While lead-acid batteries are still a common choice, in the last few years lithium-ion batteries slowly became a new standard for solar systems. Lithium-ion batteries count as deep-cycle by default — all of them are meant to be discharged and then charged back again. Their popularity is not a surprise: they are better than traditional lead-acid batteries in almost every way.
- The depth of discharge reaches 90-95%, though on a daily basis it's better not to use more than 70-80% of their load.
- They last longer — the warranties are up to 10 years.
- They are more efficient than lead-acid batteries and lose only around 5% of energy during their work.
- They are smaller and lighter — in fact, a lithium-ion battery on average occupies 75% less space and weighs 75% less than a comparable lead-acid battery.
- They are charged faster: it usually takes about 1-2 hours to fully charge a lithium-ion battery compared to 2-4 hours for a lead-acid battery.
- Partial state of charge isn't an issue.
The only problem with lithium-ion batteries is their high cost, which can range from 7,000$ to 30,000$, while prices for a lead-acid battery vary around 5,000$-15,000$.