Does it make sense to have my BMS only connected to my battery

[severin20]

Hey fam,

So my max power spike is probably 150amps in my 24v system. But I'm too cheap to buy a 150amp BMS.

Can I just set my high/low voltage cutoffs on my charge controller and inverter, and then have the bms solely attached to the battery doing balancing as needed?

That way if the bms shuts off it doesn't leave my charge controller at risk of panels attached but not batteries, and I can pull as large of a load that my inverter can handle.

Does this idea make any sense?

 

[Topper]

Personally,
NO..Not without automatic overvvoltage & undervoltage protection, and auto shut down.. If voltage of a single cell exceeds your balancer capabilities and peaks past 3.65v, you have destroyed a battery, rendering your pack out of service.
Your pack MUST be protected with auto shut down.

 

[larfme]

You do not need to use a BMS for all of your charge and discharge. After much research and testing, I think it's actually a bad idea to run large loads through a BMS. Way too many people have burned up their $200+ BMS units.
If you top balance your cells and they are good cells, you can use a small BMS.
I run my main load (inverter) direct from the battery and have an SSR shut down my inverter if the BMS shuts down from low voltage. I also have a SSR shut down one of my charge controllers if the voltage is too high. I have a second charge controller connected to the BMS directly because I found that the 60A Daly BMS does not cell balance if there is not a charge controller connected to it.
If you do not have a charge controller yet, I would highly suggest getting one with a built in relay for turning it off. It will make this setup much easier.
Also your inverter will need to have a relay or have a physical power switch. Inverters with a soft start switches will not work as well.
If the inverter does not have a relay built in, you will need to put the SSR (solid state relay) in line with the power switch.

 

[lonertic]

Can I just set my high/low voltage cutoffs on my charge controller and inverter, and then have the bms solely attached to the battery doing balancing as needed?

That's what i started off trying, and at first it seemed to make sense since the inverter can shut off in an under voltage event. The problem is that no cells will be identical, even brand new ones, despite best manufacturing efforts. If you look at LFP's discharge curve, the voltage drops sharply at close to 0%.. The issue then is one cell may drop well below it's safe 0% voltage (2.5v) yet the overall bank is still above the cutoff threshold... say you set your inverter cutoff at 20V (8x2.5v), you can have 6 cells still at 5% (6*2.7) but 2 other cells WAY below safe lower limit at 2V each.. And the overall bank will still read 20.2V to the inverter so protection doesn't kick in yet. But by that stage you've already permanently damaged 2 of your cells. I learned that mistake the hard way, my cells were $200/each... So the lesson costed me a lot more than the BMS, which I've now bought.

What the BMS does is it monitors individual cells, so then it cuts off the whole bank as soon as any individual cell drops below threshold.

If you really wanted to save money, you can mimic the same by using an Arduino or similar to monitor individual cell voltage, and as soon as one cell falls out of acceptable range, you send a command to the inverter to cut off discharge immediately. One of the most expensive components of a BMS are the MOSFETs that handle the high current but can also cut power when needed, so if you could get the inverter to do that job, then yes you can get away with it. But you'll need to create the monitoring for the inverter.

 

[Steve_S]

Let me phrase this another way...
Would you choose not to have working brakes on your car ?
That's the BMS' job, not only to monitor the entire battery pack but also the cells within. Even perfectly Matched/Batched & Binned cells will go out of balance with some cells reaching Full or Empty before the others. This can result in "Cell Damage" which affects the entire pack and ultimately lead to a critical failure.

Consider the "Investment" in batteries & assembling all of that. Then consider if the whole installation (building or vehicle etc) is worth putting at risk over a couple of hundred dollars ? Then ask yourself, how much is your life & the lives of your loved ones worth ?

An 8S-200A SmartBMS is NOT that expensive in reality when compared to everything else...
Only you can gauge what risks your willing to take... so seriously, think it through.

 

[Diysolar123]

The BMS is protecting your individual cells as other have pointed out.
You can try and go the DIY route by getting a very low capacity smart bms that will provide you all the data from the individual cells, and then add in an arduino to extract that data from the smart BMS, aannnddd then add in some DC contactors to disconnect the battery bank when either charging or discharging if any of the cells hit their limits... however, when you add all that up, well, you may as well just get a 200A smart BMS.

 

[larfme]

The BMS is protecting your individual cells as other have pointed out.
You can try and go the DIY route by getting a very low capacity smart bms that will provide you all the data from the individual cells, and then add in an arduino to extract that data from the smart BMS, aannnddd then add in some DC contactors to disconnect the battery bank when either charging or discharging if any of the cells hit their limits... however, when you add all that up, well, you may as well just get a 200A smart BMS.

I am not sure why he would need an Arduino. That seems very complex (depending on skill set) for a simple task. Just hook up the BMS as normal. Connect the Inverter directly to the battery (bypass the bms) and if wanted, connect the charge controller directly to the battery as well.
Connect the negative side of the SSR's to the BMS P- and the other side of the SSR's to the Inverter switch and to a relay on the input side of the charger.
The BMS performs all its normal functions and shuts down based on its parameters. This in turn turns off the Inverter and charging.

If your BMS is shutting down due to a runaway cell, it's time for a top balance or cell replacement.

 

[lonertic]

I am not sure why he would need an Arduino. That seems very complex (depending on skill set) for a simple task. Just hook up the BMS as normal. Connect the Inverter directly to the battery (bypass the bms) and if wanted, connect the charge controller directly to the battery as well.
Connect the negative side of the SSR's to the BMS P- and the other side of the SSR's to the Inverter switch and to a relay on the input side of the charger.
The BMS performs all its normal functions and shuts down based on its parameters. This in turn turns off the Inverter and charging.

If your BMS is shutting down due to a runaway cell, it's time for a top balance or cell replacement.

I'm still new and not familiar with all the inverter types but the issue if that were my setup (hybrid PV inverter) is that when there's PV again, the inverter will be able to charge the batteries but it's got no way of waking up the BMS again since it's technically on a separate circuit holding the relay open, separating the inverter charging source from the battery. Whereas if the BMS is connected to the inverter directly it'll sense the incoming charging voltage and engage the batteries. That's the part that, if it were separated, I'd imagine will require some programming to programmatically tell the BMS or inverter what to do.

 

[severin20]

Thank you everyone for taking the time to respond. Feels like I should just run a 48v system and go for a BMS, makes my life easier and no need to fiddle with the SSR etc.