ASK THE EXPERTS: Replacing 30-Year-Old Batteries

Intermediate

I have a 360-square-foot cabin with a 12-volt off-grid PV system and a Trace 612 inverter. There are three balcony-mounted 40 W Kyocera PV modules charging the system. The 30-year-old reconditioned nickel-cadmium batteries are pretty well shot at this point, and I am considering a new set of Iron Edison industrial nickel-iron to replace them.

If I have either 100 or 200 amp-hours with the new batteries, would you recommend replacing the charge controller? If so, which one would work best in this small, simple system? Should I replace the PV modules, too? I’d like to know what other considerations (and costs) there might be with new batteries. Thanks very much. I have been loving Home Power all this time!

Mary Jensen • Twisp, Washington

First, congrats on keeping your system working for 30 years! The simplest thing would be to replace just the batteries with new nickel-iron batteries—you can expect another 30+ years from them. If the old inverter is meeting your needs, and the old panels and old charge controller were still working with the old batteries, there is no reason that they shouldn’t work with the nickel-iron batteries. While the manufacturer recommends a bit higher charge voltage for the nickel-iron batteries (16.5 V), which your old controller may not do and your inverter may not like, the batteries can be used at lower voltages, just with a little lower capacity.

The Trace U-series inverter was a very good inverter in its day. However, in the last 15 years, sine wave inverters have become the norm. Switching to a modern sine-wave inverter gives you the possibility of serving more AC loads.

If you’re running more loads, you may also need more solar-charging capacity—especially if the cabin is in full-time use. When those 40 W Kyocera modules were new, having only 120 W of charging on a 200 Ah battery bank was pretty normal. However, it also meant that recharging took longer after a cloudy period. In some cases, it could take a full week of sunny days to recharge the battery bank.

With reduced PV module prices (a 270 W to 300 W module is about $200), it is now affordable to put in much larger arrays. Doing so in your location will allow for significant charging—even on cloudy winter days. With a 12 V, 200 Ah battery bank, 600 W of solar-charging capacity could charge as well on a mostly cloudy day as the old 120 W array did on a fully sunny day. This can be a big improvement in the winter. In the summer, surplus energy could be used for water pumping or running a small fridge.

If you upgrade the PV array, you’ll also need to upgrade the charge controller. Most new module voltages aren’t compatible with 12 V battery banks, so you’ll need an MPPT controller that can step down the modules’ voltage to battery voltage. Blue Sky Energy, MidNite Solar, and Morningstar all make small MPPT controllers that would be appropriate for your system. Ones with digital displays will help you monitor the system more easily.

Some of the charge controllers (such as MidNite’s) can be intentionally overloaded; for example, you could use a 30 A controller on a 600 W array, which could potentially generate 50 A to the battery under certain conditions. This would be well-suited for cloudy conditions and limited under full-sun conditions. Not all controllers can current-limit to keep from overloading themselves, so verify before choosing this design path.

While you don’t necessarily need to swap out balance-of-system equipment or upgrade your modules when you replace your batteries, equipment advances may lend to a more adaptable setup.

Zeke Yewdall • Ward, Colorado

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