There are often posts on 4×4 groups where people inquire about purchasing a battery. Sometimes the advice given makes no sense at all, which motivated DIRK VAN REENEN to consolidate the key elements around how a battery, charger and charge controller work.

For the purpose of this article, I am going to write about standard batteries, which are based on the principle of lead and sulfuric acid. This relates to most batteries used these days, whether it is a lead-acid, absorbent glass mat (AGM), gel battery or whatever else it is called.

Any battery consists of six cells of 2V each, which are in series – therefore 12V. The basic principle is that you convert the sulfuric acid to lead sulphate by applying electrical power across the poles, in other words charge the battery. When you use the battery, the lead sulfate will break down again to lead and sulfuric acid.

The 2V cells operate at 1.75V when depleted and 2.25V when fully charged. The midpoint between the two is 2V – hence the 12V classification. When a battery is depleted, it is therefore 10.5V. If you continue to draw power from this battery, the lead will break down and calcify and warp the plates inside. The cell will die, and the battery is basically destroyed. Similarly, a fully charged cell is 2.25V. So, a fully charged battery should read 13.5V across its terminals when no load is connected. The capacity of the battery is the Ah (Amp-hour) rating of the battery. It is indicated with the letter ‘C’ and is usually also given as a 20-hour reading. So, a 100Ah battery will deliver 5A for 20 hours. However, it will not be able to deliver 10A for 10 hours and certainly not 20A for five hours.

To convert it to the load you apply a formula: Watts = Amps x Volts. If the battery can therefore deliver 5A, you can draw 5A x 12V (or 60W) for 20 hours.

What is the difference?

All batteries use lead plates and sulfuric acid for the process of generating electricity. The difference is how the electrolyte (sulfuric acid) is stored.

In older batteries, the electrolyte is loose inside. Many people mistakenly believe that only these are lead-acid batteries. They usually also have six plugs where the electrolyte can be topped up or monitored. They also tend to give off hydrogen when being charged and you will be able to see and hear the bubbles.

Then there are also VRLA – or vent regulated lead acid – batteries. These also contain loose electrolyte, but the batteries are sealed and have a special ventilation system that ensures that the hydrogen can escape without the battery leaking. In AGM batteries, the electrolyte is absorbed in a fine glass paste so that less hydrogen is released. In gel batteries, the electrolyte is simply dissolved in a silicone gel. In AGM and gel batteries, the electrolyte makes better contact with the lead plates and there is less oxidation and calcification on the plates.

Know your cycles

There are two types of batteries. The first is what you will typically find in your car – a so-called cranking battery. It consists of very thin lead plates so that it can deliver a lot of current for a short time. A vehicle’s starter typically draws 800A to operate, but only for the few seconds it takes to start the engine. After that, it charges back to 13.5V. If you have trouble starting the car and the battery becomes depleted, you are basically destroying the lead plates. Do this a few times and the battery as a whole is destroyed. This type of battery cannot provide continuous current and becomes depleted faster than a deep-cycle battery.

Deep-cycle batteries, on the other hand, have large lead plates in them that will deliver less current, but over a long period of time. The cells are exactly the same and operate between 1.75V and 2.25V. The big difference, however, is the volume (mass) of lead that the sulfuric acid can absorb. A typical 100Ah deep-cycle battery will weigh 33kg, compared to the approximately 15kg of a normal car battery.

Deep-cycle batteries are not made to deliver a high current and should not be used to start your vehicle. The lead blocks will heat up and deform, destroying the battery.

If you draw a deep-cycle battery below 50 percent (12V), you shorten its lifespan. Each battery can deliver a certain number of cycles. Every time you use the battery to below 50 percent, it loses a cycle. So, if you use a deep-cycle battery with 1 000 cycles to below 12V every day and recharge it again, it will last about three years. If you use it to just 60 percent, the lifespan will be five years or more.

Be aware that a high-cycle battery is not the same as a deep-cycle battery. It is more of a hybrid between a normal vehicle battery and a deep-cycle battery and will weigh between 22kg and 25kg.

Charging batteries

Charging a battery is typically done in two phases – bulk and float. So-called smart chargers use up to eight phases, which helps to charge the battery a bit faster, but for the purpose of this article we are focusing on two phases.

To quickly charge the battery to about 85 percent capacity, a voltage of 14.4V is applied across the poles. The internal resistance of a depleted battery is low, which means that the Amp will be higher. So, the current initially required to charge the battery must be high. As the battery charges, the resistance increases and the current decreases accordingly, but the voltage remains constant at 14.4V.

When the battery is charged beyond 85 percent, phase two – or float charging – kicks in. The voltage is reduced to 13.6V and the battery now charges more slowly during the last 15 percent until it is fully charged.

Your vehicle’s battery can typically be charged at C/0.8, but to charge a deep-cycle battery the charger you use must not deliver more than C/10. If it is more, the electrolyte inside the battery will start to boil and it will give off hydrogen gas. It is not only dangerous but will also shorten the life of the battery.

The capacity of lead-acid batteries is measured in Ah and that of Lithium batteries in Watts. The latter’s cells are also 3.6V and should not be charged with ordinary chargers. So, if you connect a 60W load to your 100Ah battery and it runs for 12 hours, you have used 60Ah (60W = 12V x 5A and 5A x 12hours = 60Ah). The battery is now at 40 percent and you have lost one of the cycles. To fully charge it, you must put the 60Ah back again and if you use a 10A charger, the process will take about six hours.

Chargers and charge controllers

I have already referred to chargers. Invest a little extra money and buy a decent one. This will extend your battery life. Cheap chargers are not stable with their voltage, nor can they maintain a constant current.

Charge controllers are intelligent chargers, which handle and stabilise DC-DC conversion. There are two types – PWM and MPPT. The former is cheaper, but also less stable and quite sensitive to the input voltage. It will typically not be able to handle more than 18V to charge a 12V battery. The rest of the power is simply lost. MPPT controllers are of better quality and can handle an input of 150V.

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