Battery Charging Do’s and Dont’s

General Charging Advice - Do’s

• Batteries will self-discharge over a period of months even without a load. Many GEL, AGM and Calcium's are better than regular lead-acid batteries but even so you should charge them back up regularly, or better still use a trickle charger (or solar panel) to keep them in top condition and extend their life.
• Use the right charger for the battery Chemistry e.g. GEL cells require a lower charging voltage, while Calcium cells need a much higher "stratification" charge from time to time. Ideally use a charger with temperature compensation and a temperature sensor which can be attached to the batteries being charged.
• Make sure your charging current is big enough to cope (the rule of thumb is 10% of the amp-hour rating of the battery / battery bank as a minimum: i.e. a 10A charger for a 100Amp-Hour battery).
• Automatic, multi-stage chargers are worth using to extend battery life. Most Vehicle alternators, especially in modern ISS vehicles will never properly and fully charge up a battery so invest in a good charger or a solar arrangement with a charge controller/regulator with at least a three stage charging control (many chargers are now 6 or 7 stage affairs).
• In an automatic 3 stage charger typically you get Bulk, Absorption & Float phases.
• Bulk charging puts in the rated maximum current while maintaining a constant voltage.
• Absorption charging then "tops up" the battery to 100% with a constant (elevated) voltage while monitoring the reducing current.
• When the battery is fully charged Float Mode is a reduced voltage which avoids overcharging the battery but just maintains the battery in a non-sulphating state. Some chargers periodically also "trickle" or pulse current into the battery to counter self-discharge.

General Discharging info

• Don’t flatten the battery, as you may strip active material off the battery plates. Deep Cycle does not mean "100% Discharge", and flattening drastically reduces their useful life (i.e you'll get 3 - 10 times fewer cycles from the battery than if you only discharged to the 50% level).
• A rule of thumb is to recharge when there is 40% - 50% or more capacity left in the battery.
• You can estimate the battery charge level by first taking any surface charge off the plates (e.g. switch on lights for 20sec) then disconnect any chargers. Measure the voltage across the battery terminals. For a guide, at normal temperatures:

1. Standard lead-acid battery: 12.6V = 100% charged (For AGM or GEL battery: 12.8V = 100%)
2. For all types 10.5 = 0% (i.e battery fully discharged)
3. Always try to keep above 12 Volts minimum (=20% capacity approximately when battery is not loaded).

• If you do flatten the battery get it back onto charge as soon as possible to avoid sulphation.

Use the Right Charger

• The rule of thumb is 10% of the amp-hour rating of the battery / battery bank as a minimum: i.e. a 10A charger for a 100Amp-Hour battery).
• Flooded batteries often cannot accept high currents for sustained periods and using too large a charger can damage them (strip the plates and boil off the acid!).
• Good quality AGM and GEL batteries can often accept much higher currents for rapid charging. See Optima and Lifeline pages for more information, but they can often be charged in an hour or so if the right charger is available and battery temperature is monitored.
• Ideally check the specification sheet for the battery and use the correct settings (AGM/GEL/Flooded) which will set up the Bulk, Absorption and Float voltages appropriately.
• Ideally monitor the battery temperature. Warm is ok, hot is not!
• Most GEL batteries need 14.2V maximum for bulk charge.
• Most AGM batteries and Flooded are ok up to 14.4 - 14.6V
• Calcium batteries can require 16V for short periods.
• All should be maintained at a lower "float" voltage (13.1V typical). Keeping them at levels above this can over-charge them.

Topping up Levels

• "Flooded" or Wet Lead-Acid batteries have removeable caps on the cell tops to allow topping up with distilled water as required.
• Keep the plates covered in electrolyte at all times!
• The impurities in tap-water etc do not help the battery, but may be better than nothing in an emergency, and do not top up with battery acid. Battery acid is only used when the battery is first filled.
• Sealed batteries (AGM AND Gel) are sealed at the top, and thus won't leak acid when tipped over or give off gas while charging under normal conditions.
• Note some Sealed "Maintenance Free" types still use wet electrolyte, but simply have a sealed top. Their performance is similar to standard  "Flooded" lead-acid batteries however they can’t be  topped up with water.
• The little "magic-eye" indicators can often be useful, but remember they only tell you the condition of 1 cell (out of 6) so don't completely rely on them.