Major update March 2016

Battery charging and battery chargers is often misunderstood, resulting in batteries caused to die before their time. This article explains how batteries are best charged and what battery charger to use.

All batteries are charged by applying a voltage across them that is higher than the battery’s existing voltage. The greater the difference between the charging voltage and the battery’s existing voltage, the greater the charging current that will flow, and the quicker the battery will be charged. That charging voltage must be tightly controlled. If it is too high it will damage or wreck the battery.

Historically, vehicle alternators and (still made) low-price battery chargers generate a more or less fixed 14.2-14.4 volts for charging lead acid and AGM batteries. As the battery charges, its voltage slowly rises towards that fixed voltage. As it does so, however, the voltage difference between the battery and the charger constantly reduces and the charging rate falls accordingly.

It’s like filling a small tank from a pond of exactly the same height by running a hose between the bottoms of each. The water level in the small tank will slowly rise, slowing continually until the water level reaches that of the pond. Just as with the water example the alternator does not need to know (and usually has no way of knowing) the battery’s state of charge. The charging battery simply rises in voltage. As it does so, the charging rate tapers off until its voltage approximates the alternator(or cheap battery charger) voltage. Then charging finally ceases. RV batteries charged this way (which is most of them) can take many hours to fully charge. Most never do. Given several days continuously however they may even overcharge.

The starter motor is designed to work at about 70% battery charge. The energy drawn whilst starting is typically replaced by the alternator within two-three minutes. This crude but cheap and simple system works well enough for starter batteries but less so for deep cycle auxiliary batteries. These too are limited to slow charging. Many never reach anywhere near full charge.

This charging has been known for over 100 years. Since the early 1900s, serious battery charging has been done by charging at a constantly increasing voltage. This maintains a constant rate of charge throughout 80-90% of the charging cycle. A final stage is usually done at constant voltage. There variations of this but all work much as outlined below. All aim at ensuring a battery is not charged too quickly or beyond safe levels. Conventional lead acid, gel cell and AGM batteries are charged in similar ways. Lithium-ion batteries require a different regime (described later in this article).


Multi-stage charging

Since charge rate depends on the difference between the charging voltage and the battery voltage, the first ‘Boost’ stage constantly increases the charging voltage as the battery voltage rises. It attempts to keep charging current at the maximum rate that is safe for that battery. For a lead acid deep cycle battery that is typically 20% of the battery’s amp/hour capacity. In many cases the limit will be the charger’s ability to do so. The Boost stage typically continues until the voltage across the battery’s terminals reaches about 14.4 volts. Whilst this is the voltage of a charged battery, the battery is not yet fully charged!

Battery charging is an electro-chemical process and, like many such, can be slow. The charge can in effect be seen as held within the water/acid electrolyte. At this stage the charge is concentrated in and around the battery’s lead plates (so it’s only 14.4 volts around there. To enable the charge to be evenly distributed, the charger moves on to a so-called ‘Absorption’ stage. This is typically done at a fixed voltage during which charging current is about half that of the previous stage and takes about two hours.

Following Absorption,  the charging current is reduced to that which just counterbalance the battery’s internal losses. This is called the ‘Float’ Stage and, depending on ambient temperature and battery type, will be from 13.2-13.6 volts (AGM/gel cells) to 13.6-13.8 volts (conventional lead acid batteries). As with the Absorption stage, the charger will revert to Boost if the battery voltage drops because of a heavy applied load.

Lead acid deep cycle batteries need to be kept as close to 100% as possible. Their life is shortened if left other than that. If the RV is not used at least every month or so it is advisable to keep its batteries on Float charge.

AGM batteries hold 50%-60% of their charge for a year or more. As these are damaged by even minor long-term overcharging many AGM makers recommend they be left fully charged – and again only after 6-12 months. But not to leave them on ‘float charge’.

Some chargers also have (usually manually selected) ‘Equalising Cycle’. This cycle overcharges the battery for an hour or two. It is the lead-acid equivalent of giving the rig a good thrash down the expressway to blow the carbon out. Few battery makers recommend it (and particularly not for AGMs or gel cells). My own advice is never to even think of using it.

Different battery types require different voltage/current settings. All good quality three-stage battery chargers have programs for standard lead-acid, sealed lead acid, gel cell, AGM batteries etc. A few are beginning to for lithium-ion batteries that have a full inbuilt battery management system. See ‘Lithium-ion battery charging’ below.


Caution when buying a battery charger

Those who know about battery charging will advise buying a high quality multi-stage charger. But time and again people will suggest  ‘there’s no need to pay the $350-$1000 that XYZ is talking about – ‘Joe’s Hardware has the equivalent for a third of that price.’ It never has. If it had, companies like Ample Power, Cetek, Latronics, SEA, Selectronic, Redarc, Victron, Xantrex etc would rapidly be out of business.

A multi-stage charger brings a battery up to charge very much faster (and deeper) than does a conventional charger. As a rough guide, a 10 amp multi-stage charger will outperform almost all ’20 amp’ conventional chargers and many a ’25 amp’ cheapie. The apparently much higher prices of multi-stage chargers are far less than they appear. Good ones start at about $250.


Lithium-ion battery charging

A lithium-ion (LiFePO4) cell is nominally 3.2 volts. A 12 volt battery thus has four of these cells connected in series. Charging is typically at constant current – requiring 13.2-13.4 volts. This will charge the battery to about 80%-90% charge. Many users do not attempt to exceed that. Many settle for about 80%.

Unlike most other batteries, it is vital to ensure that each cell of a LiFePO4 battery maintains equal voltage. All must have cell management system that does just that. The system may also ensure that charging voltage is not exceeded – nor allow the battery to be used below a preset state of charge. This is often available from the battery vendor but not necessarily within the battery price. It is totally essential it be included.

The actual state of charge of a LiFePO4 battery is difficult to assess by voltage measurement. When 100% charged a 12 volt such battery may be 13.4 volts. In typical RV use this drops to 13.1-13.2 volts at 90% or so charge. It then remains virtually constant until some 10% charge – when it rapidly drops.

The industry however suggests (and some charger makers include) a final charge (at higher voltage) that brings the battery close to 100%. This is claimed however by many DIY users to substantially shorten battery life. This may well be so but reliable (non-anecdotal) evidence is not readily available. See for an overview of the technology.

The chargers referred to so far are mains (110/230 volt) operated.


Solar Regulators

Good quality (plus $275) solar regulators have multi-stage charging as  described above. The better ones include MPPT (multiple power point tracking) that recovers from10%-15% of energy  otherwise lost in regulators that lack it. (An article on MPPT will be posted on this site shortly.)

See also Charge Batteries Faster and Deeper – also Speeding Battery Charging from a Generator and also Lithium-ion batteries in caravans

f you liked this article you are likely to also appreciate my books on RVs and solar. Batteries and battery charging are covered in greater depth in Caravan & Motorhome Electrics. Solar That Really Works! is for cabins and RVs. My book Solar Success relates to home and property solar systems. There is also the Camper Trailer Book.