Deep Cycle Batteries - Overview and FAQ

The Deep Cycle Battery "Frequently Asked Questions" list is below.

Visit our Deep Cycle Batteries Index for prices or to select your battery by application.

For "less technical advice" see our General Deep Cycle help page.

How are Deep Cycle batteries different to car starting batteries?

AGM Deep Cycle Battery

  • Deep Cycle batteries are designed for slow discharge at fairly low current levels of 10's of Amps (e.g. to power fridge/lights), compared to starting batteries which are designed for short bursts of high current (100's of Amps).
  • Starting engines requires a short burst of energy, which initially comes off the surfaces of the battery plates. So, for starting engines the designer goes for thin plates with maximum surface area.
  • For deep cycling the energy is created via a sustained chemical reaction, so the designers prioritise the battery plate thickness over surface area to allow for this. Thick plates give Deep Cycle batteries longer life.
  • There are many types of Deep Cycle construction & chemistry but the basic choice is "Sealed" (normally GEL or AGM) versus "Access" batteries where you can access the cells to top them up.
  • Access batteries (also called Wet or Flooded Lead-Acid) give off gas when charging, so need good ventilation. Sealed batteries do not gas (normally), and are suited for use inside passenger areas (e.g. under a caravan seat).

How should I charge my Deep Cycle Batteries

projecta IC battery charger

  • 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 will never properly and fully charge up a Deep Cycle 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, Absorbtion & Float phases.
  • Bulk charging puts in the rated maximum current while maintinaing a constant voltage.
  • Absorbtion 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.

How deeply can I discharge my Deep Cycle Batteries?

projecta battery charger

  • 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 (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% capacity left in the battery if you can (or at least 20% left for any reasonable battery life).
  • 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 (inc solar). Measure the voltage across the battery terminals (we sell a good Digital Voltmeter for about $15!). For a guide, at normal temperatures:
  1. Standard lead-acid battery: 12.6V = 100% charged
  2. For AGM or GEL battery: 12.8V = 100% Charged.
  3. For all types 11.8V is fully discharged = 0%.
  4. Always try to keep above 12 Volts minimum (off load).
  • If you do flatten the battery get it off load and back onto charge as soon as possible to avoid damage and rapid sulphation.

What type of Battery Charger should I use?

ctek battery 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, Absobtion and Float voltages appropriately.
  • Ideally monitor the battery temperature. Warm is ok, hot is not!
  • Most GEL batteries need 14.1V 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 the fluid Levels?

Century deep cycle battery

  • "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. Boiled and cooled water would be better though, and do not top up with battery acid. Battery acid is only used when the battery is first filled.
  • Sealed batteries 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 electrolytic fluid, but simply have a sealed top. Their performance is similar to standard "Access" or "Flooded" lead-acid batteries however they don't need topping up with water as they use a re-combinant agent inside to turn the normal gasses produced under charging conditions back to water.
  • The little "magic-eye" indicators can often be useful, but remember they only tell you the condition of 1 cell (our of 6) so don't completely rely on them.

Flooded or Sealed Batteries?

Trojan deep cycle battery

  • "Flooded" or Wet Lead-Acid batteries have removeable caps on the cell tops to allow topping up with distilled water as required.
  • Sealed batteries 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 electrolytic fluid, but simply have a sealed top. Their performance is similar to standard "Access" or "Flooded" lead-acid batteries however they don't need topping up with water as they use a re-combinant agent inside to turn the normal gasses produced under charging conditions back to water.
  • Sealed batteries generally use a low pressure safety valve to "vent off" in the case of a battery failure or over-charging. This valve only lets the gas out quickly then automatically re-seals, sometimes resulting in a rather "sucked-in" or convex side to the battery if it has been chronically over-charged.
  • Which is better, Flooded or Sealed? That's the subject of a whole separate FAQ but in general a good quality flooded (Trojan, Century for e.g.) will be less expensive than the equivalent quality Sealed battery so if ventilation and access to "top-up" is not an issue then they are still a good choice.

Wet Lead-Acid or GEL/AGM Dry Batteries?

AGM deep cycle battery

  • Flooded Lead Acid batteries, whether sealed maintenance free or accessible types, can give years of good service if charged and maintained well. Trojan is a good example and many owners report 5+ years of service. However they are charge-rate limited, with higher internal resistance meaning they can not accept a charge rate (and hence charge up) as quickly as the GEL or AGM types.
  • Both GEL and AGM use electrolyte suspended in a medium (Gel paste or an absorbed glass mat respectively) instead of a liquid. They are sometimes referred to as "acid starved" and without the liquid acid they are safer in an accident situation, and of course do not give off gasses while charging normally.
  • Good quality GEL and AGM batteries also display much lower internal resistance than standard lead-acid batteries for rapid charging and the ability to power high bursts of power.

AGM or GEL Deep Cycle Batteries?

Gel-Tech GEL deep cycle battery

  • AGM batteries use an absorbed glass matting material to hold their electrolyte and are becoming the more popular choice for sealed deep cycle batteries in RV & Marine use. The fact that leading brands such as LifeLine, Optima and Ultimate all use AGM is testimony to its advantages. AGM also performs better than GEL in very cold conditions and AGM is often less expensive than GEL like-for-like.
  • GEL batteries use a gel paste in which the electrolyte is suspended. When a high quality GEL battery from a manufacturer such as Sonnenschein (Germany) or GEL-TECH (USA) is used then very long life-spans can be achieved, and if space constraints (e.g. golf-carts) dictate that flattening the battery will happen then GEL technology recovers better than most AGM or regular Lead-acid batteries.
  • The downside for GEL, cost aside, is that it requires a lower charging voltage than typically produced by vehicle alternators and over-charging can permanently damage them so under the bonnet applications are generally not possible.
  • AGM and Lead-Acid batteries both work fine on the higher voltage levels that alternators produce, unlike GEL. This makes them a safer choice although many AGM batteries should also not be fitted under-bonnet due to case material/heat constraints.