1/ LIMITED “USEABLE” CAPACITY

It is typically considered wise to use   just 30% – 50% of the rated capacity of typical lead acid “Deep Cycle”   batteries. This means that a 600 amp hour battery bank in practice only provides,   at best, 300 amp hours of real capacity.
  If you even occasionally drain the batteries more than this their life will   be drastically cut short.

2/ LIMITED CYCLE LIFE

Even if you are going easy on your   batteries and are careful to never overly drain them, even the best deep   cycle lead acid batteries are typically only good for 500-1000 cycles. If you   are frequently tapping into your battery bank, this could mean that your   batteries may need replacement after less than 2 years use.

3/ SLOW & INEFFICIENT CHARGING

The final 20% of lead acid battery   capacity can not be “fast” charged. The first 80% can be “Bulk Charged” by a   smart three-stage charger quickly (particularly AGM batteries can handle a   high bulk charging current), but then the “Absorption” phase begins and the   charging current drops off dramatically.

Just like a software development project,   the final 20% of the work can end up taking 80% of the time.

This isn’t a big deal if you are charging   plugged in overnight, but it is a huge issue if you have to leave your   generator running for hours (which can be rather noisy and expensive to run).   And if you are depending on solar and the sun sets before that final 20% has   been topped off, you can easily end up with batteries that never actually get   fully charged.

Not fully charging the final few percent   would not be much of a problem in practice, if it wasn’t for the fact that a   failure to regularly fully charge lead acid batteries prematurely ages them.

4/ WASTED ENERGY

In addition to all that wasted generator   time, lead acid battery suffer another efficiency issue – they waste as much   as 15% of the energy put into them via inherent charging inefficiency. So if   you provide 100 amps of power, you’ve only storing 85 amp hours.

This can be especially frustrating when   charging via solar, when you are trying to squeeze as much efficiency out of   every amp as possible before the sun goes down or gets covered up by clouds.

5/ PEUKERT’S LOSSES

The faster that you discharge a lead acid   battery of any type, the less energy you can get out of it. This effect can   be calculated by applying Peukert’s Law (named after German   scientist W. Peukert), and in practice this means that high current loads   like an air conditioner, a microwave or an induction cooktop can result in a   lead acid battery bank being able to actually deliver as little as 60% of its   normal capacity. This is a huge loss in capacity when you need it most…

The above example shows specification of   Concord AGM battery : this spec states that the battery can provide 100% of   it’s rated capacity if discharged in 20 hours (C/20). If discharged   in one hour (C/1), only 60% of rated capacity will be delivered by the   battery. This is direct effect of Peukert losses.

At the end of the day, an AGM   battery rated for 100Ah at C/20 will provide a 30Ah usable capacity when   discharged in one hour as 30Ah = 100Ah x 50% DoD x 60% (Peukert   losses).

6/ PLACEMENT ISSUES

Flooded lead acid batteries release   noxious acidic gas while they are charging, and must be contained in a sealed   battery box that is vented to the outside. They also must be stored upright,   to avoid battery acid spills.

AGM batteries do not have these   constraints, and can be placed in unventilated areas – even inside your   living space. This is one of the reasons that AGM batteries have become so   popular with sailors.

7/ MAINTENANCE REQUIREMENTS

Flooded lead acid batteries must be   periodically topped off with distilled water, which can be a cumbersome   maintenance chore if your battery bays are difficult to get to.

AGM and gel cells though are truly   maintenance free. Being maintenance free comes with a downside though – a   flooded cell battery that is accidentally overcharged can often be salvaged   by replacing the water that boiled off. A gel or AGM battery that is   overcharged is often irreversibly destroyed.

8/ VOLTAGE SAG

A fully charged 12-volt lead acid battery   starts off around 12.8 volts, but as it is drained the voltage drops   steadily. The voltage drops below 12 volts when the battery still has 35% of   its total capacity remaining, but some electronics may fail to operate with   less than a full 12 volt supply. This “sag” effect can also lead to lights   dimming.

9/ SIZE & WEIGHT

A typical 8D sized battery that is   commonly used for large battery banks is 20.5″ x 10.5″ x 9.5″. To pick a   specific 8D example,  BP AGM weighs 167lbs, and provides just 230   amp-hours of total capacity – which leaves you with 115 amp hours truly   usable, and only 70 for a high discharge applications!

If you are designing for extensive boon   docking, you will want at least four 8D’s, or as many as eight. That is a LOT   of weight to be carting around that impacts your fuel economy.

And, if you have limited space for   batteries on your rig – size alone of the batteries will limit your capacity.