Battery Selection & Care Tips

  • As a rule we would work out the capacity required for the application then double this figure to ensure the batteries are not discharged too heavily. For example, if an application called for a 200 Watt load to be driven for four hours without recharge the required battery could be worked out as follows. At 200 Watts the inverter would draw 20 Amps from the battery, multiplied by four hours gives us 80 AH. From this we would say a suitable battery would be 160 AH, this is much larger than for example a typical car battery however two heavy duty car batteries in parallel would typically have this capacity. This may seem excessive but bear in mind many loads would not draw their rated power continuously, for example power tools would not run continuously under load however a television or lights would typically be used for many hours at a go. Microwave ovens draw a fairly large current but only for short bursts, kettles are a massive load and to boil even a single one is not practical with this type of equipment or batteries.

    Typical Capabilities

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  • The relatively high capacity of leisure batteries would make them appear an ideal choice for running inverters, however they do not have the peak current capability we are looking for. They are designed to deliver a relatively low current over a prolonged period such as the 12 Volt flourescent lighting in caravans and small portable televisions. This means their output voltage falls under heavier loads and will appear to the inverter as a discharged battery causing the alarm to sound or the unit to switch off, in extreme cases the leisure battery can suffer permanent damage. Car type batteries have been designed primarily to deliver a heavy current for a short time to crank the engine over which when started will rapidly recharge the battery and supply all auxillary power whilst running. This short heavy duty means they are constructed with heavy plates but don't have particularly high capacities as they are not normally expected to be discharging for long periods.

    Leisure Battery Voltage Falling

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  • The heavy currents drawn by an inverter are not likely to be matched by the charger used to replenish the battery. In a car the modern alternator can recharge the battery at good heavy currents but mains powered and solar or wind driven chargers are less likely to reach these levels. Consequently we must spend longer charging the battery than draining it so for applications were a daily schedule of work is required we must work out these times to fit a 24 hour cycle. It is often preferred to spread out the recharge time and charge at lower currents than a heavy bulk charge particularly were sealed batteries are used. In our example situation with a 200 Watt inverter running for four hours with a 160 AH battery we would have in theory twenty hours left in the day to recharge to the original level. To be practical we could say we will recharge the battery over sixteen hours at a nominal current of 10 Amps. This would be ample as in fact the battery would only draw the 10 Amps initially for the bulk then gradually fall. It may stay near this figure for days but it is still topping up, the important thing is that the battery returns to a state were it can provide another four hours work from the inverter. Batteries that fall far below their normal charge can become permanently damaged so having ample capacity helps prevent this.

    Good Battery Charge

    A typical well charged 12 Volt battery will read about 12.8 Volts when sitting idle (not connected to charger or inverter). Connected to an inverter with a known load a safe length of operating time could be established within it's capacity.

    Another 12 Volt battery that has been used within its capacity but now needs recharging would typically have fallen to 10.5 Volts. At this point the inverter alarm circuit would begin to sound alerting the user to act and recharge the battery.

    Low Battery Charge
    Discharged Battery

    Although inverters with alarms switch themselves off at around 9 Volts failure to recharge the battery or draining it further can cause permanent damage.

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  • When charging regular lead acid batteries gasses are evolved which should be allowed to ventilate to atmosphere where possible. The higher the charging currents the more actively they will give off gas. It is dangerous to allow charging gasses to build up in an enclosed space as any source of ignition, in particular an electrical spark may trigger an explosion.

    Explosive Gasses

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  • The construction of these batteries is such that they require much tighter tolerances in the charging process. Because of this only battery chargers specified as suitable for these type of batteries should be used and the charging times followed accordingly. Failure to follow this can lead to venting and possible permanent damage to the battery.

    Venting Gel Battery

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