Updated: Aug 19, 2020
We live in interesting times where there are both older proven battery technology (such as lead-acid, gel, etc.), great options where the more advanced options have matured (like lithium iron, lead carbon, super-capacitors, etc.), and brand new technologies seem to be not too far on the horizon (such as solid state, or glass substrate batteries). With the green power and electric vehicle revolution, more money is being poured into energy storage research than ever. There are many options and the technologies are all interesting!
I specifically want to outline some of my thoughts on why Lithium Iron Phosphate batteries (Lifepo4 in short... the chemical components abbreviated), is currently ideally positioned and should be your number one choice at this point in time, specifically for residential solar.
When I installed my first solar system for my house, I was on a tight budget. The "new" batteries then was also WAY too expensive. (I am talking 8 years back or so.) I really had no choice but to buy deep cycle 6 volt golf-cart batteries (8 of them), which came at a very attractive price / performance point. I loved them. I researched in depth how to handle them for optimal lifespan, I managed their temperature (built a beautiful wooden battery box with automatic fan temperature control!), I measured their voltages precisely almost every day, I rotated them in the battery bank, and of course regularly filled them with distilled water. I even measured their specific gravity using a hydrometer and kept a chart.
All went well for about 3 years. I cycled them daily down to about 50% capacity, which gave me about 5 kilowatt-hours of usable power. At the start of year 4, things suddenly (without warning) took a turn for the worse. I noticed one of the cells on my charts that were starting to dip... its voltage struggled to get up to full charge, and it used more water than the other batteries, and it got hot when charging. It went down-hill and fell apart quickly and spectacularly... a month later that battery started to bring the whole bank down and I was forced to replace it with a second-hand battery. (I won't go into the detail of my failed battery restoration attempts.) Not long thereafter the other batteries also started to give issues, and that was the end of my lead-acid bank. It gave me almost 4 good years. I realised at the end of all this, keeping a lead-acid bank happy and maintained is hard work... no regular person out there want to spend so much time on batteries. And if you cycle them every day, they will fail sooner than what you think.
The first time I plugged LiFePO4 batteries into my upgraded solar system, it was like day and night... I could not believe the difference! Here is very shortly what I found:
I had lots more power available. Since you can discharge lithium down close to zero vs. lead-acid's 50% at best.
I had LOTS more power available. Lithium phosphate's efficiency in charging and discharging is above 96%. Lead acid is around 85%. That means I lost about 30% of power when charging and then using my lead-acid batteries. Suddenly the same capacity of LiFePO4 batteries charged quicker and lasted longer. (You effectively need fewer solar panels!)
They did not heat up. Even charging at high amps, they remained cool and odor free (no gassing).
They needed zero maintenance. No longer filling with water and checking often.
They took up very little space. (My lead acid wooden battery box almost took up half my garage.)
Their voltage curve is flat and power delivery is stable. Even if they are near empty or I put them under high load there are no big voltage drops, and my inverter is not complaining.
On top of all this, if you look after them they will easily last 10-15 years, even if you cycle them every day. It is well proven.
Also, when they eventually do wear out, it is usually not as drastic as with lead-acid batteries where they completely die. They rather gradually lose capacity over time at a fairly linear rate. That is why you can often buy second hand LiFePO4 batteries and still get good life out of them. They very seldom crash and "burn out". Theoretically, you could still be using them after 20 years if you are happy with reduced capacity.
The question of cost often comes up. Interestingly, LiFePO4 is much cheaper than lead-acid per unit of usable electricity over its lifespan if you start doing some calculations. There are many research articles and cost analyses on this. I am not going to get into the detailed calcs, but in summary, 10 years+ at almost full discharge and 96% efficiency gives you 5x+ more power than a similarly sized lead-acid battery bank. So sure, LiFePO4 may cost you 3 times more, but you get 5 times more in return. You save lots of money although the initial cost is a bit high.
Then there are some of the newer technologies, such as super-capacitor hybrids and so on. They are starting to get some traction, but they are still significantly more expensive, and to some extent at least not field proven. There are promises of further improvements and other new battery technologies, but realistically they are 2 years out at least, and I am sure will be astronomically expensive at first.
For these reasons, lithium iron phosphate really is currently the best choice by a long-shot. Also, as other battery technologies mature, lithium should further drop in price, putting a reliable and very good battery technology in reach for more households. I really believe lithium iron phosphate batteries are going to be with us for a long time to come still.