Battery Sag and Charge settings help

[djvdberg]

Hello,

Just want to make sure I'm starting off correctly and not killing my batteries..

I'm running:

1. Axpert 3KVA PWM inverter - https://www.sustainable.co.za/axpert-mks-3-24-3kva-3-0kw-24v-hybrid-inverter.html
2. 2 x 12 180 Ah Omnipower Batteries - https://sinetechstore.co.za/shop/ba...cle-rechargeable-agm-gel-hybrid-vrla-battery/
3. 4 x 350Watt Sharp Solar panels
4. Batteries are connected to eachother and the inverter with thick 6 gauge wire, very short.

What I need help with is firstly the voltage sag, I use these as backup when power goes out and work from home, they carry around 600W power. When I put on 600W load the voltage sags from 27.5 to 25.5 (Drawing +/- 20Amps) almost instantly, then gradually goes down, is this normal? You can see when it reaches 22.5 and goes back to grid, it equalize back to +/- 25v.

Volts:



Amps:



Secondly, below is my inverter settings, are these correct? The back to grid and battery cutoff is low because of the voltage sag, but If I set it to let's say 24.5v back to grid, I don't think I'm getting much capacity out of them.



So... Is all of this normal, or what should I change/look at?

Thanks!
Dion

 

[gnubie]

Lead acid batteries are well known for sagging under load and the amount of sag gets worse as current drawn goes up. You have a notional 4.3kWh of storage in the batteries. Suppose you only want to draw down to 50% for longevity. From the looks of it you can't measure amp hours taken out of the battery to use that as a cut off point so you have to use voltage.

Since that varies under load my advice is that you set the inverter so that it only draws from battery, say turn the mains input off, and run your 600W load for about 3.5 hours (2.1kWh of power drawn) and with the load still operating check your battery voltage. That becomes your low cut off voltage / transfer back to grid voltage. That's pretty crude and relies on your 600W load actually being there and will change as the batteries age and drop capacity but it's a good starting point. At least that's how I handle this situation, someone else might have a better idea (apart from amp hour / watt hour counting).

Bulk charge voltage is too low. Manufacturer recommends 29.4. Acceptable range is 28.8 to 30.
Float charge voltage is a little high. Manufacturer recommends 27.6. Acceptable range is 27 to 27.6
Equalisation voltage is a little high. Manufacturer recommends 28.2. Acceptable range is 27.6 to 28.2

To be honest those equalisation voltages are odd. Equalisation is normally recommended to be higher than bulk. It might pay to check with the manufacturer on that. Normally I say to not equalise an AGM battery (and definitely not a gel) but I'm not familiar enough with hybrids to say if it's a good idea or not. That the manufacturer has the spec says it's OK though.

battery datasheet

*edit to add missing words*

 

[djvdberg]

Thank you good advise!

What would you recommend for measuring amp hours?

BMV700? And then how do you disconnect? Relay?

Thanks!

 

[RCinFLA]

Check voltage on top of battery terminals avoiding connection lugs and wiring. Then check voltage at inverter terminals.

You have to be very careful about parallel battery wiring matching and any connection resistance.

The lower the voltage the inverter sees, the more current it draws to provide the required output AC power.

Don't try to do any capacity evaluation on lead acid battery. You can do a load test on individual batteries. Get a cheap 300 watt 12v inverter to load test (several light bulbs for AC load) with about 25 amps load on single battery for about 30 minutes.

If they have been float charged for a long time there will be an initial dip in voltage then gradually come up a bit.

Attached is a table of 25 amp load test I did on 16 of my 105AH lead acid batteries that were almost four years old.

Battery Voltage at Load test of 25 Amps
Start	30 sec	1 min	5 min	10 min	15 min	30 min		after 30 min. no load Recovery time
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

 

[Steve_S]

Equalisation is normally recommended to be higher than bulk.

These are VRLA AGM so their chaging & equalization will be different from conventional FLA.

@djvdberg, always use the Battery Manufacturer Specs & Recomendations for charge discharge settings. Also, be aware that for longlife & maximized usage of the batteries, do not go below 50% DOD (Depth of Discharge). Unfortunately with Lead Acid / AGM it is very difficult to determine the SOC / DOS by simply looking at Voltage because the chemical state within is what is the true guage is. AGM is more stable in this regard. The SCC (Solar Controller) should have some form of bi-directional monitoring with regards to how much goes in and how much comes out, I am not familiar with your equipment brands.

For Battery State monitoring there is many options and I'll just cut to the chase. The better solution to apply, is to use a Shunt Based Active Monitor that is situated between the Battery Bank and the Inverter/SCC. The shunt will see the power coming in from the SCC as well as what is pulled by the Inverter and will be able to monitor and calculate what is going back & forth. There are LOTS of them on the market and many are, simply put, "junk" which have limits. The Victron Battery Monitors are TOP NOTCH and priced accordingly. That being said, there are some Much Less Expensive Chinese solutions that people are using with great success and no issues. Will has reviewed several good ones. There are monitors with a Hall Pickup instead of a Shunt, MANY of those are UNIDIRECTIONAL and most are unsuitable foe mobile application as movement can throw off the sensor (they are finicky to say the least) BUT if used in a Fixed ESS ( Energy Storage System) they are generally no problem. NOTE the Fine Print, for Unidirectional OR bidirectional, you will want BiDi ! NOTE ON HALL PICKUP, there are high capacity versions available but not common above 100A so you really have to look hard. Shunt Monitors are limited by the amperage of the shunt only which can go up to 1000A.

Here is Will's Page on his recomendations:

Off-grid Solar Battery Monitors

Building a vehicle mounted solar power system? Let me help.

www.mobile-solarpower.com

#2 on his list is a Great Value BUT that link takes you to a 100A model.
Here is the link for the same BUT 350A capacity (link has Will's Aaffiliate Link).
https://www.amazon.com/AiLi-Voltmet...dp/B07FGFFHC6/ref=psdc_14244451_t1_B07CTKYFTG

Hope this helps a bit, Good Luck.
Steve

 

[djvdberg]

Thanks guys,

So at least what I'm reading is I'm not doing anything completely stupid..

I've been looking at a Victron BMV-700, can interact with my setup, maybe that's then the best solution then, will have a look at the Amazon version allso.

Regarding voltage at the terminals and voltage at the inverter, the cables are literally 40cm long and 6 gauge, so no drop.

Thanks guys!
Dion

 

[gnubie]

These are VRLA AGM so their chaging & equalization will be different from conventional FLA.

The voltages in their spec sheet are unusual in my experience with AGM batteries, but these are hybrid AGM gel batteries, not just AGM so it's outside my area of experience.

 

[djvdberg]

BMV-702 on the way... ?

It talks to ICC Software and can then better handle charging, going back to grid etc..

Thanks guys!

 

[denarius]

It talks to ICC Software and can then better handle charging, going back to grid etc..

Solar assistant also supports victron BMV with similar power grid/battery switching as ICC: https://solar-assistant.io/help/power-management/axpert