Battery bank discharging faster than anticipated

Tnarg224

New Member
Joined
Jul 14, 2021
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5
Hello everyone,



I'm new to this forum and have browsed through old threads briefly trying to find someone having the same sort of problem I am, but to no avail. My girlfriend and I are currently building a trailer to cruise around the US in and while building it are living in an older camper on my father's property in Vermont. We are piecing together a solar system and currently have as follows:





2x 12V 200AH Renogy AGM batteries wired in series to make a 24V bank. We figured that staying above 50% SOC we have about 2400 wH, and total battery bank storage of 4800 wH



Midnite BRAT 30A Charge controller



Gopower 2000w pure sine wave inverter



IOTA Engineering DLS-27-25 25A, 24V power converter with the Q4 smart charger



Midnite solar battery SOC meter

And a mismatch of PV mods wired in parallel that are:



Canadian solar

Pmax 330 W

Vmp 37.2 V

Imp 8.8 A

Voc 45.6 V

Isc 9.45 A



Astron Energy

Pmax 260 W

Voc 37.78 V

Isc 8.93 A

Vmp 30.9 V

Imp 8.48 A



We just got a pair of Trina 415W panels that we have not set up yet.





Now, we calculated our loads (pretty severely overestimating our time using various devices to be much higher than we really anticipated) to come out to be 2800wH for 24 hours. While living in the camper in the meantime, we are hooking up the shore power cord into our inverter. I know we are going through a couple steps that will be losing efficiency (namely going from DC from the battery bank back to AC for the inverter and then back into DC again to run 12V appliances through the camper.) But to find out how much power we are using after being plugged into the inverter, I hooked up a kilowatt meter in between the inverter and the shore power cord, and it says that we have a constant 50W draw. This accounts for our 12V propane refrigerator and the power converter inside the power center of the camper. There was a battery charger but we disconnected the battery and it isn't sending any power to it any more.





We figured that here, in the height of summer, we have about 12-14 hours that are not making solar production to charge the batteries and are running just off our battery storage. The problem is, when we wake up in the morning, our battery monitor is usually telling us that we are at 50-60% SOC, which is where I'm confused. The manual for the inverter said it draws 1.5A when converting power. For us, the inverter runs all the time, not just when we use AC appliances, because we are plugged directly into the inverter with an extension cord that runs to the power center in the camper. Adding the current draw from the inverter to the 50w load that we have coming from the refrigerator and power converter overnight (well say 14 hours to be safe), it totals to be about 1200wH, which should only be about 50% of our total usable wH capacity. (Usable meaning staying above 50% SOC) This should mean the battery meter should be telling us 25%



The documentation that comes with the voltage reader says that the voltages from 10-100% SOC are from 10% meaning 23.3V up to 100% meaning 25.5%. Once the sun sets and we turn on any devices that aren't lights, we have been running our generator through our converter charger until we go to bed to get back up to 100% to last throughout the night without dropping down below 50% SOC. Take tonight for example. It is 9:00 PM as I post this, the array has not been making any major power for about 3 hours, and our battery bank is already at 70% SOC. My voltmeter reads 25.1 when touched to the batteries with the inverter off. There is a load of 50w when only running a small 12V fan and the refrigerator.



Our inverter, charge controller and PV mods are the only things that we have that are not new. Could it be inverter inefficiency that is adding to how much we are really drawing out of our bank at night? How can I investigate that further? I know there can be some inverter inefficiencies, and since the inverter has to be on all the time, I could see how that would add up to have a pretty significant impact on energy usage. It seems like maybe there could be more energy loss before the inverter (resistance?) that I'm not seeing since the kilowatt meter isn't telling me how much energy is being used before the inverter converts the electricity. Or could it be that my storage calculations for our battery bank is wrong? (2X 12V, 200AH batteries=4800wH) I just can't believe that with all of our battery capacity, we can be that low in the morning, based on having a 4800wH total storage capacity. I just don't know what I'm missing here. I appreciate any input anyone has and if there is any more information needed, I'll be happy to provide it.
 

Bud Martin

Photon Sorcerer
Joined
Aug 27, 2020
Messages
2,984
Do you have clamp on DC amp meter to check the current draw from the battery?
What is efficiency rating of the inverter? 90% peak (it means the it will 1000W on the DC input of the inverter to produce 900W on the AC output side, you will also have system loss?)
The SOC meter based on Voltage readings are just estimate, you need coulomb counter.
 
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Sojourner1

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Sep 30, 2019
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678
Location
USA
What is your charger setting. Meaning what do you charge to? If you're charging to 25.5v as 100% SOC you're not at 100%.

Your 25.1v or 70% (24v system) would be 12.55v with my 12v system which I've never seen. To give an example... when my SOC is 36% drawing 114a my batteries are at 12.94v or 25.88v if it was 24v system.
 
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chrisski

Solar Boondocker
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Aug 14, 2020
Messages
3,249
IMO, the only way to check battery state of charge is with a shunt and battery monitor. The one you seem to be using seems only to measure voltage. I’m not sure what a Columb Counter is, but I think that’s a fancy name for a battery monitor with a shunt.

To me your panels seem way to mismatched to be used in parallel and you are losing a lot of production that way. Your charge controller should tell you how much its producing. THere’s some math on a link in my signature block about mismatched panels. I ask that you stop mismatching panels until you do some math. I can see at three panels mismatched by enough, it could produce less than any one panel. You’re certainly not making 330 watt + 260 watt as is.

I’m not following you about DC to AC to DC. WhenI first hooked my inverter up, I plugged it into the SHore power port, but I forgot to shut the breaker for the converter off. THat cost me 700 -1100 watt draw as my batteries tried to charge themselves. I shut the breaker off.
 

robby

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May 1, 2021
Messages
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IMO, the only way to check battery state of charge is with a shunt and battery monitor. The one you seem to be using seems only to measure voltage. I’m not sure what a Columb Counter is, but I think that’s a fancy name for a battery monitor with a shunt.

To me your panels seem way to mismatched to be used in parallel and you are losing a lot of production that way. Your charge controller should tell you how much its producing. THere’s some math on a link in my signature block about mismatched panels. I ask that you stop mismatching panels until you do some math. I can see at three panels mismatched by enough, it could produce less than any one panel. You’re certainly not making 330 watt + 260 watt as is.

I’m not following you about DC to AC to DC. WhenI first hooked my inverter up, I plugged it into the SHore power port, but I forgot to shut the breaker for the converter off. THat cost me 700 -1100 watt draw as my batteries tried to charge themselves. I shut the breaker off.
I agree with the panel mismatch problem and also he does not have enough panels to charge those batteries.
 

Sojourner1

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Sep 30, 2019
Messages
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USA
After the OP replies with charging regimen the picture will get bigger. Mismatch panels have nothing to do with the 24V power converter with the Q4 smart charger they said they useafter the sun goes down. Unless they have massive loads which appear they don't the batteries shouldn't be running down in 3 hours and need a boost to get through the night unless....the batteries aren't fully charged.
 

Alphacarina

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Feb 22, 2021
Messages
214
I’m not following you about DC to AC to DC. WhenI first hooked my inverter up, I plugged it into the SHore power port, but I forgot to shut the breaker for the converter off. THat cost me 700 -1100 watt draw as my batteries tried to charge themselves. I shut the breaker off.
It sounds like he doesn't have a 24 to 12 volt DC to DC converter for his 12 volt loads - He's running a 120/12 volt charger off his 24 volt inverter to get 12 volts, 24/7 - That is certainly part of his problem, IMO

I agree he really needs a matched set of solar panels to get the best production - Paralleling what he has up there now isn't going to work well

Don
 

Tnarg224

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Joined
Jul 14, 2021
Messages
5
Ok seems like I am definitely going to need to set up my new panels then. I have 3 415W panels that I haven't put on because I thought I had enough with those 2 mods to charge my batteries.

Specs for new mods are:

Trina solar TSM 415

Pmax 415W
Vmp 40.9
Imp 10.15
Voc 49.6
Isc 10.6


I was going to wire them in parallel since the BRAT is a PWM controller. For the trailer I was going to get an MPPT charge controller. Good or bad idea? Down the road I was going to put 2 of them on my roof of the trailer and leave the third off so I can set it up in the sun if we are parked in the shade for extended periods of time. Will I be needing all 3 to have enough voltage (or wattage or amperage?) To reach full charge? What is the voltage I should be seeing to be fully charged with my bank? Am I affecting the longevity of my batteries by not reaching full charge? What is the difference between a battery monitor and a state of charge meter? Is a battery monitor reading voltage as well? I'm not 100% sure that the power converter we have has gotten to it's float setting yet. It doesn't list it in the manual but it says on the side what each cell will be charged to for bulk, absorb and float. I'll post back when I get back home.

To answer the charger setting questions, I set the BRAT up for the sealed profile #2 which is bulk and absorb at 14.4 and float at 13.5. It says equalizing N/A which I understand is a setting just for sealed batteries.

The inverter has an efficiency rating of >80%. But it isn't new and I suspect that could also be part of my issue. What kind of voltages am I looking for with the clamp on meter? I can get my hands on one..

Thank you all very much for your help and I'll post back in a few days when I get back from a long weekend in the white mountains. Life is good.
 

fratermus

Solar Addict
Joined
Mar 19, 2021
Messages
328
discharging faster than anticipated... 2x 12V 200AH Renogy AGM batteries wired in series to make a 24V bank

AGM can exhibit capacity walk-down (actual or apparent) when charged with gentle current. Is the bank getting the 25A Iota regularly, like at least 1x/week?

Even 25A might be marginal. Minimum charging currents vary by battery/manufacturer, but some common ones:
  • 0.15C = 30A
  • C/5 = 40A
  • C/4 = 50A
IIRC these batts have 60A max printed on the case, although this data sheet shows 80A. I don't know if Renogy publishes a minimum charging current.

Could you borrow a ~45A charger and give the bank a couple rounds of firm charging to see what happens? If they snap back to expected performance then Iota and others make 45A-50A 24v converters. Could sell off the 25A to help recoup the expense.
 

Tnarg224

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Joined
Jul 14, 2021
Messages
5
AGM can exhibit capacity walk-down (actual or apparent) when charged with gentle current. Is the bank getting the 25A Iota regularly, like at least 1x/week?

Even 25A might be marginal. Minimum charging currents vary by battery/manufacturer, but some common ones:
  • 0.15C = 30A
  • C/5 = 40A
  • C/4 = 50A
IIRC these batts have 60A max printed on the case, although this data sheet shows 80A. I don't know if Renogy publishes a minimum charging current.

Could you borrow a ~45A charger and give the bank a couple rounds of firm charging to see what happens? If they snap back to expected performance then Iota and others make 45A-50A 24v converters. Could sell off the 25A to help recoup the expense.
I don't think it will be possible to get my hands on a bigger amperage charger without buying one. I could possibly wire those Trina mods I have to get a higher amps rating? I still need to stay within the charge controllers amperage setting though right? You are spot on on the rated charging amperage of our batteries. The data sheet you posted could be for a newer version of our battery, I can't say for sure, but it looks just a little bit different.




We have had this setup for a few months now. We bought the batteries last September and were trying to use just the 330W panel to charge the bank. We did this for about a month, and then left for the winter after leaving them fully charged. I can't remember the voltages but I remember checking them with a multimeter and being satisfied that they were charged enough. Coming back 6 months later, we moved on to two mods in our array because we were having the same problem during those first two months. We got the power converter about a month ago and have been running it almost daily since then, sometimes for several hours at a time, just to be sure to allow enough time to recharge the bank. What should a single battery be showing for voltage when fully charged? Or should I be checking something else to verify? Should I check them one at a time or should I check the bank as a whole? Is this something that could potentially affect the battery capacity?

If I need a higher charging amperage, I will have to wire my panels in series as opposed to parallel, correct? Does this mean I'm going to need a bigger charge controller than the BRAT? I was looking at an MPPT charge controller that I think was rated to 50A.

Thanks for the response. Much appreciated
 
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2009Bounder2020

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Dec 28, 2020
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Yes your thinking right on the mppt... its really worth it, 50a or higher is a good starting point.

another cheap option is to keep what you have and add more than one charge controller.
you could just run one charge controller per panel (of if you have 2 of the same panels put them in series on thier own controller)
 

fratermus

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Mar 19, 2021
Messages
328
What should a single battery be showing for voltage when fully charged? Or should I be checking something else to verify?

The data sheet says charging for cycled batteries is "14.4-15V", so I'd expect them to be ~charged when acceptance for the series bank falls to <=2A (the C/100 rate) at 28.8v.


I still need to stay within the charge controllers amperage setting though right?

MPPT controllers can be overpaneled to some degree, but AFAIK PWM should be run within their amp rating. I've abused one with overcurrent (emergency situation), and it would error and go offline for a bit when exceeding 105% of rating. I don't know what other PWM would do. Within sane limits MPPT will hold the line at their rated output.

I agree with @2009Bounder2020's suggestion about multiple controllers. Maybe pick up a ~60A mppt for the new trinas and leave the mismatched CS and Astron paralleled on the Brat. I am pathologically averse to running mismatched panels on the same controller, so I don't know if a separate PWM for the Astron would make sense or not. Onlookers who do know such dark magics please chime in! :)
 

Tnarg224

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Jul 14, 2021
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I was planning on using only two of the new Trina's for most of the time. 2 for on the roof of the trailer and 1 to drag out into the sun if we are parked in the shade for extended periods of time. Sounds like only having one mod to charge this bank won't be quite enough, will it? I remember estimating we would need about 800W of solar, am I wrong on the number?

4 hours of sun in the winter X 800W of array wattage=2400W. Is this not a correct assumption, since it won't be enough amperage to actually charge the batteries? I will only have 20A, if I wire in series, coming from the array since the IMP is 10.15. Or do I have that wrong? I can never remember the effects of series vs parallel. I thought just matching the wattage would make enough power to recharge the bank.
 

Tnarg224

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Jul 14, 2021
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The 2 new PV mods are bringing me up to what the BRAT says is float charge. I believe it is at 13.5V for float. Still waking up to 50% SOC according to my meter. Will get myself a battery monitor with a shunt. Any recommendations on space conscious units?
 

Mia

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Jun 21, 2020
Messages
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After the OP replies with charging regimen the picture will get bigger. Mismatch panels have nothing to do with the 24V power converter with the Q4 smart charger they said they useafter the sun goes down. Unless they have massive loads which appear they don't the batteries shouldn't be running down in 3 hours and need a boost to get through the night unless....the batteries aren't fully charged.
Or could the batteries possibly be bad/failing? Were they ever discharged below 50% when you were away? if so, maybe they aren’t holding the charge?
 

12VoltInstalls

…myself everything do I…
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I’m also in Vermont

did you get an mppt?

we have a constant 50W draw. This accounts for our 12V propane refrigerator and the power converter inside the power center of the camper.
LS-27-25 25A, 24V power converter with the Q4 smart charger
I think you are losing a lot right there^^^
Still waking up to 50% SOC according to my meter. Will get myself a battery monitor with a shunt. Any recommendations on space conscious units?

This is one of those circumstances where it just makes sense to charge from solar at higher voltage but use a 12V battery bank. While 24V is better in several aspects getting you running successfully can be done quick with 12V storage; you could also ‘fix’ things so 24V works but your instantaneous loads don’t appear to require 24V.

Power Suck Solution One:
Eliminate the converter and other chargers/etc and just use 12V from 12V. (New inverter needed though)


Power Suck Solution Two:
Older RV fridges are fine on propane but many (all?) of the newer ones use power even on propane- and A LOT of power on electric. Ditch that and go all electric 5-10CF from Home Despot or walmartha with something 60- 90W $140-$500

Power Suck Solution Three:
Get two more trinas if you can: run 2S2P on mppt. Use your pwm for the mismatched ones (inefficient) or get a second small mppt (better) for those

I think you also need more battery; I have -330Ah useable at 12V with (currently) 400W input with an electric fridge 24x7 on a 1200W inverter. I have no phantom loads, use 12V led lighting, 600W coffeemaker, 12V hood fan….

Your setup isn’t working because of sucking leach loads, concentrated charge hours, and insufficient battery. It will and can work as well as mine. My camper is wicked old- I didn’t have a converter to bypass, and my shorepower cord hasn’t been unplugged from the inverter for 3+ years… and I don’t have a generator

You can do it, too.



(Where in Vermont are you?)
 
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