How low can I discharge my 48 volt battery system safely on a daily basis?

tonygcan

Hello, my system is composed of the following:

12 PV panels Sharp 185 watts (total of 2220 watts)
Xantrex XW MPPT SCC
Xantrex XW 4548 inverter
8 6-volt Rolls batteries, 450 amp hours

My system is programmed to cut off supply when the voltage of the batteries reaches 44 volts. However, The deep cycle batteries FAQ states that it is not advisable to allow your batteries to discharge below 50% of the batteries capacity. That would be below 48.24 volts.

My system starts with a full charge (about 51 volts read from the XW SCP) and by the time morning comes the voltage all read about 46 volts. Is it okay to Lola my batteries to discharge to 46 volts on a daily basis or should I switch to the grid late at big to conserve the batteries?

Thanks for any insight.

Cariboocoot

Re: How low can I discharge my 48 volt battery system safely on a daily basis?

There's a difference between Voltage at rest and Voltage under load, especially considering what (how much) the load is. The figures in the battery FAQ are at rest Voltages; no loads.

Using nominal system values while under load guarantees the batteries won't be drawn below 50%, but there can be a margin for lower Voltage; when the load is removed the Voltage 'springs back up' and could then be above 48 Volts resting, meaning the battery is still above 50% (although just barely).

So the question is: how much load is being supplied when you have this 46 Volt reading? If it is only the inverter consumption the batteries are being drawn down too low. This will shorten battery life, but is not likely to cause a sudden catastrophic failure. (42 Volts would be "dangerously dead".)

If possible, don't take them that low. If it's practical to stop discharge at 48 go for it.

Blackcherry04

Re: How low can I discharge my 48 volt battery system safely on a daily basis?

46 volts is low depending on the load. This is the problem with a On / Off Grid system, your right in between what a true Off Grid system would require in capacity for what you use. The best ROI is between 40-50% DOD on flooded Batteries by my numbers. Your Batteries should go 7-9 years if cared for, but here is where the trade off's come in. If you Raise your storage capacity, then you have to raise your re-charge capacity or reduce your loads, It's all about power management. If your using your system everyday, then it's about life cycles of the batteries and they become more of a justifiable expense.

Your 44 Volt cutoff is the emergency stop, It should not be considered as the voltage that you make decisions about recharging or switching to another power source. If you set it to high the Inverter will kick out on heaver loads, to low and you damage your batteries.

What I do is at 9:00 evening I look at my voltage, I know my over night loads. If I am low I'll fire the generator up for a hour or what ever it takes to get up where it needs to be till 8:00 in the morning and it all starts over again for the next 24 hrs. It's all Power Management, ain't it fun. As your batteries age, the weather, loads and everything changes, you make adjustments. Once your battery bank maintained capacity get's below 80% or so they are not worth charging for what you get back out of them.

Eric L

Re: How low can I discharge my 48 volt battery system safely on a daily basis?

Since you have the grid available, I would do some tests in the morning. When you see that 46 volt reading one morning when you're going to be home, take all loads off of the inverter (switch them to the grid), and turn the inverter off. Wait a couple of hours for the bank voltage to stabilize with no load on it, and then measure voltage. This will give an indication of what the resting voltage is and what the actual voltage is in the morning. Ideally, you should at the same time do a specific gravity reading as a double-check on your voltage meters; the two should be pretty close.

FWIW, I have a very similar system design to yours, with grid support. I have my inverter switch to grid at 48.8 or sometimes 48.4 volts. With my loads, this is typically enough to carry the house from dusk to dawn, and keeps the batteries from falling below 75% SoC. I have the inverter reconnect set at 52.4 volts, which means that if my house does go to grid power, the system switches back to off-grid operation fairly quickly the next morning, as long as there is some sun.

I also suspect you're pushing them a little too far.

westbranch

Re: How low can I discharge my 48 volt battery system safely on a daily basis?

Tony, can you add a bit more history of your system. It will help assess if you have gone too far too long and damaged your bank.
ps I see you have Rolls batteries, did you do a commissioning EQ charge?

tonygcan

Re: How low can I discharge my 48 volt battery system safely on a daily basis?

Thanks for all the advice guys. My system is young. It went online last December 18 only. Yes, I did do a commissioning equalization. I don't always go down to 46 volts. For example today, when I checked my SCP this morning the batteries read 48.1. I was a little unsure about the cut off voltages but thanks to you guys I'm pretty clear on that now.

Blackcherry, would you know if there's a way for me to program my XW4548 to cut off automatically at 48.6 and cut in at 52.5 volts or do I need o do this manually? I'm trying to understand the grid support feature without selling to the grid since we don't have that facility here in the Philippines.

I set up my solar system instead of spending money to buy a back up gen set. I figured I'd rather be using a system that would give back everyday rather than a generator hat I would be using only about 2-3 times a year when we have blackouts during typhoon season. I have the batteries supply all my lighting requirements for the house which makes the family more conscious about what lights they keep on.

westbranch

Re: How low can I discharge my 48 volt battery system safely on a daily basis?

tony can you describe your overnight loads? and the specific gravity of your cells now and after the commissioning charge?

Blackcherry04

Re: How low can I discharge my 48 volt battery system safely on a daily basis?

tonygcan wrote: »

Blackcherry, would you know if there's a way for me to program my XW4548 to cut off automatically at 48.6 and cut in at 52.5 volts or do I need o do this manually? I'm trying to understand the grid support feature without selling to the grid since we don't have that facility here in the Philippines.

Tony, I am not familiar enough with that inverter to tell you how, but I am sure someone will be. Poster " JoeB " seems to have a lot on knowledge about them, you might want to PM him.

tonygcan

Re: How low can I discharge my 48 volt battery system safely on a daily basis?

Blackcherry04 wrote: »

Tony, I am not familiar enough with that inverter to tell you how, but I am sure someone will be. Poster " JoeB " seems to have a lot on knowledge about them, you might want to PM him.

Thanks Blackcherry.

tonygcan

Re: How low can I discharge my 48 volt battery system safely on a daily basis?

westbranch wrote: »

tony can you describe your overnight loads? and the specific gravity of your cells now and after the commissioning charge?

Hi West, i apologize but I don't own a hydrometer and so have no way of measuring SG. My loads consist of the following. From 6pm to 10 pm about 600 watts per uh our or a total 2.4 kwh. Ten from 10 pm to 6 am about 175 watts per hour or 1.4 kwh. So far the highest harvest I've had in one day is 7.8kwh. Somehow, it doesn't seem like I am getting the rated 450 amp-hour of my batteries. Somehow, if I start with full batteries at 6 pm of the night before or about 51.8 volts I wi ll end up with 48 volts in the morning.mthis is after consuming about 5-6 kwh overnight. Atm450 amp-hours x 48 volts I should have a capacity of 21,600 watts and at 50% DOD I should have about 10,800 watts available. However, my voltage goes down to 48 or below 48 volts in the morning.

BB.

Re: How low can I discharge my 48 volt battery system safely on a daily basis?

Assuming you have flooded cell batteries, you really need a hydrometer of some sort... Perhaps you can find one at an automotive or truck supply shop in the Philippines?

-Bill

jeffkruse

Re: How low can I discharge my 48 volt battery system safely on a daily basis?

Please forgive me if someone has suggested this already but I only spent a few minutes reading this thread. It seems like you might benifit from using the GRIDSUPPORT function on your 4548. I had a simalar system. The Gridsupport did not work exactly like I thought it should have but it still was beneficial. I had Sell off. With Gridsupport on the system would use from 10 to 30% of the power from the grid all the time and 100% from the grid when your battery got low (you specify the voltage).

Rngr275

Re: How low can I discharge my 48 volt battery system safely on a daily basis?

tonygcan wrote: »

Hi West, i apologize but I don't own a hydrometer and so have no way of measuring SG. My loads consist of the following. From 6pm to 10 pm about 600 watts per uh our or a total 2.4 kwh. Ten from 10 pm to 6 am about 175 watts per hour or 1.4 kwh. So far the highest harvest I've had in one day is 7.8kwh. Somehow, it doesn't seem like I am getting the rated 450 amp-hour of my batteries. Somehow, if I start with full batteries at 6 pm of the night before or about 51.8 volts I wi ll end up with 48 volts in the morning.mthis is after consuming about 5-6 kwh overnight. Atm450 amp-hours x 48 volts I should have a capacity of 21,600 watts and at 50% DOD I should have about 10,800 watts available. However, my voltage goes down to 48 or below 48 volts in the morning.

Coming from an "Off-Grid" perspective... If I/we were using 1.4Kw over night (while everyone was sleeping) I would be looking for the culprit. Even the 600w/hr from 6-10 would put me on the war path....

westbranch

Re: How low can I discharge my 48 volt battery system safely on a daily basis?

I concur, hopefully it is not that inverter, otherwise if it is you will have to set it up as JeffKruse said

Ralph Day

Re: How low can I discharge my 48 volt battery system safely on a daily basis?

I don't think you can "safely" discharge your batteries like you are without a hydrometer. The cheapest ones bought at an auto supply house will do (I've found them as accurate as my good one). You're on the road to destruction of those batteries without one. I know. I started out without one, just used the battery monitor meter, but it drifts out of alignment over time. You really need to know your state of charge. I have some 8v CS type surrette batteries (820ah 20hr rate) which are 9 years old but because of bad treatment early on some cells are considerably "older". Spend $5, get a hydrometer.

Ralph

tonygcan

Re: How low can I discharge my 48 volt battery system safely on a daily basis?

Thanks for all the tips and the warnings. I'll make a trip to the local True Value store to see of they have. Hydrometer available.

I tried to use the grid tie support facility on the XW 4548 and also enabled the charge function othe inverter. However, after a couple hours I had a fault in the system that said that my AC1 was over voltage.

Is there a way to check if my batteries are giving out their rated amp-hour capacity?

I suspect that my batteries are giving me less than its rated 450 amp hour capacity.

Blackcherry04

Re: How low can I discharge my 48 volt battery system safely on a daily basis?

tonygcan wrote: »

Thanks for all the tips and the warnings. I'll make a trip to the local True Value store to see of they have. Hydrometer available.

I tried to use the grid tie support facility on the XW 4548 and also enabled the charge function othe inverter. However, after a couple hours I had a fault in the system that said that my AC1 was over voltage.

Is there a way to check if my batteries are giving out their rated amp-hour capacity?

I suspect that my batteries are giving me less than its rated 450 amp hour capacity.

Tony, get the hydrometer and then do a full charge ( Bulk, Absorb ) and let them rest for at least 3 hrs without a load on them, then post the readings. That will tell where they are as far as capacity. Your batteries are in series, that total does not mean you have 450 Ahr in capacity, you would only have 225 Ahr @ 50% DOD. I would suspect that your going to have to Equalize them if you haven't.

You can adjust the incoming Grid voltage limits for the inverter.

Offgridpower

Can someone explain to me in baby language, like a 4 year old, how come a 48V system cannot be drained to 24V, which would be a 50% discharge and why my generator should kick in at 46V or 44V?  Much appreciated.  Just moved into an off grid house, with 64 batteries. My Xantrex readout is 51-52V once the generator stops or the sun goes down, then the drain down starts and by middle of night my generator kicks in again to charge up.  Then usually goes for 2-3 hours for the full cycle, bulk - float - absorb, before it shuts down.  Its its charging nothing and just wasting propane as the battery sometimes already fully charged.  So, I was thinking about lowering where the generator kicks in to about 40V, so that the generator kicks in less and when it does kick in, charges up the battery more.  Thanks for the input. 

BB.

Keep all replies to "Offgridpower's" questions in his own "new discussion" at this link:

https://forum.solar-electric.com/discussion/358218/how-low-can-i-discharge-my-48-volt-off-grid-battery-system-safely-on-a-daily-basis#latest

-Bill "moderator" B.