There are a lot of battery options on the market now for off-grid solar. It can be confusing trying to understand what options are best, especially with new technology being offered and confusing marketing tactics. This article goes over all the kinds you might see on the market, but if you just need help choosing from our options, read this.
Flooded Lead Acid
Flooded lead acid (FLA), is the most traditional battery technology and has had great success in off-grid solar use. It comes with a few downsides, and must be monitored and maintained to get the most out of them.
- Long service life, but must be consistently checked and maintained to achieved this long life.
- FLA batteries are the most resistant to damage when accidentally over charged
- More expensive to ship and insure as they are classified as a dangerous good
- They release hydrogen gas when charging (which is explosive), so extra care needs to be taken no spark can occur near the batteries
- They must be in a well-ventilated area, and cannot be in a living space
- They support high rates of charge and discharge
- Low cost for the storage capacity compared with other technologies
(Photo Credit: Home Power Magazine)
VRLA (Valve Regulated Lead Acid) Battery
There are two major types of VRLA (Valve Regulated Lead Acid) Battery, AGM and GEL. Both batteries are very comparable, and there are both AGM and GEL batteries that are specifically designed for solar usage. It is much better to select a VLRA battery based on what application it is designed for, than by which technology it uses.
AGM vs GEL:
- GEL batteries typically offer longer service life and allow deeper levels of discharge without permanent damage, but require more specific charging methods.
- AGM batteries typically offer higher discharge current, which allow them to be used as starter batteries (like the one in your car).
Key Points about VRLA batteries:
- Require no maintenance
- Won't last as long as perfectly maintained FLA, but will last much longer than a poorly maintained FLA setup
- Still a low cost option
- Cheap to ship and safe to handle (un-spillable)
- Have the widest operating temperature window
- Safest battery design - only let off gas when overcharged far beyond limits (which won't happen with a reliable charge controller)
（Photo Credit: Home Power Magazine)
Advanced Lead Acid Technology
There are several new battery technologies that are based on lead acid. These include lead crystal batteries, lead carbon batteries, super capacitor batteries, and many others based on lead plates with sulfuric acid electrolyte. A lot of these batteries have very similar claims: greater cycle life, ability to discharge down to 0 volts, no degradation from partial charging - which have traditionally been the downsides of lead acid batteries.
These are based on AGM batteries, but have some additives to the electrolyte that are designed to reduce sulfation on the plates and increase the batteries' service life. This technology has been around since the 1980s, but is yet to see mainstream adoption despite appearing to be better.
An Ultra Battery is a type of lead carbon super-capacitor battery that has a separate negative electrode made from carbon, which is meant to give the battery superior performance with extremely high current draws. It was originally designed as a battery technology for electric hybrid vehicles, where it needed to operate at traditional car voltages of 14.4 volts, discharge a lot of current even when only partially charged, and sit for long periods of time at a partial state of charge without degrading. Initial testing of the batteries appeared to be positive, but they are yet to see much adoption. They have had some usage as grid-tied storage to smooth out power requirements, however it seems most of these have since been replaced with lithium iron phosphate.
Carbon impregnated negative electrode
Several manufacturers offer AGM, GEL and FLA batteries with carbon-lead alloy negative electrodes. This carbon additive is designed to increase the cycle life on the battery by reducing permanent sulfation by changing the way the negative plates react with the sulphuric acid electrolyte. This technology has been around for over 10 years, with the Trojan offering, “Smart Carbon” FLA batteries being one of the first. These batteries have proven advantages over traditional FLA batteries in PSoC (Partial State of Charge) applications, when the batteries are never able to reach 100% charge.
Key points about Advanced Lead Acid Batteries:
- Often have no proven track record (yet)
- Designed for very specific applications
- Potential to offer much longer service life over traditional lead acid technology
- More expensive when compared to established lead acid batteries, however can be viable when considering their extended service life
- Need to look beyond the marketing to establish if they really are suitable for small scale off-grid applications
Lithium Iron Phosphate
There are a few variations of the lithium chemistry, but the most suitable to off-grid usage is the iron phosphate. This technology offers great capacity and current output for a lightweight and small size. The only downsize to this technology is the high cost due to use of rare earth metals. LiFePO4 is the safest of all the lithium technologies - as safe as the battery in your phone.
- Operates in similar voltages to lead acid, but REQUIRES a charge controller designed for lithium iron phosphate chemistry
- Has extremely good cycle life
- Can deliver very high current outputs
- Lightweight and small for the storage capacity
- Much more expensive than other technology
(Photo credit: Australian Academy of Science)
What do we recommend?
In small scale off-grid applications, we recommend using a VRLA battery that is designed for solar. Our most popular battery for off-grid use is the 200ah GEL VRLA battery, as it offers a great balance. It has a high service life, can handle being deeply discharged a few times without significant damage, is very safe, compatible with almost all charge controllers and inverters, and has a cost comparable to FLA and AGM batteries.
We also offer lithium iron phosphate batteries, which in many circumstances are worth the extra cost. If you have space and weight constraints such as boats or RVs, but you still require a lot of storage capacity, lithium iron phosphate can be a good solution. We also recommend these for larger scale applications (when you'll have upwards of 8 solar panels), or to anyone wanting to maximise lifespan and storage capacity. You will be paying 2-3x as much as GEL for this advantage, but with the longer lifespan this is the more economical choice in the long term.
If you have high current or PSoC requirements we can look into lead carbon, however we often find the switch to 2V batteries as a better alternative, as lead carbon batteries require being discharged to a lower voltage than most inverters support to extract the available capacity.