Recommendation for a BMS for large 2V 1800 AH AGM battery.

bryhan67 · February 2023

Have a unique situation. My main goal is track and log battery voltage on my PC. My batteries are UPS 2V and weigh 300lbs each. I have a main bank of 24 cells I run the house with. I have a spare set of 12 Batteries. This spare set I would like to charge full then us a BMS like a Daly LiFePo4 12s 36v 250A or a 30A unit if I cant charge.

I am trying to keep a max 2.30v to 2.35v over several days to try and equalize the cells. I have a lot of different voltages. I am also trying to identify bad and good cells. Kinda tricky to spot them. I was thinking if I can view all 12 cells or 24 cells I can see voltage out of wack. I would put a inverter on them and run a heavy load to spot bad cells.

1# How low can a Daly BMS go on voltage per cell. I need 2.25 to 2.35

2# Which BMS unit is best to view remotely on my PC?

3# I see the tiny wires on these BPM and cant imagine if can put to much power in a single battery. Is it a waste of time to get a 250A or 500A bms? Of if its not possible to charge I can get cheapest 30A BMS just to monitor.

4# If I get a 24S BMS that says 72v will it even turn on if I only put 48V to it ? Or should I get a 13S that uses 48V and turn the charge output down to 2.25V.

Thanks for the help.

BB. · February 2023

What brand/model of batteries do you have... Does the Mfg. have a suggested charging process for stored batteries?

Perhaps you are overthinking this? If you charge to ~2.35 volts per cell (aka 14.2 volts for 12 volt bank or 28.4 volts for a 24 volt bank) and hold the charge for a day or so--And measure the voltage of each 2 volt cell and see how out of balance they are...

And do the charging once a month (AGMs typically should be OK with charging every 3-6 months).

Marc Kurth has more AGM experience than most of us here... A couple his related posts:

https://forum.solar-electric.com/discussion/comment/423743#Comment_423743

At the risk of being repetitive, all AGM batteries are not the same:

Fullriver charging: 14.7v Absorb and 13.65v Float @ a maximum initial charge rate of .25C to .30C. Chronically charging at lower than the specified values will result in sulfation and early battery demise.
...
Concorde AGM: 14.3v Absorb and 13.3v Float (+/- .1v) @ a maximum initial charge rate of 3C to 5C. (.2C Minimum) Exceeding these values will gradually dry out the batteries.
...
I can tell you from experience that operating outside of these parameters for long periods will shorten the battery bank lifespan.

https://forum.solar-electric.com/discussion/comment/423715#Comment_423715

Greg, Dave's point above is critical to understand. I cannot count the number of battery monitoring systems that have helped to destroy expensive lead-acid battery banks. They have to be calibrated on a regular basis.

One AGM battery manufacturer flatly says that your battery bank is fully charged ONLY when it will accept less than 0.5 amp per 100 ah of its C/20 rating while charging at the Absorb voltage. (1/2 of 1%) Example: A 12v, 400 amp hour battery bank is fully charged only when it draws less than 2.0 amps while charging at 14.4v. Terminating charging before reaching this point, will result in undercharging and sulfation. Other AGM manufacturers have different numbers, but I see around 1% (1 amp/hundred) most often.

Remember that a sulfated battery bank will reach this low current more quickly because it cannot absorb a full charge. Typical Absorb times are 2-4 hours depending on how deep the discharge was. You will often see the internet megaphone screaming about avoiding overcharging AGM batteries, but 99% of the early battery demise that we see is caused by sulfation caused by undercharging. A quick way to spot sulfation is a low standing voltage after fully charging. (Should be measured 4+ hours after charging.)

AGM batteries (as I understand) can be EQ Charged by ensuring that they are held at their recommended charging set point (or specified EQ set point for some brands) for a few hours to 24 hours. Make sure the batteries don't gas or bubble or get overheated.

If your cells are always in balance, or you can bring them back into balance with 4-24 hours of "low voltage" (lower than FLA battery) EQ charging, then a BMS is not really needed--Especially for your "stored" cells.

The issue with stored lead acid cells (of any type)--Is they do age even if not cycled... Other than keeping them cool/cold, there is nothing you can do to reduce the effects of time...

FYI... The typical aging formula is aging slows by 1/2 for every 10C (18F) reduction in temperature. If "room temperature" is 75F/25C, then storing at 57F or 15C will slow aging by 1/2 (or give you 2x longer "aging life").

Conversely, storing at +10C/+18F over room temperature will increase the speed of aging by a factor of 2 (or 1/2 the aging life at room temperature)... So keeping your batteries cool is a big help for longer life.

-Bill

bryhan67 · February 2023

EnerSys PowerSafe DDm100-33 UPS batteries. They are 2V each but huge about 300lbs.

BMS is more to spot bad cells when in use and rate them just how bad. When you have 24 batteres its a moving target to get readings. espacialy when I use them to power all the house. The battery really bad is easy to spot they voltage goes way up under charge and low under load. But thats not the case when its a little bad verses really good. Hard to explain but hard to chase accurate numbers. But having a BPS if just to read voltage and take a screen shot under extreme load or charging can help me a lot. If they can help foat the celles same voltage once fully charged then a bonus. Being 300 lbs its not easy to move them around.

BB. · February 2023

There are Utility Scale Lead Acid (and other?) monitoring systems. Looks interesting, but probably not cheap:

https://eepowersolutions.com/products/battery-monitoring-systems/vigilant-next-generation-battery-monitoring-system/

Question about your batteries... Typically UPS batteries (a few 5-15 minute discharges per year) vs daily cycling for off grid solar power.

Do you have any cycle life data for your batteries? I did not find any with a quick search.

-Bill

Marc Kurth · February 2023

EnerSys is going to be the best source of information on their batteries.

We do a lot of 900 ah to 2400 ah 48v systems using 2v cells with Concorde and Fullriver. I have no hands-on experience with UPS batteries but they are generally designed for occasional deep cycles at a very high rate. They typically do not publish any ratings for the charge/discharge cycle life, only that they have a 20 year design life in standby applications. That is why you see their ratings expressed as C/5 or C/8 instead of C/20, C/24 or C/100. You also don't see them marketed for use in off grid solar.

An important factor is that they spend the bulk of their service life in Float, allowing ample time for the cells to balance to each other. Telecom battery banks can often involve a large number of parallel batteries and operate successfully for years. Again, they spend their lives on Float and balance out better.

In our 2v, 48v banks, I teach my customers not to get wrapped around the axle watching the cell voltages while charging and discharging - it is a moving target. Variations in manufacturing and recombination efficiency will shift individual cell performance. Especially after a few years of service. I DO teach them to check the voltage AFTER the batteries have been in Float for good while. For example, Concorde AGMs recommend equalization when there is more than 0.10v between 2v cells, during Float. The equalization/conditioning method and settings vary with how far they are off, but it is generally around 2.58v/cell (temperature compensated) for 4-8 hours. One way to get an early warning is to watch cell temperature during charging. Get an infrared gun to make it quick and easy. If the same cell is consistently hotter than it should be then take a closer look. To achieve a long service life with an AGM battery bank, it is critical that the system design provides lots and lots of hours in Float.

For your battery bank, my best advice: Go back to my first sentence!

Dave Angelini · February 2023

Use them up and replace with a cycling battery. Offgrid is hard enough ! You did not buy these new correct?

bryhan67 · February 2023

These were almost free to me and have been working great off grid for 3 years. I can run air condition all night no issues. I even run a AC in the battery room to keep batteries, inverter and controllers cool to last for many years. I know what they were not designed for solar. Not here to debate what kinda battery I should or shold not have.

My question is " Recommendation for a BMS. Or a smart way to monitor voltage. Like a Daly BMS and just turn charging off if it wont work if it cant keep up with float. I did check EEPower, nice stuff but expansive.

I do watch temps and that to is a good way to spot a bad cell. I am trying to kind rate different batteries so I can mix them and keep voltage simular.

THe extra 12 batteries have been putting on a nice float for a month and soon I will swap put other 12 and try do the same. The 24 batteries I have to use every days so tough to get a good long float on them. For sure I can see the longer I can do a float they get closer in voltage when it come to these.

Recommendation for a BMS for large 2V 1800 AH AGM battery.

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