How do I correctly set Battery Low Voltage Cut Off?

**SunEagle** · 10-19-2017, 11:33 AM

Maybe a better solution is to size your battery system so that you never use more than 25% so it never gets to 50% SOC.

IMO when it gets below 70% SOC it might be the time to recharge it using a generator or another power source than the solar pv. Keeping it above 70% will extend the life and allow more cycles.

The LBCO is a last ditch action to save the battery from becoming a door stop.

**Matrix** · 10-19-2017, 12:13 PM

Thanks ... I am seeing that there is a difference between DC and AC amps. (DC being much higher than a set AC amp load). This Calculator states 5 AC amps at 115v would be 575 watts ... and on a 24v system would be 26.45 DC amps. Is this about correct? And does that mean that 5 AC amps is drawing 24.45 DC amps off my batteries?

If I take that number of 24.45 dc amps to this Calculator ... and using 435@20 and 483@100 for the Ah rating (from the Trojan spec sheet) ... if I use 24.45 amps as my DC load ... is that calculator saying that is 24.45 amps per hour? I assume so and it says at that load (24.45 amps and 575 watts per hour) I could get 8 hours and 50 minutes from my 435 Ah battery bank before I reached 50%. I AM GETTING NO WHERE NEAR 8 HOURS ... Even at 125 to 350 watts. I get More like 6 hours or less before I hit 50-60%

I did extensive load testing before I purchase the batteries ... my total need WH's per day should be 3400 (with no fudge factor). But I can regulate that by picking and choosing what loads to run off grid. My biggest loads are the well pump (550 WHs per day) and Fridge (2400 WHs per day). Lights and other things throw in an additional 300 WHs per day (tested also) and I pull a max total per day of 3400 WHs per day. But I am not seeing this. Even with a watt load of about 125 to 300 watts per hour ... my batteries are lasting about 5-6 hours. Most calculators I used said the batteries would be good for 1 day autonomy. I would not be able to get 1 autonomy at 3400 WHs from my little 435Ah 24v battery bank.

So what am I missing?

**SunEagle** · 10-19-2017, 12:33 PM

It might be easier to use "watt hours" for your battery usage instead of amps or amp hours. The problem is that the "amps" value will change with the voltage but the "watts" and "hours" are the same for any voltage.

If you have an estimated daily load of 3400 watt hours then you work backward to determine the battery system Ah rating.

Using a 24volt battery and 3400wh you will need about (3400wh / 24v = 142Ah x the number of days you want to have the battery in reserve.

If you decide to only discharge 20% you use 5 days. For 25% discharge use 4 days. Going to 50% discharge you would use 2 days but more than likely you have underestimated your actually daily watt hour usage and will use more than 50%.

So IMO a practical battery system that can use 3400wh day with a 25% discharge would be 142Ah x 4 = 568Ah 24volt. And to add a fudge factor go up to 600Ah. That should safely get you 3600 watt hours a day as long as you put back more watt hours then you take out to make sure the battery gets back to 100% SOC.

**SunEagle** · 10-19-2017, 12:36 PM

Originally posted by Matrix

Thanks ... I am seeing that there is a difference between DC and AC amps. (DC being much higher than a set AC amp load). This Calculator states 5 AC amps at 115v would be 575 watts ... and on a 24v system would be 26.45 DC amps. Is this about correct? And does that mean that 5 AC amps is drawing 24.45 DC amps off my batteries?

If I take that number of 24.45 dc amps to this Calculator ... and using 435@20 and 483@100 for the Ah rating (from the Trojan spec sheet) ... if I use 24.45 amps as my DC load ... is that calculator saying that is 24.45 amps per hour? I assume so and it says at that load (24.45 amps and 575 watts per hour) I could get 8 hours and 50 minutes from my 435 Ah battery bank before I reached 50%. I AM GETTING NO WHERE NEAR 8 HOURS ... Even at 125 to 350 watts. I get More like 6 hours or less before I hit 50-60%

I did extensive load testing before I purchase the batteries ... my total need WH's per day should be 3400 (with no fudge factor). But I can regulate that by picking and choosing what loads to run off grid. My biggest loads are the well pump (550 WHs per day) and Fridge (2400 WHs per day). Lights and other things throw in an additional 300 WHs per day (tested also) and I pull a max total per day of 3400 WHs per day. But I am not seeing this. Even with a watt load of about 125 to 300 watts per hour ... my batteries are lasting about 5-6 hours. Most calculators I used said the batteries would be good for 1 day autonomy. I would not be able to get 1 autonomy at 3400 WHs from my little 435Ah 24v battery bank.

So what am I missing?

With a 435Ah 24v battery you should be able to harvest (435ah x 24v / 4 = 2600 watt hours) a day using 25% discharge. If you are using 3400wh then you are discharging that battery more than 33% and likely not getting it back up to 100% SOC from your solar pv system.

But if you are seeing a discharge of more than 50% it could be due to you using more than 3400watt hours or the battery is not back up to 100%.

**Matrix** · 10-19-2017, 01:05 PM

Yes I am beginning to question my system set up. I realize I may be under sized (OK I am undersized) ... but now after reading several threads here today, I am beginning to wonder if I messed up buying the Trojan L16H-AC 435 instead of something in Trojans RE series line. My reading is that the L16H may not be as suited for the higher amp draw spikes. And will not last as long.

So that leads me to wonder. Do I go out and purchase 4 more L16H-AC batteries to increase my battery bank size to 870 Ahs ... or buy 4 more batteries in the RE line to add to my existing L16H batteries (but that would be mixing Ah ratings and batteries types) ... or do I want 2-4 years and baby my existing bank of 435 Ahs for now and purchase 8 in the RE line when I kill my existing L16H 435 set.

I really cannot undo the purchase of the 4 L16H batteries from last week. But I do realize I can either. limit my load, or increase my battery bank size.

Are the L16H-ACs a bad choice for off grid usage?

Should I add 4 more L16H's or just wait it out?

I am grid connected with an off grid sub panel. So I can toggle various loads back and forth as needed.

**SunEagle** · 10-19-2017, 01:45 PM

IMO I would keep the existing batteries and try to find ways to not over discharge them. Sometimes the first set of batteries is the "learning set" and then the second is either better type or better taken care of. Never mix battery types or different ages in the same system. The result is the worst battery will bring down the rest to it's level shortening the life of all.

Without being blunt, if you have grid power why would you spend the money to have an off grid system? Every time I do a calculation I come up with a cost per kWh generated from the solar / battery system that is 4 to 5 times the cost to purchase the same kWh from the grid.

Being off grid totally is a matter of personal decision but having both a grid tie and off grid system seems strange to me. A grid tie solar pv system will pay for itself and in some places a hybrid grid/battery system will be ok but unless you are paying about $1.00/kWh from your POCO off grid does not make sense to me.

**Matrix** · 10-19-2017, 02:05 PM

Simple ... the Zombie Apocalypse. LoL.

But seriously, I did it to have off grid options while not having to have inspectors coming thru my home to inspect everything. Did not want to jump thru the hooks of doing a Grid-Tie system with battery back up and having inspections. I basically wanted a simple solar generator to last me thru a power outage like we had last month during the hurricane here in FL.

Did I design it to code? Well, everything I used is UL 1741 certified (inverter/charger, panels and CC), wiring is all in conduit and is over sized and grounded to NEC spec. Battery cables are 4/0. So I would say yes. I was not doing it for savings ... simple for sustainability in the event of natural disaster or emp, etc. Wanted to design a system that would get me some lights, fans, fridge and water from the well at least in the day time. I pretty much have this except for possibly running the well pump at night.

The whole "plan" came about 4 days before Hurricane Irma hit when the wife said ... how we gonna charge our cell phones. Looking at harbor freight for a solar cell phone charger let to the 100 watt 4 panel set up they had ... and 2 used AGM 6v batteries with a 1500w inverter back fed into a receptacle of my house. This got us thru 5 days with out power with lights and fans. Used a generator for the fridge and pump. So I thought ... If I was to design a system, why not make it large enough to do that AND the pump and fridge. One thing lead to another ... and here we are.

I guess I could grid-tie it and reap some of the benefits ... but then I would have to buy a grid tie inverter (I think). And would have to have it all inspected and carry 100k of insurance (which I think my home owners already has) before the local Electric Co-op would issue me an interconnect agreement.

**SunEagle** · 10-19-2017, 02:23 PM

Originally posted by Matrix

Simple ... the Zombie Apocalypse. LoL.

But seriously, I did it to have off grid options while not having to have inspectors coming thru my home to inspect everything. Did not want to jump thru the hooks of doing a Grid-Tie system with battery back up and having inspections. I basically wanted a simple solar generator to last me thru a power outage like we had last month during the hurricane here in FL.

Did I design it to code? Well, everything I used is UL 1741 certified (inverter/charger, panels and CC), wiring is all in conduit and is over sized and grounded to NEC spec. Battery cables are 4/0. So I would say yes. I was not doing it for savings ... simple for sustainability in the event of natural disaster or emp, etc. Wanted to design a system that would get me some lights, fans, fridge and water from the well at least in the day time. I pretty much have this except for possibly running the well pump at night.

The whole "plan" came about 4 days before Hurricane Irma hit when the wife said ... how we gonna charge our cell phones. Looking at harbor freight for a solar cell phone charger let to the 100 watt 4 panel set up they had ... and 2 used AGM 6v batteries with a 1500w inverter back fed into a receptacle of my house. This got us thru 5 days with out power with lights and fans. Used a generator for the fridge and pump. So I thought ... If I was to design a system, why not make it large enough to do that AND the pump and fridge. One thing lead to another ... and here we are.

I guess I could grid-tie it and reap some of the benefits ... but then I would have to buy a grid tie inverter (I think). And would have to have it all inspected and carry 100k of insurance (which I think my home owners already has) before the local Electric Co-op would issue me an interconnect agreement.

I understand. I had about 36 hours without power due to Irma. I used a combination of a 5500 watt gas generator during the day and my two small battery banks at night. I never set up the solar panels because I just charged the batteries during the day when the gen set was running. My future plan is for a whole house gen set run from bottled gas along with my small "off grid" solar/battery systems until the batteries fail.

I figured I spent over $3000 for my two small "off grid" systems which can supply maybe 800 watt hours daily and then decided batteries were not the way to go.

**organic farmer** · 10-19-2017, 02:51 PM

Regardless of which battery you bought, battery life will be a feature of how many times the batteries have deep-cycled and how deeply you cycle these batteries.

How deep you let them cycle is determined by how you set the LBCO preset. As this setting goes lower, the system will bring the batteries to deeper cycles. Thus shortening their lifespan.

It is really a determination that you must make.

**Sunking** · 10-19-2017, 03:14 PM

Matrix tap the breaks. Honestly if all you want is Emergency Temp Power solar is not the way to go. No professional or even informed DIY would use solar for Emergency Power. To limited, expensive, and unreliable.

Here is what you do first. Look around and decide what you must have power for like a fridge, a light or two, and say an outlet to charge phones with and a TV. Next determine how many watt hopurs that would require in a day. I can tell you roughly 2 Kwh unless your fridge and TV are antiques

OK if we use 2 Kwh/day you want a battery sized to about 4 to 6 Kwh. At 4 Kwh means you have roughly 1 full day before you need to worry about recharging. A 4 Kwh battery at 12 volts = 333 AH or just round up to 350 AH @ 12 volts, or even better 175 AH at 24 volts. With me so far?

So you already have 4 fantastic batteries. I believe you said 4 Trojan L16H or 6 volts @ 435 AH. Wire them in series and you have 24 volts @ 435 AH or 10.44Kwh or 5.2 Kwh usable. That is a lot of power. More than enough to run a fridge a few lights, charge everyone's cell phone in the neighborhood and watch TV for 2 to 3 days without a recharge. .

Ok with the 4 batteries you have here is what smart money would do. Buy you a 24 volt 40 to 60 amp Battery Charger and a 24 volt 1000 to 2000 watt Inverter and forget all about solar. But if you insist on throwing away money on solar, just based on the 4 batteries you have now will require a 1000 watt Solar Panel ($1500 to $2000) plus a 40-Amp MPPT Charge Controller ($400). Do you have $2000 to spare for a solar battery charger that you will only use once or twice a year? That is the definition of an insane democrat.

So get a good 24 volt battery charger (40 to 60 amps), and a 24 volt Inverter up to 2000 watts and call it done. If you want long term outage say more than 2 days that the batteries can run without a charge, then buy a damn generator to plug the charger into. It would take 3 to 5 days to recharge your battery with a 1000 watt solar panel, or 8 hours with a less expensive generator that does not need bright sunny days with no clouds to run. It can run in the dark or cloudy days.

So do not do something ignorant like use solar. You already have more than enough battery, and forget 12 volts and solar.

**AzRoute66** · 10-19-2017, 03:55 PM

Somehow I think this thread didn't take the right fork in the road right from the start.

Originally posted by Matrix

[...] I have 4 new Trojan L16 H batteries (6v 435 aH with less than 4 cycles) in series for a 24v system. I am able to fully charge them via solar in 1/2 a day.

OK, that should be 435 Ah x 24V = 10,440 Wh.

My question, Where should I set the LBCO in my Conext SW 4024 Inverter/charger? According to what I read, 50% at Open Circuit for a 24v battery is 24.2v The Inverter will only go as hi as 24v for Inverter LBCO. But the 50% 24.2v Open Circuit is with no load (I assume). Inverter shut off I believe assumes a load. So is it OK to have the inverter shutting off at 24v or less under load? 24v would be somewhere in the 45% range of Open circuit battery voltage.

The key, as you suggest, is the Open Circuit proviso. What I would say is that if 50% is represented by 24.2V at no load, then it would be represented by something less at load, and something even lower at higher (surge) load.

And what about momentary surges from pumps or refrigerators? if I am at 24.7v in the middle of the nite with about 150 watts on the system ... and my pump or fridge kicks on ... I can run them once they level off and settle in

Again, to be expected, remember you were at 24.7 before the surge...

... but their start up momentarily drops the voltage to the point I can get an Inverter shut off if the inverter LBCO is set to 24v. The start up surge lasts 15-25 seconds (yes I have tested this). And then the voltage of the batteries come back up to the 24.7v's with the fridge or pump running.

... and 24.7V after the surge. I think I would say that the momentary loss of voltage is not only to be expected, that is it not representing any problem with battery bank capacity, only with the internal resistance being high enough to cause a transient voltage drop.

- I can set a delay in the inverter so that the LBCO of 24v only occurs after 25 seconds or more or less. But ...

If I were you, I would set the LBCO delay to 30 - 40 seconds. If the inverter routinely handles the surge and returns to 'normal operations' it sounds like the exact scenario for which the programmable delay was implemented in the first place.

Is the standard of not discharging past 50% (I would like to be closer to 65%) stated for Open Circuit, no load, resting of the batteries ... OR is the LBCO suppose to be set for 50% when there is a load on the system? (or higher - which i cannot do because the SW will only set the LBCO as hi as 24v)

The 50% is 50% of the battery stored energy, not a function of either load or voltage. This is why you cannot "set the LCBO for 50%" - you have to get that notion out of your mind. The trouble is you are trying to approximate 50% energy with a voltage reading. If you really want to know, put on your slippers and go check the specific gravity. You could do that the next time it trips out on LBCO, or just before dawn. As you have a 10,440 Wh battery and know that your well measured usage is about 1/3rd of that, I would be shocked if your new battery bank was anywhere near 50%. If that doesn't appeal to you, you might want to put an energy meter on that battery bank and see if the readings closely correlate to the specific gravity true SOC measure. The worst that can happen is that you find out that you have mis-estimated your loads.

As I've said before, I don't know squat about inverters, but perhaps the comments above will get the conversation going on a different track.

Addressing some of the later points covered...

In particular, I don't think the 'design to 25%' or the number of days of autonomy is relevant. The battery is what it is, the voltages and loads are what they are. Does anyone think that he mis-estimated his loads so badly that he is actually using over half of his 10.4 kWh battery that is fully charged every day by noon, especially considering the voltage does rebound nicely after the surge is over?

I also think his calculators are right, and that his capacity is fine, and that the fact that the voltage is dropping during surge is solely a function of the internal resistance of the battery bank. It may be possible that there is a connection problem, not sure how to verify that other than to feel around for excess heat.

The OP didn't buy bad batteries, and since 435 Ah is about 3x the 142 Ah needed for a days load, I suggest setting the LCBO delay for 30 - 40 seconds to endure the surge and finding out more about 'capacity issues' after that via specific gravity and/or a good energy meter. Then he can decide whether to replace or augment the current battery bank. Or - of course - leave it as is.

**Matrix** · 10-20-2017, 12:46 AM

Originally posted by Sunking

Matrix tap the breaks...do not do something ignorant like use solar. You already have more than enough battery, and forget 12 volts and solar.

Great Advice ... guess I was too late in asking the question. I purchased and installed the solar system before I found the forum

Here is the system as it stands right now
- 24v system
- 9 REC Twinpeak 2 285 watt panels
- Array: 2,565 watts
- Midnite Solar Classic 150
- Conext SW 4024 Inverter / Charger and MidNite E-Panel
- 4 Trojan L16H-AC 435Ah batteries wired in series for 24v
- all going into a Reliance 306crk sub panel that is basically a 6 circuit generator transfer switch that I am using to transfer between Solar and AC line. Gives me Line / Solar / off control of 6 circuits which is my primary lighting, a few receptacles, the TV, the Router, and the Fridge and Well pump. which all can be toggle Line Solar / off in any combination.

Originally posted by Sunking

Here is what you do first. Look around and decide what you must have power for like a fridge, a light or two, and say an outlet to charge phones with and a TV. Next determine how many watt hopurs that would require in a day. I can tell you roughly 2 Kwh unless your fridge and TV are antiques

Did that thru a Kill-a-watt meter for about 2 weeks before purchasing.
- fridge actually runs at between 110 watts and 165 watts when running. Start up is about 400 watts (it uses 2400
- I figured my load to be about 3400 Wh's per day
- what I did not figure was heavy load draw for say 15 or 20 minutes at different times of the day, and what impact that might have on an overall single charge. The batteries seem to be drawing down faster than expected. I am only getting about 6-8 hrs from them and using on average about 125 to 150 watts per hour

Originally posted by Sunking

OK if we use 2 Kwh/day you want a battery sized to about 4 to 6 Kwh. At 4 Kwh means you have roughly 1 full day before you need to worry about recharging. A 4 Kwh battery at 12 volts = 333 AH or just round up to 350 AH @ 12 volts, or even better 175 AH at 24 volts. With me so far?

Yes. And those are the calcs I was pretty much getting except I was building it around about 3400 Wh's per day max.

Originally posted by Sunking

So you already have 4 fantastic batteries. I believe you said 4 Trojan L16H or 6 volts @ 435 AH. Wire them in series and you have 24 volts @ 435 AH or 10.44Kwh or 5.2 Kwh usable. That is a lot of power. More than enough to run a fridge a few lights, charge everyone's cell phone in the neighborhood and watch TV for 2 to 3 days without a recharge. .

Ok with the 4 batteries you have here is what smart money would do. Buy you a 24 volt 40 to 60 amp Battery Charger and a 24 volt 1000 to 2000 watt Inverter and forget all about solar... If you want long term outage say more than 2 days that the batteries can run without a charge, then buy a damn generator to plug the charger into.

This is pretty much what I have done, I also have generator support access going into the main load panel. So using the SW Inverter/Charger I can charge thru the generator if there is no solar available.

But I see your point. Well taken. I am just further along and probably not where the smart money went, but it has been a fun project.

I am just trying to protect my batteries and do not want them to discharge to deeply. I am using them every day. The Invert is also connect to AC Support. And can supply a Support AC while the batteries are powering the off-grid load. I set a threshold in the inverter to say, 24.5v, and when that hits, the inverter kicks off, the batteries go to 0 amp output and the AC line input starts suppling the entire load. I can set the charger in the inverter to say, 24.7v and the charge will kick off the inverter and start charing the batteries back up to 100% and not allow the inveter to kick back on until the batteries are fully charge or 2 hours which ever comes last.

I just wanted to know in the original post, where to set my invert LBCO in the case of NO AC IN (like in a black out) so that the batteries dont ever discharge to deeply.

The inverter will only allow for a max of 24v for LBCO. That is lower than what Trojan says the 50% SOC mark is for voltage. So I did not know if that idea of "Do not discharge batteries below 50% (and ideally 60 or 70%)" if measured by voltage (as the inverter is doing for LBCO) was an under load voltage, or an open ciruit voltage. And then would the momentary sags of 15-30 seconds where the battery voltage would drop down below 24v when an inductive load kicks in ... would that be seriously damaging the batteries.

And also what voltage would be a good place to let the inverter turn off and kick the entire load to the AC Line in. I can set that to just about voltage I want. I was thinking about 24.7 so then I would never hit LBCO before the inveter kicks off. It would kick off and transfer over to AC before the LBCO was ever reached.

Whew.

**Matrix** · 10-20-2017, 01:19 AM

Originally posted by AzRoute66

Somehow I think this thread didn't take the right fork in the road right from the start.

OK, that should be 435 Ah x 24V = 10,440 Wh.

The key, as you suggest, is the Open Circuit proviso. What I would say is that if 50% is represented by 24.2V at no load, then it would be represented by something less at load, and something even lower at higher (surge) load.

... and 24.7V after the surge. I think I would say that the momentary loss of voltage is not only to be expected, that is it not representing any problem with battery bank capacity, only with the internal resistance being high enough to cause a transient voltage drop.

If I were you, I would set the LBCO delay to 30 - 40 seconds. If the inverter routinely handles the surge and returns to 'normal operations' it sounds like the exact scenario for which the programmable delay was implemented in the first place.

That is what I was thinking ... but did not want to ruin some new batteries because of faulty logic on my part.

Originally posted by AzRoute66

The 50% is 50% of the battery stored energy, not a function of either load or voltage. This is why you cannot "set the LCBO for 50%" - you have to get that notion out of your mind. The trouble is you are trying to approximate 50% energy with a voltage reading. If you really want to know, put on your slippers and go check the specific gravity. You could do that the next time it trips out on LBCO, or just before dawn. As you have a 10,440 Wh battery and know that your well measured usage is about 1/3rd of that, I would be shocked if your new battery bank was anywhere near 50%.

So are you saying that 24.2 may or may not be 50% SOC? I know my daily usage of the loads I have off grid are nearly close to 3400 Wh's per day. But what was kinda concerning me yesterday was I was seeing that voltage drop faster than I expected. I went from 100% charged and 25.5v's at 4pm down to about 24.8 by 11pm and at 8am the next morning the pump kicking on kicked the inverter off. The voltage on the batteries was about 24.5v, but the surge dropped it. And when the pump went off the voltage returned to 24.5v but the inverter had already kicked itself off due to LBCO set to 24v and not enough delay time.

Originally posted by AzRoute66

Addressing some of the later points covered...

In particular, I don't think the 'design to 25%' or the number of days of autonomy is relevant. The battery is what it is, the voltages and loads are what they are. Does anyone think that he mis-estimated his loads so badly that he is actually using over half of his 10.4 kWh battery that is fully charged every day by noon, especially considering the voltage does rebound nicely after the surge is over?

I also think his calculators are right, and that his capacity is fine, and that the fact that the voltage is dropping during surge is solely a function of the internal resistance of the battery bank. It may be possible that there is a connection problem, not sure how to verify that other than to feel around for excess heat.

The OP didn't buy bad batteries, and since 435 Ah is about 3x the 142 Ah needed for a days load, I suggest setting the LCBO delay for 30 - 40 seconds to endure the surge and finding out more about 'capacity issues' after that via specific gravity and/or a good energy meter. Then he can decide whether to replace or augment the current battery bank. Or - of course - leave it as is.

I'm interested to see what the consensus is on this last statement.

Thanks for the well thought out answers from all. As I learn more, maybe I will be able to frame the question better to better explain what I am seeing with the batteries. As it stands right now, i do not think I am horribly exceeding my load, but my batteries are not seeming to hold up as well as the various battery bank sizing calcs suggested they would. Like I have stated, I am seeing about 12 hours of autonomy and 1/2 or more of that time was spent sleeping or not at home.

**AzRoute66** · 10-20-2017, 01:28 AM

Originally posted by Matrix

So are you saying that 24.2 may or may not be 50% SOC?

What I would say is that if 50% is represented by 24.2V at no load, then it would be represented by something less at load, and something even lower at higher (surge) load.

Could I suggest that when you have it charging (or discharging) and taking a lot of amps, to touch every battery terminal and cable connection related to the battery bank to see if any are warmer than the others?

How do I correctly set Battery Low Voltage Cut Off?

How do I correctly set Battery Low Voltage Cut Off?

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