# Battery help

Posts: 3Registered Users
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.

• Posts: 2,222Solar Expert ✭✭✭✭✭
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 .
1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank

• Posts: 2,852Registered Users ✭✭✭✭✭
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.
Off-grid.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
• Posts: 4,709Solar Expert ✭✭✭✭
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.
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
• Posts: 3Registered Users
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 .

• Posts: 3Registered Users
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.

• Posts: 7,923Solar Expert ✭✭✭✭
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.
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Posts: 4,021Solar Expert ✭✭✭✭
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!
"we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
http://members.sti.net/offgridsolar/
E-mail [email protected]

• Posts: 4,021Solar Expert ✭✭✭✭
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?
"we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
http://members.sti.net/offgridsolar/
E-mail [email protected]

• Posts: 3Registered Users
all of your information has been so helpful thank you. I do have someone to help reconfigure our panels thank you.