Update June 2017

Battery charging and battery chargers are often misunderstood – causing batteries to die before their time. This article explains why and how to avoid it.

Batteries charge by applying a voltage that is higher than that existing. The greater the voltage difference, the faster and deeper it will charge. That voltage must, however be tightly controlled. If, too high, it damages or wrecks batteries.

Historically, vehicle alternators generated 14.2-14.4 volts. Cheap battery chargers still do.

As the battery charges, its voltage rises towards the charging voltage. The voltage difference between the battery and the charger thus constantly reduces. Charging rate falls accordingly.

Battery charging and battery chargers – like filling one tank from another that’s much bigger

Charging is like filling a small tank from a huge one (of similar height) via a hose between the bottom of each. The water level in the small tank slowly rises until levels equalise. As with ponds, alternators need not know battery state of charge. The charging battery simply rises in voltage. As it does so, charging tapers off. Eventually, voltages are equal. Charging then ceases.

Many RV batteries charge this way. They take many hours to fully charge. Most never do. Given many days continuously, however, they may even overcharge.

Battery charging and battery chargers – starter batteries

A starter motor draws surprisingly little energy. Following engine starting, the alternator replaces it within two to three minutes. Such charging is crude but cheap and simple. It works well enough for starter batteries, but less so for RV auxiliary batteries. These are limited to slow charging. Few reach full charge.

Constant current charging

Serious battery charging is done at constantly increasing voltage. This maintains a constant rate of charge current throughout 80-90% of the charging cycle. A final stage is usually done at constant voltage. There are variations. All, however, work much as below. Conventional lead acid, gel cell and AGM batteries are similarly charged.

Lithium-ion batteries, however require a different regime. This is described later in this article).

Battery charging and battery chargers  – boost stage

The initial ‘Boost’ stage constantly increases charging voltage as battery voltage rises. Its intent is to keep charging current at the battery’s safe maximum. For a lead acid deep cycle battery that’s typically 20% of its amp/hour capacity. For large batteries, that limit may the charger’s ability to do so.

Boost typically continues until battery voltage reaches about 14.4 volts. That battery is nevertheless not yet fully charged.

Battery charging and battery chargers  – absorption stage

Battery charging is an electro-chemical process. Like many such, it is slow. The charge, in effect, is held within the water/acid electrolyte. At this stage, however, the ‘charge’ is uneven. It is concentrated in and around the battery’s plates. Evenly distributing the charge requires ‘absorption’.

Absorption is typically at voltage ensuring charge current is about half that previously. It typically require two or so hours.

Battery charging and battery chargers  – float stage

Following Absorption, charging current reduces such that it counterbalances battery internal losses. This stage is called Floating. It is 13.2-13.6 volts for AGMs and gel cells. Conventional lead acid batteries require 13.6-13.8 volts.

As with the Absorption stage, charging revert to Boost if battery voltage drops. This may happen if there’s a heavy load.

Keep lead acid deep cycle batteries as fully charged as possible. Their life is otherwise shortened. If an RV is unused for more than a week or two – keep its batteries on Float charge.

AGM batteries, however, hold 50%-60% of their charge for a year or more. Whilst rugged, even minor long-term overcharging damages them. Unused AGMs need to be initially fully charged – then only after 6-12 months. Do not leave them on ‘float charge’. It may ruin them.

Battery charging and battery chargers  – equalising

Some chargers have (usually optional) ‘Equalising’. This heavily overcharges the battery for an hour or two.

The original idea was to equalise cell voltage. Technology changes, however, render it unnecessary. Most battery makers now recommend against it. Never do it AGMs, nor gel cells. Nor, in my opinion, with any battery.

Different battery types require different voltage/current settings. All good quality battery chargers are programmable accordingly. Currently, only a few have programs for LiFePO4s. See ‘Lithium-ion battery charging’ below.

Caution when buying a battery charger

Always use a high quality multi-stage charger. Cheap ones sooner or later wreck costly batteries. A multi-stage charger brings a battery up to charge rapidly, deeply and safely. A 10 amp multi-stage charger will thus outperform almost all ’20 amp’ conventional chargers. And many a ’25 amp’ cheapie. Good chargers start at about $250.

Lithium-ion battery charging

A lithium-ion (LiFePO4) cell is nominally 3.2 volts. A 12 volt such battery thus has four such cells. Charging is typically at constant current. It requires 13.2-13.6 volts. This charges the battery to about 80%-90%. Many users settle for about 80%.

It is vital that each LiFePO4 cell maintains equal voltage. Ensuring this requires cell management. This also prevents current draw below a preset state of charge. These systems are available from LiFePO4 vendors. They may not, however, be included with the battery. It is essential one be used.

LiFePO4 state of charge

LiFePO4 state of charge is difficult to assess by measuring voltage. A 100% charged 12 volt LiFePO4  battery may be 13.4 volts. In typical RV use this drops to 13.1-12.9 volts at 90% or so charge. It’s then virtually constant until 10% remaining. It then drops rapidly.

Some charger makers include a final voltage charge. This brings a LiFePo4 close to 100%. Many users, however, claim this shortens battery life. This may well be so. Reliable evidence, however, is not readily available. See Lithium ion batteries in caravans for an overview.

Solar Regulators

Good (plus $275) solar regulators have multi-stage charging. The better ones include MPPT (multiple power point tracking). This recovers 10%-15% of energy otherwise lost.  

Battery charging and battery chargers  – further information

See also DC-DC charging – also Speeding Battery Charging from a Generator and also Lithium-ion Batteries in Caravans

If you liked this article you will like my books on RVs and solar. Batteries and battery charging are covered in  depth in Caravan & Motorhome Electrics. Solar That Really Works! is for cabins and RVs. Solar Success relates to homes and properties. See also the Caravan & Motorhome Book and the Camper Trailer Book.