"PowerSafe" battery performance - voltage drops causing inverter abnormalities

CKNPROSOLAR

Running a test site system that has 120KWH usable battery power (80%). The resulting voltage outgoing from the batts is seemingly very, very load dependent and seems to be increasingly unstable. To the point that now I am getting what looks like inverter low battery cutouts at voltages that just a couple months ago caused no problem. These are lead AGM batteries on Conext XW+ 6848 inverters (running 4). This is hitting the 40KVA backup generator but instantaneously the voltage goes up 4 volts and restarts the inverter and the generator goes directly into a cool down. This is currently happening anywhere from 4 to 10 times a night. The battery SOC reading never drops below 40 however, actually stayed above 50% last night. I have an Automatic Generator Start box but it is not what is cranking the gen. Something is happening in front of it.

Is this a sign of damaged batteries, inverters, battery monitor....??  

The inverters list range of 40 - 64VDC. I currently have the Low Voltage cutout set to 43 on the inverters showing a 2 VDC hysteresis. Could I lower the LVcutout to 41? 

Estragon

I'm not sure where the 80% usable number came from.  Lead acid is generally ~50% usable in regular use for a decent balance of cycle life vs cost.  They can be taken down further occasionally, provided they're recharged promptly.

Assuming 150kw rated bank (120 ÷ .8), 50% would be ~75kw (likely a 20hr rate).  4 x 6.8kw inverters could be a sustained load of ~25kw, at which rate (more like 5hr rate) the batteries could be near 50% in a couple of hours. 

As well as the discharge rate, apparent capacity is reduced by temperature.  My flooded banks lose ~30% of apparent capacity in cold (freezing) temps.

Under ~42v, the batteries are nearly dead, with little remaining power.  I doubt lowering LBCO from 43 to 41 will help much.

If the bank has been taken to a 20% SOC regularly, and especially if left there for long, permanent loss of capacity is likely.

That said, with a bank that size, I imagine there are a lot of individual batteries and interconnects, which means lots of potential trouble spots.  I'd start by measuring individual battery voltages, ideally resting, at low and high bank voltages.  My guess is some are weaker than others, and being chronically undercharged.  I'd also check interconnect currents with a clamp-on DC meter, looking for any higher resistance connections.

BB.

Do you have a link to the battery manual (charging instructions)?

-Bill

ElectricLove

I agree with @Estragon that the performance expectation of AGM (Lead) should not be 80% of nameplate capacity, 50% is best case scenario.  Another big warning to keep in mind using AGM is to be very careful of NEVER equalize charge them, if you do an equalize mode where the voltage is same as absorb, this isn't true equalizing and would be fine with AGM's (provided it isn't done too long) but a FLA Equalize ramps the voltage above absorb voltage and this would destroy your AGM bank very swiftly!

With so many batteries you could definitely have some connection troubles, corrosion and any Lead battery go hand-in-hand...

I'll make a suggestion from my experience which is with an admittedly smaller bank (25kWh) and smaller system (just 2 of the XW inverters) but I'm getting really rock solid performance; go with Lithium batteries, I am using these ones from Ohmmu Energy (www.ohmmu.com/group31).  Although they cost more up front ($/kWh "rated" capacity) they make up for it in a few ways:

1. The "nameplate" capacity is real and performance will fall in line with it (unlike Lead), you could consistently do 80-90% Depth of Discharge cycles and still get something like 3000+ cycles with them.  For comparison's sake your 120kWh bank of AGM really performs at about 50% so you have an effective capacity of about 60kWh, to get an effective capacity of 60kWh from the Lithium you would only have to install about 70kWh of batteries (or install 120kWh and get double the performance).  This can save time, space and materials cost (less racks, less wires, less disconnects, less weight etc)

2.  If you analyze the "true cost" you would need to break it down to $ per kWh stored and retrieved over time ($ per usable kWh of capacity); if an AGM bank can go 1000 cycles at 50% capacity and costs $120 per kWh you could calculate this as ($120 * 120kWh) / (1000cycles * 0.50performance * 120kWh) and you would get $0.24 per kWh of storage utilized...  Now do same for a Lithium bank that goes 3000 cycles at 90% capacity and costs $530 per kWh ($500 * 70kWh)/(3000cycles*0.90performance*70kWh) = $0.196 per kWh of storage utilized... And again this is just banking on the energy not to mention the savings in space, racking, wiring, installation, etc.

3.  Round-trip efficiency; due to Peukert's losses a Lead battery bank loses significant energy as "waste" when discharging over a "short" period of time (anything under 10 hours); so you spend 120kWh fully charging up a battery bank and then dump the power in 3 hrs effectively only getting about 60kWh out of that bank before it is empty; you just lost 60kWh of energy... Lead is good when the discharge rate is going to be 20hrs or more, the round-trip efficiency is quite good in those scenarios.  With the Lithium bank you spend 70kWh charging it, dump all the power in 3 hrs and effectively still got 60kWh; so you only lost 10kWh of energy... This is yet another savings as that excess 50kWh of waste came from somewhere (an oversized solar array?  Grid-charging?  Unless this is true waste energy it costs something)

I used to work in the critical power field and it is amazing how once you sit down and do the math the "expensive" Lithium batteries are actually quite a bit cheaper in reality.  In UPS equipment sometimes weight is a big factor (if not in basement the loading of the UPS+Battery bank is substantial and using Lithium cuts the Battery weight to about 30-40% of the AGM, this can make a huge difference during a system design!

Anyways, you already have your batteries but I'd admonish you to reconsider and look at Lithium options!  The ~ $530/kWh is about the "sweet-spot" in my eyes which is why I chose Ohmmu as that is their target as well, and of course they are local for me in Arizona!

ElectricLove

I should have mentioned one other thing...  Voltage sag...

Again, Lithium beats out Lead in performance; under "significant" loads Lithium experiences very little voltage sag compared to Lead.  This is one of a billion reasons why no EV would use Lead (modern EV's), A Tesla can pull 700kW from a 100kWh battery pack, sure it'll sag from 380VDC down to 340VDC during this draw but if you did that with Lead it would sag far below 300VDC.  

clockmanfran

I have used 'PowerSafe' AGM battery's in the past, and still have a few operational ones for test purposes only.

In 2007 I had 48off 12v AGMs in 12 strings.

My Observations.

Firstly these 'PowerSafe' are true AGM, ie. they are damp woven/felt material separator between plates, No liquid acid.

I obtained them from Telecom Mobile Mast back up UPS systems.

In strings of 4 for 48v, if one cell is damaged in any of the 4 12v batteries then that string drags down the battery bank, then the other 3 batteries in that string never get fully charged properly and the other 3 batteries become permanently damaged.

Reason for shorting internally with AGM,? There is only about 6 to 10mm clearance between the positive and negative bus bars for each cell, (i had one apart to examine, BUT PLEASE Don't DO THIS AT HOME), and over time the cell expands upwards and causes a short and that cell is damaged.

Conclusion with AGM 'PowerSafe batteries.

There life is limited by time, (the cell expands) and amount of use.

With my old 'PowerSafes', If I charge a individual 12v battery and leave for 24 hrs, I then look at the holding voltage, and the last 3, attached photo. are now holding at about 12.83v  12.86v and 12.90v. 

I used to take them out of service when each AGM battery holding voltage is down to 12.86v.

They are not suitable for use that most Off Gridders put them through as they need to be monitored regularly.

They are expensive for what they are and do and asked to do.

I No Longer use AGM batteries for Battery banks.

My Photo shows AGM batteries for my OzInverter Test Rig 2019. There are now only 3 Operational 12v 105ah 'PowerSafes' left from the originals, the other is even older 'Marathon' .

I trust this helps? 

Heres a Utube Vid showing those batteries in use...........  at 3.12mins    .... https://www.youtube.com/watch?v=5s9B3RTF1QA