Battery Questions

[Andy Bajc]

Hi there
New to the off grid scene. Just purchased a home with solar panels, deep cycle lead acid batteries, small wind turbine and diesel backup generator. I currently have 12 2V Rolls-Surrette batteries (KS21T) with 1104 Ah 20 hr capacity. They are in series for a total of 24V. I have 13 solar panels totaling 2985 watts. First off, is this enough solar panels to keep the batteries fully charged (to float at least once every 2 weeks)? If not, how many more watts of solar panels would be required? The previous owner did not seem to take care of the batteries and now they seem to be sulphated. I don't think he knew what equalization was or the importance of fully charging regularly. Specific gravities when the batteries are in float range between 1.210 and 1.224. I am working with Surrette to try and bring them back up by charging at a higher voltage. The battery bank voltage drops quickly each day as the sun goes down. Luck if I get half way into the following day if its cloudy without the generator kicking in to charge through bulk. If I were to require switching out the battery bank, is twelve 2V batteries the way to go or is six of the 4 volt batteries preferable. What would be the reason for going either way. It's pretty clear to me that I may need to upgrade my power system down the line. Just looking for some guidance moving forward.

[Joe Blake]

Hi Andy,

Welcome to the forum. I have two sets of batteries and panels. The older set is not grid-tied so relies entirely upon the panels to maintain charge. The answer to your first question about how long to get to float would I suspect depend upon the load you place upon them. This set of batteries consists of four 6 volt deep cycle in series/parallel to give 12 volts and only has about 500-800 watts of panels. Its load is a couple of 12 DC/24 DC/240 v AC freezers which have been going for over 8 years now. I've replaced the batteries once in that time.

The second set of batteries is fed by 3,200 watts of panels and runs the entire house, other than the two freezers (although I sometimes swap the freezers over to the "house" batteries if the voltage on the 12 volt batteries is down a bit.) For this system I use sealed lead acid cells (12x2 volt) in series to give 24 volts. Excess power is fed back to the grid, and also I can call upon the grid to recharge if necessary. As it's the dead of winter in the southern hemisphere, this happens pretty regularly, to the extent that I draw about 3-4 kWh overnight to bring up 100%. The charger is set to kick in at about 85% SoC. However, on a good day I can export almost exactly the import.

I can't give a definitive answer as to which is the "better" system as each performs a different set of functions, but from what you say I'd (marginally) tend to the 2 volt cells, but that's simply my personal preference.

[Rob Beckers]

Andy, welcome!

I've written an article about battery care that could help: https://www.solacity.com/lead-acid-battery-care/

Surrette has been slowly raising their recommended bulk/absorb Voltage over the years, and for a 24V bank it's currently 30.0 Volt. We used to raise the absorb Voltage a little above the recommended setpoint for sulphated banks, to over time whittle away at the sulphate. However, at 30.0V there's not much headroom left to raise it for daily use without running into serious water use and heat generated during charging. You could try 30.4V, but watch the water levels!

Also check that your charge controller is set to re-start a bulk-absorb-float cycle every morning when the sun comes up. This is an optional setting on some controllers and you really want this switched on.

We've had customers pooch expensive Surrette battery banks by not properly keeping them charged over the winter. I've been amazed at how well they've bounced back with a diet of higher-than-usual absorb Voltage plus a number of equalize cycles.

What you want to do is slowly work away at it. Running multiple equalize cycles in quick succession is not going to do it (and hell on the batteries!). Run with higher absorb Voltage for a week, do an equalize cycle, run another week, another cycle etc. With a little luck the SG will start to come up (if they're not too far gone).

There's no specific advantage of 4V over 2V cells, it's a matter of physical size and weight. At some point the 12V batteries just get too heavy, and you move to 6V units. For higher Ah values they too get too heavy and then 4V or 2V units come in. At the end of the day you have the same number of cells.

There is most definitely a recommended value of solar PV vs. battery bank size: What kills lead-acid batteries in short order is when they sit at partial charge for long periods of time. Having a large battery bank with little solar PV will do that; the solar panels are never able to fully 100% recharge the batteries within a reasonable amount of time.

What you want is for the solar PV to be able to recharge the batteries completely on an average day (coming out of the night), within a reasonable amount of time. What works depends on your location (how much sun), your loads, orientation and shading for the panels etc. Generally the regular calculations for off-grid system sizing work well: The solar panels have to generate enough per average day to make up for all the usage and losses, and the battery bank is sized to hold 3 days of energy if nothing came in, where at the end of the 3 days it's down to 50% state-of-charge.

-RoB-

[Ralph Day]

That's a nice little link Rob. Now I know why my Surrette's are 25 years old after 13 years. My calculated absorb time is over 13 hours! Need (and have always needed) more PV. But they are tough to have put up with poor charging regimen for as long as they have.

Now, unless I expect enough wind overnight, I leave my inverter passing through to carry loads from 7pm to 7am (cheapest utility rate). I won't get as large a set (820ah, 20 hr rate) for my next bank.

Ralph

[Rob Beckers]

Ralph, your situation really is the no. 1 reason I see with people living off-grid that have battery issues. Especially somewhat older systems when PV was still VERY expensive, they tend to have large battery banks and very little to charge it.

You could try a few equalize cycles. Those Surrettes really are well-build, and respond well to equalizing. I've brought back a number of battery banks from the brink this way.

Upping your charge Voltages (15.0/30.0/60.0 Volt absorb) for PV will help a bit too. At least that gets the chemistry going when there is sun.

Good luck!

-RoB-

[Ralph Day]

Hi Rob
I generally run an eq charge every 3 months (if Janet is not in the house...she can smell the acid vapour even though there's a fan on the battery box running 24/7)

They seem to react well after that for a few weeks. This is not the "recovery" eq, just preventative maintenance eq. With grid power cheap as it is it's tough to shop for Thousands of dollars in replacement stock.

Ralph