Lithiumbatterien- 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 main advantages of LiFePO4 batteries is the longer cycle life and durability compared to the current deep cycle lead-acid batteries. They also weigh about a quarter of the equivalent usable capacity compared to their lead-acid counterparts such asgeneral assembly, 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, so it is easy to accidentally damage the battery through improper use.
- A lithium battery (almost) always has a built-in battery management system (BMS). This can be sophisticated and have functions that protect the cells from short circuit and overload, measure and display the current, state of charge (SoC) via Bluetooth and also balance the cells, or be simple and only disconnect the load/charge when a cell voltage is low/ high.
- Lithium battery BMS separates the load/charge by using MOSFETs, these MOSFETs are usually the limit of the battery's continuous current. If the battery is rated for 100A continuous and you have a 2000VA load, you will draw about 167A from the battery and destroy the MOSFETs almost instantly.
- Deep cycle lead-acid batteries are rated for their 20 hour rating, i.e. H. if you discharge a 100Ah battery at 5A it would be completely discharged in 20 hours. If you discharge it at 20A, you will discharge it at the 5 hour rate and its usable capacity will be reduced by about 20%. Lithium batteries do not suffer from this problem, but depending on the internal resistance of the lithium cells, discharging at a high current towards empty can cause the BMS to disconnect the battery to allow 90% usable capacity.
- Most lead-acid batteries are rated for 600 cycles at 50% depth of discharge (DoD), while many lithium batteries are rated for 2000 cycles at 100% DoD. This means that after this number of cycles the battery will be at 80% of its original capacity, of course these numbers are highly variable based on factors such as discharge/charge rates, temperature, vibration etc.
- Lead-acid batteries like to be full at all times, so you should always leave them on trickle charge when storing them. Lithiums are preferably stored at 40% SoC.
- You can use lead-acid chargers with lithium batteries, but the lifespan of the lithiums will be reduced because the lead-acid batteries require longer absorption times (the point near the end of the charge cycle that keeps the voltage higher to ensure battery life becomes full). While lithium batteries have very high charging efficiencies and do not require these long absorption times, the absorption voltage causes voltage-induced stresses in the cells that negatively impact their lifespan. In fact, lithium batteries last the longest when used between 40-60% rather than 0-100%.
- Since the SoC and voltage of lithium is exponential when approaching full or empty and has a very small voltage change between 20% and 80%, it is difficult to determine the battery SoC by measuring the voltage, a battery monitor is required. This also means that if a cell has a few percent more charge than the others in the bank as soon as it approaches 100% capacity its voltage would be close to 3.65V, the upper limit of LiFePO4 cells is 3.65 V is While the other cells would be around 3.45V at 96%. Therefore, the BMS will disable charging and you will not be able to use the entire battery and the cells will degrade unevenly.
- A circuit that measures the voltage of each cell and burns off the excess charge from the most highly charged cells to bring them to the same level as the rest is called a passive balancing circuit.
- Active balancing is very efficient in distributing charge, but is not essential. Since the equalization only starts when the cells are above approx. 95% SoC, when a tiny part of the power is burned away while the charger limits the power because it is almost full, you have not lost any speed when charging and only a few milliWhof- Power consumed, which often comes from the sun. For large banks, e.g. B. 800 Ah or so, active balancing has the advantage that it can distribute large amounts of charge without generating much heat.
- If the cells are well balanced when the pack is assembled, it can take years before they start to become noticeably unbalanced.
- The BMS is the most common cause of lithium battery failure, abused cells suffer reduced lifespan but rarely fail suddenly.
- When a lithium battery seems cheap, there are usually two reasons: A BMS can cost up to a third of the cost of a lithium battery pack, so cheaper, lower amperage BMS units save money. B-grade or recycled cells can be used to save costs.
- Prismatic (rectangular) or cylindrical shaped cells are not necessarily better than each other. The prismatic cells are significantly simpler and lighter for DIY battery banks. Packs with very long warranties often use cylindrical cells because if one cell fails, the cell fuse will blow rather than disable the entire pack, while prismatic cells will not melt every cell.
- 100Ah Lithium is rated at 12.8V and therefore 1280Wh compared to 100Ah 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 dominant term in the automotive industry.
- A 1200Wh battery could be 100Ah at 12V or 25Ah at 48V and have the same number of cells inside but 16 cells wired in series (48V) or in a combination of 4 parallel 4 series and therefore have the same weight as costs.
- Newer lithium batteries can be charged between -5 and 55°C and discharged between -30 and 60°C and are therefore well suited to 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 50 Ah usable capacity per day with mainly lighting (i.e. use of the AGM 20-hour tariff).
- AGM - 50Ah usable requires 100Ah capacity as AGM should only be discharged to around 50% to maintain its lifespan over the long term. A decent quality 100Ah AGM battery can cost around $300-$400, has a 1-2 year warranty and is rated for 600 cycles and has a float life of up to 12 years. If solar or grid chargers keep the battery full, in 12 years the battery has only used 96 cycles and therefore still has almost 80% of its original capacity.
- Lithium - 55Ah usable required. A lithium battery of this size can cost around $500-$1,000 and typically comes with a 3-year warranty rated at 2,000 cycles and has a shelf life of 10 years.
In this situation, the lithium batteries offer hardly any advantages over the AGM batteries, apart from weight and size.
Situation 2: Daily off-grid use, 288 Ah required per day, two days of autonomy (two days with no available charging sources). Often large loads such as air conditioning are used and 12v is not required so 48v is an advantage.
- AGM - 4x 360Ah (17.28kWh) batteries (taking into account the 5-hour tariff) and 3 battery balancers required. Costs from around $3,200 to $4,200. These last less than 2 years at 365 cycles per year.
- OPzV (often referred to as long-life lead-acid) – 6x 360 Ah (17.28 kWh) cells required. Costs from around $5,000 to $7,000. These last about 8.2 years at 365 cycles per year.
- Lithium - 32x 80Ah (8.192kWh) cells and a matching 16 cell BMS required. Cost of about $3,100. These last about 8.2 years at 365 cycles per year.
In this situation, the lithium batteries offer clear advantages over the AGM and OPzV batteries.
Are LiFePO4 Batteries Safe?
Various compositions of lithium such as lithium ion, lithium iron phosphate and lithium polymer are available. The LiFePO4 batteries are the safest type of lithium batteries because there is no risk of the battery bursting into flames like lithium-ion. They will not overheat or catch fire even if punctured.
The other benefit of LiFePO4 batteries is that they have no negative health or environmental risks as the cathode material is non-hazardous.
There are many other chemicals on the market, but at the time of writing, LiFePO4 still has the most favorable properties.