New Batteries - Maximizing Service Life

Leomania · September 2016

I just replaced the four ancient 255AH AGM batteries in my solar system with 125AH units. My original purpose for having a battery backup was to have stable power during rolling blackouts when the system was installed 15 years ago, but that isn't really a concern now. The original batteries had physical signs of distress at 10 years (one had a serious buildup of corrosion that I had to clean up a couple times per year) so I'm not going to claim that the batteries lasted 15 years; they probably were compromised in terms of capacity at 5-8 years.

So with the new batteries, I'm going to do a better job of following the recommendation that I draw them down a bit every couple of months. I'd also like to understand whether the charging configuration makes sense. Per the Trace manual for my old SW4048 inverters, since the batteries are 125AH and this is a 48V system, I should set the charge rate at 12A AC (125 * 0.2 = 25A DC, 25/2 = 12A AC). But since both inverters are configured to charge, and they are both connected to the same points on the battery pack, this needs to be halved to 6A. Do I have this correct?

The installers made a few recommendations to change some of the inverter default charge settings. These are:

Low Battery cut out VDC - 45 (default 44)
Bulk Volts DC - 57.2 (default is 57.6)
Equalize Time - 0 (default is 2 min)
Max Charge Amps AC - 10 (that was for the 255AH batteries, I am using 5A)

The reasoning regarding the bulk volts DC setting change was 'Setting Bulk Volts at 57.2 ensures the C40 charge controller will always see an "empty battery" and not restrict PV current during a bulk charge'. I'm just assuming they knew what they were doing, and I've restored this setting on both inverters.

Are the bulk/float charging characteristics of the SW4048 good enough that I shouldn't need to do anything special to make these batteries last?

Thanks,

- Leo

Dave Angelini · September 2016

One would have to know what is reccomended by the battery manufacturer. It is the first step

We don't assume if this is offgrid. None of the "pros" are there when the lights go out....

BB. · September 2016

Hi Leo,

What kinds of batteries do you have (make/model)? AGM, flooded cell deep cycle, GEL, or what?

In general, for flooded cell batteries, a 4kWatt AC inverter really needs about a 400 AH @ 48 volt battery bank. Two of those inverters would want 800 AH @ 48 volts (to run at full power and full starting surge capabilities). A 125 AH flooded cell battery bank--The inverter(s) will only reliably run ~1.25 kWatt / 2.5 kW surge.

I like to design a "balanced" system... The load drive the battery bank, the battery bank drive the charging settings/solar array size. Where you live (hours of sun) and how you use the system (backup power/seasonal usage vs full time off grid usage every day...

At this point, your system seems to be really unbalanced (very large AC inverter(s) and a very small battery bank). And, generally, most people do well with a battery bank designed to support ~2 days of stored energy (for bad weather/no sun days) and 50% maximum discharge (for longer battery life)--A pure emergency backup system can use different ratios, as well as using AGM batteries--But I would very much like to hear about your usage/needs to figure out what would be best for your needs.

-Bill

Dave Angelini · September 2016

Morning Bill, How is the coffee?
Another problem for the OP is the C40 solar controller is really wild on regulating the set-point. Not a real good choice for long life on an AGM!

BB. · September 2016

Cooling down now after a few days of mid-80's... Don't know what battery type now--Was AGM...

-Bill

Leomania · September 2016

Good morning!

I put this info into a box on my profile page thinking it was a signature, but I don't see the info in my post; maybe it's not a signature line after all:

Siemens SM100 x48 (3x16 panels) : Trace SW4048 Inverters x2 : Trace C40 DC Controller x3 : Lead Acid 125AH x4
Utility Interactive Mode : Time of Use Metered : 15 Years and Counting

The batteries are VMAXTanks SLR-125 AGM: https://www.vmaxtanks.com/SLR125-AGM-Solar-Battery-_p_38.html

Something curious on the status indicators this morning: one inverter says it's in float mode, the other says it's still in bulk. I put a voltmeter on the batteries and the voltage is hunting around, seems like it's varying between 53.8 and 54.6V.

Bill, I'm in the SF bay area - Belmont, CA. My three 16 panel groups are on three different sections of low-pitched roof, with variable amounts of shading throughout the day due to neighbors trees and angle of the sun. One panel set gets just a few hours of sun due to a redwood tree that has grown a lot in the last 15 years.

I don't think we'll ever really get out of PG&E's Tier-1 usage; no pool pump, no A/C. Gas central heater and gas dryer. We do have two 220V heaters installed 6 years ago to heat bedrooms that were added on during an upstairs addition before we bought the house with just a gas wall heater in the hallway. Those bypass the inverters, though. The reason for two inverters was to have both 120V phases connected to the solar system, not just one. So it wasn't a large load that drove the decision to have these two inverters.

I chose to put in smaller batteries purely out of cost considerations due to how often the batteries are actually called upon to provide backup power. I expect these batteries to be in float mode 99.9% of the time, power outages are so much less common than they had been in the early 2000's.

Thanks,

- Leo

Leomania · September 2016

A quick update: Both inverters are in float mode now. I had set the bulk voltage to 58V (not sure if that was well-advised but it's in the range recommended by VMAXTanks) and the sun hit all three panel groups and a short time later the second inverter went from bulk to float.

Checked the TOU meter and it showed I'm back to selling, so things at least appear to be back to normal operation now.

Now to keep these batteries happy and well-fed! ;-)

- Leo

BB. · September 2016

Hi Leo, I am just the next city up in San Mateo (lived a couple of decades in Belmont before here).

Watch the battery temperature--As you say, they will be spending most of their time in float--Not bulk charging. AGM are a bit more sensitive to over charging--And if they vent (over heating, over gassing), they will not last long.

Also--The batteries do act like a capacitor--So, having too small of battery bank can cause accelerated aging with the battery bank (micro cycling--discharging for part of an AC cycle then charging for part of the AC cycle). More or less, for long battery life, the rule of thumb around here has been ~100 AH (on a 48 volt battery bank) per 1kWatt of solar panels. You have 4.8 kWatts, and should have ~480 AH minimum.

As long as you are selling power, and the batteries stay relatively cool--I guess it is working.

However, eventually you will probably need to replace some equipment--Past ~10+ years, power electronics tend to fail (inverters, charge controllers, and such) and not be worth repairing (no factory support, spare parts are difficult to non-existent). Watch the batteries, the C40's have been known to fail without warning (over charge)--However, it is really your hybrid AC inverters that control the battery voltage (they "divert" power to the grid to keep the batteries at proper float voltage during the day).

If/when the major components fail (the inverters are typically the most expensive/labor to fix/replace--But matching/rewiring your solar array to the GT Inverter or new charge controller may be needed too)--You may wish to revisit your whole design and emergency power assumptions.

I would suggest that you may wish to go directly to a grid tied inverter (solar panels=>GT Inverter)--And forget about backup power... Or make a second, much smaller, solar backup power system... Or just go with a natural gas powered genset (or gasoline, if you assume that the next "big one" is an earthquake that takes out natural gas distribution too).

As an engineer (now retired--I guess)--I have always loved the idea of battery bank AC inverters + Solar power. But as you say, the last power outage here that was over a few hours was about 60 years ago when the power in parts(?) of Pacifica/coastal region was knocked out for over a week. My parents survived that just fine (with a very young yours truly) without any gensets or power (candles and flashlights). The big issue was the water tanks just about ran out before power was restored.

I would end up replacing batteries more often than they would cover any power outages.

-Bill

Leomania · September 2016

Hi Bill,

Great info - thanks. Given that my panels are variably shaded, the three sets have different angles towards the sun, and the panels are now 15 years old, I think it's safe to consider the true capacity of the system to be considerably less than 4.8KW. Still, this means my batteries are under-powered. Hopefully that won't limit their lifetime too much.

My system has 5 thermal sensors, I put them all on one battery as recommended by Trace and as I had found them on the original batteries. I'll contemplate some thermal and perhaps voltage monitoring to see how they fare in normal operation and when I do the monthly/bi-monthly load test (by turning off the solar breakers).

Are there any critical parameters I should ponder changing in the SW4048 charging profile? And what are the implications of the "C40 solar controller is really wild on regulating the set-point" that Dave mentioned in his post above?

I have to admit, I'm going to nurse the electronics along for as long as the components have a prayer of continuing to work. A big investment to get the old hardware replaced (whether or not it's due to a failure) is probably financially a net negative. Like you, I was enamored by the independence the solar + battery backup mix provided, but it's not like I'm a Tier 3 user who could justify a larger cash outlay for a more near-term payback. I replaced the batteries because the SW4048 can't operate without them, and that will probably take more than a year to pay itself off.

Thanks,

- Leo

BB. · September 2016

Sounds like you have gotten good life from the batteries with your present settings... So, I am not sure I can give you any better ones.

Mostly, just keep an eye on the system and think about what you want to do next eventually. As you know, PG&E is slowly reducing the "subsidies" for GT systems--Eventually it will become questionable how much money any GT system will save a person (I too am in the Tier one rates too).

Conservation for now is the best investment. In 5-10 years--Perhaps it will be going off grid completely and a natural gas genset as backup (if not as primary power).

We too have a growing redwood tree in the neighbors back yard--What will eventually happen--Don't know (we cut a redwood down in our old house because it was encroaching our foundation (plus nothing grows under a redwood).

The system design options and economics for solar panels has changed a lot in 15 years ($10+ per watt for panels, now less than $1 per watt). And if you have variable shading--possibly micro inverters (one inverter per panel) may make sense for your home (micro inverters do have their own issues--But so do central inverter systems).

-Bill

New Batteries - Maximizing Service Life

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