Midnite Solar Classic BMS communications

[OffGridInTheCity]

Offgrid-I’m curious what happens on a cold, sunny day when the BMS shuts down the batteries and the controller is left in an extended high voltage condition.

In my case / climate - the batteries never go below 5C/51F so this isn't an issue.
But if it were to occur - the BMS disconnects the batteries from the the 'system' via the shunt-trip breaker, so the Charge Controllers are no longer connected to them and can't try to charge them.

Are Midnites uniquely able to handle this? It seems to violate the rule of “hook up the batteries first.”
Or can the Midnite tell itself to shut down the input from the panels before the battery BMS calls it quits?

Midnites are fine when disconnected from the battery as described above - they just stop charging regardless of incoming PV. They are made to shunt-off the incoming PV, no problem. Midnites even allow you to set the max charging amps (going out to the battery) OR PV amps coming in - regardless of PV array itself


I use this because I have 80a breakers at the battery control box but my array can occasionally generate up the 86a the 150s allow - and in fact caused my 80a breakers to trip. So rather than buy 100a breakers I just set the max output to 80a you see above.... since it rarely happens.

 

[toms]

I forgot to mention that @toms point about slowing the charge (reducing the current) near the end to give the BMS an opportunity to balance the cells near the full charge point is a good one. Much better reason than anything else I've heard about why an SCC and BMS would communicate. The only issue I would have with it: How are existing loads that are taking advantage of the PV harvest affected? In a system with no communication between the charge controller and the battery, the SCC simply provides as much as it can, limited by what current everything will take, including both the battery and whatever loads there at the time. If the BMS says it wants less current (even though the attached battery will happily take more), does the SCC simply cut the current back to that point? What happens to the loads?

If the battery is balancing at 3amps, and a 10amp load is applied, the BMS calls for 13a from the SCC. Same scenario when in “float” mode.

 

[Horsefly]

In my case / climate - the batteries never go below 5C/51F so this isn't an issue.
But if it were to occur - the BMS disconnects the batteries from the the 'system' via the shunt-trip breaker, so the Charge Controllers are no longer connected to them and can't try to charge them.


Midnites are fine when disconnected from the battery as described above - they just stop charging regardless of incoming PV. They are made to shunt-off the incoming PV, no problem. Midnites even allow you to set the max charging amps (going out to the battery) OR PV amps coming in - regardless of PV array itself

View attachment 81481
I use this because I have 80a breakers at the battery control box but my array can occasionally generate up the 86a the 150s allow - and in fact caused my 80a breakers to trip. So rather than buy 100a breakers I just set the max output to 80a you see above.... since it rarely happens.

So @OffGridInTheCity, your situation makes me expand the question I asked above. Let's say that in your situation the "shunt-trip breaker" (not sure what that is) disconnects the charge controller from the battery. I would assume that the disconnect not being directional MOSFETs, the Inverter is also disconnected from the battery at that point as well, right? So in this hypothetical, are all loads cut off near freezing as well, even though the battery can safely discharge at these lower levels?

 

[OffGridInTheCity]

So @OffGridInTheCity, your situation makes me expand the question I asked above. Let's say that in your situation the "shunt-trip breaker" (not sure what that is) disconnects the charge controller from the battery.

A shunt-trip breaker is a regular breaker that can be turned on/off but it can also be tripped (turned off) by an external voltage - e.g. apply 24v or 120v for a couple of seconds or continuously (like a relay coil) and the breaker will turn off. It must be reset/turned on manually. For larger powerwalls - ABB SACE used breakers can be purchased for a good price on ebay - like this 200a with 2 poles and a 120v coil - https://www.ebay.com/itm/144120566891?hash=item218e41546b:g:3ogAAOSw6Itg9wSq. I use a 2 pole 400a one for my system.

I would assume that the disconnect not being directional MOSFETs, the Inverter is also disconnected from the battery at that point as well, right? So in this hypothetical, are all loads cut off near freezing as well, even though the battery can safely discharge at these lower levels?

Yes! In my case I have it set to go critical for both charge and discharge at 5C/41F.....


However, Batrium can distinguish between between charging (a higher cut-off) vs discharge (a lower cutt-off) if needed AND you can have multiple breakers (shunt-trips) - one between Charge Controllers and battery and one between the Inverter and the battery. To me, 40F is just 'hard' on the cells even if they have specs for lower and I should never get that low under the house.

 

[OffGridInTheCity]

I forgot to mention that @toms point about slowing the charge (reducing the current) near the end to give the BMS an opportunity to balance the cells near the full charge point is a good one. Much better reason than anything else I've heard about why an SCC and BMS would communicate.

This whole thing about 'top balancing' doesn't work for me.

First - with healthy packs, I only need to touch up balancing every 6-9 months or so. I have 84 packs of 260ah each of lithium-ion and once balanced the max difference will drift 20-30mv in 6 months. One does not *need* daily balancing at all.

Second - since I'm 100% off-grid I have a fixed inverter cut-off at 49.5v (3.54v/cell) and I allow it to charge up to 56v (4.0v/cell) but it rarely get's all the way up. Only in spring/summer will it reach 4.0v/cell... as the inverters turn on at 51v (3.65v/cell) and start delivering power. There is usually excess PV compared to the load so the batteries will charge up to 52v of 53v etc... but rarely get to 56v because if they charge all the way up then I'm actually 'losing power' from the PV. The battery and operating parameters are sized to allow all this.

Bottom line - top charge/balance as a regular occurrence just doesn't make sense in my operation.

 

[OffGridInTheCity]

Example of BMS talks to the rest of the system

Was watching a youtube series (@Lumnah Acres) and they went with a 17.8kw PV array (40 x 445 dual side panels) and a complete Lion Energy Solution using 3 x "Delux" https://lionenergy.com/pages/sanctuary#why-lithium This is 3 x (8kw PV + 8kwh Inverter? + 27kwh battery) = 24kw PV/Inverter and 81kwh battery bank and the components are integrated all together.

Around 29:00 minutes in, he says he chose this solution because it was the only one he could find where it all talks to each other.

If your interested. Its light on technical detail but gives a good high level flavor.

He didn't mention price - I wonder!!

 

[toms]

Bottom line - top charge/balance as a regular occurrence just doesn't make sense in my operation.

I agree totally with this. That is the whole point of a well integrated system.

My balancing function often goes years without activating. You say you “touch up” the balancing - a well integrated system will give a cell a bit of leeway (eg 100mv) at its upper set point, then rein it in over a controlled period.

This is difficult to achieve unless you have accurate control of balancing charge current.

 

[OffGridInTheCity]

I agree totally with this. That is the whole point of a well integrated system.

My balancing function often goes years without activating. You say you “touch up” the balancing - a well integrated system will give a cell a bit of leeway (eg 100mv) at its upper set point, then rein it in over a controlled period.

Yes sir. In my case I'm retired and monitor things as part of my hobbies - so I can use the Batrium auto-level manually every 6-9 months.

This is difficult to achieve unless you have accurate control of balancing charge current.

The Batrium auto-level allows balance mid-cycle based on pack differences rather than a fixed voltage. This can be on all the time but seems a waste so I leave it off until needed and just run it manually for a few days when desired.

If I wasn't retired/able to monitor closely - I'd leave it on all the time but widen the "Different Gap" to something more lenient as you suggest.

Why do my packs drift? Could be 'just a touch' of self-discharge but I'm not sure - its very subtle.
The overall battery is a mix of cells but the fact that the packs go 6-9 months with only 30mv max difference drift over that time is operationally OK for me.

The oldest battery (packs #15 -> #28) below is at 1,226 cycles counting and at this point in the charge/discharge cycle it's near the max of 70mv for the day. At the low cut-off of 3.54v/cell it will reduce to 40mv max difference. This 40mv -> 70vm -> 40mv is a daily cycle and about as close as I can make it.

 

[Horsefly]

Bottom line - top charge/balance as a regular occurrence just doesn't make sense in my operation.

I think your logic is sound. The lesson here is that no one should ever say "always top balance" and no one should ever say "don't ever top balance". I think your situation sounds pretty unique. I don't often hear someone say that their batteries don't get fully charged because it would mean wasted PV. Interesting way to think about it, but probably not common.

In any case, your situation is yours and not trying to be balanced at the top (or ever even get to the top) is your choice. For most of the folks that I see posting here, I think an initial top balance makes perfect sense. Passive balancing (by a much less expensive BMS than the Batrium) is often sufficient, and active balancing by either a BMS or a separate device can be useful.

 

[OffGridInTheCity]

I think your logic is sound. The lesson here is that no one should ever say "always top balance" and no one should ever say "don't ever top balance". I think your situation sounds pretty unique. I don't often hear someone say that their batteries don't get fully charged because it would mean wasted PV. Interesting way to think about it, but probably not common.

This comment is interesting because I don't see how anyone can reach 100% charge on a daily basis for an off-grid system (no grid-tie) with steady consumption.

If you optimize for spring/summer (max PV) to just reach 100% charge (nothing wasted); consume thru the night to the next day where you can repeat with the inverter never stopping - this is 'an ideal' where nothing is wasted and you use the full SoC (voltage range/capacity) of the battery. However - on cloudy days or in winter when PV reduces by 50% or 75% - you'll never reach 100% charge till next spring/clear days and max PV returns.

If you optimize for 24/7 operation in winter with 100% charge vs consumption - and you're off-grid - then yes, you'll hit 100% charge every day (almost) but waste 30-40-50% of spring/summer PV power because you didn't consume it.

To me - it seems like reaching 100% charge every cycle is abnormal for an off-grid system with the goal of using all the PV that can be created.

One's operating goals make all the difference here. I started off to achieve 100% PV consumption for best ROI but have found this means being underpowered in winter, having to rely on propane/generator in a true emergency. The other goal is to be viable off-grid all year round! So I'm in the process of re-thinking to waste/divert power in summer for the sake of being successfully off-grid with PV in the winter.

 

[Horsefly]

This comment is interesting because I don't see how anyone can reach top charge on a daily basis for an off-grid system (no grid-tie) and have constant (steady) consumption.

If you optimize for spring/summer (max PV) to just reach 100% charge (nothing wasted); consume thru the night to the next day where you can repeat with the inverter never stopping. This is ideal - nothing wasted and you take full SoC of the battery. However - on cloudy days or in winter and PV reduces by 50% or 75% - you'll never reach 100% charge till next spring / max PV unless....

1) You dramatically lower consumption in winter? so that you continue to reach 100% charge on 30% of the PV you did in spring.
2) You dramatically change the DOD / operating voltage of the charge/discharge curve.

To me - it seems like reaching 100% charge every cycle is 'abnormal'

Well, since you asked I'll be a bit more pointed.

I think that every since solar panels became relatively (maybe ridiculously) inexpensive, it has been really easy to over-panel. That is, create a PV array that is always going to fully charge your battery bank in moderately good conditions. For some of us (including me), even a moderately good day of sun is going to fully charge the batteries.

My thinking is that it is easy to over-panel (not expensive). To me, it sounds like you are under-paneled. Batteries (with the included BMS) are generally more expensive than PV to charge them. If you seldom fully charge your batteries... well, sounds to me like you made a choice I would never make: over battery, but under panel.

Anyway, I think NOT reaching 100% most days is 'abnormal' but to each his own.

 

[OffGridInTheCity]

Well, since you asked I'll be a bit more pointed.

No offense - I appreciate the discussion!

I think that every since solar panels became relatively (maybe ridiculously) inexpensive, it has been really easy to over-panel. That is, create a PV array that is always going to fully charge your battery bank in moderately good conditions. For some of us (including me), even a moderately good day of sun is going to fully charge the batteries.

But this means that PV is lost and ROI degraded. I agree this is perfectly OK but part of this was selling the ROI to my wife and seemed like such a waste in the beginning.

My thinking is that it is easy to over-panel (not expensive). To me, it sounds like you are under-paneled. Batteries (with the included BMS) are generally more expensive than PV to charge them. If you seldom fully charge your batteries... well, sounds to me like you made a choice I would never make: over battery, but under panel.

I'm not under-paneled because of battery - I choose large battery (18650 / lithium-ion) for extra long life... e.g. 10,000 cycles I hope. So setting the battery size aside, I agree it's about over-panel or not. Over-panel = wasted PV unless you sell back to the grid. And it's the amount of acceptable over-paneling.

I have 45 panels = 13kw PV array. This power's my home 'acceptably' for 8 months of the year. But to cover the other 4 months I need 90-105 panels! - a 13-17kw over-panel situation. That means that for 6 months of the year I'd be wasting upwards of 1500-2000kwh/month - a large amount. I don't have enough diversion consumption to make dent in that amount of over-paneling and I don't want to grid-tie. But I may get there if I truly want to live comfortably all year round on PV. I'm noodling on it

Anyway, I think NOT reaching 100% most days is 'abnormal' but to each his own.

I appreciate your comments as it helps explain.

 

[Horsefly]

But this means that PV is lost and ROI degraded. I agree this is perfectly OK but part of this was selling the ROI to my wife and seemed like such a waste in the beginning.

ROI is time based. I won't challenge the statement, but in order to evaluate you need to consider that the lifetime of PV is at least 20+ years. I think the lifetime of your 18650 cells is probably less than that (maybe quite a bit less). I don't know what your assumptions are, but... Again, sounds like a choice I would not make.

 

[toms]

In Australia, PV is ridiculously cheap. I just picked up a 4kw system including all racking/cabling and a SunnyBoy 4.0 inverter for $400AUD

A days work to add to my system.

My batteries used to go long periods without seeing 100%. These days not so much ?

 

[Steve_S]

Quickly skimmed through the rest of the responses...

LFP behaves like LFP and not like other Lithium Batteries, they all have their own "personalities & quirks". So people NEED TO BE CERTAIN which sub-chemistry they are talking..

Between March to End of September, my solar Can and often Does charge the bank to 100% (Weather permitting of course). But My 100% is 3.425Vpc for the cells across the bank. Considering my Bank is quite oversized for my Solar Input that can be tricky if I have run heavy loads and go too deep to recover it within a day. This is simply because I have a large reserve due to my locale and situation to cover myself in Winter.

 

[Jbbonnette]

% I dunno anything about the combox... There's more info & discussion on all of this at http://midniteftp.com/forum/index.php and such questions are likely better there... They also watch closely and pickup things that are needed/wanted. I love the folks at Midnite they really are Great but they are not the speediest by any means when it comes to putting product or even updates out.

Steve,

Trying to understand the AUX2 Logic Input-2. I tryed with (resisted) 12v signal from the Batrium BMS. The idea was to have the batrium send a 12v signal to "stop charging" when the Batrium Shunt sees 90% SOC.

I'm missing something- I put a resistor on both +/- of 12 as a test, but the negitive got so hot to the touch I x'ed the test right there. Is this not a means to force the Classic into "resting"?

The link below does state "Many Lithium Battery Systems with intelligetn BMX have this signal available" Follow-on question: Is this "signal" the 12v such as my test? Is there a different/better way? Or am I completely off?

Thanks,

MidNite Solar Help

customer support platform

www.midniteftp.com

 

[Steve_S]

I use it clean and to my qualified 100% which is 3.425Vpc in the battery packs.

I can't help with combo box or extra twiddles with the aux subsystem.