Battery help

Vanna

We currently have 6 -305Watt Silfab solar array, running 8 Surrette batteries (6CS-25P), 12 volts, We have an OUtback inverter 2712 and 2 MX60's. We would like to keep our solar array, but change to a 3524 inverter and therefore need to know how many batteries do we require as we will be going to 24 volts. Thank you.

mcgivor

Upgrading to 24V nominal is usually a wise move over 12V, battery capacity will depend on daily loads along with how many days of autonomy are required, having a history of useage is helpful. If existing loads are to remain, along with all criteria being met with the current batteries, then the Ah capacity would be half the 12V capacity, if the change is it increase useage then this would naturally require a larger capacity. Depending on the capacity required it is best to keep the batteries in series for proper ballance, using 12v two would be required, 6V ×4, 4 V ×6 and 2V ×12. Could you post your actual, or proposed consumption, this will help in determining what the best configuration would be. Another thought is if the array is not to be enlarged then this would limit the battery capacity, in a full time  offgrid situation, 10% charging current of the battery nominal capacity would be about as low as one would go, so 1800W × 0.75 = 1350W, 1350W ÷29 V = 46.5 A, so a 450 Ah 24V nominal battery would be about as large as one should install. The batteries you currently have are rated at 820 Ah @ 20 hour, so 4 in series is over the recommended for 24V and way over 12V .

Estragon

So you currently have 8 x 6v batteries wired as 4 strings of 12v in parallel? You want to rewire as 2 strings of 24v in parallel? Still 8 batteries, 4 in series for 24v and 2 strings in parallel.

Photowhit

Pretty much agree with the above statements. You appear to have an out of balance system, with too large a battery bank for your array, but you might have other sources of charging, or it might be a weekend use cabin in the sunny southwest...

The good news is that the charge controllers are rated at 60 amps with out regard to the voltage. So they can handle 2x the amount of array on a 24 volt system and a 12 volt system! So now you could put the current array on a single charge controller and add as much array as you have now on the other one.

Vanna

mcgivor said:

Upgrading to 24V nominal is usually a wise move over 12V, battery capacity will depend on daily loads along with how many days of autonomy are required, having a history of useage is helpful. If existing loads are to remain, along with all criteria being met with the current batteries, then the Ah capacity would be half the 12V capacity, if the change is it increase useage then this would naturally require a larger capacity. Depending on the capacity required it is best to keep the batteries in series for proper ballance, using 12v two would be required, 6V ×4, 4 V ×6 and 2V ×12. Could you post your actual, or proposed consumption, this will help in determining what the best configuration would be. Another thought is if the array is not to be enlarged then this would limit the battery capacity, in a full time  offgrid situation, 10% charging current of the battery nominal capacity would be about as low as one would go, so 1800W × 0.75 = 1350W, 1350W ÷29 V = 46.5 A, so a 450 Ah 24V nominal battery would be about as large as one should install. The batteries you currently have are rated at 820 Ah @ 20 hour, so 4 in series is over the recommended for 24V and way over 12V .

Vanna

Thank you for this information. It came to our attention that we were undercharging our batteries.   Our batteries are 12 years old now so we do need to upgrade them anyhow. We also have to go up to 24 volts. I am not sure of our power usage but will get that information . I just want to make sure we have the correct number of batteries or Ah to support what we have. We live modestly but do have a propane boiler that pulls power. We are also considering to change this to Hot water on demand for both radiant floor and hot water needs. Our well pump is also needing to be changed as we are having issues with it now. We are thinking of installing a Grundfos soft start AC submersible well pump as our well is a dug well. 15 feet. A friend of ours also suggested a NEMO submersible well pump instead of the other one.  All of our lights are LED . Use a propane cook stove and clothes dryer. Fridge is a Sunfrost but again its needing to be replaced and since they are not available anymore we were thinking of getting a regular fridge as they have come a long way. We heat with wood all winter. But like I was mentioning I really need to make sure we are getting the most out of our batteries and need to understand how to figure that out. We would also like to have little maintenance as we have the lead acid ones now. I appreciate your time and information.
 

mike95490

If your batteries lasted 12 years, you were not treating them very badly at all, and I would not be in a rush to change things.

Dave Angelini

One way to find out how much energy you are using now is to read the harvest data from the MX60's. On days that you finished charge and floated, add the reading from each MX-60s total for the day at happy hour or 5 pm. This number is how much you used to charge from the previous night + the energy you used up till happy hour.

When you buy a new 24V inverter, you should look into a new outback charge controller as the mx-60 is pretty old and all of the outback solar charge controllers are much better at sweeping or capturing the max. You may only need 1 going to 24V !

Last thing is your concern about the number of batteries. As long as you replace the current set with the same amp hour battery and the same number of batteries, you will have the capacity you have now. It will be more actually as the 12 year old ones are not what they use to be. Good Luck!

Dave Angelini

Adding that you should look at measuring the energy to helping you pick the new AC electric refrigerator. If you are careful you should be able to get one with similar usage as you old DC refrigerator. If you go crazy here with a big 25+ cu ft refrigerator you may need more solar.

I take it you have some help here as you probably will have to time the inverter and battery cable change with also  possibly changing the wiring on your solar panels to be correct for 24V?

Vanna

all of your information has been so helpful thank you. I do have someone to help reconfigure our panels thank you.