Lithium-ion vs AGM Battery has been a very popular topic in independent power circles in recent times In light of my last post concerning the use of the DC or Hybrid concept for electrical power, it occurred to me that the system could also have used monobloc AGM/Gel batteries or indeed a bank of long life 2 volt gel cells. In that case why was Lithium chosen? Hopefully this post may go some way to highlighting that decision process.

Across all markets over recent years Lithium-ion batteries have been gaining in traction . To the uninitiated it is easy to dismiss Lithium-ion as an expensive alternative to VRLA (valve regulated lead acid) technologies such as AGM (absorbed glass mat), if simply looking at the amp-hour (Ah) rating. This was the initial mistake I made a few years back. Digging deeper it became clear to me that there is a lot more than Ah ratings to consider, when choosing the best batteries for your application.Lithium-ion vs AGM Battery

In the marine world (which is where I have the most experience) the choice these days and especially with higher loads – often simply comes down to Lithium-ion vs AGM Battery. In the comparisons below whilst Gel batteries are shown, they do have a lower effective capacity at high discharge currents. They cost about the same as AGMs, assuming both types are monoblocs, as opposed to 2 V long life gel cells. Wet cell or flooded lead acid (FLA) batteries whilst referred to are not considered for the crux of this particular comparison, primarily due to maintenance and safety considerations in the marine environment. This of course may not apply to other markets.Lithium-ion vs AGM Battery

https://www.youtube.com/watch?v=pnCJFXWEVYs

Useable energy and cost

It is generally accepted that the most economic and practical depth of discharge (DOD) for an AGM battery is 50%. For Lithium-iron-phosphate (LiFePO4 or LFP) which is the safest of the mainstream Li-ion battery types, 80% DOD is used.

How does this work out in the real world? Let’s take two Victron 24V battery examples and compare useable energy for a small yacht:

1 x Victron Lithium-ion 24 V 180 Ah

The nominal voltage of the LFP cell is 3.3 V. This 26.4 V LFP battery consists of 8 cells connected in series with a 180 Ah rating. The available energy is 26.4 x 180 = 4. 75 kWh. Useable energy is 26.4 x 180 x 0.80 = 3.8 kWh.

2 x Victron AGM 12 V 220 Ah

The nominal voltage of the lead-acid cell is 2.0 V/cell. Each 12 V monobloc battery consists of 6 cells connected in series with a 220 Ah rating. Connecting 2 x 12 V 220 Ah batteries in series to give 24V and 220 Ah, the available energy is 24.0 x 220 = 5.28 kWh. Useable energy is 24 x 220 x 0.50 = 2.64 kWh.

This begs the question, what Ah rating of AGM batteries would be the equivalent of the 3.8 kWh useable energy of the Lithium-ion battery? To get 3.8 kWh of useable energy from an AGM battery it would need to be twice that size to start with due to the 50% DOD economy rule i.e. 3.8 x 2 = 7.6 kWh. At 24V that would mean 7,600/24 which gives us a battery rating of 316.66 Ah, which is moving closer to twice the rated capacity of the Lithium-ion 24 V 180 Ah. Note this does not take into account, the ageing of the batteries, temperature derating or the effect of higher loads. For AGM batteries, higher loads have a greater effect than on Lithium. See the section – Useable energy: effect on discharge capacity and voltage with differing loads, below. Based on all this it is reasonable to say that an AGM battery will need to be twice the Ah rating of a Lithium one.

What about price? Using the Victron price list we see that a 12V 220 Ah AGM is € 470 ex VAT or 2.136 €/ Ah. For 316.66 Ah that is the equivalent of € 676.50 at 12V or € 1,353 at 24 V. The 24V 180 Ah Lithium is € 4,704 for the same amount of useable energy and is therefore 4,704/1,353 = 3.48 times more expensive (or less if we consider the factor of 2 referred to above) when comparing Ah ratings.

Based on this you might immediately conclude that Lithium is not cost effective, however useable energy compared to price is only part of the story.Lithium-ion vs AGM Battery

Weight

Most Ah ratings of batteries regardless of type are specified at the 20 hour rate. This was fine in the days of light loads, but as the number of loads and the size of loads has increased over time, we also need to look at high short term loads, medium and longer term ones for differing types of equipment. This can mean a large battery pack. At the extremes we might have air conditioning running for 10 hours using 10 kW, compared to an LED light using 100 Watts in that time. Balancing these differing requirements and all the loads inbetween becomes key. With a large pack as shown below to achieve this, it becomes clear just how heavy Lead Acid can be compared to Lithium. 1360/336 = 4 times heavier.

Useable energy: effect on discharge capacity and voltage with differing loads

As stated earlier most batteries Ah rating are quoted at the 20 hr rate. In the image below for the lead acid battery, if that were a 100 Ah battery at the 20 hr rate, you can see that 0.05C means 100 x 0.05 = 5 Amps for 20 hours = 100 Ah available until the battery is totally flat. As we use only 50% of the battery we can see that the voltage will still be 24 V at 50% DOD for a 5 Amp load over 10 hours, and therefore we would have consumed 50 Ah.

Increasing the current draw (as the graphs below show) can affect the useable energy available and battery voltage. This effective shrinkage in the rating is known as Peukert’s effect. With lead acid the higher the load, the more you need to increase the Ah capacity of your battery to help alleviate this. With Lithium however a load of even 10 times greater at 0.5C can still have a terminal voltage of 24V at 80% DOD/20% SOC, without going up on the Ah rating of the battery. This is what makes Lithium particularly suitable for high loads.

Note: In the graphs below Discharge Capacity vs Terminal Voltage is shown. Usually you will see AGM graphs as Discharge Time vs Terminal Voltage. The reason we plot Discharge Capacity (instead of Discharge Time) is that Lithium has a higher and more stable terminal voltage than AGM, so plotting the curves with Discharge Capacity in mind gives a more accurate comparison of the chemistries, showing that Lithium increases useable energy at higher loads due to higher and more stable terminal voltages. Whilst you may consider this a grey area (in part too due to the varying internal resistance of batteries also) it is probably the only true way to compare the technologies. This is further demonstrated in the images below the graphs.

Lithium – Discharge Capacity vs Terminal Voltage

Lead Acid – Discharge Capacity vs Terminal Voltage

Useable Energy (Lead Acid)

Useable Energy (Lithium)

Charge Efficiency

Much that we have seen in the discharge process is also true in the converse process of charging. Don’t be put off by the large generator sizes shown below, as this blog merely shows a range of scenarios. Solutions are scalable in principal. First let’s compare charge efficiency of Lead Acid on the left to Lithium on the right, during the complete charge cycle. Charging the last 20% of a lead acid technology battery is always slow and inefficient when compared to Lithium. This is borne out in the fuel costs (or whatever charging source you use) in the images further down. Note the difference in charge times too.

Note: Charge rates

The recommended charge rate for large size AGM batteries is 0.2C i.e. 120A for a 600A battery consisting of paralleled 200Ah blocks.

Higher charge rates will heat up the battery (temperature compensation, voltage sensing and good ventilation are absolutely needed in such a case to prevent thermal runaway), and due to internal resistance the absorption voltage will be reached when the battery is charged at only 60% or less, resulting in a longer absorption time needed to fully charge the battery.

High rate charging will therefore not substantially reduce the charging time of a lead-acid technology battery.

By comparison a 200Ah Lithium battery can be charged with up to 500A, however the recommended charge rate for maximum cycle life is 100A (0.5C) or less. Again this shows that in both discharge and charge that Lithium is superior.

Battery choices, markets and cycle life

Depending how you treat a battery you can reasonably expect the range of cycles below, subject to the DOD and the battery banks being properly sized for the loads. Operating temperature also comes into play. The hotter the battery the less time it will last. Battery capacity also reduces with ambient temperature. The baseline for variations due to temperature is 25 degrees Centigrade.

Conclusions

Clearly AGM batteries will need to be replaced more often than Lithium. It is worth bearing this in mind as this entails time, installation and transportation costs, which further negates the higher initial capital cost of Lithium as does the lower cost of recharging Lithium.

No matter what battery choice you make there is also both a capital cost and technological risk at the outset. If you are in a position of having the capital for the higher upfront costs of Lithium, you might find that life is easier and that choice is a cost effective one over time. Much of this depends on the knowledge of the operator and how they treat a battery system. There is an old saying that batteries don’t die, they are killed. Good management practices are your insurance against early failure, regardless of the technology used.

Lithium-ion vs AGM Battery? The choice is yours. Personally I think the time is right to consider Lithium in the marine industry as a cost effective, reliable, high performance solution. Last week (it was only out of curiosity you understand) I went for a test drive in a Lithium-ion powered Tesla Model S – and as we know, no self-respecting electric vehicle manufacturer would still use lead acid based battery technologies today. Time for the marine industry to catch up with the Lithium-ion vs AGM Battery debate?

John Rushworth

Credits

Thanks to Reinout Vader and Johannes Boonstra for the images and technical advice in writing this blog.

Further reading

Whitepapers, inc Energy Unlimited by Reinout Vader: https://www.victronenergy.com/support-and-downloads/whitepapers

Battery choices: https://www.victronenergy.com/batteries

Continue reading →

Why your Lead Acid Battery is all Swollen Up? Working in the solar Energy industry in Ghana, I often come across several batteries that are swollen up .These mostly lead acid batteries have often than not, been purchased at very high prices not too long ago. On this particular occasion our team was conducting a survey at a prospects home in Tema when I noticed that all of her eight 100Ah batteries were swollen.

Typically a 100Ah battery will cost between $200-$300 depending on quality .In addition to this, most suppliers in Ghana give little or no warranty even though some global brands like Victron Energy give up to two year warranty on their batteries .This article aims to reveal to the public why lead acid batteries swell-up and how to avoid the problem.

Sealed lead acid batteries – both AGM and gelled electrolyte can swell up and expand sometimes. This happens due to the construction of lead acid batteries which is referred to as “recombinant”. They are constructed in such a way to allow absorption of gasses released during the chemical process inside the battery.

The positive and negative plates are placed very close together with only the thickness of the divider separating them. They are tightly secured in the cell cavity resulting in very little extra space inside the battery. When the cell plates expand, it exerts pressure on the inside walls of the battery. This situation can cause the battery case to swell resulting in possible splits and cracks at various points of the battery.

Why Do Battery Cell Plates Expand?

The cell plates most often expand due to overcharging of the battery. The battery may also expand due to shorting of the terminals of the battery. Both these situations results in heating up of the cell plates inside the battery. The lead of the cell plates has a high expansion rate when heated.

The outcome is that the battery experiences extreme pressure inside that swells up and deforms it. The swelling-up of the battery may also cause great damage to the internal components and parts.

Why your Lead Acid Battery is all Swollen Up ,How to Avoid Swelling Up of the Battery?

Overcharging or short-circuiting of the battery is the only reason for swelling up of the lead acid battery. The problem is not inherent in the battery itself. In order to avoid swelling up of the battery you need to tackle the underlying cause of the problem.

You need to follow proper instructions in charging the battery. The culprit may be that you are using a wrong charger when charging the battery. If the charger is providing too much current, this may be the cause for battery swelling up. For instance, if you used 24V charger to charge a 12V battery it will most probably result in overcharging of the battery.

Whatever the reason for overcharging of the device, the end result is the swelling up of the battery. To avoid the prospect of overcharging or short-circuiting of the battery, you need to take the following precautions:

Use the right type of charger that is fully compatible with the battery.
Ensure proper polarity when connecting the charger to the battery
Shield the battery terminals to avoid short-circuiting of the battery
Use a charger whose maximum charging capacity is lower than the battery
Using a good quality charger

Victron Energy Blue smart charger is a good choice for small battery banks in Ghana

Battery charging tip: increase battery life with Victron 4-step adaptive charging

Victron developed the adaptive charge curve. The 4-step adaptive charge curve is the result of years of research and testing.

The Victron four-step adaptive charge curve solves the 3 main problems of the 3 step curve:

Battery Safe mode

In order to prevent excessive gassing, Victron has invented the ‘Battery Safe Mode’. The battery Safe Mode will limit the rate of voltage increase once the gassing voltage has been reached. Research has shown that this will reduce internal gassing to a safe level.Thereby preventing swollen battery condition

Variable absorption time

Based on the duration of the bulk stage, the charger calculates how long the absorption time should be in order to fully charge the battery. If the bulk time is short, this means the battery was already charged and the resulting absorption time will also be short, whereas a longer bulk time will also result in a longer absorption time.

Storage mode

After completion of the absorption period the battery should be fully charged, and the voltage is lowered to the float or standby level. If no discharge occurs during the next 24 hours, the voltage is reduced even further and the battery goes into storage mode. The lower storage voltage reduces corrosion of the positive plates.

Once every week the charge voltage is increased to the absorption level for a short period to compensate for self discharge (Battery Refresh mode)

The above tips will help you to prevent your battery from getting swollen up and expanding. Taking precautions will not only protect your battery from being damaged but it will also minimize the threat of fire caused due to overheating of the battery.

Click here for more information on Victron Energy AGM & Gel batteries