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AndrewHighlands
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# Posted: 3 Sep 2021 05:36am
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Can someone please advise on how to determine whether a 24 V battery is in a good state of charge, and ideally someone who is familiar with Morningstar Tristar charge controllers?
In April I installed a new off-grid solar PV system:
I have a 1kWp solar array supplying 4x new wet deep cycle batteries wired together at 24V. There is a Tristar charge controller in-line. I monitor status with an analogue voltmeter and ammeter, and by qualitatively assessing how much power I use in relation to the size of the battery. I have a 400W inverter, but mostly use DC. All components were new in April.
My four batteries give me, I calculate, 460Ah (at 24V). So, at 50% I have access to a store of 230Ah or approx 5kWh. I assume that my battery is in a very good state because I use little energy (ca. 0.5kWh per day) and because the charge controller is mostly in float mode drawing hardly any amps. But I have a concern:
After sunset, with what I expect is a fully charged battery (voltmeter at 25.6V-ish), my family use about 0.1kWh. When we finish and I check the charge controller I see that it is showing a single yellow light which the manual says means 35 to 60% state of charge, and the voltmeter shows between 24.9 and 25.2V. I don’t understand why this should be because I have used only a hundredth of the power available.
In the morning, at sunrise, the bulk charge is less than an hour, then into PWM and float, which suggests to me that the battery needs little topping up.
So, can someone tell me:
1. Is the yellow light telling me that the battery is not in as good a state as I think? Or is it that the charge controller is inaccurate? By the way, next morning, the yellow light is still on, but even in cloudy weather it takes less than an hour for PWM to start, as described.
2. How does one use the voltmeter to determine when the battery is in very good, good, bad, state of charge? I expected that in a good state the voltmeter would show about 25.5V, but after using this 0.1kWh, it shows ca. 25V. Is this voltage reading anything to worry about.
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Brettny
Member
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# Posted: 3 Sep 2021 05:58am
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You can use this chart from Trojan to figure out volts/state of charge. But that way is prety crude and you need to allow for no loads for a period of time so the voltage can come back up.
You could also put a prety cheap battery monitor in.
https://www.amazon.com/gp/product/B089KH2NXF/ref=ox_sc_saved_title_6?smid=A2M49RHWISC A2E&psc=1
Did you download the manual for that charge controller? It has alot of good info in it.
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Steve_S
Member
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# Posted: 3 Sep 2021 05:59am
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Sounds like you have not Equalized the batteries which should happen monthly regardless. Equalize = DeSulphation. What you have now is a Surface Charge and the electrolyte is more than likely stratified. So a Good 2 Hour EQ should kick it up.
CHECK WATER BEFORE EQ !
Make sure that battery cells are topped up with Pure demineralized distilled water ONLY ! Ensure 3/4-1" space from top (see battery data sheet ! )
Voltage is a terrible indicator for Lead. Using a Hygrometer is the only way and that is testing the Acid Gravity in each cell.
The Care & Feeding of your (Expensive) Lead-Acid Batteries
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AndrewHighlands
Member
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# Posted: 3 Sep 2021 07:38am
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Thanks for both replies, and the useful info.
Brettny: I have looked in detail at the charge controller manual. Someone told me that battery monitors aren't very good, but I'll look again.
Steve_S: What you say is very interesting because I have never equalised the batteries. As mentioned, they have been connected up since April, and it has been concerning me that I ought to do it, for the Tristar manual says that I should do it three-monthly. But, the person who helped me design the system says that I should have the automatic equalisation switch (on the Tristar) switched off. There is a manual equalisation method but I have not done it because the same man said: "If you avoid discharging them deeply and leaving them to sulphate then an equalize charge will not be needed, and is likely to shorten their lives if you do one.
Thanks for the links which I have printed off.
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Brettny
Member
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# Posted: 3 Sep 2021 08:18am
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Yes voltage is a bad Indicator of SOC but right now hes really just trying to figure out what's wrong with the system not check state of charge super accurate.
Is your charger hitting 28v before going into float? Also what voltage are your pannels?
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Steve_S
Member
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# Posted: 3 Sep 2021 08:31am
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Sorry but Blunt is here. The FOOL who told you not to equalize Lead Acid is just that and he's played with YOUR Investment and that's your wallet.
Discharging any Lead Battery below 50% DOD (note I do not use SOC) you are causing harm, regardless if Heavy Deep Cycle OffGrid Lead or a simple light car battery.
When I dropped just under $4K on my Lead I studied everything about them using the Manufacturers datasheets & info which is Rolls Surette. I was not about to let "someone" tell me what and I looked at Credible Real Info as posted above.
I am now Very Deep into Lithium Iron Phosphate LFP Batteries and now my old Lead Bank is my Secondary Backup to my 30kWh of LFP which is now my primary battery system. Irony is, My LFP is 30kWh and my lead was 11kWh Usable. For the cost of of that Heavy Lead 5 years ago, I've got all LFP (DIY'ed) and 3x the stored capacity, today.
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AndrewHighlands
Member
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# Posted: 3 Sep 2021 08:44am
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Yes, the charger goes to 28.8V in PWM. This is its setting. When it is in PWM and float mode everything is fine (according to the voltmeter). The morning bulk charge is rapid also (less than an hour on a cloudy day) before the controller goes to PWM mode. It is just that I don't understand why the battery voltage drops quickly after dark when only 0.1kWh are used.
Maybe it is as Steve says. I'm just reading up on the literature he sent and will likely give the battery an equilisation tomorrow.
The panel voltage: according to the documentation that came with them, each is 34V nominal and 41.4V open circuit. There are four panels, each at 335 Wp.
Thanks again.
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gcrank1
Member
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# Posted: 3 Sep 2021 11:08am - Edited by: gcrank1
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My system is 24v to the scc, then 12v, 1 big ups bat and 1 'marine deep cycle' in parallel (yeah, mismatched, OLD...but they are still working!).
If you are checking the supposed SOC, and taking the clue from the 'yellow light' Right After Using The Bats then you have not given them time to sit and settle down from the discharge. I wait At Least 1hr. for my 1st check, then several hours later again With No Use.
Just last month I unhooked the array in the afternoon (my solar charging is basically over by 1pm), ran on the bats that evening all we wanted and in the morning before the sun really got above the tree line, and before we ran more than the morning lightbulb much at all, my bats were still at (12v system) 12.4v.
As you, by mid-morning my scc read-out has topped the bats and gone to absorb/float even on an overcast day.
This indicates to me that our usage to recharge balance is quite good.
Fwiw, we are easy on usage, heavy loads are done with the 1700/2000w inv/gen for short periods. I used to have a smart charger run to the bat-bank whenever the gen ran but have quit with that to see if my 9am to 1pm 600w of array can keep up. YEP, no probs.
The normal lights, usb ports, fans, cordless tool chargers, etc. all run off the solar as needed on a 300w inverter to 120vac at the bat-bank. We use what we want when we want and the system keeps chugging happily along 
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Brettny
Member
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# Posted: 4 Sep 2021 07:44am
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From reading your original post at 60% charge your batteries not under load should be about 24.5. I would say the meter on your CC is wrong. 25.4 would be a fully charged battery. I have noticed that with my 12v GC2 bank once your down to say 80-90% state of charge it takes a while to drop below 70-80% . The output is not very linear compared to time.
Let us know how you making out doing an equalization charge. I need to do that with my setup still.
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gcrank1
Member
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# Posted: 4 Sep 2021 11:10am - Edited by: gcrank1
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Im not up on Eq because my bat-bank was 4 big ups agm's so Eq was, and is still disabled even though #2 is a FLA marine now. I just started with each load tested then fully charged for several days off system with my grid smart charger; probably should do it again before winter.
Btw, the load tester from a friend is the big load automotive type, maybe the wrong thing but it sure showed why my bank wasnt working with 3 big agm's in parallel, 1 was waayyy Bad, 1 middlin and 1 pretty good. By pulling the Bad 1 out things improved a lot, then after about 1 month a friend gave me the good marine bat so I pulled the middlin agm off and things have been running great since.
So, you might want to check each bat to find out if 1 or some are pulling down the rest. That is what they do, the bank is only as good as the worst battery.
Once you have known good bat(s) fully charged as baseline to start from you can better judge the system performance.
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