Bulk / Absorb question

n0eggs · February 2019

Hello all!

My system has been running for about two weeks now and it seems (to me) to be running well. So far I'm only dipping into the batteries about 10% on average overnight (being overly conservative at first).

My bank is two 24v parallel strings of Rolls s550's and I've got them connected to a Midnite Classic 150.

Rolls states that the Bulk phase typically charges a bank to 80%. This has me slightly concerned since the default program of the classic has Bulk beginning in the early AM sun on a bank that is already at 90% SOC. Rolls also recommends the formula T = 0.42 x C /I for absorb time calculation - with the ~100a output of the classic, that puts my Absorb cycle at roughly 4 hours. This combination seems like it could be too much to replenish 10% usage.

Hoping for some opinions on whether or not some fine tuning would be a good idea or if all is a-o-k.

Thanks!

Estragon · February 2019

The bank will take a diminishing current as absorb progresses and the bank gets closer to full. At close to full, current will be down to ~1-2% of bank capacity. On a 700ah bank, for example, that would be 7-14amps, much of which is going to heating and gassing rather than increasing SOC.

The classic should normally have "end amps" set to ~10a, so it will go to float when the absorb timer times out or end amps is reached, whichever comes first. As the bank ages, it may be necessary to increase end-amps a bit.

If your classic has a WbJr installed, it can tell actual amps into the bank (irrespective of loads). If not, the classic can only see amps at its output, so end amps may need to be adjusted to account for loads.

I find (on a good sunny day) my banks usually get to end amps in 2-3 hrs.

n0eggs · February 2019

The bank has 856ah and I do have a WBjr. Oddly I just checked that the Classic ending amps setting shows 0 with the use WBjr for End Amps setting unchecked.

I also see a "days between bulk" setting.

mike95490 · February 2019

The Classic also has a Skip A day feature, to not charge a lightly used battery daily.

With many new batteries, you need a dozen or so deepish cycles to get the plates condtioned for full capacity. Shallow cycling delays that process

Vic · February 2019

Your Surrette batteries will need about 50 - 80 cycles to break in. Until the batteries are broken in, you will need to watch the SG readings on your good, high quality Hydrometer.

Identify at least one battery's cell as a Pilot Cell (usually one of the cells with the lowest SG reading, when fully-charged). You will want at least one such cell per string of batteries.

Number each battery (with a unique number), and letter each cell -- A, B, C, for each battery.

Create a battery LogBook. Record SG readings there, your charge settings, EQ Date, duration, and other observations about the batteries.

EA settings will probably change as the battery cells becomes more fully Formed (broken in).

You will want to cycle the battery more deeply than to 90% State Of Charge (SOC) to break them in.

EDIT: FYI, here is a Surrette article on Measuring SGs:

http://support.rollsbattery.com/support/solutions/articles/4347-measuring-specific-gravity

Here is a Link to the Surrrette Battery Manual, use the Flooded Battery section:

http://support.rollsbattery.com/support/solutions/articles/654-battery-user-manual-flooded-agm-gel

Vic

n0eggs · February 2019

Sounds like I should skip a day or two of charging and begin SG logging time!

Although I do have one, any suggestions on a tried & true meter?

Recommendations on how deep of a discharge and how often to do it would also be appreciated.

Lastly, what readings would one expect to see as a new bank gets broken in?

Estragon · February 2019

I found weather made going to deeper discharge happen anyway. I use ~15-20% on a normal day, so a couple days of cloudy weather got it down to ~60% SOC. Anthing under ~85%SOC means the bank will take close to full current for at least part of the cycle. I'd personally avoid going below 50%, and would avoid having it sit anywhere near that for more than a day or so.

It apparently takes ~30-40 cycles for mine to get to full capacity, Surrettes could be more.

Definitely start SG logs. If cells vary by more than .02-.03 or so, I'd do an EQ.

As Vic suggests, check pilot cell(s) every few days for the first while, until you get charging parameters dialled in for your specific bank and usage. When the bank goes to float after charging, you should get consistent readings of about what you got in the EQ. If you find water levels dropping much, absorb voltage can be lowered a bit. If SG is slowly dropping, raise voltage a bit.

I found my glass meter tended to stick a lot and was hard to read, and ended up switching to this one:

https://www.solar-electric.com/midnite-solar-battery-hydrometer.html

Nice big numbers for these old eyes to see though a face shield :-)

Vic · February 2019

n0eggs,

The Hydrovolt is a good Hydro:

https://www.solar-electric.com/midnite-solar-battery-hydrometer.html

Have not found the Surrette Formula for Absorb time to not be of much use for off-grid systems.

You may find that required charge voltages to need changing to reach full charge will change as the batteries break in. Also, the EA value needed to reach full charge could also change, also.

Just be certain to RINSE, RINSE, and RINSE your Hydro after each measuring session, else, in time the Hydro will become inaccurate. Use only Distilled Water for this rinsing and for topping up your batteries.

When you get a chance, could you compose a Signature, that gives more details of the PVs and other parts of your system? This will allow more timely and accurate suggestions regarding the system that you have, not what we guess it might be.

Skipping Days is one way to try to allow deeper discharges of the battery, BUT, considerable battery charging does occur on Skip Days that have reasonable sun. So, sometimes purposely discharging the battery further at night, shutting off the Classic (Mode = Off, or shutting off the PV in breaker), and perhaps running one or two portable electric heaters with the Classic sleeping can accomplish some of this. On one system here, we Skip one day, and another, three days are Skipped twix recharges. This minimizes the number of recharge cycles, and does help lower the SOC on the day of recharge.

Many/most deep cycle Flooded batteries (FLAs) like to be discharged below about 90% SOC for most charge cycles. Sometimes in inclement weather, one might want to fully charge a battery that is at a bit higher SOC, when the weather for the following days is guessed to be low on sun.

FWIW, Vic

mike95490 · February 2019

Here's a handy hint regarding Bulk and Absorb times.

record the time the battery spends in BULK, for a couple days, and average that time to set your Absorb timer. That's what the Magnum PT100 controller does, if the batteries were shallow cycled, they get a short absorb, if a deep cycle and a long bulk, they get a long absorb.

https://www.magnum-dimensions.com/pt-100-mppt-charge-controller

Vic · March 2019

Hello n0eggs,

How are you doing with your system?

How are your SG readings, etc.

Thanks for an update, Vic

Bulk / Absorb question

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