The battery is the most expensive consumable in a solar system, and it dies from how it is treated, not from age alone. Two chemistries matter on a farm: lead-acid, cheap to buy and costly per use, and LiFePO4 (lithium iron phosphate), costly to buy and cheap per use. Which one is appropriate depends on your budget's shape, your winters, and how deeply you plan to work it every night.
The job, measured honestly.
A battery's job is to hold tonight's and the gray days' energy, and the honest unit for that is the watt-hour, not the amp-hour on the sticker. Amp-hours only mean something at a stated voltage; watt-hours travel across systems (amp-hours × volts). A "100 Ah" 12 volt battery holds about 1,200 Wh on paper.
On paper, because you cannot use it all. Depth of discharge (DoD) is how much of the tank you actually draw, and it is the biggest lever on battery life: chemistry sets the sane limit, and every cycle past it is borrowed from the battery's future. The companion number is cycle life: how many charge-and-discharge rounds the battery delivers before it fades to 80 percent of its capacity. Divide price by cycles, not price by sticker, and the two chemistries trade places.
Lead-acid.
The century-old workhorse, in two farm-relevant forms: flooded (open cells you top up with distilled water) and AGM (sealed, the electrolyte held in glass mats, maintenance-free and spill-proof). Both share the same habits. Draw past about 50 percent DoD routinely and their cycle life collapses, so half the sticker is spending money and the rest is structure. Left sitting partly discharged, the plates grow crystals (sulfation) and capacity quietly walks away, so lead-acid wants to be brought back to full promptly and held there by a proper float stage. And flooded cells vent hydrogen while charging, which is why battery boxes get vents and why a sealed cabinet full of flooded batteries is a bad idea.
Lead-acid remains appropriate where the budget is tight, the temperature swings are wild, the load is occasional rather than nightly, or the battery lives somewhere a maintenance visit is easy.
LiFePO4, and the winter catch.
LiFePO4 is the stable lithium chemistry that has taken over farm and off-grid storage, and its advantages are blunt: 80 to 90 percent usable DoD, several times lead-acid's cycle life, half the weight, no watering, no venting, and a flat voltage curve that holds equipment happy until nearly empty. Priced per cycle it is usually the cheaper battery, even at twice the sticker.
It has one non-negotiable winter catch: lithium must not be charged below freezing. Charging a LiFePO4 cell below about 0 °C plates lithium metal onto the anode and permanently damages it, so an outdoor battery needs either a BMS with a low-temperature charge cutoff (many budget packs skip it) or a self-heating pack, or a home indoors. A battery that spent the summer flawless and died over January was usually charged frozen.
The flat voltage curve also means a plain voltmeter tells you little about how full a lithium battery is; state of charge comes from the BMS or a shunt monitor, not the number on a multimeter.
The BMS is the last line.
Every lithium pack contains a battery management system: circuitry that balances the cells and disconnects the pack when something goes out of bounds: too full, too empty, too hot, too cold, too much current. It is essential, and it is not your protection plan. BMS quality varies enormously, especially in budget packs, and when a BMS transistor fails it can fail closed. The fuse on the cable protects your wiring and your barn; the BMS protects the cells. You want both, and you want the fuse to be the one you are counting on.
Building a bank.
Batteries gang together like panels do: in series, voltages add (two 12 volt batteries make 24); in parallel, capacity adds. Two habits keep a bank healthy. Match the batteries: same chemistry, same capacity, same age, because a bank works at the level of its weakest member and drags the others down to it. And protect both paths: the charge side and the load side each get their own overcurrent protection, with lugs torqued to spec, because a loose battery lug is a slow-cooking heater exactly where you least want one. The details live on the safety page; how big to build it lives in Sizing a System.
The shortest version
Measure batteries in watt-hours and price them per cycle. Lead-acid: cheap up front, use half the sticker, refill it promptly, vent it. LiFePO4: costs double, delivers several times more, but must never be charged below freezing, so insist on low-temperature cutoff. The BMS protects the cells; your fuse protects the barn. Match bank members and fuse both paths.
Words to work from
Take these terms with you. They are what battery listings quietly assume you know.
- Watt-hour
- The honest capacity unit: amp-hours times volts. Travels across system voltages; amp-hours alone do not.
- Depth of discharge (DoD)
- How much of the tank you draw each cycle. The biggest lever on battery life.
- Cycle life
- How many rounds a battery delivers before fading to 80 percent. Divide price by this, not by the sticker.
- Flooded
- Open lead-acid cells you top up with distilled water. Vent hydrogen while charging.
- AGM
- Sealed lead-acid, electrolyte in glass mats. Spill-proof and maintenance-free, same 50 percent DoD manners.
- Sulfation
- Crystal growth on lead plates left partly discharged. How a lead-acid battery loses capacity sitting still.
- Float
- The controller's gentle hold-at-full stage. Lead-acid wants to live there.
- LiFePO4
- Lithium iron phosphate: the stable, deep-cycling lithium chemistry farm storage uses.
- Low-temperature cutoff
- The BMS feature that refuses to charge below freezing. Non-negotiable for an outdoor lithium battery.
- BMS
- The management circuit inside a lithium pack: balances cells, trips on abuse. The last line of defense, never the first.
Frequently asked questions.
Lead-acid or lithium for a farm solar system?
Price it per cycle, not per sticker. LiFePO4 costs roughly twice as much and delivers several times the usable cycles, so for a battery worked nightly it is usually the cheaper battery. Lead-acid still makes sense for tight budgets, occasional loads, and batteries that live where winters are harsh and maintenance is easy. If the battery lives outdoors in a freezing climate, lithium needs low-temperature charge protection.
Can I charge a lithium battery in winter?
Not below freezing, unless the pack protects itself. Charging LiFePO4 below about 0 degrees C plates lithium metal inside the cells and permanently damages them. Outdoor packs need a BMS with low-temperature charge cutoff or built-in heating, and budget packs often skip both. Discharging in the cold is fine; it is charging that does the damage.
How long will a solar battery last?
Count cycles, not years. Lead-acid worked to half depth typically gives several hundred to around a thousand cycles; LiFePO4 worked to 80 or 90 percent commonly gives several thousand. One cycle a day makes those roughly two to four years and eight to ten, but the real answer follows treatment: depth of discharge, prompt refilling, and winter charging manners matter more than the calendar.
Can I mix old and new batteries in a bank?
Avoid it. A bank behaves like its weakest member: the tired battery drags the voltage, the fresh ones overwork to compensate, and the whole bank ages toward the oldest unit. Match chemistry, capacity, and age. When a lead-acid bank's members start failing, plan on replacing the set; with lithium, per-pack BMSes make mixing slightly more forgiving, but matched is still the rule.