DEEP CYCLE – WHAT IT MEANS
When you own and drive a car, 4WD, truck, etc, you hardly think about the battery that starts the motor. This is because the automotive engineers who designed the engine have made the autoelectrics, motor and battery system very, very reliable. As a result you hit the ignition system, crank the motor and in a second or so the motor starts.
Even though the label on the side of your battery says some vaguely impressive things, much of this is fairly meaningless. The construction of this battery is brutally designed down to the cheapest price possible. As you know, these batteries have lead (Pb) in them and lead is expensive. The lead on this battery is bonded to a lattice – like frame to maximise the surface area of the lead to the liquid electrolyte (sulphuric acid). This surface area equates to an ability to generate a lot of amps for the starter motor. In a healthy starter system you can hear the starter motor really zip.
Once the motor has started, the alternator immediately begins to recharge the battery power lost to the starter motor. Before you know it, the battery is fully charged again.
This starter motor – battery discharge – alternator – battery charge sequence is called a Shallow Cycle system. It keeps the cost of the battery as low as possible.
Shallow cycle batteries, however, are hopeless for any other application than starting cars and trucks. When you discharge a shallow-cycle battery below about 70% of its rated capacity it can be damaged. The flimsy metal lattice frames can heat up and buckle. Lead on the frames can dislodge and fall to the bottom of the battery. There may be insufficient lead anyway to still hold a decent charge after a 50% reduction in stated capacity. Poor, high resistance internal electrical conductors can heat up.
The battery in your “house power” bank (see “Tech Talk”, How Do I Know What To Do About A Battery Bank?) goes through a much different cycle than a car start-up. Firstly, it does not need several hundred amps for a few seconds to power a starter motor cranking a cold engine. Secondly, it does not need to be instantly recharged at high charge rates from a powerful alternator. In a battery bank this is what happens.
You have shut down for the night. The solar panel array, wind generator or vehicle propulsion motor has charged your Deep Cycle battery bank. You may even have a 240V supply on the post at the caravan park which has a trickle charger charging the house power batteries. Either way, your battery bank is being recharged SLOWLY.
And then it’s the turn of the Deep Cycle battery bank to do its job. You turn on an electric kettle, microwave, TV, etc, to settle in for the night. The electric fridge is running, the battery bank, depending on its size and capacity discharges, 20, 40, 60 or 80 amps to the inverters to make that 240V quietly contributing to domestic bliss. The batteries will happily discharge down to 10-20% of their total capacity without stress to the batteries themselves. They love to be steadily charged and steadily discharged, over and over – and deeply – again and again. They have much more lead inside on far sturdier frames with low resistance internal connections. Shallow cycle batteries will simply fail in these conditions.