OptiMate UK

OptiMate Battery Chargers and Maintainers

Technical terms explained

Constant Current: The first & main stage of the charging programme, or ‘boost‘ stage, during which the current stays constant. (Unlike many competitor products TecMate’s chargers do deliver true constant current).  To maintain a constant injection of current requires application of a rising voltage (‘pressure’) to overcome the steadily rising resistance as the battery’s charge rises.  The circuitry required is complex.

Constant Voltage: "Constant Voltage" means that the voltage at which the charger delivers current to the battery remains fixed or is limited, and the current passed into the battery varies according to the battery's internal resistance, falling progressively as the battery's resistance (to accepting further current input) rises with it’s state of charge. This progressively decreasing charge current is also known as the ‘logarithmic’ charging characteristic. This circuitry is simple & cheap, so nearly all‘common’ chargers are made thus.

Battery Resistance: A large battery has less resistance than a smaller similar battery if both are in the same condition or status. Think of eating: a really big man eats more & can eat faster than a small kid.  A fully charged battery has higher resistance than when part-discharged just as you eat more slowly when you are full. A deep-discharged or sulphated battery has very high resistance & is therefore extremely difficult to recharge again – it is blocked. Constant voltage (‘logarithmic’) chargers cannot recharge such batteries because their voltage pressure is limited. But constant current chargers often can because the voltage they apply can temporarily be allowed to rise enough to unblock the battery before again being limited to a safe maximum. Constant voltage is useful for the absorption stage & the maintenance or float stage when the battery already has enough charge to start the engine.

Absorption stage: Where the Constant Current stage can be thought of as the boost stage, the Constant Voltage absorption stage can be thought of as the fill stage. The voltage gradually rises during the constant current stage as the battery becomes charged. When the battery is from 70% to 85% fully charged (70% for small motorcycle batteries, 85% for automobile batteries) the voltage reaches the 14.3 V level at which prolongation of the constant current stage may risk gassing the battery so the charge voltage is now limited at not higher than 14.3V. (Some battery manufacturers suggest 14.4V, but allowing for charger manufacturing tolerances we have taken 14.3V to be wiser). This second stage of charging is called the absorption stage because it allows the battery to absorb further current according to its need before progressing to the maintenance stage (float charge mode). The absorption stage continues (at the voltage limit 14.3V) until the current absorbed by the battery has fallen to about 200mA to ensure a thorough charge replenishment. Note that automotive & truck batteries which have discharged to being flat might need even a day or more in the constant current & absorption stages before reaching a good state of charge when connected to OptiMates 1-4. OptiMate 5 and above are better suited for larger sized batteries.

Maintenance charge mode: When, during the absorption stage the current absorbed by the battery has reduced to about 200 mA the green LED indicates that the battery is now effectively fully charged & ready to use. At this stage, OptiMate first does a check now for battery voltage retention without charge support. The voltage limit is now automatically reduced to 13.6V for the medium to long-term maintenance of the battery, without gassing. The battery can draw current according to its needs to support small loads (or current leaks in the vehicle wiring circuit) varying from zero to up to 200 mA. A number of battery manufacturers recommend a float Voltage of 13.5V minimum, 13.8V maximum for a 12V battery.

Standby feature: During the maintenance mode OptiMate continuously monitors the current drawn by the battery. This is especially important when the battery remains connected within a wiring system such as of a vehicle or in a power support system. For example in classic cars the vehicle may still have its original wiring with deteriorating insulation & corroded or loose electrical contacts. The current the battery needs to support such leaks or loads may rise right up to the 200mA limit allowed in float mode.