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Lithium battery technicalities
The low-down on lithium batteries vs traditional types

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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.

 

 

The following technical article has been kindly provided by mobile power specialist Solar4RVs – http://www.solar4rvs.com.au/  – listing the key benefits of LiFePO4 batteries when compared with deep cycle lead acid batteries.  They have longer shelf and charge life, and are also approximately one quarter of the weight of the equivalent useable capacity in comparison to lead-acid counterparts, such as AGM, lead-carbon and gel – an important consideration for mobile applications.

 

Important considerations

A 12V lead-acid battery has six cells and there are no protective electronics inside the battery, so it is easy to accidentally damage the battery through improper use.

 

Custom-built CALB lithium battery – 380Ah and ideal for use with a 3000W inverter

 

A lithium battery almost always has a Battery Management System (BMS) in-built.  This can be sophisticated, with features that protect the cells from short circuit and overload, measure and display the current, State of Charge (SoC) by Bluetooth and also balance the cells. A simpler BMS only disconnects the load/charge when a cell voltage is low/high.

The BMS on lithium batteries disconnects the load/charge by the use of metal–oxide, semi-conductor, field-effect transistors (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, so, for example, if you discharge a 100Ah battery at 5A it will be completely discharged in 20 hours.  If you discharge it at 20A, you’re discharging it at the five-hour rate and its useable capacity will be reduced by around 20-percent. 

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 may cause the BMS to disconnect the battery so it should be rated at 90-percent useable capacity.

Most lead-acid batteries are rated for 600 cycles at 50-percent Depth of Discharge (DoD), whereas many lithium batteries are rated at 2000 cycles at 100-percent DoD.  This means the battery will be at around 80-percent of its original capacity after that number of cycles, depending on factors such as discharge/charge rates, temperature and vibration.

Lead-acid batteries prefer to always be full, so you should always leave these on a float charge in storage, if possible.  Lithiums prefer to be stored at 40-percent SoC.

You can use a lead-acid charger with a lithium battery, but the lifespan of the lithium will be reduced, because the lead-acid battery requires a longer absorption period – the point towards the end of the charging cycle that holds the voltage higher to ensure the battery gets full. In contrast, a lithium battery has very high charging efficiency and does 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 percent, rather than 0-100 percent.

 

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 percent, it is difficult to know the SoC of the battery by measuring the voltage, so 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-percent capacity its voltage would be close to 3.65V – a LiFePO4 cell’s upper limit is 3.65V – whereas the other cells at 96-percent would be approximately 3.45V.  Therefore the BMS will disable charging, limiting battery capacity and the cells will degrade unevenly.

A circuit which measures each cell voltage and burns off the excess charge from 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.  Balancing begins only once the cells are above approximately 95 percent SoC, so if a tiny portion of the power is burned off while the charger is limiting the power, being close to full, no speed in charging is lost and only a few milliWh of power is burned off, which is often ‘free’ from solar.  On large banks of around 800Ah, active balancing’s advantage is that it can distribute large amounts of charge without creating much heat.

If cells are well balanced when the pack is assembled, it can take years before they start to be noticeably out of balance.

The BMS is the most common cause of failure of lithium batteries, but abused cells suffer from a reduced lifespan. However, they rarely fail suddenly.

If a lithium battery seems cheap, there are normally two reasons. A BMS can cost up to one third of the price of a lithium battery pack, so cheaper and lower-current-rated BMS units save money.  B-grade or recycled cells can be used to reduce costs.

Prismatic – rectangular shaped – or cylindrical shaped cells aren’t necessarily better than one another. 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 separately fused.

A 100Ah lithium battery is rated at 12.8V and therefore 1280Wh, compared with 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 four-parallel, four-series combination and therefore be the same weight and cost.

Newer lithium batteries can be charged between -5C and +55C, and discharged between -30C and +60C, so are well suited for Australian conditions.

 

Cost penalty, or not

 

Lithium’s up-front cost is higher than AGM and gel…or is it?

Consider the following two situations:

Situation One: Weekend use; four trips per year, with 50Ah useable capacity required per day, powering mostly lights (utilising the 20-hour rate of an AGM).

AGM – 50Ah useable requires 100Ah capacity, as AGM should only be discharged to approximately 50% to preserve its long term life.  A 100Ah AGM battery of reasonable quality may cost approximately $300 – $400, have a one- to two-year warranty and be rated to 600 cycles, with up to a 12-year float life.  With solar or mains charger keeping the battery full, the battery will only have used 96 cycles over 12 years and therefore still have almost 80-percent of its original capacity.

Lithium – 55Ah useable required. The cost of a lithium battery about this size may be approximately $500 – $1000 and typically come with a three-year warranty, be rated to 2000 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 Two: Daily off-grid use, 288Ah required per day, two days autonomy (two days allowing for no charging being available). Large loads are often used, such as air conditioning and no 12V is required, so 48V is beneficial.

AGM – four 360Ah (17.28kWh, allowing for the five-hour rate) batteries and three battery balancers are required, at a cost of approximately $3200 to $4200. These will last less than two years, with 365 cycles per year.

OPzV (often called long-life lead-acid) – six 360Ah (17.28kWh) cells are required, at a cost of approximately $5000 to $7000.  These will last about 8.2 years, with 365 cycles per year.

Lithium – 32 x 80Ah (8.192kWh) cells and a suitable 16-cell BMS are required, at a cost of approximately $3100. 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, including Lithium-ion, Lithium Iron Phosphate and Lithium Polymer are available.  The LiFePO4 batteries are the safest type of Lithium batteries, because there is no danger of the battery erupting into flames, as with Lithium-ion. They will not overheat and they will not catch 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.

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