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Collyn Rivers Discusses Lithium Batteries - AAEN

Thursday, 26 February 2015

 

Lithium Batteries

Collyn’s office

 

Lithium batteries are far smaller and lighter than lead acid batteries of similar capacity, but apart from both having positive and negative electrodes and electrolyte, they are almost totally different.

They were first offered in locally made RVs by Kimberley Kampers in 2012. Off-road caravan builder Bushtracker claims that 98% of all buyers now specify a lithium battery system. There are also a number of DIY systems (of varying success).

 

Lithium Battery Types

The original LiCoO2 (lithium cobalt oxide) batteries store a lot energy for their size and weight. In early 2013 however, one ignited in a (empty) Boeing 787, and another two days later. All were grounded - and Boeing ceased deliveries. Fires also occurred in electric and hybrid cars.

 

As these incidents were widely reported many assumed lithium batteries were affected. This is not so. Those in RVs and boats etc. use the later developed LiFePO4 (lithium iron phosphate).

 

LiFePO4 Batteries

LiFePO4 batteries are larger and heavier than LiCo02 batteries but are still only 25% of the volume and weight of any battery of similar rated capacity.

 

Each is built from nominally 3.2 volt cells - four provide 12.8 volts at about 97% charge falling to about 12.7 volts at 20% SoC and then falling in a knee-like curve.

 

If fully discharged, most such batteries cannot be recharged. Usage must be automatically curtailed (typically at 20% SoC). All must have a system that monitors individual cells, and also disconnects the load below a preset voltage (see Battery Management Systems later in this article).

 

The better known brands currently cost several times more than a lead acid deep cycle battery of the same nominal Ah capacity) but offset by a typical 2000 cycles from approx 100%-20% SoC.

 

LiFePO4 batteries can supply huge current. A 100 Ah unit readily churns out 200 amps or more with no effect on its life span - nor causes extra capacity loss in doing so. Its Peukert’s constant (an indicator of energy lost through internal resistance to current drawn) - is far less than other batteries (e.g. under 1.1).

 

Such low internal resistance also enables charging efficiencies of plus 90%. Makers typically claim 92.5-95%. (That of deep cycle lead acid batteries is about 80%.)

 

Battery Management Systems

The industry’s use of this term confuses as it denotes something vaguely similar but not identical to BMS in general.

 

When a LiFePO4 cell is fully charged, input current ceases. This limits overall capacity of those series-connected unless all are equally balanced. Individual cell management is essential but must be done at the battery. Control of the charging/ discharging current/voltage levels too is essential.

 

These functions are included in some commercial LiFePO4 batteries.

 

DIY Systems

There are a number of RVs (mostly motor homes) that have LiFePO4 battteries self-assembled by their owners from individual cells. They have a boughtin or self-made battery management system that whilst simple and effective are often specific to the battery used.

 

Many such systems are charged at about 13.8 volts, (80% or so charge). If so, problems are less likely, but far from common agreement on charging causes problems if working on such systems.

 

Charging from 230 Volts

Providing the battery’s management system is well designed and installed, there are no big problems in charging from a high quality 230 volt charger. The industry is mostly adopting constant current to 80-90% (SoC) at about 50% of the Ah capacity. Some settle for about 90% but most add a constant voltage stage for the final few per cent.

 

That 50% of AH current typically requires about 13.6 volts at 10-15% SoC, slowly rising to 14.4 volts at 90% SoC. LiFePO4 charging totally ceases at about 16.8 volts but most charger makers’ cut off at 14.65-14.7 volts.

 

That cut-off voltage determines the SoC but, as that required for 99% may be only 0.01 volt higher than for 95%, it is not practically feasible to accurately assess a LiFePO4 battery’s final SoC by measuring its voltage.

 

LiFePO4 battery makers and makers of RV lithium battery chargers stress that tight control of final charging voltage is crucial (as is the minimum charge).

 

Sydney based GSL Electronics’s 30 and 60 amp LiFePO4 (18-95 volt dc input) solar chargers use constant current for most of the charge, and tightly controlled constant voltage thereon.

 

Even with the facilities at his disposal, GSL’s design engineer Daniel Rubinstein makes no absolute claims for final charge. It’s almost certain to be around 95% but, Daniel says, ‘there are too many variables to make engineering-based claims closer than that’.

 

Some lithium battery makers say that most two-stage high quality chargers can be used, but warn against any with automatic desulphation (the voltage is too high). They also advise against chargers of lower final voltage as that may preclude reaching full charge.

 

Used as above most LiFePO4 makers suggest typical battery life is 1500-2000 cycles - two to four times that of lead-acid batteries discharged only to 50%.

 

Allowing for discharging to 20% SoC, a LiFePO4’s capacity is a realistic 75-80%. As that is delivered at close to constant voltage that energy is greater than from the same rated capacity lead acid battery. This is clear if calculated on the (correct) watt hour basis (not simply amp hours).

 

A LiFePO4’s almost constant voltage assists RV owners: there are no issues of fridge performance dropping, nor lights dimming as the battery discharges. (Lead acid RV batteries are rarely fully charged so their effective capacity (at 5% discharge rate), is well under half their nominal rating.)

 

LiFePO4 batteries run better at higher temperature. Their capacity is increased by up to 10% due to higher lithium ionic conductivity. Caution is nevertheless advisable for under-bonnet mounting as heat may affect the inbuilt battery management systems.

 

Alternator Charging

Some LiFePO4 resellers are alleged (by RV owners) to claim ‘normal alternator charging is fine’ - but ‘normal’ alternators ceased to exist around 2000 - outputs thereon varied from 14.7 volts plus to 12.7 volts (some with load).

 

As a LiFePo4 will charge at 14.4-14.6 volts some charger makers suggest their products are suitable for their charging (as long as the battery has full internal management) but as with all RVs it’s preferable to use a dcdc alternator charger designed for the purpose.

 

REDARC's LFP 1225 and 1240 vehicle chargers use constant current, plus constant voltage to attain the last few per cent charge. The units switch from one mode to the other by monitoring current acceptance and voltage. Timing parameters preclude premature switching as minor loads vary.


The units can be installed up to accept alternator input whilst driving and solar whilst not.

 

Company CEO Anthony Kittel advises that ‘both units achieve 100% SoC for the batteries tested.’ Mr Kittel adds that, ‘once installed they continue to perform a maintenance charge as the vehicle is driven.’ This reverts to constant current charging when a load is applied.

 

REDARC stresses its units must be used only with LiFePO4 batteries that have an inbuilt battery management system that includes under- and over-voltage cell protection, cell balancing functions and able to handle the respective 25 and 40 amp charge current. Redarc advises to contact them if in doubt.

 

CETEK has a lithium (XS) charger (but limited to 5 amps at a maximum 14.4 volts). Sterling (UK) has a range of chargers that include a lithium-ion charging cycle. This article is copyright (2015) Caravan and Motorhome Books, Church Point, NSW 2105.

 

Collyn Rivers’ main books are Caravan & Motorhome Electrics, and Solar That Really Works.

 

They are available to auto electricians and TAFE colleges at the discount shown (right). Editorial and ordering details are on our now all-new website caravanandmotorhomebooks.com - or via telephone - 02 9997 1052.

 

Click here to see the full article

We would like to acknowledge Collyn Rivers and AAEN (www.aaen.com.au) for kindly allowing reproduction of the article from the Feb/March 2015 edition.