Lead-acid vs Lithium
Lithium Batteries - Are They the Right Choice for You?
Lithium Iron Phosphate (LiFePO4), also known as lithium and LFP batteries deliver high energy and power density for mobile applications. They often have the best combination of performance, safety, cost, reliability and environmental characteristics for RV and marine applications.
One of the key benefits of LiFePO4 batteries is the longer cycle and shelf life when compared to the current deep cycle lead acid batteries. They are also approximately one quarter of the weight of the equivalent useable capacity in comparison to their lead-acid counterparts such as AGM, lead-carbon, gel which is an important consideration for mobile, and in some cases, marine applications.
Some other important considerations and notes:
- A 12V lead-acid battery has 6 cells. There are no electronics inside the battery, therefore it is easy to accidently damage the battery through improper use.
- A lithium battery will (almost) always have a Battery Management System (BMS) in-built. This can be sophisticated and have features that protect the cells from short circuit, overload, measure and display the current, State of Charge (SoC) by Bluetooth and also balance the cells or be simple and only disconnect the load/charge when a cell voltage is low/high.
- The BMS on lithium batteries disconnects the load/charge by the use of MOSFETs, these MOSFETs are commonly the limit on the continuous current rating of the battery. If the battery is rated for 100A continuous, and you have a 2000VA load, then you will be drawing approximately 167A from the battery and you will destroy the MOSFETs almost immediately.
- Deep cycle lead-acid batteries are rated at their 20 hour rate, i.e. if you discharged a 100Ah battery at 5A it would be completely discharged in 20 hours. If you discharge it at 20A, you'll be discharging it at the 5 hour rate and it's useable capacity will be reduced by around 20%. Lithium batteries do not suffer from this problem, however depending on the internal resistance of the lithium cells, discharging at a high current towards empty may cause the BMS to disconnect the battery so allow 90% useable capacity.
- Most lead-acid batteries are rated for 600 cycles at 50% Depth of Discharge (DoD) whereas many lithium batteries at 2000 cycles at 100% DoD. This means the battery will be at 80% of its original capacity after that number of cycles, of course these figures are highly variable based on factors such as discharge/charge rates, temperature, vibration etc.
- Lead-acid batteries prefer to always be full, you should always leave these on a float charge if in storage if possible. Lithiums prefer to be stored at 40% SoC.
- You can use lead-acid chargers with lithium batteries, however, the lifespan of the lithiums will be reduced as the lead-acid batteries require longer absorption periods (the point towards the end of the charging cycle that holds the voltage higher to ensure the battery gets full). Whereas lithium batteries have a very high charging efficiency and do not require these long absorption periods so the absorption voltage will cause voltage induced stresses within the cells negatively affecting their life. In fact, lithium batteries will last longest if used between 40-60% rather than 0-100%.
- As the SoC and the voltage of lithiums is exponential when approaching full or empty and has a very little change in voltage between 20 to 80%, it is difficult to know the SoC of the battery by measuring the voltage, a battery monitor is required. This also means if one cell has a few percent more charge than the others in the bank, once it nears 100% capacity its voltage would be close to 3.65V, LiFePO4 cells upper limit is 3.65V. Whereas the other cells at 96% would be approximately 3.45V. Therefore the BMS will disable charging and you are not able to utilise the entire battery and the cells will degrade unevenly.
- A circuit which measures each cell voltage and burns off the excess charge off the highest charged cells in order to bring them down to the same level as the rest is called a passive balancing circuit.
- Active balancing is very efficient as it distributes charge but is not essential. As balancing only begins once the cells are above approx. 95% SoC, if a tiny portion of the power is burned off while the charger is limiting the power due to being close to full, you have not lost any speed in charging and only burned a few milliWh of power which is often from solar. On large banks i.e. 800Ah or so, active balancing has an advantage that is can distribute large amounts of charge without creating much heat.
- If cells are well balanced when the pack is assembled, it can take years to start to noticeably be out of balance.
- The BMS is the most common cause of failure of lithium batteries, abused cells suffer from a reduced lifespan but rarely fail suddenly.
- If a lithium battery seems cheap there are normally two reasons, a BMS can cost up to one third of the cost of a lithium battery pack, therefore cheaper and lower current rated BMS units save cost. B grade or recycled cells can be used to save costs.
- Prismatic (Rectangle shaped) or Cylindrical shaped cells aren't necessarily better than one another. The prismatic cells are significantly simpler for DIY battery banks and lighter. Packs with very long warranties often use cylindrical cells because if any cells fail, the cell fuse will blow and won't disable the whole pack, whereas with prismatic cells each cell is not fused.
- 100Ah of lithium is rated at 12.8V and therefore 1280Wh compared to 100Ah of AGM which is rated at 12V and therefore 1200Wh. It makes more sense to speak in Wh or kWh than Ah, but Ah at 12/12.8V is the predominant term in the automotive industry.
- A 1200Wh battery could be 100Ah at 12V or 25Ah at 48V and could have the same number of cells inside but be wired 16 cells in series (48V) or be wired in a 4 parallel 4 series combination and therefore be the same weight as cost.
- Newer lithium batteries can be charged between -5 and 55C and discharged between -30 and 60C so are well suited for Australian conditions.
The upfront cost is higher than AGM and gel... Or is it?
Consider the following two situations:
Situation 1: Weekend use, 4 trips per year with 50Ah useable capacity required per day with mostly lights (i.e. utilising the 20 hour rate of the AGM.)
- AGM - 50Ah useable requires 100Ah capacity as AGM should only be discharged to approximately 50% to preserve its life long term. A 100Ah AGM battery of reasonable quality may cost approximately $300 - 400, have a 1 to 2 year warranty and are rated to 600 cycles and have up to a 12 year float life. With solar or a mains charger keeping the battery full, the battery will only have used 96 cycles over 12 years and therefore still be almost 80% of its original capacity.
- Lithium - 55Ah useable required. The cost of a lithium battery about this size may cost approximately $500 - $1,000 and typically come with a 3 year warranty, be rated to 2,000 cycles and have a 10 year shelf life.
In this situation the lithium batteries offer little advantage over the AGM batteries apart from weight and size.
Situation 2: Daily off-grid use, 288Ah required per day, two days autonomy (two days allowing for no charging sources being available). Large loads are often used such as air conditioning so 48V is beneficial.
- AGM - 4x 360Ah (17.28kWh) (allowing for the 5 hour rate) batteries and 3 battery balancers required. Cost of approximately $3,200 to $4,200. These will last less than 2 years with 365 cycles per year.
- OPzV (often called long-life lead-acid) – 6x 360Ah (17.28kWh) cells required. Cost of approximately $5,000 to $7,000. These will last about 8.2 years with 365 cycles per year.
- Lithium - 32x 80Ah (8.192kWh) cells and a suitable 16 cell BMS required. Cost of approximately $3,100. These will last about 8.2 years with 365 cycles per year.
In this situation the lithium batteries offer significant advantages over the AGM and OPzV batteries.
Are LiFePO4 batteries safe?
Several chemistries of Lithium such as Lithium-Ion, Lithium Iron Phosphate and Lithium Polymer are available. The LiFePO4 batteries are the safest type of Lithium batteries as there is no danger of the battery erupting into flames as with Lithium-Ion. They will not overheat, and they will not catch on fire, even if punctured.
The other advantage of LiFePO4 batteries is they do not have any negative health risks or environmental risks as the cathode material is not hazardous.
There are many other chemistries on the market, but at the time of writing LiFePO4 still has the most favourable features.