.
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#6
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.
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#7
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.
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#8
(05-17-2020, 10:18 PM)gpn Wrote: https://www.solar-electric.com/learning-...lar-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.


(05-17-2020, 09:32 PM)OffGridInTheCity Wrote: 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.

(05-17-2020, 09:32 PM)OffGridInTheCity Wrote: 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 single BMS with low amps (cheaper) and have it turn on/off a contactor rated for 500a - maybe something like a Gigavac GV200 - https://www.waytekwire.com/item/77078/50...gJsbvD_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?"  https://youtu.be/J2KH21Fwi9o

If you do 5 separate 4s batteries, then you need 5 separate 4s 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 get answers 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?

(05-18-2020, 05:10 AM)daromer Wrote: 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 or a contactor if I have 5 BMSs (one for each battery)?
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