Head check on bulk & float voltage for Renogy AGM

[krby]

TL;DR:
For various reasons, I'm using 14.4V bulk, 13.4V absorb, and 13.0V float voltages for an AGM SLA battery bank. Is this ok? I'm concerned the batteries might never get fully charged.


Full background and question below:
Background
I have put together a DIY UPS for my home network rack. Max load is no more than 300W on the AC side, typical is 150-200W. If I shut down the NAS, it's about 50-60W. I sized (accounting for various losses and the 20% DoD in the next paragraph) this DIY UPS to run about 2.5 days with the NAS off. I cobbled together an Arduino + sensors that pretends to be a USB UPS interface and alerts the NAS when the AC goes out so it can shut down.

I chose AGM SLAs for this. The usage pattern for these batteries is they sit on low garage shelf so weight is not a concern They are 100% charged nearly all of the time, but during an power outage will drain down to 20% remaining or deeper. I know the deep discharge is going to shorten their life of the batteries. But, this will happen maybe 4-6 times a year, and not every time will be down to 20%. Also, the discharge current is about 5A for a 400Ah bank, so I'm not asking a lot. I expect to get at least 5 years out of them this way. If I'm wrong, I'll replace them when I discover that or some new battery tech tempts me.

The equipment is

• Two 12V Renogy 200Ah AGMs wired in parallel (got a good deal a while back)
• AIMS 12V 600W Pure Sine Inverter (I had this already and know the network rack electronics run fine on it)
• Powermax PM4 55A converter/charger

The Powermax has a Bulk, Absorb, Float charge profile. With a pot on the Powermax, I can set the peak voltage for bulk mode, then absorb is -1.0V from bulk peak, and float is -0.4V lower than absorb. The charger does CC until it hits the bulk peak, then CV at absorb and float. I don't know if the Powermax uses time or tail current to switch from bulk -> absorb, float. It wasn't expensive, so maybe just time?

The Powermax defaults to 14.6V for bulk (so 13.6V absorb and 13.2V for float). It turns out my AIMS inverter has an input range of 10-15VDC, but that 15V is +/- 0.5V. When bench testing a few months ago, wiring the AIMS inverter directly to the Powermax charger tripped the inverter's overvoltage protection often (not every time). I want to be sure the inverter doesn't turn off when AC mains power returns after an outage, so I turned down the bulk peak voltage down to 14.4V (checked with a known good multimeter when powering on the charger with no load)

Finally, my question!
So, I ended up with 14.4V for bulk, 13.4V for absorb, and 13.0V for float. Is this ok? Looking at the Renogy voltage chart 13.0V is just at fully charged for their AGM batteries.

 

[mikefitz]

So, I ended up with 14.4V for bulk, 13.4V for absorb, and 13.0V for float. Is this ok?

This is far from ideal. The concept of having absorbtion voltage 1 volt lower than bulk is a strange concept The default 14.6/ 13.6/ 13.2 would be prefered over your settings. Since the batteries will mostly be in standby this charge profile seems suitable and is within the range of Renogy recomended values


The Powermax is really designed for RV applications where the second stage of charging is actually a power supply providing power to RV 12v system. Only when the current falls to a low value does a change to storage voltage occur.
So in effect you have a bulk stage charging to 14.6 volts followed by a constant voltage period of 13.6 volts, this you could regard as an absorbtion and float period to bring the battery to full charge. A maintenance voltage of 13.2 is considered ideal.

 

[krby]

Thanks for the reply!
I agree the lack of flexibility isn't great. It was cheap and I knew it would likely handle 24/7 DC output because as you said, it's intended to both charge and power the DC side of an RV or boat from shore power for long periods of time.

With my settings (14.4, 13.4, 13.0V: (which I have picked in order to keep the inverter from tripping it's overvolt protection) I'm going to be at 13.4V for many hours. I was working on this system yesterday so just got it reassembled and plugged in about 5-6hrs ago and the DC circuit is now at 13.40V (I have just voltage monitoring of the DC circuit, nothing sophisticated enough to try and compensate for voltage sag under load or similar effect of charging) so given that I'm using about 110W of AC from the inverter, I'm probably using ~10A of DC. If the Powermax is just going by output current, it may never fall to "Storage Mode", right? If so, what's the effect of keeping my batteries at 13.4V for long periods of time?

Assuming it does fall to Storage (13.0V for me) eventually is that OK for the batteries?

 

[krby]

Quick update: As of this morning. The voltage is at 12.91 under load, so it appears the Powermax stepped down to Storage Mode sometime overnight. But it's a bit lower than I'd like.

 

[mikefitz]

But it's a bit lower than I'd like.

Yes, a bit low. I have no experience with the charger power supply but would expect at some voltage level it will revert to charge mode.

 

[krby]

Ok, I bumped the peak bulk voltage up a bit, tested with the inverter to check the OVP wouldn't trip, then just let the Powermax charger/convert run with only the batteries as a load (inverter turned off). Since I thought the battery was nearly full before this, I was hoping to see if the Powermax switched between boost, normal, storage based on time or (hopefully!) current. It just time.

After power on, Voltage worked it's way up to about 14.55 quickly, held there for 30min (29m 50s +/- about 20sec). It then held at 13.55 for 12 hours (12h 0m 22s +/- about 20s) before dropping down to ~13.05, (13.02-13.03 under load)

So, I guess I'm complaining about a not very smart charger (I got what I paid for) and a not very flexible inverter (I wish the DC input range was a bit higher, like 15.5 or 16V). Off course, the charger isn't doing temperature compensation, so I checked out the battery specs. Assuming a worst case of 35C in the garage, the Renogy data sheet says that's -0.240V for float and -0.180V for bulk.

Bulk max (from the same data sheet): 14.8V - 0.240V = 14.56V. That's close to what I'm using, But I guess ok given how rarely a bulk charge will happen. If I'm home and the garage is really hot, I can charge the battery slowly with some other method.

Float max: 13.8 - 0.180 = 13.62V. That's above the 13.55 value I'm getting from the Powermax, so I guess I'm ok there.


So, I guess this will be fine for me.