Battery Voltage Reaction

[Darin O.]

Brand new to solar. Just bought the Renogy 200 Watt 12 V Kit with Wanderer Controller and added a bayite power meter between battery and TechOn 1500W Inverter connected to a Sterling sealed lead acid 125ah deep cycle battery. I've done tons of research and understand that to protect the battery I shouldn't use it below 12.2 volts, which, as I understand it, would constitute using only about half the available battery power. So here's my questions that I can't find any answers on:

When I put a decent load on the inverter, something greater than charging my cellphone, I see on the power meter the volts begin to drop and they will drop below 12.2 volts. However, as soon as I remove the load, the meter will show the battery going back up to something well above 12.2 volts. So is going below 12.2 volts during the load hurting the battery?

While a load is on the battery, should I never allow the load to take the volts below 12.2?

Or can I allow the load to continue to pull from the battery even though it takes the battery during the load well below 12.2 as long as I turn the load off when the actual state of charge of the battery goes no lower than 12.2 volts?

If the answer to this last question is "yes", how can I know when the actual state of the battery is 12.2 volts when the meter only reads what the load is causing the battery to do under load?

So far, this is the only piece of solar usage that I can't find any answers on and don't understand, at least on a basic level. Please educate me!!

Thank you in advance for taking the time to read, understand, and respond to my questions!!

 

[snoobler]

Are you certain the battery is fully charged? SLA typically requires charging to 14.2-14.8V with that voltage held until the current drops to about 3A. It should be fully charged before first use.

Are your cables between the inverter and battery firmly attached? Loose connections can cause a notable voltage drop.

Your inverter may consume 15W just being on (idle power consumption).

Voltage drop with load is normal. The charts you find indicating voltage vs. SoC require the battery be at rest for about 24 hours with no load or charging for those to be meaningful.

The most effective means of determining SoC is with a battery monitor that counts current. The most accurate are something like a Victron BMV unit that also factors in Peukert's Law (if you draw more than the rated current, you will get less than rated capacity). They also are programmed with the fully charged criteria, so they can reset themselves to 100%.

 

[Darin O.]

It may not have been fully charged the first time I tried to put a load on it when I saw the reaction. It appears you're saying that would make a difference in dropping below 12.2V?? I immediately turned the load off because I was afraid I may be damaging the battery.

The cables are firmly attached.

When you say "Voltage drop with load is normal", is that to say even a drop below 12.2V is normal? That doesn't damage the battery then as long as when I remove the load the meter states the battery immediately goes back above 12.2V?

(I apologize for my ignorance)

 

[snoobler]

Please read the battery FAQ in link #4 in my sig (turn phone/tablet sideways if you can't see it).

12V batteries aren't empty until they hit about 10.5-10.8V under load. You'll want to stop well before that.

Voltage is an essentially useless measure of SoC unless it's sat for 24hr without being loaded or charged.

 

[Hedges]

Here's the manual for my AGM battery.

http://sunxtender.com/pdfs/Sun_Xtender_Battery_Technical_Manual.pdf

Page 35 shows 12.2V is 10% DoD to 50% DoD, depending on current draw.
At room temperature.

Have a meter? Measure voltage directly between terminals of battery. If you get a different reading from what inverter shows, measure voltage across each connection and each wire, find if anything is showing too much voltage drop.

 

[Darin O.]

Thank you for the information. I've gone through everything you've given me a few times, trying to digest and understand it, including link #4. It's been very helpful to understand the differences among batteries, even different batteries of the same type (not all brands of FLA are equal obviously), and various other data points. At the end of the day though it looks like, and I'm hoping you can confirm this, or straighten me out, that there's really no way to know the SoC or DoD on a battery under load except to have something like the Victron BMV unit properly hooked up to the battery or battery bank. Is that the case?

 

[snoobler]

Thank you for the information. I've gone through everything you've given me a few times, trying to digest and understand it, including link #4. It's been very helpful to understand the differences among batteries, even different batteries of the same type (not all brands of FLA are equal obviously), and various other data points. At the end of the day though it looks like, and I'm hoping you can confirm this, or straighten me out, that there's really no way to know the SoC or DoD on a battery under load except to have something like the Victron BMV unit properly hooked up to the battery or battery bank. Is that the case?

Voltage can give an approximation, but that varies based on your specific battery and loads. The resource @Hedges provided is not particularly common. On page 35, his specific battery shows SoC for various loads:



If at a 1 hr rate, 12.2V may indicate 85% SoC. At a 24 hr rate, it may indicate about 50%. This is illustrative of how greater loads cause greater voltage drop.

Since the table is in terms of various rates, you could probably interpolate that to apply to your battery provided you can identify BOTH voltage AND current at any given time. As your current changes, so does the column you would use. This is probably confusing, and that's why voltage is a poor indication of SoC UNLESS it's open circuit voltage. The following table from the same resource only requires a 4 hour sit since last use:

 

[RCinFLA]

You should not pull more than about 30 amps for extended time from a single 125AH lead-acid 12v battery. That is about 325 watts AC from inverter. If you draw 30 amps you will run battery completely discharged in about 2.7 hours.

Below is actual load test done on sixteen 105 AH lead acid batteries. They were all 105 AH sealed lead-acid starting from full charge state.
Load was 25 amps for 30 mins which was about 12-15% DOD at 25 amp draw. Test was only for battery health check.

Often when float charged for long period there is an initial dip in voltage for first couple of minutes as excess lead oxide coating is burned off of positive plates. This is due to an initially higher series resistance due to the thicker lead-oxide coating. After the 30 min discharge the batteries were no-load rested for an additional 30 minutes. The recovery voltage was taken after 30 min. rest period.

105 AH Battery Voltage at Load test of 25 Amps		30 min. Return
	discharge	time ->						no-load
Start	30 sec	1 min	5 min	10 min	15 min	30 min		30 min rest recovery
12.83	11.92	11.96	12.07	12.06	12.04	11.98		12.66
12.82	11.96	11.99	12.11	12.11	12.10	12.05		12.68
12.82	11.93	11.98	12.08	12.07	12.05	11.99		12.66
12.83	11.98	12.02	12.13	12.13	12.11	12.05		12.69
12.80	11.95	11.99	12.10	12.10	12.09	12.03		12.67
12.80	11.92	11.97	12.08	12.07	12.05	11.99		12.66
12.80	11.95	12.00	12.10	12.09	12.08	12.00		12.67
12.80	11.95	11.99	12.10	12.09	12.07	12.00		12.67
12.93	12.06	12.07	12.16	12.15	12.13	12.07		12.75
12.92	12.04	12.07	12.16	12.15	12.13	12.07		12.74
12.96	12.09	12.08	12.18	12.17	12.15	12.09		12.78
12.89	12.03	12.05	12.15	12.14	12.12	12.06		12.73
12.93	12.05	12.07	12.16	12.15	12.14	12.08		12.76
12.94	12.07	12.09	12.19	12.18	12.16	12.11		12.78
12.89	12.04	12.07	12.17	12.16	12.15	12.08		12.74
12.91	12.05	12.07	12.16	12.15	12.13	12.08		12.74