Hi, I'm the proud but overwhelmed owner of a new off-grid solar house, and of course there are teething problems. May I ask your comments on two of them? One is that a Surrette technical support manager I've phoned recommends a 6.3 hr absorb charge, compared to 3 hrs recommended by the inverter/charger technical support staff and two local solar companies.

I have sixteen S600 batteries, two banks of eight = 48 V and 900 Ah @ C20. The Surrette guy says the ideal absorb charging is 57.6 V at 100 Amps. But because my Magnum 4448 inverter charges at 60 A max (30 A per string), he says it'll take a lot longer than ideal to charge my batteries. Using the Surrette charging equation, the Absorb time should be 0.42 * C20 / I, where C20 is the bank's Ah at the 20 hr rate, I is the current, and 0.42 is 20% / 50% + 5% losses (assuming that over the time of the absorption charge that 50% of the maximum current will be available). This is 0.42 * 900 / 60 = 6.3 hrs for my bank.

Has anyone else ever had such a long absorb time recommended? The Magnum inverter's battery monitor kit shows a 100% state of charge within the 3 hr period, so I wonder if this is just a battery manufacturer's perfect ideal and a generator fuel waste for me.

The time would be much lower with a higher charging rate on the inverter/charger. Do all 48 V chargers have this low charging current and thus long charging time?

As well, the Surrette guy also says I can only equalize with the generator, not the panels. That's because my 2.5kW PV array can only output 43 ADC (2560 W / 57.6 V), so charging the batteries by sun will take even longer than charging by generator/inverter (43 A vs 60 A charging). This comes out to 10-11 hrs, which of course is longer than a day (or two, in winter)'s sunshine. He also says this means that almost all of my solar charging cycles won't charge the batteries past 90% full, so I should equalize monthly (at least in winter). Anybody care to support or comment on these recommendations?

Thanks, and have a great day.
...Terry
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12 REC AE-US 215 W, Magnum 4400 W inverter/charger, Outback FM80 controller, 16 Surrette S600 batteries, Generac Guardian 10 kW generator

The battery reps will always tell you to treat your batteries better..... In a solar off grid home it is nearly impossible to treat the batteries the way the manual expects you to. The manuals are written for the batteries assuming they are used in floor scrubbers, or fork lifts. These all see nightly charging with weekly/ monthly equalization charges from a grid powered charging system.

That being said, you should not have to set the absorb time to 6hrs, this is an absolute waste of fuel and $. I would set it to 3 hrs as you installer recommended.

Where is this home? I ask because it seems to me that the solar array is almost 1/2 the size it should be for the battery bank you have if it were in my area. But my relative sizing is based on my climate here in Ohio (very cloudy winters). Off grid homes are notoriously hard to size unless a VERY detailed analysis is done on expected loads during the winter months. Maybe you can give us some insight on how the array / inverter / battery bank were sized. Budget, expected loads, future expansion planned,...?

SolarOH

I forgot to address your last question. I always recommend equalizing once a month, summer or winter. And especially since you array is so small compared to your battery bank, I would recommend you use your generator to equalize every time during the winter, and every other month during the summer or shoulder seasons.

SolarOH

Thanks for that useful info, SolarOH. I'm getting the same advice from two others that my panels are undersized for my batteries. Here's how I got what I got:

My load estimates were for 6.7 kWh for careful winter use, and 10.5 kWh when there's enough summer sun to afford the microwave oven, bread machine, coffee maker etc. My installer chose 16 Surrette S600 batteries on the argument that it would give me 980 Ah/day, which on a 48 V system would be 4700 W, so 50% usage would be 23,500 W. Thus using 10 kW/day I'd get 2.3 days of battery use down to 50% if no charging at all, and more likely 3-4 days.

I'm in eastern Ontario, with cloudy winters like you. I've heard estimates of 2.5 sun hrs/day in Dec, 3 in Nov & Jan, up to 9 in July. I can't find my notes explaining why the system vendor recommended 12 215W REC AE-US panels, for 2560 W total. It could be that was the max I could fit in a single row on the roof and still be reachable by my roof rake from the ground. Or it could be that he thought this was the max a single Outback FM80 could handle; I've since had another vendor suggest the FM80 can fit six more such panels, which is now where I'm tempted to go should I find the money to upgrade before next winter. However I'm lacking roof-rake-reachable roof space as well as sunny ground space.

Or maybe he thought that the cost was near my limit. The panels, racks, inverter, e-panel, charge controller, auto-gen-start, battery monitor, batteries and installation (everything but the generator) added up to $30k Cdn (about the same US). Note that our nasty Cdn government has no rebates at all on new solar installations.

He chose a 48 V inverter rather than 24 V for thinner wiring and less loss; it's a Magnum 4448. He recommended a second one if growth is planned. It's a 240 VAC inverter/charger just in case I want 240 in future; the price was the same as a 110. I just need 110 now, having a 110 VAC Grunfos Softstart well pump.

So where I stand is that I'm going to take your recommendation of monthly equalizing, and 3 hrs of absorb charging first (at 58.2 V). I'll hope to add six more panels this summer/fall, but I'm not sure if I need a second inverter or charge controller for that. And the new panels likely won't get snow removal as often as the first 12.

Any thoughts on this system sizing? Thanks again,
...Terry

Adding 6 more modules would take it to the limit. This is what I'd reccomend as well. In reality the array would still be undersized, but this will help matters.

And congrats. Living in an off grid home can be a very rewarding experince.

terry, wicked system.

My question for you ... how often do you need to clear the panels after snowfall? That's something (else) I hadn't thought of.

Hi, I am up where you have to brush the snow off your panels. My solution is stand them almost vertical . You lose some harvest having them that steep but I think more is lost if you let the snow get on them and wait to have a good hot cup of coffee before you get all suited up and strap on your snowshoes and stroll over. Anyways, just stand them up and enjoy a second cup of coffee

Hi, I am up where you have to brush the snow off your panels. My solution is stand them almost vertical . You lose some harvest having them that steep but I think more is lost if you let the snow get on them and wait to have a good hot cup of coffee before you get all suited up and strap on your snowshoes and stroll over. Anyways, just stand them up and enjoy a second cup of coffee

To answer the real question here, it is called a battery bank. You have to put in what you take out just like the bank. There are interest charges of about 15%. I have a 2160 AH @48 volt, flooded battery bank and charge with a 2400 watt solar array. I have no problems with charging them up. I hardly scratch the surface every day and put it right back the next day. I often take them up to 62 volts for 4 to 6 hours and it hardly warms them up to be a concern. I think the whole problem everyone is having is batteries have a limit of how many amps they will accept.So trying to pile drive lots of current in them isn't the answer. Those Surrettes that were mentioned are likely sugared and should be taken into the city and put on a charger hooked up to the grid 24/7 at 57 to 57 volts for a few days. Check the SP every one in a while and they should come back. It might take a week, but if sulphated it should work. They should give and receive power much easier without the layer of suphate in the way.

Ancient post, but it's true - some batteries require a long absorb time to reach full charge, sometimes more hours than there are sun on a winter day. And it's true that lead acid batteries need to reach a full charge every so often (perhaps 2 weeks). So for optimal battery life, you can either run a small generator on a regular basis, or you can implement some scheme where every day or so you rotate one battery out of the pack and use the remaining batteries to provide it with a full charge. Then discharge it back to the level of the other batteries before putting it back in (so the pack stays balanced*). Lithium batteries simplify things.

* - with any pack, using a balancing circuit is almost always a good idea.