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-termovercharging 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. SUNJACK USB BATTERY CHARGER will charge battery cells safely and quickly. It is one of the best selling battery charger on the market.
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.
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.