How much battery to battery voltage difference before equalizing ?

Halfwalker · Feb 1, 2021

Our setup here in CT consists of a Schneider XW Pro inverter, 8x Crown CR-430 batteries for 48V. The XW Pro is a replacement for a pair of ancient Trace SW4024 workhorses that were killed in the hurricane last year. 4x of the batteries are 1 year old, used with the SW4024 setup in 24V. They saw maybe 7 discharge/charge cycles. When we got the XW Pro we got 4x more of the exact same CR-430 battery to make up the 48V bank.

System went live last Nov, and I've done 2 full battery capacity tests, taking it down to 55% SOC and recharging, plus another 2 smaller tests only down to maybe 80% SOC. We get about 8 hours runtime on battery, and it takes the generator about 3 hours to fully recharge from 55%.

I built a small meter to monitor the individual batteries ... The middle 4 voltages are the older batteries. String goes from upper-left negative down, then to lower right and up to upper right positive.

The individual voltages are within 0.02V. I've checked the batteries several times over the weeks with two different multimeters, and they might say one battery is 6.72V when the monitor here says it's 6.74V. So fairly good accuracy. The widest battery-battery difference in this image is 6.68-6.81, or 0.13V +- 0.02V. So the max variation might be 0.17V, but might be as low as 0.09V.

The question is, at what point in that variation should an equalization run be performed ?

snoobler · Feb 1, 2021

Welcome to the forum.

Have you been equalizing in accordance with Crown's directions?

https://documents.unboundsolar.com/media/crown-cr430-flooded-battery-specs-2834660653-1.pdf

They recommend every 7 days.

Putting 4 new batteries in series with 4 old batteries is something to be done only when absolutely necessary. When done, it should be done when all batteries are known to be at a true 100% SoC. It's one of the few "rules" with bank configuration you should really follow. It strikes me as suspect that the middle 4 are higher, which suggests the new batteries were at a lower SoC when initially put in series, or they had been on the shelf for awhile and maybe formed some light sulfation.

Assuming you're at float, those numbers aren't alarming. More importantly, what are their voltages through the absorption phase? THAT's when they should be equal across the board.

I assume you're conducting regular specific gravity checks. That matters far more than voltage. Crown is very aggressive on their equalization recommendations. Many other manufacturers recommend it "on condition" or on 30-90 day schedules. When SG drops below a certain amount and/or the difference exceeds .03, then it's time for an equalization.

I recommend the following:

Day 1: fully charge. Once in float, top off all cells to their correct fluid level.
Day 2: fully charge. 2-3 hours after float is attained with minimal system load, check and record all 24 cells for SG. When checking, it's important to suck electrolyte in to the hydrometer and expel it twice before taking a reading.

Once you have those 24 values, you can assess their actual states of charge vs. specific gravity, which is a more accurate measure and determine if equalization is needed.

Halfwalker · Feb 1, 2021

Prior to installation I charged them in pairs of 12V with a typical charger for a good two weeks. The pairs were mixed each time, so battery A paired with B for one charge, then with C for another and so on. Each one charged with at least 3 different partners, with a small 12V load put on them after so the next pair-charge could actually put come current into them. I verified with Crown and with Solarbiz (vendor) about the mixing of old-ish and new and the charge profile, and they were both fine with it. The old ones really hadn't been exercised, mainly sitting at float 54.2V, with only that handful of discharge/charge cycles down to 55% max.

The full 8x bank has had one equalization charge, after the first capacity test. These are the charge settings in the XW Pro

That meter image is at float, where they've been for a few weeks (no outages knock wood). I don't have good SG records, but all the checks I've done showed SG to be almost the same across the board. I do need to actually *log* the numbers dangit.

snoobler · Feb 1, 2021

Ah... backup system. Far less worries about dissimilar age.

Glad to hear you made an effort to equalize them as 12V. Helps.

Probably worth doing a shallow discharge and permitting a bulk/absorption and float cycle to compare the peak voltage are where they should be, 7.38V or so.

Most settings look good. 80% max charge rate is concerning. Data sheet indicates 100A max. That should be 100/430 = 23.3%.

Personally, I'd do an equalize to 64.8V per the CR-430 data sheet if your inverter will tolerate that. I would further limit the charge rate to 15/430 = 3.5%.

Crowns have an atypical charge cycle. Per the data sheet:

Phase 1: Bulk to 56.88V
Phase 2: Hold 56.88 until drop to 15A
Phase 3: Hold 15A until 64.8V termination or TOTAL charge time of 16 hours, phases 1 through 3.

An equalization is a 4 hours extension of phase 3.

They don't even recommend a float.

Halfwalker · Feb 1, 2021

The 80% refers to the percentage of the inverter charge capacity - it can do 140A, so 80% is 112A, a bit over the 100A datasheet number. These values are from Solarbiz, who've done literally hundreds of installs of the XW-Pro with Crown CR-430 battery banks.

As it's configured now, it will charge at the Absorption voltage (59V) starting at 112A until the current drops to 2% of battery capacity (8.6A) or the 300min/5hr timeout.

Here's from a discharge/charge test in Dec. It was taken down to about 80% SOC, and around 3:10pm AC was restored. Battery current (green) jumps to 112A, voltage to about 55.8V. When voltage hits 59V at 3:40pm it holds there (mostly) and current starts to drop off. At 4:45pm current hits 8.6A so it moves to float, dropping voltage to 54.2V, current drops off slowly after that.