LifePo4 Batteries

Our New Lithium (LifePo4) Batteries

We had wanted lithium batteries since we bought the boat, but had to put lead acid in as an interim while we learned what we changes to make to the boat to manage and control them appropriately.  Lithium Iron Phosphate (LifePo4) is the safest of available Lithium Ion technologies and is NOT prone to fire like some of the others.  It is regarded as the most appropriate type to use on a boat or in a caravan.

Our house batteries were Trojan lead acid batteries, with 900Ah capacity.  They weighed 250kg and took up a huge amount of room.  The other issue was the voltage sag.  If we tried to run a load on the inverter the voltage would drop below 12v.  They also took a long time to charge.  They have to sit at the absorption stage for 2 hours everyday at 14.8 volts.  If they don't get fully charged at least a few times per week they can fail prematurely.  Lithium batteries don't mind being a bit undercharged, if anything they last better if not fully charged each day.

This means that the voltage can vary between 11.7 and 14.8 volts.  This can impact on voltage sensitive equipment like our variable speed water pump.  

The voltage variations upset the watermaker as well.  It is a simple unit with manual pressure control.  The change of voltage from going from float to absorption and back again is enough to change the pressure by 50 to 100 psi.  LiFePo4 would also give us the option to run the watermaker when sailing or at night, knowing that it won’t matter if we are not fully charged.

Even the electric toilets pump to the overhead holding tanks better when the voltage is high. 

The fairly flat voltage curve of LiFePo4 will prevent these issues.

Changes To The Electrical System

Lead acid batteries are fairly tolerant to charging or being left on float for a long period of time.  LiFePo4 have a more precise charging regime, and can not be left at high float voltages for a long time.  When motoring with standard alternator setups the batteries would be held at 14.0 volts for hours.  Battery chargers with too high settings for absorption or float can also damage LifePo4.  All power sources must also be controlled.  

Solar Controllers

VE Direct Smart  Network

The first thing we did when we bought the boat was add an extra 1000 watts of solar panels and a programmable Victron MPPT 150/100 solar controller.  This is now our primary means of providing power to the boat.  We had 200 watts of solar already with an old solar controller that we used for the first few years.  We now have a smaller Victron 100/30 controller for the original 200 watts, plus we have added an extra 120 watts to feed into this new unit.  Both of these MPPT controllers are supplied with the exact battery voltage from a bluetooth link to a Victron BMV700 battery monitor.  This ensures they both see the same voltage at the battery rather than the lower voltage caused by voltage drop through their supply cables.

The Victron Solar controllers are easy to change parameters as they are controlled by a bluetooth connected smartphone app.  Solar is our primary source of charging, and the boat is a liveaboard.  Victron have recently updated their firmware to give a lot more control of the charging parameters.

 The default LifePo4 settings from Victron are Absorption at 14.2 volts for 2 hours, with float of 13.5 and a rebulk voltage of offset of 0.1 volts.

These settings keep the LifePo4 batteries at a very high state of charge, something they don't seem to like, and can shorten their life and reduce capacity.

Some people set absorption as low as 13.8 volts.  Some manufacturers still recommend absorption at 14.6 volts.  As our cell balancers need a cell voltage of above 3.5 volts to start balancing then we need to get to around 14 volts as we have 4 cells in series to make our nominal 12 volt system.    So in order too ensure the cells see some balancing we charge to 14.2 volts but with only a 5 minute absorption time.  At the end of this time the current has dropped to about 2% of the battery capacity, that is 12 amps for a 600 Ah battery.  This gives the batteries about 10 minutes or so above 14 volts to do any balancing.  Hopefully it will be enough.

We have float voltage at 13.4 volts which is very close to the resting voltage of a fully charged battery.  A float voltage of 13.6 volts will hold the batteries at or above 100% state of charge which is undesirable.  At the end of the absorption period the batteries drop to float.  The battery state of charge rapidly drops until the battery monitor reads about -7Ah consumed.

Victron have recently enabled the ability to set a rebulk voltage on their MPPT controllers.  The default for their LifePo4 profile is 0.1 volts below the float.  This means that when a load is applied and the batteries drop 0.1 below the float they will then return to bulk charging and absorption.  We found that every time we used a high current appliance, such as a 1000 watt kettle, the batteries would charge again.  We wanted to stay away from constant high state of charge so this value has been lowered to 0.5 volts.  When the high current appliance has finished, the solar will still put most of the amp hours back in while in float, until it settles at about -7Ah again.

We have also used the equalisation setting to allow an extra charge period if it is desired to give more balancing time or to fully charge before an overnight passage.

If the boat is to be left without any loads running for a length of time the MPPT controllers can easily be adjusted to keep the batteries at a lower state of charge to improve their longevity.

Inverter/Chargers

Our boat came from the the US and had 110 volt systems.  The orignial Trace inverter/charger has only limited voltage control and float charges by turning pulsing on and off.  This is the worst possible way of floating as it means the voltage goes up and down.  Currently we still have this unit but no longer use it for charging except if needed for an hour or two with our 110v generator.  In a marina it is turned off and not used to float charge.

Last summer we fitted a new Victron Easyplus 1600 VA inverter charger in addition to the existing 110v system.  Again as with most of the Victron equipment this unit is fully programmable and lithium compatible.  We used this unit as it simplified the installation as it comes prewired with an RCD and breakers.   The Victron Inverter/Charger has been changed to have very conservative charging parameters.  It will now only charge to 13.6 volts.  Unfortunately it can not be programmed for less than 1 hour of absorption.  At 13.6 volts and 1 hour of absorption the batteries will be about 95% charged.  It requires a computer and cable to change the parameters, so by being conservative it shouldn't have to be changed if the boat is left unattended.

Alternators

The alternators are more of a problem, but unlike most boats, because we have so much solar we don't rely on the engines much for charging.  Because we are a catamaran we have two engines.  Each one had a diode splitter that divided the charge between the start batteries and the house batteries.  

The ideal solution is to fit large alternators with external regulators and heavy duty serpentine belts.  On some engines it is fairly easy but on our Yanmar 3 cylinder 3JH3E engines it requires a serpentine belt kit, external regulator and new alternator. About $4000AUD per engine!  Condsidering that we don't need much from them, the decision was to protect the batteries from them rather than optimise their output.

Victron BMV700 relay control settings

The diode splitters have now been replaced with Victron Cytrix Li Ct relays.  These only close to supply power when either the house or the start batteries are above 13.4 volts. The port engine also has a manual battery switch to enable it's battery to be used to run the house if the LifePo4 batteries are turned off for maintenance or if the boat is left with no load.  This will prevent unnecessary cycling of the LifePo4 batteries.  But we needed to stop the problem of overcharging by floating for hours at 14 volts if motoring for a long time.  

To control the alternators we use the relay on the BMV700 to disconnect the positive wire to the Cytrix relay causing it to open and stop charging the house batteries.  The relay is programmable to open on state of charge.  The BMV relay is a just a switch and requires a 12 volt feed on one side, which when the relay is closed supplies power to the Cytrix relay.  As it is currently programmed, when the BMV700 reaches 95% state of charge the relay will open, stopping the charge to the house battery. 

BMV700 syncronise to 100% settings

This also allows the alternators to contine to charge the start batteries to 14.2 volts to fully charge them.  It will resume charging the house batteries when the charge drops below 90%.  These figures are all user configurable.  There is also a switch to turn the relay off, preventing charging to the house batteries.  The victron units can be configured by smart phone app via bluetooth, which makes it much easier than pressing tiny buttons in sequence, the alternative way of programming the BMV700.  When the solar is charging the house bank it will also float charge the start batteries as the Cytrix relays work in both directions.

To ensure that the relay opens at a safe charging level the BMV700 has been adjusted to reset to 100% when the voltage is above 13.8v and the tail current is less than 3%.  This means that the net amps going into the battery is 3% of the 600Ah battery capacity, that is 18amps.

The downside to this system is that the existing 90 amp alternators only put out about 40 amps each into the house bank.  With an external regulator they could put out more initially and then back off to perhaps 60 amps each.  They also have no real protection against overheating.  LifePo4 batteries have very low resistance and will draw a lot of current when charging.  But as they are in parallel with the start batteries this shouldn't be an issue unless the LifePo4 batteries are heavily discharged.  

 LifePo4 Batteries

EVPower 300Ah Pack using GBS cells.

We looked at lots of options for LiFePo4 batteries.  Some might say it became a bit obsessive.  In the end we decided to go with a quality Australian supplier using cells made into packs and ready to go.  These are perhaps half way between some of the so called drop in batteries and the DIY from scratch that others have done.  In the end we live on the boat and couldn't afford to be without our batteries for too long.  Carefully balancing individual cells in series for 3 days might be ok if you have a workshop at home, but we wanted a quick changeover.  

We bought two of the 300AH packs from EV Power in Western Australia as it was the best use of space in our battery compartment.  At 46kg, they were the right size and weight for once person to lift in to place.  We had 900Ah of lead acid batteries, but LifePo4 can be drawn further than lead acid so have a more useable capacity.  600Ah seemed about the right size.  300Ah would not be enough as we wanted to avoid drawing them down too much on a regular basis.  We use 120 to 150Ah overnight, so drawing down to only 75% state of charge on a regular basis should give a very long life.  The full capacity can be used on occasion without any huge impact.

Unlike lead acid batteries, LifePo4 batteries don't self balance between their cells.  So some electronics are required to prevent imbalance.  Our  EV Power packs came with LFP 08 Cell Balancers which are passive balancing circuits mounted on top of the pack between each cell.  These units bleed a small amount of current off when the cell voltage goes above 3.5 volts, or about when the battery voltage gets above 14.0 volts.  The amount of current bled off is proportional to the voltage.  So if one cell gets too high, it will bleed off more quickly until all cells are close in voltage.  They seem to balance well at  a pack voltage 14.2 so there is little need to go higher where imbalance issues can become a problem.  At 14.3 volts some of the red LED indicators came on as the cells approached 3.6 volts to show that some balancing was taking place.

Extra storage space!

 As can be seen in the photo, we now have extra storage space as the batteries take up less than half the space.

This space can now be used for storage and for a new future larger inverter located close to the battery.

Installing the wiring and the first tie down strap.

 Each pack has its own Battery Control Unit (BCU).  Normally this has a push button and  indicator light on the front but as these ones are hidden in the battery compartment they have been relocated inside to the navigation station.

Each battery has a 250 Amp circuit breaker before the BCU.  Each BCU then feeds into the boats existing wiring.

In the event of a fault, either pack can be run on it's own.

Protective cover to prevent accidental knocks

The batteries sit underneath the existing green timber battery compartment cover.  A small frame was built in front of the batteries to protect them from anything that moves in the storage area.  The 250amp circuit breakers are still easily accessible without removing the cover.

Does it Work?

First impressions are certainly positive.  The batteries charge very quickly, and hold their voltage overnight even when using large appliance. 

The ability to get up and make a piece of toast in the electric toaster without having to wait for the sun to hit the panels is nice.  The batteries just stay above 12.9 volts even on heavy current draws.  Having the variable speed water pump behave itself is nice too.  We have shed 140kg from the back of the boat and the transom steps are now out of the water.