Energy Management Part 2 - Batteries

Admittedly lead acid batteries have advantages. They are quite cheap and essentially are available all over the planet. Charging lead acid batteries is dead simple, and including lead acids in yachting electrics has been true and proven for decades.

However, they do have a lot of disadvantages in a cruising scenario:

• deep cycle lead acid batteries can only be discharged to 50 % state of charge. Deeper discharge damages the battery, effectively limiting the usable capacity to 50 % of the rated value.
• lead acid batteries really like to be kept at 100 % state of charge. Storing the batteries below 100 % for a prolonged time effectively reduces the battery capacity over time.
• when charging the batteries the charging current diminishes quickly. The charge cycle starts with high charging current, but with increasing state of charge the internal resistance of the battery increases and as a consequence charging current decreases.
  This means that charging a lead acid battery from 60 to 80 % is much quicker than charging from 80 to 100 %. As a result fully charging a lead acid battery (to 100 %) takes a long time, so most cruisers stay between 50 and 80 % most of the time for efficiency reasons - which inevitably will lead to battery damage over time (see above)
• discharging and charging of lead acid batteries is subject to substantial system losses. Charging current is dissipated partly as heat, discharge current and effective battery capacity is subject to losses dependent on discharge current ("Peukert Law").
  
• lead acid service batteries typically last a few years and a maximum of a few hundred charging cycles (depending on how they are treated)

It is the combination of these properties that make lead acid batteries a less then optimal choice for energy storage on crusing yachts:

When charging a lead acid battery from an energy source that is expensive or annoying to operate (generator, engine driving alternator) it takes a lot time to fully charge the battery. The engine must be running for hours only to fill up the final 10 %, tempting the owner to stop charging before the battery is full.

Solar charging efficiency is also affected by the "diminishing returns" of a slowly filling lead acid battery bank. The battery simply cannot accept the charging current while it is available.

Lithium chemistry based batteries are widely used in many applications today and are mature enough to be used on boats.

In maritime use a variant called Lithium Iron Phosphate (LiFePO4) batteries is used which has some interesting properties: 

• very safe technology, LiFePO4 batteries do not explode or otherwise decompose in exothermic reaction even if abused heavily
• very low internal resistance
• high acceptable charging/discharing currents (1 C nominal)
• high charge acceptance rate and very low charging losses: effectively LiFePO4 batteries absorb any charging current offered at almost 100 % efficiency. This also means the batteries stay cold while charging.
• large usable capacity range: LiFePO4 batteries can be discharged to 20 or even 10 % state of charge without any problem
• long life expectance (~8000 charging cycles or about 10 years) if treated properly

There are some disadvantages, of course:

• considerably higher cost than lead acid
• very sensitive to overcharging and sensitive to deep discharging, therefore requiring special battery protection circuitry ("Battery Management System")
• special requirements to charging sources due to extremely low internal resistance

The combination of the advantages provide huge benefits to cruisers. 

However, not many boat owners actually consider Lithium batteries as a replacement for the service/house battery bank.

The question is: why is this the case?

An important reason might be that simply all the people involved with yacht electrics are so used to the simple, proven lead acid batteries that they do not consider any (more expensive and complicated) alternatives.

In addition, cruisers with their special needs for prolonged independence from shore power are not the majority. Most boaters are actually not having problems with lead acid simply because they are not away long enough from shore power to "feel the pain". Lead acid is "good enough" for most of them, I guess.

Lithium batteries are also not a drop-in replacement for lead acid, no matter what vendors are claiming. A system with Lithium batteries must be designed properly. This requires designing proper charging, distribution and battery protection systems.

Designing a good cruising boat energy management solution is not a small task, and most marine electricians will simply not be up to the task.

Well. Luckily I happen to be an electrical engineer.

Let's build a LiFePo4 based energy management system!

The following articles will outline how I designed the energy management system for our boat, for our needs. Please note that I do not claim that this is the ideal solution for everyone, in particular when staying offshore for much longer time I would certainly reconsider charging sources and possibly add additional ones.