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The Battery Management System I really need (mix)

jall

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Jul 12, 2021
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Hi pals.
This is my first post here. I hope not repeating any already discussed matter in other threads.
I wonder if someone here has tried the following setup:
1- Using a low amp BMS (60A for instance) connected to a 12v 300Ah LIFEPO4. That BMS ONLY would feed current to the low amps loads in the vehicle (but NOT charging).
2- High AMPS (3kw inverter) is directly connected to the battery (inverter has iits own shutoff -adjustable- protection)
3- Charge wuld be directly connected to the battery (not through the BMS) and comes from alternative 3 ways:
a- MPPT solar regulator (correctly programmed)
b- Utility AC charger 75A (correctly programmed)
c- Engine´s 250A alternator (trough a cutoff protection relay at 14,2 v)

Since this is my first solar system, I supose is better to ask to the experts. I know that probably the first answer could be "buy a 500A BMS", but I am reluctant to pay for the BMS almost the same the cost of the battery. My idea is to use a Daly smart 60A since what I want is the live data information (BT) plus balancing (underdischarge protection for low amps loads is aldo desired).
The best BMS I would like to use is a 60 amps with active balancer (1A min) with BT link, but I can´t find such product in the usual online channels.

So, let me know if you see something stupid or it has sense
 
2- High AMPS (3kw inverter) is directly connected to the battery (inverter has iits own shutoff -adjustable- protection)
Not good enough. It needs to monitor each cell and shutoff when individual cell voltages are out of bounds.
Your battery could have cells 3V, 3V, 3V, 4V and the 3Kw inverter would still be charging.
 
The BMS can only cut off the load if it's between the load and the battery. So if the directly attached devices fail in a destructive manner, the BMS isn't going to be able to help. You are then at the mercy of the quality and reliability of the 4 devices and the BMS which are directly connected.

Most people install a BMS to provide one protection system as an additional point of protection above and beyond the protections built into individual units. This means two things have to fail before battery destruction: the BMS and the failing device.

In your situation you have a lot of single points of failure, which would work fine.

Until it doesn't.

It's just a question of cost/benefit. If you can afford a new battery pack and the associated downtime, then maybe you'd rather hold onto the money and use the cheaper BMS, knowing that a slightly higher chance of destructive failure is worth the money (to you).
 
Using a BMS with a contactor or relay seems to be what you are looking for. The BMS Will shut off a high amperage relay switch that's connected to all your loads. It's much cheaper to use a small BMS and a high amperage relay than it is to use a high amperage BMS. For a 3k watt, 12 volt inverter, you should use a relay that can handle 300+ amps.

Honestly, if you need more than 2000 watts, I would go with a 24-48 volt system. A 12 volt system can put out 3000 watts at about 250 amps, a 24 volt system will do 3k watts at ~125 amps, and a 48 volt will do the same at ~62 amps. Higher VOLTAGE BMS units don't generally cost that much more, it's the higher AMPERAGE that increases the cost substantially.

Like @MisterSandals said above, the BMS will cut off charging and discharging if a SINGLE cell voltage gets too high or too low, the inverter will not, which could easily overcharge / over discharge your cells and damage them. Don't rely on a low voltage disconnect for the entire pack.

You said you were looking for active balancing @ 1 amp per MIN. Most cheaper BMS units only have passive balancing and the ones that have active are normally 1-5 amps per HOUR. If your cells are properly top balanced, the passive balancing available on most cheap BMS units should do just fine. If you feel like you need more, Heltec makes a 5 amp, 4s, balancer that's about $30.

Also, don't charge LiFePo4's directly from an alternator. They can take pretty much any amount of power you throw at them, and will easily overload your alternator, it thinks it's basically a dead short and pumps out it's max potential. They aren't designed to be used (high power) while the vehicle is stationary, or for charging large lithium batteries. A quick search for "alternator charging" on here will give you lots of good info, but I use a DC to DC charger to limit the current drawn from the alternator to 40a.
 
The 3 answers are really contributive, and makes me sure I have to think more about. All your sugestions are good points to take into account.
Remember that we are talking about a VEHICLE (so, 24 V is not suitable)
The problem with the Hi AMPS is that a reasonable 250 Amps BMS will only admit 120 amps at charge, but I have an upgraded 250A alternator which is intended to feed the power directly to the inverter if needed (not at night, of course).
Higher amps BMS are also available, but at 400 USD or more (remember I want live data display, so, "smart" is required).
40A for charging the batt is aceptable for the PV panel, but not for the alternator.
The iinverter is 3kW continous (9kW 9 minutes), but such power will be only needed during minutes (basically cooking). What will be more constant is a 1kw load (air conditioning), and that load is what would drain the battery if the alternator doesn´t feed enough.

Thanks again. It is clear that this site is the correct place to ask experienced people :)
 
Hi Mistersandals.
Sorry for the confusion. No. The brand of the inverter is "NAT Power System" RP series, pure senoidal wave low frecuency (toroidal transformer). It also has 75A charger and is fully parametrizable via LCD display, even with "user defined battery" option. Has a "transfer relay" and automatic switch for blackout.
Happy with such inverter since I tried others before and the experience was quite frustrant. Also price is excellent
There are some reviews in the internet (I plan to do my own in a few days).
You can see here a video of the checking stage at the factory:
 

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Using a BMS with a contactor or relay seems to be what you are looking for. The BMS Will shut off a high amperage relay switch that's connected to all your loads. It's much cheaper to use a small BMS and a high amperage relay than it is to use a high amperage BMS. For a 3k watt, 12 volt inverter, you should use a relay that can handle 300+ amps.
That is exactly whay I would like, specially because the cut-off is symetrical (same current in charge and discharge). The issue here is how to drive the relay . Maybe using the gate signal from the power Mosfet to drive a smaller one driving the coil?
Like @MisterSandals said above, the BMS will cut off charging and discharging if a SINGLE cell voltage gets too high or too low, the inverter will not, which could easily overcharge / over discharge your cells and damage them. Don't rely on a low voltage disconnect for the entire pack.
Yes, very good point !
You said you were looking for active balancing @ 1 amp per MIN. Most cheaper BMS units only have passive balancing and the ones that have active are normally 1-5 amps per HOUR. If your cells are properly top balanced, the passive balancing available on most cheap BMS units should do just fine. If you feel like you need more, Heltec makes a 5 amp, 4s, balancer that's about $30.
Sorry for the confussion. I said balancing at 1A min ("at least" or "minimun"), not "per minute". Most BMS have ridiculous 30 mA or so balancing current, what I think is not functional for a 300Ah batt .
As you said before: modest "smart BMS" with relay is the master solution.

I titled this thread: "the BMS I really need" because is not the usual one. Taking in account the coments received, the idea is:
to use a low amps smart BMS driving a relay. That performs the functions of:
- low current balancing
- live data information with BT dongle
- overcharge and overdischarge cut-off protection
- unbalancing cut-off protection
For more serious balancing maintenance I could use a capacitive active balancer with manual switch. Its action can be monitorized via the BSM BT live data monitoring.
That setup would fit perfectly my needs.
Any proposal for the base BMS? Daly for example?
 
That is exactly whay I would like, specially because the cut-off is symetrical (same current in charge and discharge). The issue here is how to drive the relay . Maybe using the gate signal from the power Mosfet to drive a smaller one driving the coil?

Yes, very good point !

Sorry for the confussion. I said balancing at 1A min ("at least" or "minimun"), not "per minute". Most BMS have ridiculous 30 mA or so balancing current, what I think is not functional for a 300Ah batt .
As you said before: modest "smart BMS" with relay is the master solution.

I titled this thread: "the BMS I really need" because is not the usual one. Taking in account the coments received, the idea is:
to use a low amps smart BMS driving a relay. That performs the functions of:
- low current balancing
- live data information with BT dongle
- overcharge and overdischarge cut-off protection
- unbalancing cut-off protection
For more serious balancing maintenance I could use a capacitive active balancer with manual switch. Its action can be monitorized via the BSM BT live data monitoring.
That setup would fit perfectly my needs.
Any proposal for the base BMS? Daly for example?
Chargery BMS uses a contactor I believe, and there are other brands that do, but I can't find any at the moment.

I have heard that the Chargery is a good brand, and runs about $100.
 
Sorry for the confussion. I said balancing at 1A min ("at least" or "minimun"), not "per minute". Most BMS have ridiculous 30 mA or so balancing current, what I think is not functional for a 300Ah batt .

I have 280Ah batteries using the Overkill Solar 120 amp BMS. My cells are staying very well balanced. I think you're making too much out of cell balancing, unless you plan to purchase low quality cells.
 
1) Charging & Discharging from an LFP pack and not using a BMS is simply put really not smart. The BMS is the Brakes & Emergency brake like on your car... Car with No Brakes = ????
2) LFP Rules: LFP can discharge up to 1C and charge to 0.5C MAXIMUM. 280AH battery can output 280A for 1 hour and take 140A charge for 2 hours. Attempt to push more than the capacity and you WILL have major issues !
3) Passive Balancing is virtually useless on Large Capacity cells (that was not their design intent which is for smaller cells) and this even includes properly Matched, Batched & Binned cells. Active Balancing with >5A capacity will work on these large capacity cells. The higher the Active Balancer amperage is the higher the cost.
4) Alternators (wrong kind) can & will destroy Lithium Batteries, All lithium based batteries are VERY Voltage & Amperage sensitive. The Marine folks have gone down that dark path and have solutions that work well.

 
Thanks all for the help on my desk. Your opinions and experience are very valuable and really appreciated.
Following the clue from @A:Justice, I took a look at the CHARGERY website and they have EXACTLY what I need: a professional BMS with separate display, external shunt (up to 600 A), smart (programable and displaying all the needed parameters at your hands), and commanding TWO SEPARATE relays (one for charging and the second for discharging). In adition, the balancing current is 1.5A. it has no BT nor app, but the included display unit (can be disconnected on the fly and let the main unit stand alone) is more than enough. Also 2 extra seral COM are included. Price in Aliexpress is afordable for a profesional solution as this, and have an impresive technnical manual covering all subjects in 67 pages full of diagrams, screen captures, data, etc. This manual (already printed in my hands) is really GOOD, and is much more than usually we can get from a manufacturer. Also there are described the changes in the software versions up to date...
The product is BMS8T, and the link: https://www.aliexpress.com/item/329...exp_id=8aeaf362-920a-41c8-a53d-48f12d28a101-0
So, I never heard before about Chargery company, but they have what I need and I have discovered it thanks to this thread. Thank you very much folks, Jose
 
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