LiFeP04 Batteries for Solar & BMS

[northerner]

LL FWIW I know you are not looking at Midnite Solar Classic Controllers but any of the Classic controllers have an Auxiliary Input (Aux 2) that allows an external signal to shut the charge off. Perfect for using a BMS.

Classic Manual

Does it shut off charging or force the controller into a lower float voltage? My understanding is it would be ideal to have the charge controller drop down to a lower float voltage. This way the charge controller will still provide power to the loads without unnecessarily cycling the battery up and down. Just trying to comprehend what goes on after charging a LiFePo4 battery is complete?

I know that some users make use of opportunity or waste not modes using the Midnite Classic, where any power beyond what the battery uses is made use of. If the solar controller shuts off charging, would the extra power from waste not or opportunity modes still be available? I could see that being the case if the Classic dropped down to a lower float voltage?

Or maybe it is ideal that you set up the Classic to go to float after reaching a preset bank voltage, and having a HVD event is something that shouldn't normally happen? I also take it that the controller would resume charging once the HVD signal is no longer present?

Update: Looks like the charging function is disabled as long as the signal is present on the AUX 2 input to the Midnite Classic. When voltage of the cell that triggered the HVD drops down, it would no longer give a signal to disable the CC, and the Midnite Classic would then resume charging. This could alternate back and forth.

[Living Large]

Does it shut off charging or force the controller into a lower float voltage? My understanding is it would be ideal to have the charge controller drop down to a lower float voltage. This way the charge controller will still provide power to the loads without unnecessarily cycling the battery up and down. Just trying to comprehend what goes on after charging a LiFePo4 battery is complete?

I know that some users make use of opportunity or waste not modes using the Midnite Classic, where any power beyond what the battery uses is made use of. If the solar controller shuts off charging, would the extra power from waste not or opportunity modes still be available? I could see that being the case if the Classic dropped down to a lower float voltage?

Or maybe it is ideal that you set up the Classic to go to float after reaching a preset bank voltage, and having a HVD event is something that shouldn't normally happen? I also take it that the controller would resume charging once the HVD signal is no longer present?

This discussion is about HVD, a failure mode. Would not normally happen.

Regarding the MS Classic, I could not find the described functionality of shutting down based on an input at Aux 2 in the manual, and I have an email into MS to confirm if it does or does not support this. The Aux ports have a lot of options, so I may have missed it.

Regarding your UPDATE. So there evidently is a function to support this. I need to go back and look again. My impression was this event shouldn't normally be happening in my setup, and my inclination would be a shutdown that requires intervention. The signal could be latched externally if this was desired.

[northerner]

This discussion is about HVD, a failure mode. Would not normally happen.

Regarding the MS Classic, I could not find the described functionality of shutting down based on an input at Aux 2 in the manual, and I have an email into MS to confirm if it does or does not support this. The Aux ports have a lot of options, so I may have missed it.

Would not normally happen, but if it did happen, it looks like it could alternate back and forth between charging and not charging. Would it not be better to have it forced to a lower float voltage, if possible?

I looked for this function as well and according to Midnite it is there, but I don't see a description on how to use it?

[northerner]

Regarding your UPDATE. So there evidently is a function to support this. I need to go back and look again. My impression was this event shouldn't normally be happening in my setup, and my inclination would be a shutdown that requires intervention. The signal could be latched externally if this was desired.

Yes, latching would work, and then one could unlatch it at a lower voltage. It could easily happen with a slight imbalance with the cells, and differences in charging rates, but depends on the margins of safety you put in as well.

If you disable charging with an HVD, with no one around at the time, you could run the batteries down and wind up with an LVD to follow.

[Living Large]

Yes, latching would work, and then one could unlatch it at a lower voltage. It could easily happen with a slight imbalance with the cells, and differences in charging rates, but depends on the margins of safety you put in too.

If you disable charging with an HVD, with no one around at the time, you could run the batteries down and wind up with an LVD to follow.

It depends what you want it to do. Based on the discussion we have been having, I was anticipating that for me it would not be a typical event. But hey, if it could be an added part of normal operation, that would add flexibility in what is possible.

I saw it as a hard error, and was thinking of shutting the whole system (MPPT, inverter/charger) down for either an LVD or an HVD.

[Sunking]

Does it shut off charging or force the controller into a lower float voltage? My understanding is it would be ideal to have the charge controller drop down to a lower float voltage. This way the charge controller will still provide power to the loads without unnecessarily cycling the battery up and down. Just trying to comprehend what goes on after charging a LiFePo4 battery is complete?

That is where you have to get out of the Pb box thinking. When the first cell reaches 90 to 95% you terminate charge period.

Now you could switch to float providing the float voltage is lower than the pack resting voltage so all charging stops. The LFP battery would have to drain down to Float Voltage before the panels would supply any power. Ideally if you have a day with very little use say gone over night somewhere, is just turn the controller off and not recharge the next day. All that can be done with BMS. Its one less cycle or nail in the coffin. LFP does not need to be 100% SOC. In fact better if it is never 100%.

[Sunking]

It depends what you want it to do. Based on the discussion we have been having, I was anticipating that for me it would not be a typical event. But hey, if it could be an added part of normal operation, that would add flexibility in what is possible.

I saw it as a hard error, and was thinking of shutting the whole system (MPPT, inverter/charger) down for either an LVD or an HVD.

LL we are talking two separate issue. LVD is physically either a Relay or Logic signal to operate a relay to disconnect a load, or a signal to Inverter to turn it off. There is no HVD in our discussions. Just a signal sent to the CC to turn off. If that cannot be done then a HVD relay to disconnect the panels. However I am confident th eMS controller can be controlled by aux inputs. Sounds like Xantrex can also be controlled. Let us know what they say. I already know what MS is going to say.

[northerner]

That is where you have to get out of the Pb box thinking. When the first cell reaches 90 to 95% you terminate charge period.

Now you could switch to float providing the float voltage is lower than the pack resting voltage so all charging stops. The LFP battery would have to drain down to Float Voltage before the panels would supply any power. Ideally if you have a day with very little use say gone over night somewhere, is just turn the controller off and not recharge the next day. All that can be done with BMS. Its one less cycle or nail in the coffin. LFP does not need to be 100% SOC. In fact better if it is never 100%.

I agree that you only want to get to 90% or so on the first cell that reaches 90%. All I was suggesting is that it's better if you can force the charge controller into a lower float voltage, rather than shut off the charge controller or panels.

It looks like the way the Aux 2 input works is it will disable charging as long as a voltage signal (ie from the BMS) is present. I know I wouldn't want to permanently latch off charging, but also would not want it alternatively switching from able to disable, as the voltage hovers around the HVD trigger point.

[northerner]

About LVD

About LVD, I think that if your power system is essential, it may be worthwhile to have the LVD switch to a smaller temporary backup battery, and then disconnect the main battery from the system. I know that would apply to me who is dependent on a furnace, etc... to prevent the house from freezing.

If you happen to have an LVD event, could you not be stuck down a creek without a paddle if not prepared? It could prevent one from switching on the charge controller, the inverter/charger, plus you would have no power. I would take it that the battery pack voltage may recover enough to be able to run your inverter/ charger to be able to run the generator, in order to recharge the pack. An alternate plan would be to have a separate charger that can recharge the batteries from the generator.

[Sunking]

In off grid system requires a generator.

[northerner]

In off grid system requires a generator.

Yes, but the inverter/charger won't work if the LVD has shut the system down, unless the voltage of the cell recovers?

[Living Large]

LL we are talking two separate issue. LVD is physically either a Relay or Logic signal to operate a relay to disconnect a load, or a signal to Inverter to turn it off. There is no HVD in our discussions. Just a signal sent to the CC to turn off. If that cannot be done then a HVD relay to disconnect the panels. However I am confident th eMS controller can be controlled by aux inputs. Sounds like Xantrex can also be controlled. Let us know what they say. I already know what MS is going to say.

Dereck, here was my thinking as a noobie, again with no equipment or experience (please point out where I went wrong). First of all, my misuse of terminology HVD and LVD may be part of the problem.

Here goes. I was hearing that you want to never charge a cell over a certain voltage, or drain below another certain voltage. Inside of this window would be the normal operating range, with a higher low and a lower high threshold - these are programmed into the CC and the inverter/charger. Back to the larger window, the Orion BMS (for example) will detect the two "never exceed" events for the first cell that it happens to.

So my thinking was the CC or charger might be charging when the HVD level is exceeded, and I would want it to stop charging. So while not literally a "disconnect", by issuing a signal to both the CC and inverter/charger via the Aux ports, charging would stop.

Likewise an LVD signal could also be issued to both, whatever condition causes it to arise. In either case, this rare event would cause me to find the system shutdown, and I'd have to perform a repair/replacement.

Please explain how this possibly simplistic thinking falls short. Is this insufficient to accomplish what is needed? Do you really need a hard disconnect in addition to shutting down the CC and/or inverter/charger?

ps Did you see that Northerner also didn't see the Aux port shutdown in the Classic manual? I need to look again. I didn't hear back from them yet.

[Sunking]

Here goes. I was hearing that you want to never charge a cell over a certain voltage, or drain below another certain voltage. Inside of this window would be the normal operating range, with a higher low and a lower high threshold - these are programmed into the CC and the inverter/charger. Back to the larger window, the Orion BMS (for example) will detect the two "never exceed" events for the first cell that it happens to.

So far so good.

So my thinking was the CC or charger might be charging when the HVD level is exceeded, and I would want it to stop charging. So while not literally a "disconnect", by issuing a signal to both the CC and inverter/charger via the Aux ports,charging would stop.

OK this is where you start to go off track. During charge cycle be it from solar or generator the BMS is monitoring all cell voltages. If the pack has been bottom Balanced properly, the weakest cell reaches 90 to 95% SOC before any other cell. When that condition is met the BMS sends a signal to the charger to terminate the charge. Nothing gets disconnected, thus nothing is at High Voltage, you are just done charging and everything is normal.

Likewise an LVD signal could also be issued to both, whatever condition causes it to arise. In either case, this rare event would cause me to find the system shutdown, and I'd have to perform a repair/replacement.

OK this is where the wheels fly off the track. There is nothing to repair or replace. The LVD is a fail safe to prevent any one cell from being over discharged. If a LVD is issued means you have discharged one or more cells to the SAFE LIMIT, and it disconnects to prevent damage. All it means is you need to recharge before it reconnects, there is no damage.

So the real question is how do we take equipment made for Pb and make it work on LFP. HVD is just a borrowed term from EV's and is a bit misleading. In EV's they use Regen Braking which under certain conditions could lead to a Over Charge. That condition could happen when you take off with a fully top balanced pack and live on top of a hill and use Regen immediately before the pack has had enough discharge to handle the REGEN Braking. Most BMS made for EV's have what is Called a CAN BUS that talks to the charger, motor controller, and all systems on an EV. So as you can see HVD is misapplied to RE systems as we will not physically disconnect the batteries from the charger, we just terminate the charge cycle when the first cell hits the set point. So from this point forward let's call it Charge Terminate or CT. OK?

So how do we implement CT? I can think of two ways. Both involve using the the BMS signal of when the first cell reaches the set point. Either to an Auxiliary Input to a CC if available, or a relay switch to disconnect the panels from the CC.

Next is the LVD. Same thing really we use the BMS to issue the LVD signal to one of two place. Your Inverter if it can accept such a signal, or a relay inserted between the batteries or inverter. There is even a 3rd method that does not need any equipment or BMS signal bu tI would not trust it, and use the Inverter built-in LVD.

If were were to use the Pb mentality and assume all cells are equal voltage, the inverter trips at 42 volts or 1.75 vps on Pb. If we apply that same logic to LFP we disconnect at 2.6 vpc or 41.6 volts. So in theory the INVERTER should trip before the BMS.

Let's go one step further. What if we buy an Integrated Inverter with built-in genny support and charger?

[Sunking]

Yes, but the inverter/charger won't work if the LVD has shut the system down, unless the voltage of the cell recovers?

If it were me I would send a signal to the genny before that happens. But that is just me. Same consequence if it is Pb or LFP so your point is moot.

[northerner]

If it were me I would send a signal to the genny before that happens. But that is just me. Same consequence if it is Pb or LFP so your point is moot.

You could only do that if you have auto start set up. For my system where I live in a very cold environment, it can take me half an hour or longer to get the generator started manually. My procedure now is to preheat the carburetor with a hair dryer set to high, for 20 minutes or half an hour. Then it's a delicate procedure to get the generator to start on gasoline. After it starts, I switch the gasoline fuel off and switch to natural gas before it quits. Then warm it up for 5 or 10 minutes before I can finally make use of the power.

I may look at preheating the shelter in the future, but issue with that is I have to have it ventilated really well at the same time. But at this stage, auto start with a generator is just a dream!