What you need to know about battery chargers to keep your RV’s batteries fully charged
WORDS AND PHOTOGRAPHY BY GRANT HANAN ADDITIONAL PHOTOGRAPHY BY CARAVAN & MOTORHOME
Today’s RVs are designed to supply all of your power needs using a 12 volt DC power supply when travelling. This means you can tap into portable stored power from batteries when not connected to 240 volts, but there are times where you may need a 240 volt power supply as well. In this Tech Talk, we’ll take a look at how it all works.
Simply put, the batteries we have in our RVs go flat so they need to be recharged. But do you know whether your RV’s battery charger fully charges the batteries? And if you’re buying an RV, do you know what system is being used—or have you assumed that the RV manufacturer knows what they’re doing and installed the best that money can buy?
Technology has made significant progress in recent years. A basic charger will restore some energy into a battery, but may not do it very well. In fact, it could be shortening the life of those expensive batteries you have just purchased to replace the ones that gave up the ghost!
FULLY CHARGING A BATTERY
For batteries to be fully charged, the battery charger applies a minimum of three basic stages using a range of different voltages and currents. Some manufacturers will label these differently for commercial reasons. Within the industry, they have common names.
This is where the charger applies the maximum amount of amps to increase the voltage quickly.
With maximum volts, the amps are slowly decreased over a long period of time. This is often referred to as “soaking the battery.”
The battery is now full and very low amps are applied to keep the battery full and ready to go.
NOT ALL CHARGERS ARE EQUAL
A simple charging system starts with a battery charger that has leads attached to the positive and negative terminals. The charger is then plugged into a 240 volt wall plate and turned on to begin the battery charging process. Depending on the charger type, it will continue to charge until it’s full or switched off.
If this approach is applied to an RV that has an integrated charger hardwired, it means every time you plug into 240 volts via a 10 amp power lead, you are in effect turning on the charger and charging the batteries. The issue here is that you need a charger that will go to Float, otherwise you risk overcharging the batteries.
LINEAR CHARGER VS STAGE MODE
To demonstrate the two charging types we have a flat battery with 10.4 volts. First we charge it with a linear charger. The maximum voltage reading is 14.1 volts. Taking the same battery and now with a stage charger, the maximum voltage is 14.7 volts.
Taking these results and plotting them over time would look something like these diagrams.
The Linear graph shows how the constant fixed amperage will increase the volts and settle out at a fixed voltage over time. This will remain until you turn the charger off. If left too long, it could “cook” the battery.
The Stage mode graph shows a similar increase, but a higher voltage is reached and held for a while. This action “soaks” the battery. A more moderate voltage is then applied in Float mode of 13.4 volts reducing any likelihood of “cooking” the battery.
INCORRECT CHARGING SET UP
Simply put, if you get the battery and charging set up mix wrong, you can expect a shorter battery life. Let’s look at what can occur with an incorrect charging set up.
When undercharging occurs frequently, the battery becomes sulphated – meaning that there is a build-up of sulphate deposits on the battery plates. This increases resistance in the battery, which in turn returns incorrect fully charged readings. If this type of charging pattern continues, what was 110 amp hour capacity will now more likely become 30 amp hours. This is the point of no return and you will need to replace the batteries.
When a battery is overcharged it becomes overheated. Essentially, the battery becomes “cooked” to the point that the electrolyte is boiled out. This isn’t good for any battery especially sealed AGM and Gel types as they have no thermal protection sensors. If left unchecked, an overcharged battery can lead to battery fires.
With excessive discharging, you run the risk of increasing sulphation. Batteries should never be allowed to discharge greater than 50% of their capacity. A well designed battery circuit system will have inbuilt voltage sensing relays (VSRs) that will disconnect the battery from the power drain at a pre-determined setting.
SELECTING THE CORRECT CHARGER
Before parting with your hard-earned cash, you need to know what you want the charger to do. Consider these points before making your purchase.
Type of Battery
All battery types require different amp rates and voltage currents throughout the charging cycle to get the best results. Many of today’s smart chargers are capable of being set for a variety of types. So be clear about what type of batteries you will be charging.
Battery Size (in Amp Hours)
When you have large amp hour capacities, you need a charger with large capacity. If you end up with a charger that’s rated incorrectly, you may find it will take days before a set of batteries is recharged.
Note: when your RV batteries are charged, you may want the battery charger to double as a power supply and keep the batteries topped up at the same time. Not all battery chargers have this capability.
If you’re looking to upgrade your RV’s on-board battery charger, be aware that some of the newer types on the market come with extra features.
Aim for a multiple-stage charger as some have up to nine stages. The charger monitors the battery and carefully changes amps over time.
Clever chargers conduct a state of charge analysis prior to commencing charging. When starting up, they pulse rapidly with high voltages and amperages. This action prepares the battery for charging by flexing the plates and creates a pathway through any sulphated plates.
At the end of the absorption stage, the charger stops for a while and measures any voltage drop. The charge is then finalised at the Float stage. Flashing lights on the charger provide an indication of the battery’s health or a report can be downloaded to a computer for viewing.
Some chargers also have an Equalisation mode. This is where the smart charger looks at each cell individually and balances out the charge.
More sophisticated chargers have a Supply mode. This means they have the ability to carry the load demand on the circuit. Switching relays then revert the charger to a battery monitoring status independently.
Other chargers provide Temperature sensing to ensure the amperage isn’t allowed to climb too high and overheat the batteries.
With all the associated 12 volt accessories that RVs have today, we rely on having a fully charged battery bank when travelling. Having the right 240 volt smart charger that suits your set up will ensure you get the maximum years of service from your batteries. And that’s a saving you can take to the bank.
Safe and happy RVing!