Battery Talk over a Beer

Last week we had a question, which turns up regularly in the interactions we have with friends and followers.

Why would you want lithium batteries, aren’t they more expensive?

Battery talk is one of the favourite topics for avid cruisers and as I usually just listen, I have been able to gauge a few strands of knowledge along the way... not at all an expert though, so this blog will simply answer in a basic way the questions we received.

Nearly a year ago now we replaced 8 Lifeline Absorbent Glass Matt (AGM) batteries with 2 Lithium Iron Phosphate (LiFePO4) batteries on Impi.  At the moment we are doing a bit of an evaluation of the system with the help of Jack Peters, Outback Marine, Southport.

So this blog will explore the basic differences between AGM and LiFePO4 batteries, more in particular the batteries as sold by EV Power on the West Coast of Australia. It is aimed at people with little knowledge of batteries and is not trying to educate electronics engineers!

Let’s take a hypothetical case of a boat where the owners want to have 300 ampere hours (Ah) per day available to use. 

In theory a 300 Ah battery pack can deliver 300 amperes (amps or A) for 1 hour or 10 amps for 30 hours.  In practice however if you let an AGM battery discharge to more than 50%, the number of cycles expected in the life cycle of the battery, reduces.  It is even better to stay over 60% meaning that you can utilize 40% of the total capacity before recharging. 

Looking at the above graph you can see that the expected cycles for an AGM battery discharged to 60% is 800.  In order to have 300 Ah available without recharging your battery you need to have a capacity of 750 Ah.

A LiFePO4 battery can safely be discharged to 80%, but let’s just say we keep it conservative and discharge only to 70%.  This means that to discharge 300 Ah from the battery, without recharging, you need a 375 Ah battery pack.  The expected number of cycles with a depth of discharge of 80% is 2000 cycles. Discharging to 70% should give you slightly more cycles but let’s keep it at 2000 cycles as stated in the specifications below.

Let’s compare the cost of the purchase of the batteries excluding the installation cost per cycle.

AGM		Batteries Direct Australia
300 amp usable	750 Ah pack	5*150 Ah batteries
	Cost for bank	5*$937 = $4685
	Weight	5*43.50kg = 217.5kg
	Cycles	800
	Cost per cycle	$5.86
		Outback Marine Southport
	750 Ah pack	5*150Ah batteries
	Cost for bank	5*$936=$4680
	Weight	5*43.50kg = 217.5kg
	Cycles	800
	Cost per cycle	$5.85

And for the LiFePO4

LiFePO4	EV Power
375 Ah pack	400Ah pack
Cost for bank in 2016/17	$3,200
Weight	52kg
Cycles	2000
Battery Management System	$484
GST	$369
Transport	$405
Total cost	$4,458
Cost per cycle	$2.23

In this comparison, it seems that excluding the installation costs, the LiFePO4 are less than half the price of the AGM batteries per cycle at a quarter of the weight. They also take significantly less space.

Now we have all heard stories of exploding lithium batteries, so are we doing something dangerous by installing these batteries?

There are several types of lithium ion batteries and it has been proven that the LiFePO4 are way safer than other types of lithium ion batteries such as for example Lithium Ion Polymer batteries.  Reasons for this are a higher thermal and chemical stability, safer cathode material and more structurally stable cells.

LiFePO4 cells are much harder to ignite in the event of mishandling, however in order to prevent failure of the battery it is important to have a Battery Management System (BMS).

What does the BMS do?

A 12V LiFePO4 battery is made up of 4 cells of which the optimum charging voltage is 3.55V for each cell.  So for the complete battery the optimum charging voltage is 14.2 V in absorption charging and 13.6 V in float charging.

It is important to understand the difference between bulk, absorption and float charging for batteries.  Simply explained the different stages of charging explain the ability of a battery to absorb charge. AGM batteries have 3 stages of charging: bulk, absorption and float. LiFePO4 have also 3 stages however the absorption stage is very short which leads to some sources saying that there are only two stages.

Bulk charging means that an AGM battery accepts as much amperage as your charger is rated for.

The number of amps going into the battery bank should however not be greater than 25% to 30% of the capacity of your battery. For example the maximum amps going into a 750Ah AGM battery bank should not be greater than about 180 amps to 220 amps. There is some disagreement on this depending on the sources.  We have found in practice however that in our AGM installation, due to battery efficiency it tended to be towards the lower number.  Charging at 180 amps to 220 amps can be achieved by having 2 Victron chargers wired in parallel.  This is the installation we have on Impi.

In the case of LiFePO4 in bulk charging the voltage increases steadily whereas the current is close to the maximum the charger can output. The battery accepts about 100 to 110 amps from a single 120A Victron charger or up to 220A from 2 chargers wired in parallel. Here we can charge a battery pack with the amperage of about half its capacity.  

During bulk charging the voltage increases up to the absorption voltage while your charger is delivering the maximum current that it can. For LiFePO4 this voltage should be 14.2V and definitely no higher than 14.6V.  For AGM batteries the voltage is between 14.3 and 14.8V.

LiFePO4 batteries have a very short absorption stage during which the voltage remains at 14.2 but the current decreases. However for AGM batteries the absorption stage kicks in when the battery is around 80% full. During this stage the number of amps absorbed, declines. The absorption stage ends when the number of amps going into the battery falls below a preset number. This stage takes more time.  The last remaining 20% of an AGM battery take much more time in comparison with the first 20% during the bulk stage assuming you started at 60% depth of discharge.

The batteries achieve float stage when they are at about 95% charged depending on the chargers. In float the voltage drops for AGM batteries to 13.2V and for LifePO4 to13.6V. Only a trickle charge is absorbed into the batteries until they are 100% full.

Most AGM batteries on boat installations also require regular equalization which is basically an overcharge for a limited time to avoid sulphation of the battery plates and which equalizes the strength of the electrolyte. This is not required for LiFePO4 batteries.

From this explanation it is clear that LifePo4 batteries have the following advantages:

Greater charging efficiency

Faster charging

No equalization

The draw back with LiFePO4 batteries is that even a single over charge or over-discharge can seriously damage your batteries. LifePO4 batteries also require cell balancing to maintain a consistent state of charge across all the cells of a battery. These roles are assumed by the BMS.

For the LiFePO4 batteries, overcharging is when the charge is greater than 15V or with a safety margin, greater than 14.6V and over-discharging is when the voltage is less than 10V or with a safety margin, less than 11.0V. 

So your Battery Management System will shut down the batteries in case of over charging or over-discharging. 

The EV Power Battery Management System consists of two parts:

The Battery Control Unit (BCU) – one per battery pack of 400AH, it monitors the battery voltage and the cell module loop and takes action to prevent charging and discharging if there is a fault.

The Cell Modules – one per cell act as stand alone shunt balances and link together to provide cell voltage monitoring.

It is also important to check that the voltage of your input currents remains below 15V. This role is assumed by the Victron inverter chargers for shore power and generator. The regulators enable us to set the voltage of the current delivered by alternators and the MPPT controller ensures that the solar power current is within the limits acceptable by the batteries.  These devices have settings for different batteries including LiFePO4 batteries.  There are other brand chargers however which would not be compatible.

So a boat converting from AGM batteries to LifePO4 can continue to use existing charging systems if they have a similar set up to Impi. 

Our installation was done by Overkill Electrics (Pittwater) at a cost of about AU$5000.  This included all additional parts such as relays, two 175A Prestolite alternators, brackets to fix them onto the engines and their installation.

We are conducting a full evaluation at the moment with the assistance of Outback Marine (Southport) who provide a similar installation using the same LiFePO4 batteries.

We hope to communicate our findings in the new year but so far things are looking good! We are particularly impressed with the lack of voltage drop when using several electrical appliances even when the battery is discharged more than 50%.  So watch this space for a full evaluation.

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