DIY Solar Power: How to Extend the Life of Your Battery Bank
Learning basic “care and feeding” of your battery bank protects your investment and adds peace-of-mind to the self-sufficient life.Posted Jun 24, 2014
Over 445,000 individual solar installations now exist in the United States alone…
with “hotspots” not only in the sunny Southwest, but also in densely populated areas like Massachusetts where consumers are concerned enough about the global energy situation to take matters onto their own roofs.
Your own home solar project can be as elaborate or as simple as your goals and budget indicate. Though systems that feed current back into the grid are popular and sensible for many urban homes, excitement and curiosity are building about self-sufficient “off-grid” systems, in which homes store their own power in lead-acid batteries to fulfill all of their own needs, and the cord to the utility company is literally cut.
Our popular article “Our Simple DIY Home Power System,” has inspired many readers to install simple alternate energy systems for energy independence and as a preparedness measure. These systems pretty much take care of themselves, but for one key component that requires periodic attention to ensure optimal performance – the battery bank.
Home installations routinely involve over 2000Ah of batteries, but we often recommend 550-1000Ah for those interested in voluntary simplicity and less battery-related anxiety. Some households running only a few lights, chargers, and light appliances can get by with as little as 220Ah. Unconscious energy wasting may be the norm in our culture, but living off your own batteries changes those habits for good. With a little attention and periodic care, you should get 10-12 years out of a set of heavy-duty Surrettes — with luck, even more! The popular and cheaper T-105s will last only half as long on average, but the cost is proportionately lower.
Here are a few battery-wellness essentials from the accumulated wisdom of our off-grid community.
Note: these guidelines are aimed at those using “flooded” style lead-acid batteries, which are recommended for most full-time use. If you’ve purchased sealed “gel” batteries as part of an occasional-use or emergency back-up system, some points will not apply to you (hydrometer use and water refilling, for instance) but the basic conditions which batteries prefer remains constant across the board.
~ Read the manual.
Read the manual provided with your batteries, and if you’re doing the installation yourself, read it a few times. If there is no paper manual, go to the manufacturer’s website and digest as much as you can stand of the nitty-gritty details of charging and keeping your batteries happy. If you use a professional installer, they should take care of the set-up details and provide you with a simplified list of tasks. Ask lots of questions, and call them if anything looks odd: that’s part of what you’re paying for. If you’re on your own, online forums can be a lifesaver.
~ Don’t buy batteries too big for your charging capacity.
This may be the most common problem we see: a homeowner wants to make sure their capacity is high enough to run their freezer, waffle iron, and table saw at the same time and invests in a big top-of-the line battery bank, expecting to get at least 10 years for their money. Unfortunately, they get cold feet looking at the budget and cut down the number of solar panels or scrimp on the inverter, and end up either buying an expensive diesel generator during a cloudy spell, or damaging their bank irrevocably and sending it to the recycler’s after 3 short, costly years.
The quality of your inverter and charge controller is crucial: the best equipment is fully adjustable for all types of battery, and this makes a huge difference in battery lifespan. Even with lots of panels, a cheap inverter and charge controller won’t be able to regulate charging adequately, and you’ll pay for it in the long run.
Rule of thumb: You must have 1 amp of charging capacity for each 10 amp hours of battery capacity. Therefore a 600 amp hour battery needs a 60 amp charging source.
~ Don’t buy batteries (or a solar array) too small for your electrical usage.
The conventional advice is to size your battery bank based on your expected usage with plenty of room for sunless days and extra for low temperatures, and there’s a handy online calculator to help you with this equation. But in our experience, this results in a bloated, pricey battery system which then requires a huge array plus a big generator to save your investment from the scrap heap. Also, it can lead those with less cash to give up, thinking they can’t afford their own system.
The folks we know, who have had time to make all the common mistakes and learn from their neighbors, are buying much smaller batteries, and spending the savings on radically efficient appliances and more solar panels. Just make sure you have surge capacity for your largest loads and energy storage for 2-4 sunless days.
~ Have at least two ways of charging your batteries, unless you live in the sunny Southwest.
We would all love to live off our solar panels alone, but don’t invest heavily in batteries thinking that will save you from buying a diesel generator. It won’t. If you live somewhere with long cloudy spells, your batteries will need an alternative charging method for the dark season. Depending on your climate and budget, this could be a wind turbine, microhydro system, or the fall-back diesel or gas generator coupled with a battery charger. Many battery salespeople recommend sizing your bank for a week of “autonomy” (no battery charging), but this results in overlarge batteries. Rather than adjusting your battery size to your typical on-grid power usage, consider adjusting your usage to your choice of batteries.
~ Avoid heat and cold as much as possible.
Batteries prefer an ambient temperature between 60-70°F (15-20°C). Long hot spells can cut their lifespan by years, and extreme cold snaps may compromise performance. Protect your batteries from temperature fluctuation by taking the time to build them a suitable insulated shelter, and situating the shelter on the shadiest side of your home. Insulation can be used around the sides of the batteries, but the tops must be well-ventilated. Never place your batteries inside your home! Harmful gases can build up, and explosion is always a remote possibility.
~ Moisture and dirt cause corrosion and seeping between terminals.
Again, the key is shelter and coverage. A snug cover will keep the rain and dust out. Building the battery shed in a sheltered location which won’t be exposed to driving rain and wind makes good sense. Use a corrosion inhibitor, such as dielectric grease or good old Vaseline, during assembly; or spray Fluid Film onto the connections after assembly.
~ Monitor usage, and stop before you hit 50%.
If you use your batteries even more lightly, they will last longer, but you’ll get the maximum use-per-dollar with 50% discharge. Batteries have a hard time recovering from discharging too deeply, and if this happens regularly they will fail much sooner than they should. On a PV system, this can mean special conservation measures on cloudy days: using your French press instead of the electric coffee maker for example, and postponing the laundry if possible.
Make it a habit to glance at your current state of charge on your battery monitor or voltmeter, imagining your voltage as water pressure in a tank that is gradually diminishing until only a trickle comes out.
Tip: Using a low-voltage disconnect takes this essential step out of the hands of distracted users. That system is built into better inverters; yet another reason not to skimp on that important equipment. Though manual checking works, installing a digital battery monitor where you’ll see it easily and often makes it easy.
~ Top up often and fully.
At a bare minimum, batteries should be charged to 100% once a month, but every few days is much better. Too many incomplete charges damage your batteries, and signify that your bank is too big for your PV array — luckily, panels are cheaper than ever.
~ Use your hydrometer.
Every battery owner should have one of these simple plastic turkey-baster-like gadgets. Set yourself a digital reminder once a month to use it to check the specific gravity of your batteries. Be aware that you will not get an accurate reading if you use your hydrometer just after adding water, in low temperatures, or during active recharging.
~ Never mix batteries.
When you buy a set of batteries, that’s it: in general, you shouldn’t add to the set during its life. All batteries should be the same age, type, brand, capacity, and state of charge. Exceptions are sometimes made for a relatively new bank to replace one failed cell, but if you find after a few years that you want more capacity, you’re better off buying a new bigger bank. If your older bank is in good condition and has some years left, someone may be willing to buy it used at a significant discount — ask local installers if they know anyone.
~ Check water and terminals twice a year. We see a lot of vacation homes with nice batteries ruined by boiling dry in the summer months, because no one was topping up the water. All battery plates must be submerged in distilled water at all times. Load-dumps are useful in avoiding over-boiling during intense sun, though a good charge-controller simply disconnects the solar array when necessary. It’s also surprising how many problems are caused by accidentally loosened battery terminals, leading to confusing symptoms such as reduced available voltage.
If we were to offer one all-around motto for battery care, it would be this: Planning and proper placement pay off. Keep it small, build a good shed, and monitor often. If you’re thinking of setting up an off-grid system for a seasonal or year-round home, either for primary use or emergency preparedness, take the time now to talk to as many experienced off-gridders and installers as possible. If your batteries are not sized appropriately for your system or are carelessly placed with exposure to unfavorable atmospheric conditions, they will be doomed from the start.
We all have days when we get carried away with the blender and the vacuum cleaner and the belt-sander, all during the same few cloudy hours, before noticing with alarm that we’re down under 50% on our battery monitor. If the forecast for the next several days shows nothing but grey, if may be time to get that back-up generator out of the shed.
On the other hand, if you’re determined, like us, to make a go of it using only your renewables, we simply treat those batteries like a family member who’s under the weather. Maybe they’ve been feeling taken for granted and overburdened — they need a day off! The kids get a kick out of reading their bedtime story by candlelight, and we can all figure out how to make toast in a cast-iron skillet.
When we have one of these days, I like to remind myself that this is what I signed up for: real-time feedback of my electricity usage, and a sense of responsibility and pride about its source.
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