Inverter with adjustable low voltage disconnect (LVD) for LiFePO4 cells?

[ckuttruff]

Hi all! First post here. So appreciate all the knowledge and experience I've already seen on this forum! As a newbie to solar tech, resources like this are invaluable. Did a bit of digging for this topic and didn't see quite what I was looking for, so here goes:

I am building a 24V (25.6V nominal technically) solar system with 4x 295W monocrystaline panels (in 2s2p orientation), 8x 100Ah CALB CA100 LiFePO4 cells in series (bottom balanced at 2.75 and looking nicely balanced even towards the top), a 60A MPPT charge controller. Inverter is where I'm a bit stuck.

i originally bought a Go Power 2000W inverter (GP-SW2000-24). Everything was looking swell until I observed the low voltage shutoff behavior. Manual says 21V cutoff, but I saw my beautiful CALB cells dip just below 2.5 (#DangerZone) by the time the inverter shutoff. Fortunately cells look fine, and I've brought them back up and been monitoring them.

When I realized I couldn't adjust this shutoff value I returned this otherwise beautiful inverter after checking with their tech support about options. Sadly I've discovered a customizable low voltage shutoff is a rare feature.

So, after research I realize that many of these inverters were engineered for lead-acid chemistries (i think), so here's the question:
what would be an invertee I could buy that supports 60Hz 120V output (for US standards), that is 24V, supports at least a 2kW load, and has a customizable low voltage shutoff (to support lithium iron phosphate chemistry)?

and if this isn't feasible where can I acquire an LVD relay thingy that support 100A and shutoff voltage around around 24V? (cause searching has yielded little results)

Thanks so much in advance!!
- Chris

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[Sunking]

i originally bought a Go Power 2000W inverter (GP-SW2000-24). Everything was looking swell until I observed the low voltage shutoff behavior. Manual says 21V cutoff, but I saw my beautiful CALB cells dip just below 2.5 (#DangerZone) by the time the inverter shutoff.

Which cells were below 2.5? Did you note what the voltage really was?

Here is why I ask. I am familiar with Bottom Balanced LFP cells. At 8S critical voltage if properly Bottom Balance is 8 x 2.5 volts = 20 volts. While 21 is to low for comfort, it is still not critical with all cells above 2.5 volts. Ideally for a 8S LFP battery you want LVD at 3.0 vpc or 24 volts using 8S.

You are right, not a lot of Inverters offer adjustable LVD set points. But there are some but you must pay up for them. 10.5 under load is about right for lead acid and will not hurt the battery much, it saves it. 10.5 is just to low for Chi-Com LFP like Calb. Or in your case 21 volts for a 24 volt battery There's not much left at 3 volts and less, and no reason to try to go lower to 2.5 and risk it. That would be 24 volts LVD using 8S LFP.

A few to look at are:

Magnum
Schneider
Outback
Xantrex
Victron

[karrak]

To add to what Sunking is saying I would look at getting an inverter that has a remote on/off switch so that in the future if you did decide to get a BMS that monitored the individual cell voltages that you could use the remote on/off to turn the inverter off rather than using a separate high current relay or Solid State switch.

I am interested to know what the charge settings are on your MPPT charge controller and what the individual cell voltages are at the end of the CV (absorb) phase of charging

Thanks
Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[ckuttruff]

Sunking, karrak thanks so much for the swift replies. All cells were below 2.5 at cutoff, a couple closer to 2.4, which is what concerned me cause CALB specs list 2.5 as discharge cutoff (http://www.evwest.com/catalog/produc...roducts_id=208). This was measured after removing load and letting cells rest. This was surprising cause the 21V listed in manual (http://gpelectric.com/files/gpelectr..._2000_3000.pdf), would mean 2.65 for each cell (and from what I understand voltage sag should be minimal cause load was small and LiFePO4 should be fairly resilient to that right? A friend is currently staying in my tiny house where the solar is going, so it's important for the system to behave well unattended.

Thanks for those suggestions! I will definitely check out those inverter brands and I don't mind paying a bit more if it meets my needs.

and karrak, I haven't hooked up panels / CC yet since I've been bottom balancing cells and figuring out all the design (gauge size (going with AWG 1/0)), fuses, panel orientation, etc. Lots to think about.

these are my panels:

https://customsolarandleisure.com/wp-content/uploads/2017/05/Canadian_Solar_SuperPower_290_W_Spec_Sheet.pdf

and charge controller:

http://m.lightharvestsolar.com/catalog/item/8735073/10334652.htm

[PNjunction]

WHOA - stop right there.

Bottom balance is fine, PROVIDED you have a circuit in place to stop charge near the top, such as detecting when the *first* cell in the bank reaches your defined upper limit voltage with no absorb. Or, using that first cell that reaches the voltage first, trigger a stop at the overall measured pack level - usually again meaning no absorb.

Your mppt controller most likely doesn't have this feature, but acts like a normal "top-balanced" lead acid controller if you will.

In other words, you may be UNbalancing your bottom balanced bank every cycle. So one has to choose - so called balanced at the top with your usual array of solar controllers, and a little ragged at the bottom, OR balanced at the bottom, but a little ragged at the top, with a smart voltage-triggering device to do so!

[ckuttruff]

WHOA - stop right there.

Bottom balance is fine, PROVIDED you have a circuit in place to stop charge near the top, such as detecting when the *first* cell in the bank reaches your defined upper limit voltage with no absorb. Or, using that first cell that reaches the voltage first, trigger a stop at the overall measured pack level - usually again meaning no absorb.

Your mppt controller most likely doesn't have this feature, but acts like a normal "top-balanced" lead acid controller if you will.

In other words, you may be UNbalancing your bottom balanced bank every cycle. So one has to choose - so called balanced at the top with your usual array of solar controllers, and a little ragged at the bottom, OR balanced at the bottom, but a little ragged at the top, with a smart voltage-triggering device to do so!

It looks like this something that should be programmable (from this page: https://szshuori.en.alibaba.com/prod...bluetooth.html)
"Various system protection functions. Including over-charge, over-discharge, over-load, over-heat, the battery reverses connection protection and so on."

I definitely haven't hooked the panels / CC up to the battery yet, but thanks for the concern and feedback. Will dig around for my manual to see if it gives details about how that's set up

----
Edit: and I don't think it does balancing cause it's just hooked up to the positive / negative leads on my 8s pack, so they'd all be charging / discharging together (granted at slightly diff rates with differing internal resistance, etc)

[PNjunction]

Could be, but unless the over-discharge is user-adjustable, that setting could be too low for normal use - just the last-ditch 10.6v disconnect like an inverter. You'd have to do your homework here.

Also note that if you are taking this system apart and back together often, attach the controller to the battery first, and the panel last in order for the controller to get it's brain together.

[ckuttruff]

Could be, but unless the over-discharge is user-adjustable, that setting could be too low for normal use - just the last-ditch 10.6v disconnect like an inverter. You'd have to do your homework here.

Also note that if you are taking this system apart and back together often, attach the controller to the battery first, and the panel last in order for the controller to get it's brain together.

Thanks for that; yeah, the connection ordering is in some documentation that came with the charge controller, so definitely going to follow that to the letter. Thanks for the reminder about the sequence there. I'll be very careful to check about those cutoff settings and monitor closely during the first couple cycles

[ckuttruff]

Sunking , karrak

thanks so much for the swift replies. All cells were below 2.5 at cutoff, a couple closer to 2.4, which is what concerned me cause CALB specs list 2.5 as discharge cutoff. This was measured after removing load and letting cells rest. This was surprising cause the 21V listed in manual would mean 2.65 for each cell (and from what I understand voltage sag should be minimal cause load was small and LiFePO4 should be fairly resilient to that right? A friend is currently staying in my tiny house where the solar is going, so it's important for the system to behave well unattended.

Thanks for those suggestions! I will definitely check out those inverter brands and I don't mind paying a bit more if it meets my needs.

and karrak, I haven't hooked up panels / CC yet since I've been bottom balancing cells and figuring out all the design (gauge size (going with AWG 1/0)), fuses, panel orientation, etc. Lots to think about.

these are my panels:
CS6K-295MS (canadian solar)

and charge controller:
SRNE ML4860 (product description on lightharvest solar (sorry, links are getting flagged)).

Thanks again for list of inverters. Going through the manuals to see which support adjustable LVD. Any recommendations about where to buy the LVD switch or relay? They seem pretty niche / pricey with limited availability

[karrak]

I had a quick look at the manual for the charge controller, it is good that you can set all the voltages via the user defined "USE" setting. They mention a preset lithium setting but don't give any details of what the voltages are!

I would set up the following voltages
Overvoltage Disconnect Voltage: 28.0V (3.5V/cell)
Equailization voltage: 27.6V (3.45V/cell)
Boost Charging Voltage: 27.2V (3.40V/cell) to start with increasing to a maximum of 27.6V (3.45V/cell)
Floating charging voltage 26.8V (3.35V/cell)
Boost Charging recovery Voltage: 26.4V (3.30V/cell)
Over-discharge recovery voltage: 25.6V (3.20V/cell)
Under-Voltage warning level: 24.8V (3.10V/cell)
Over-discharge voltage: 24.0V (3.0V/cell)
Over-discharge time delay: 5s
Equilizing charging time: 0m
Boost charging time: 15m
Equilization charging interval: 30D
Temperature Compensation: 0 ***manual says 2-5mV/C/2V, hopefully this is wrong and you can set no temperature compensation.
I would set Fault voice alarm to give you an audible warning.

I would try to find an inverter with a remote off rather than using an LVD relay or solid state switch.

If you bottom balance the battery, unless all the cells have exactly the same capacity which is unlikely the individual cell voltages will be ragged when the charge controller goes from Boost/absorb charging to float charging. Unless you have some sort of automatic cell monitoring it is most important that you monitor the individual cell voltages to make sure that none go outside the safe operating range!

May I ask how much experience and knowledge you have with electronics?

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[ckuttruff]

karrak thanks so much for that info! Going to carefully review those CC values (esp boost vs float charging since those terms are new to me). Hoping I didnt make a poor choice on my controller. Bought it from a local shop earlier before delving into more research.

I will definitely look at your monitoring README on github. Very open to such a system. My cells are all between 118.1 and 118.8Ah from the manufacturer slip that came with the cells. And internal resistance is 0.27 for all but one (0.28), so they are very well matched. Confirmed this when bringing cells up (in series) to about 3.5V. Also was planning to leave a good amount of room at top and bottom for cutoff to account for slight imbalances (and of course manually check regularly to ensure that too much drift isn't happening)

and to answer your question about experience, I am very much new to solar / electronics (so not much to speak of; computer programmer by day), which is why I'm taking my time, doing lots of research, reading manuals carefully, etc. Very keen on not blowing up batteries, starting a fire, or wrecking pricey equipment Super open to consult with a pro, so if you know any folks near Portland, OR you'd recommend, let me know.

and regarding remote off switch, I'm definitely down to find an inverter with that feature, but ideally I'd like the system to shut off automatically in case of low voltage since I won't always be the one in there to hear an alarm. Though if there are really no good options for that, I feel bad for returning the Go Power inverter since it workes well otherwise. Do you have a brand / model you'd recommend for my setup?

thanks again so much for the response. Haven't had anyone to talk to about the project and this feedback is invaluable!

- Chris

[Sunking]

WHOA - stop right there.

Bottom balance is fine, PROVIDED you have a circuit in place to stop charge near the top, such as detecting when the *first* cell in the bank reaches your defined upper limit voltage with no absorb. Or, using that first cell that reaches the voltage first, trigger a stop at the overall measured pack level - usually again meaning no absorb.

PN you know better than that. I expect such nonsense from Karrak, but not you. Stop and think about it for a moment. Why do you Bottom Balance.

Sure it could be done that way, but you defeat the whole point of going Bottom Balanced. There are two flaws in that logic. The first one is huge.

1. That weak cell is going to reach fully charged mid day. So you turn the solar system off at noon, and go on batteries until the next morning. Do you see a problem with that? Why on God's Green Earth would you want to go on battery power mid day rather than utilize power from the panels? All you are doing is cutting your expensive battery cycle life in half putting all that extra wear and tear on the batteries unnecessarily. There is no good reason to do that. Only a fool like Karrak would do that.

2. Taking that weak cell that high is just not needed, nor is the expense of all that equipment and automation required.

Bottom Balance on a Solar System is super easy and eliminates all risk of over charging and over discharging. Instead of setting the Charge Voltage to 28.8 volts, you lower it to 26.8 to 27.0 volts. You set your controller up so Bulk = Absorb = Float = 26.8 to 27.0 volts. That forces your controller to operate as CC/CV mode. The pack will charge up to less than 100% and then FLOAT until the sun goes down. That way you utilize all the solar power until dark and save your batteries for night like every solar system is suppose to do.

All one has to do is a proper Bottom Balance, then charge them up and look for the weak cell voltage to hit 3.45 to 3.5 volts, and note Pack Voltage. Say you see 26.9 volt. Now set your charger to 29.5 volts and you are set and protected from any over charge passively. No automation or extra equipment required. From that point just monitor the cell voltages from time to time.

On the discharge side, set the Inverter LVD to 12, 24, or 48 volts respectively and you just eliminated any chance of over discharge. EV builders have been doing this for 6 years now and have it down to an Art Form.

[Sunking]

All cells were below 2.5 at cutoff, a couple closer to 2.4, which is what concerned me cause CALB specs list 2.5 as discharge cutoff.

FWIW CALB and all manufactures lie about that. The real danger voltage is 2 volts and less. 2.5 is manufacture CYA. It is two volts if adjacent cells still have energy left in them that can drive the low cell into reverse polarity. That is why you Botom Balance so that cannot happen. That only happens when you Top Balance.

That is the whole point of Bottom Balancing, you eliminate the chance of over discharging. Secondary objective is never ever fully charge a Lithium battery. Guess what Top Balance does, Fully charges the batteries. Operate from 10 to 90% and you double cycle life.

[ckuttruff]

FWIW CALB and all manufactures lie about that. The real danger voltage is 2 volts and less. 2.5 is manufacture CYA. It is two volts if adjacent cells still have energy left in them that can drive the low cell into reverse polarity. That is why you Botom Balance so that cannot happen. That only happens when you Top Balance.

That is the whole point of Bottom Balancing, you eliminate the chance of over discharging. Secondary objective is never ever fully charge a Lithium battery. Guess what Top Balance does, Fully charges the batteries. Operate from 10 to 90% and you double cycle life.

wow, okay, thanks so much for the info. so charge cutoff voltage is listed at 3.6, but looking at the discharge curve from CALB there's nothing more in 'em about 3.4, so no point going above that anyway, but (0.9 * 3.6 => 3.24). That would be my target? Or is 3.3 plenty safe?

[ckuttruff]

Bottom Balance on a Solar System is super easy and eliminates all risk of over charging and over discharging. Instead of setting the Charge Voltage to 28.8 volts, you lower it to 26.8 to 27.0 volts. You set your controller up so Bulk = Absorb = Float = 26.8 to 27.0 volts. That forces your controller to operate as CC/CV mode. The pack will charge up to less than 100% and then FLOAT until the sun goes down. That way you utilize all the solar power until dark and save your batteries for night like every solar system is suppose to do.

All one has to do is a proper Bottom Balance, then charge them up and look for the weak cell voltage to hit 3.45 to 3.5 volts, and note Pack Voltage. Say you see 26.9 volt. Now set your charger to 29.5 volts and you are set and protected from any over charge passively. No automation or extra equipment required. From that point just monitor the cell voltages from time to time.

On the discharge side, set the Inverter LVD to 12, 24, or 48 volts respectively and you just eliminated any chance of over discharge. EV builders have been doing this for 6 years now and have it down to an Art Form.

This is great info and makes a lot of sense. I'll just make sure I can snag an inverter that allows for explicitly setting low voltage cutoff at 24. Would obviously prefer to limit components