Tasman Energy
A deep cycle battery installation

Deep Cycle Battery Charging

An understanding of how batteries charge is crucial to the performance of your solar power system. Deep cycle lead acid batteries of all types all follow a certain pattern when it comes to being recharged.

Lead Acid Battery Charge Times

To fully recharge a lead acid battery that has been discharged to its full extent takes about 12 hours. The thing about the recharging of a lead acid battery is that it takes charge efficiently up to a certain point which is about 90% full. The rate of charge acceptance then begins to taper off. Let's look at a few equations in relation to charging a lead acid battery:

Battery Capacity

Battery capacity determines battery charger size. If you want to recharge batteries efficiently you need a charger that is suited to the battery bank. There is a simple rule here: 10% C10 is the minimum battery charger required to recharge a lead acid battery bank efficiently. 10% C100 is about the ideal battery charger size. Looking back to the "batteries" page, there is an example and data on a battery called a 6RP1080. To save you reloading pages, the chart for this battery is copied below.

BATTERY C120 to 1.8 Vpc C100 to 1.85 Vpc C20 to 1.85 Vpc C10 to 1.80 Vpc C5 to 1.7 Vpc
6RP1080 1180 1080 772 685 618

Here we can look and see that the C100 capacity of this battery is 1080 amp/hours so the ideal battery charger for this battery would be 108 amps. Unfortunately a quality 100 amp battery charger is not that easy to come by and rather an expensive item but for the time being lets pretend we can pick one up at the local store and it is perfectly suited to the charge regime of the above battery.

Charge Acceptance

Here we look at what happens when we discharge a battery and then commence to recharge it. Lets say we started out with a battery that was 100% full of charge. Over a period of days we discharged the battery until it was only 70% full. To be precise, we have taken out 30% of the capacity. Unfortunately for us, the discharge was followed by several days of rain so instead of using our solar panels to recharge the batteries we connect out perfect charger purchased from the local store. to power the charger we fire up a petrol generator ...

Initially, assuming we had the meter to measure it, a good rate of charge would flow into the batteries. I would expect to see the full output from the charger here, 100 amps. This causes the terminal voltage of the battery to quickly rise then stabilise at a value somewhat above the normal battery voltage. In order to charge the battery efficiently we actually want the voltage to stabilise around this point for a period of time. Some technicians call this the absorption stage. The ideal battery charger to accomplish this is one sized at 10% C10 so our charger is slightly oversized but never mind, a little extra is better than a shortfall.

Once the absorption phase is over the battery has gone from minus 30% to about minus 15% and here, if the weather had improved and the sun was shining we could quit recharging happy in the knowledge that we have used minimum petrol for maximum charge. But, it is still raining cats and dogs and we want to see what happens ...

Once the absorption phase is over the battery terminal voltage rises again to a higher level and what happens is determined by the design and quality of our battery charger. If the charger we picked up at the local store was indeed a perfect example designed for our battery it would then enter a stage often marketed as the boost stage where the battery was held at a voltage somewhat higher then the absorption stage. Here, a timer would start and the boost voltage would be held and maintained for a period of time, say two hours. Once this was completed the battery would be around 95% full.

The boost stage can lead to battery gassing, loss of water and a warm battery so it is not kept up forever. After the set period of time, a good quality battery charger should then drop the voltage back to a float voltage. here at this voltage the battery would slowly obtain the remaining 5% of charge needed to bring it back up to 100% full. Here too, our generator is running in the background, using fuel but not really adding much to the battery.

The summary of charge acceptance is that we can efficiently recharge a battery until it is around 95% full using a conventional battery charger that is correctly sized for our battery bank.

Using a bigger battery charger than ideal

Before we move on, lets just say that the local store did not have a 100 amp charger in stock but somewhere in the dusty recesses was a 200 amp charger that was offered to you at the 100 amp charger price. Beauty, you think, twice as fast. Unfortunately not so. As a battery accepts charge the voltage rises (shown below). Here, when you connect up this oversized charger to your battery bank, the voltage rises too fast. The absorption stage is shortened and the voltage rises up to the boost stage. To cut a long story short, recharging with too big a battery charger will in fact work and if the designer gets the parameters exact, it will not make a huge difference BUT it will not charge your batteries any faster than the correctly sized device.

Voltage Versus State of Charge Acceptance

We have already learned that the voltage of a battery rises when it is being charged. Instead of quoting voltages relating say to a 12 volt battery (when you may in fact have a 24 or 48 volt arrangement). We also must remember that a lead acid battery is actually a 2 volt device. Here we look at the industry normal of quoting volts per cell. Simply multiply these voltage by the number of cells to obtain the voltage for your battery.

Static voltage of cell at around 30% discharged, nothing connected (no load) and no charge going in: Approximately 2.1volts

Cell at absorption stage, correct battery charger attached, battery about 20% discharged: Approximately 2.21volts

Cell at boost stage, correct battery charger fitted, about 10% discharged: Approximately 2.5 volts per cell

End of boost stage, correct battery charger fitted, about 5% discharged: Approximately 2.65 volts per cell

All voltages are listed as approximate because battery charging is not an exact science. Actual charge voltages will vary slightly due to the age of the battery, the cell temperature of the battery and the exact composition of the battery. It should also be noted that these voltages are for flooded lead acid batteries. Voltages for sealed batteries MUST be prevented from rising above a certain point.