Deep Cycle Batteries

Charging and Discharging

The Golden Rules Are:

  • Never flatten your battery, and don’t regularly take out more than 40%.
  • If you do flatten / over-discharge a deep cycle battery, charge it back up ASAP.
  • Use the right size and type of charger, and on the right setting (if in doubt contact us).
  • Leave the battery on a trickle-charge when not in use, or top-up the charge monthly.

General Charging Advice

  • 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.

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 (i.e you’ll get 3 – 10 times fewer cycles from the battery than if you only discharged to the 50% level).
  • 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 11.8V = 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, 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 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. 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 (out of 6) so don’t completely rely on them.

Charging systems

Solar Charging

If using Solar charging then you will need a Solar Controller or Solar Regulator to avoid over-charging the battery. These are normally purchased separately from the Solar Panel, with the exception of some “fold up” or “suitcase” style solar panels which have them built-in.

We have plenty of models in stock (both Panels and Regulators) and good practical experience with fitting Solar to caravans & Motorhomes so would be happy to advise. Many panels these days come with a silicon diode built in to avoid reverse discharge of the battery via the panel, but this is NOT sufficient to protect the battery from over-charging by the panel and a regulator must be used on all panels above a few watts in power.

Note that the latest MPPT (Maximum Power Point Tracking) solar regulators can provide up to 97% efficiency, which is a big improvement on older style regulators. This can be an advantage where space is tight and a larger panel is not an option.

Do remember to set the Solar Controller / Regulator to suit the battery type if there is an option to do so (as Gel batteries for example require a lower charging voltage, while Calcium batteries can benefit from a much higher (up to 16Volt) Equalisation charge periodically. This is something many Morningstar and other good quality regulators can provide. If in doubt please contact us for more information.

We stock a wide variety of fixed and folding Solar panels with associated regulators / charge controllers.


In general car or van Alternators work reasonably well with Lead Acid and AGM batteries (not GEL). They are not battery chargers however and will never fully charge a Deep Cycle battery to 100% (see table above for reasons), so its best to use a battery charger when mains power is available to top up the battery charge and avoid reduced battery life from sulphation.

Note that alternator output voltages are often reduced by cable/wiring runs and Dual Battery systems so care needs to be taken to measure the actual voltage received at the battery across its terminals to ensure its adequate to charge the battery fully.

Conversely for Gel batteries fitted close to the alternator (under-bonnet with a thick gauge cabling) there is a real risk of damage due to over-charging as the alternator output can be too high for the battery.


Many Portable Generators do not have battery charging circuitry built-in and should be used with care if they provide a DC outlet (although you could plug a battery charger into the 240V socket).

The later models (from Honda etc.) with the built in chargers can provide a reasonably quick and efficient battery top-up on sites where they are allowed (or when off-site), although many users do grow tired of the noise and migrate to solar over time.

Combined charging

Finally many combinations of the above can work well. It can be difficult to fault find on systems with combined charging, although the better Solar regulators often allow the load to be isolated from the panel which can help (and assuming it has been wired up correctly – many are not).

We would be happy to advise but in general would recommend owning a good quality fully automatic battery charger to suit the batteries you are trying to maintain and using it regularly.

We stock a wide range including CTEK, Matson , Victron and Projecta to name just a few, please contact us for advice or pricing. Note that a good quality battery charger, if used regularly, will pay for itself over time through extended battery life and by operating more efficiently than the older style chargers. Some of our wide selection of Battery Chargers are now available online.

Again we would be happy to provide advice on the right charger for your deep cycle battery. There is some more general advice in our Deep Cycle Help & FAQ section.