Dave Redvalley wrote:The biggest difference is the depth of discharge (DoD) that you can achieve with these batteries. Lead acid batteries should really only be discharged to 80% max, while the NiFe's can routinely be drawn down to 20% with no damage, thus the 3 times smaller battery bank, with better performance. I do like that math by the way, I see now problem with it at all! LOL. The only reason I have a 24v system vs. 48 or 12 is that is what it was when we bought the place. I would rather spend the money on the batteries than on replacing all of the inverters and switching gear. The 24v seems to be working good for us.
I have no doubt the NiFe batteries can take a lot of pounding, and can survive extended discharge without damage (no small feat at all!), but what you wrote is not technically true. BTW: 80% DOD and 20% SOC are actually the same thing -- these terms are inversely related.
Flooded (true) deep-cycle lead-acid batteries
(like Trojan T-105, L-16, etc...) are designed to withstand a depth of discharge -- or DOD -- of up to 80%. That means a state of charge -- or SOC -- of 20%, but they should be promptly re-charged, and the longer they sit in a discharged state (below their float voltage), the more sulfation occurs. Sulfation is normal, and can be reversed if the batteries are charged soon after discharge, but the sulfate crystals become more permanent the longer they are allowed to sit on the plates. After a while, they are more or less permanent, and can lead to reduced storage capacity and premature battery "death". If you size the solar array and so your loads only discharge the batteries by 50%, the batteries will last a lot longer. Another critique of the flooded lead-acid batteries is that even though the can handle this rough treatment, the more often it happens, the sooner they die. Sealed lead-acid batteries
, either AGM or VRLA, are more like what you describe. They should never go below 50% and really should stay at 70% SOC or higher (30% DOD or lower) for good battery life.
can be very deeply discharged, and can be left at a low SOC for extended periods without significant damage, and that is a really
big deal. They are clearly better batteries, and we do need more companies to manufacture them.
As for the question about the battery bank voltage:
most inverters are more efficient at 24 volts, and even better at 48 volts. Higher battery voltage affects the way the solar panels and batteries are wired, but higher voltage systems need smaller gauge wires and smaller gauge battery cables, and smaller gauge wire means a lot less copper or aluminum. If your solar panels are a long distance from the batteries and charge controller(s), you can save a LOT on wire by designing your system to operate at higher voltage. I have a few customers with solar panels about 300' away from the house, and when voltage drop and all that are factored in, they saved about $1000 on copper wire by using a 48-volt inverter.