Is there a best battery?
Creating the best battery still remains a dream of top scientists in the world. Batteries are key for the energy supply in the future and there has been good progress in recent time. Different types of batteries have made it to the market but each solution comes with its downsides. The fact that there is no battery winning in all categories means that the engineers have to find the best compromise for each application. To select your best best battery we would need to know your preferences and the scope of its use. There are certain batteries more suitable than others:
How to select a battery
Before you make a decision, you should ask yourself about your primary goals. Long service life, resistance on temperature, high performance, price, cycle depth, maintenance, safety and reliability are normally the parameters to consider and trade against. For stationary systems in most of the cases, in New Zealand, the best choice is a lead-carbon battery. It wins in economics, safety, reliability, service life despite the lower efficiency, difficult cycle window and weight handling efforts compared to a lithium solution. The best cycle characteristics has the REXC series but when weight matters, the EVF series is the better choice. Full family homes prefer a 48V REXC1000 system with a C-10 capacity of 48kWh but cycle about 15kWh daily. Tiny homes, caravans or telecom mountain stations tend to deploy the EVF series. A lead-carbon always provides a scrap value.
A lithium battery is the best choice when it comes to weight, volume, currents and efficiencies. This is normally required in mobile applications like phones, laptops, torches and cars. A growing number of off-grid systems are also equipped with lithium batteries and selected when the advantages in weight and space requirements are preferable. That can be small systems in camper-vans, condominiums or tiny homes.
The failure rates of lithium installations is way higher than lead-carbon batteries and the service life is less. A well maintained REXC system can service up to 25 years whereby lithium batteries are about 8 years (Lithium nickel manganese cobalt oxides “NMC”, LiNixMnyCozO2 :LG Chem) 12 years (Lithium nickel cobalt aluminium oxides”NCA”, LiNiCoAlO2: Tesla) and 15 years for LFP solutions (Sonnen). Before choosing a lithium battery, it is suggested to clarify the disposal cost after the service life. This might come with an unexpected disposal fee.
In harsh climate conditions and super heavy duty applications, the alkalic battery would be the best choice overriding the higher costs and maintenance downsides. When the complete maintenance free solution is available, the alkalic battery might win because it can be installed inside the roof where the temperatures raise above 60°C in summer times.
Lithium battery
Lithium has the highest electrochemical potential and is, with only three protons, the lightest metal. That makes a lithium ion so suitable to be used in batteries. A lithium battery is indeed a group of batteries working with a lithium ion walking between the poles. The pure electrical characteristics (cell voltage, inner resistance) of lithium batteries are very good but lithium batteries come with significant shortfalls such as inextinguishable fire with toxic fumes, intrinsically instable chemistry, recycling and price. For the off-grid market are only two groups of lithium batteries relevant – the Lithium Iron Phosphate (LFP) and the Lithium Ion (LIo). LFP comes with 3.2V and LIo from 3.6V-3.85V normal voltage.
The intrinsic risk of a thermal runaway requires a battery management system (BMS). Since a BMS is an active system; a failure of a BMS is anytime possible and needs to be hedged to limit unwanted outcomes (steel case). All this precautions have to be factored into the equation.
Lithium batteries are interesting to look at when space, weight, high discharge and super high efficiency maters. All these arguments are in off-grid systems normally secondary and therefore it is worth to look to other battery technologies when building a stationary system.
Lead acid battery
Lead acids working on Pb2+ and Pb4+ electrical potential, emerged in a water diluted H2SO4 acid (Pb + PbO2+ 2H2SO4 → 2PbSO4+ 2H2O). The rated voltage of all lead-acid cells is 2.05V. There are manifold versions of lead-acid batteries in design and chemistry. Each built has its own application. Different chemicals can be added to achieve different characteristics. Antimony makes a battery last longer, calcium reduces gassing and carbon reduces sulphatation. The lead plates can be designed tubular or in plates and the electrolyte can be liquid (wet) or fixed (AGM or gel). A starter battery comes with many thin plates and a deep cycle battery with rather thicker plates but less numerous. Lead acid batteries do not have the problem of a thermal runaway but produces gas while charging or fast discharging. Only a severe shortcut can lead to a mild explosion but not end in a fire like lithium does. Lead-acid batteries are intrinsically safe in use.
Lead-Carbon battery
A Lead-Carbon battery is not an own battery class but is a revolutionary improvement of lead-acid batteries. Carbon can be graphite or fibre or nano tubes. The intended advancement was to get a capacitor function into the battery but it shows many unintended positive improvements in sulphatation reduction, efficiency improvement, durability and useable capacity. This can be explained by two characteristics of carbon; its high electrical and high thermal conductivity. The measurable improvements are not due to a direct chemically effect of carbon – it is more the physically improvements of the lead-plates. The sulphatation of lead is not reduced chemically but the carbon does not allow thermal hot spots to occur during the cycle process on the microscopical level. The result is like a new battery class – the presence of carbon allows higher charges/discharges currents, improves the inner resistance, reduces sulphatation, allows deeper discharge and significantly increases the batteries service life. These improvements combined with the advantages of the lead-acid chemistry makes this battery the better choice than lithium in many off-grid applications.
Alkalic battery
The alkalic battery is the opposite of lead-acid batteries, since the plates are emerged in a hydroxide and not in an acid. The cell voltage is about 1.2V. But here as well; alkalic battery is a group of batteries. The most well known alkalic battery is the Nickel-Cadmium battery, which is due to its toxicity only allowed to be used in commercial applications. There are many more chemical sub-types like the NiMH battery, NiFe battery or the Metal-Hydroxide battery. This chemistry has a significant advantage over all others – there is no chemical deterioration that theoretically would allow an infinitive cycle number. Another advantage is the electrical and thermal robustness. These made them used in trains, industrial applications, military and even aircrafts. The Airbus A350 deploys a NiCd battery learning from the severe fires from the Boeing 787 lithium solution. Alkalic batteries are also intrinsically safe in use.
The disadvantages are the relative low round trip efficiency of about 65% and the high installation efforts. The battery requires re-watering and splits water on a higher rate than Lead-acid.
Check out the article about alkalic batteries on this webpage.
Sodium Sulphur battery
This battery type is relatively unknow but we from off-grid.nz are keen to explore this special chemistry. Main differentiator is that this battery requires liquid sulphur at about 300 deg C.
This battery would erase pretty much all downsides of other technologies but also comes with inherent shortfalls like having masses of sulphur and liquid sodium under safe conditions operating. Main advantage are the very high round trip efficiency, high cycle life and can be build with ultra low negative environmental impact. Sodium and Sulphur are not hazardous and available in mass. The poly sulphur build up is a valuable material in the chemistry industry and the ceramic separator has no hazardous components. Such battery would be only in commercial applications of 1MW or more to be an option and can revolutionize the grid stabilisation market by solving the energy storing problem of the renewable energy sources in total.