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Charging a Leisure Battery Bank | RV Battery Charging

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Charging a leisure battery bank in motorhomes, RVs, campervans and caravans is essential.

Without an effective means of replacing the energy used, the batteries will flatten. 

Not only is it important to recharge them so you can continue to use them, but if left too deeply discharged for too long, their lifespan will shorten.

This post is an introduction to the 3 ways of charging a leisure battery bank.

Build your own camper quick start guide

Overview of Campervan Batteries

The leisure batteries we install in our campers aren’t the same as the starter battery under the hood.

Starter batteries are designed to provide a big burst of energy, fast and use as much as 20% of their total capacity per start.

Once the engine starts up, the alternator recharges the starter battery so it’s primed for the next time you start it up.

Unlike starter batteries, a deep cycle battery (or leisure battery) is designed to release energy in a steady flow over a longer period. 

They can discharge a lot of their energy in a longer, deeper energy drain, something that would kill a starter battery in quick time.

This post focusses on ways to charge these leisure batteries, often referred to a house batteries, 12v batteries or deep cycle batteries, just to keep us on our toes!

There are 4 types of leisure battery on the market:

  • Flooded lead acid (FLA)
  • Gel batteries
  • Absorbed glass mat (AGM) batteries
  • Lithium-ion batteries

Each of the 4 battery types uses slightly different chemistry so the way they work and perform is different too. 

For the purposes of this post, that’s all you need to know but for more information on the differences between them and joining multiple batteries to create a battery bank, check out our article on campervan batteries.

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How Does a Battery Get Charged?

Lithium batteries are similar to your laptop battery - charged or not, on or off

A battery is an electrical store of DC power measured in amp hours. 

Some of that electrical store is used up by your camper’s 12v appliances. To replace the used energy, we need to recharge it.

Batteries draw current from an available voltage supply, replenishing the energy.

Think of battery like a sponge.

Place a slightly damp sponge on water and it quickly soaks up a lot of water.

A deeply discharged battery behaves this way too. Provide it with a voltage supply and it draws as much amps as it can.

As a sponge becomes more saturated, the rate at which it continues to absorb water slows. 

A battery behaves this way too. The charging rate slows the more charged it becomes.

Once a sponge is completely saturated, it doesn’t take on anymore water. Left in the water though, it never dries out either.

It’s the same for the battery. Once it’s fully charged, even with a voltage supply, it only takes on enough current to prevent it from discharging.

Take a sponge out of water, and over time, it begins to dry out, even without wringing it out. 

Again, a battery behaves this way too. Without a constant voltage supply and even without any load, a battery will slowly release its charge over time.

If the sponge is allowed to dry completely, it becomes dry and hard. A bone dry sponge floats on water and can take a while to begin soaking up water. 

A flat battery behaves in a similar way. Depending on the type of battery, how long it’s been flat for and whether you have luck on your side, a flat battery may never recover.

These different stages of charging a battery are determined by its charging profile.

3 Stage Battery Charging

While all batteries have stages they go through to charge, these stages are slightly different between lead acid and lithium.

The principle though is the same and the detailed charging profile of each battery indicates the greater differences.

Bulk Charge – the battery is low and will take as much current available to it (upto its maximum charge rate). This stage usually brings the battery up to about 80% charge.

Absorption Stage – the battery is so full at the end of the bulk charge phase, its resistance is quite high. To absorb more current, the supply voltage needs to increase. 

It’s a bit like inflating a tyre. The more inflated it becomes, the more force is needed to pass air into it.

As the supply voltage increases, the current falls. This means that while the battery charge is increasing it is doing so at a slower rate than in the bulk charge stage.

As the battery becomes ever more charged, the voltage continues to increase, the current continues to fall and the speed of charge declines. 

The absorption phase continues until the battery is fully charged.

Float Charge – the stage basically maintain the battery in a fully charged state. Just as keeping a sponge in water prevents it drying out, providing a battery with a float charge prevents it from discharging.

3 stage battery charging diagram

Battery Charging Profiles

A battery charging profile is basically an algorithm for optimum charging of the battery.

As you can see from the graph above, there’s a smooth transition between charging stages.

Different battery types have different charging profiles. 

For the most efficient charging of leisure batteries, we ideally want to provide the battery with a voltage that closely matches its charging profile.

lead acid adn lithium battery charging profiles

While the basic principles of battery charging are similar, there are many differences between lead acid batteries (FLA, Gel and AGM) and in particular, lithium-ion batteries.

Lithium-ion batteries hold their charged voltage at about 13v until they’re almost completely depleted i.e. about 1%. 

Their charging profile is quite different to that of the lead acid alternatives.

You can use them soon as you put charge on them, even when they’re extremely low, just like a laptop battery.

Lithiums can take high currents but don’t like high voltages, so their charging profile is significantly flatter and faster.

This often means electrical components aren’t always compatible with all battery types. 

As such, always check components will work with your chosen battery.

3 Ways of Charging a Leisure Battery

Maintaining leisure batteries in a good state of charge will help prolong their life and provide the power needed to keep campervan gadgets and appliances charged and running.

Regardless of the battery type, there’s 3 ways of charging a leisure battery in a camper:

  • Charging batteries while driving
  • Campervan solar panel system
  • Hook up battery charging

A good all-round campervan electrical system set up for those who travel, use campgrounds occasionally and enjoy camping off-grid includes all 3 of these charging methods.

How to Charge a Leisure Battery While Driving

Every engine has an alternator.

It’s basically an electrical generator with a diode pack to convert its AC output to DC. 

When a vehicle’s engine is first cranked, the alternator doesn’t generate any electricity until it reaches about 2000 rpm. 

At 2000 rpm the alternator generates a high enough voltage to strike the diode pack and start outputting DC power.

It then kicks out a constant voltage (usually between 13.8v and 14.4v) until the engine is switched off, even when the revs drop to less than 2000 rpm.

A vehicle uses the generated electricity to recharge its leisure battery and power onboard electrics like the headlights, radio and windscreen wipers.

But we can tap into to this to recharge the leisure batteries too.

There’s 2 methods of using the alternator’s output to charge our onboard leisure batteries:

For more detailed information about each of these methods, click the links on the bullet above.

In summary:

Split charging simply directs the voltage generated by the alternator to the leisure batteries. Cheap, simple solutions include an isolator switch, split charge relay and voltage sensitive relay.

A battery to battery charger controls the voltage sent to the leisure batteries based on their optimum charging profile. This is a more expensive solution but more intelligent too.

So should you choose a split charging system or a B2B charger?

There are some circumstances where a traditional split charging system like an isolator switch or smart relay might be appropriate.

But there are some where they won’t work or fall short.

The type of batteries, the entire charging setup and your intended van lifestyle should help inform your decision.

Here’s a few points to consider when deciding which (if any) is best for you.

Working the alternator

Motorhome or campervan in front of snow covered Scottish Highlands

Both B2B chargers and split chargers make the alternator work harder than it was designed to. In effect this shortens the life of a vehicle’s alternator and replacing them isn’t cheap!

If you can rely on fully recharging the leisure batteries without driving, avoid split charging altogether or at least fit a component you can manually switch off.

Split charge relays don’t full charge leisure batteries

Remember we talked about the bulk stage of battery charging? Isolator switches and other split charging devices only provide a bulk charge to the batteries.

As such, you can never fully recharge the battery bank by driving alone. 

If you rely heavily on charging leisure batteries while driving, a B2B charger is probably essential because it’s the only way the batteries can be fully recharged. Without this, the battery life is negatively affected.

Battery to battery chargers

B2B chargers take the voltage from the alternator and regulate it to closely match the charging profile of the battery bank. 

They’re generally programmable components so configurable for your specific battery.

Lithium-ion batteries

Lithium-ion batteries don’t take kindly to be being overcharged and their charge profile has a much smaller voltage tolerance then the lead acid alternatives.

As such, only use the battery to battery charger recommended by the battery manufacturer. 

Lithium batteries are too expensive to take risks with. 

Smart alternators

Best van for a camper conversion to live in

Many modern vehicles are now fitted with “smart” alternators to improve the fuel efficiency of the vehicle. 

Because they don’t need to generate so much electricity from the alternator, these vehicles are more environmentally friendly. 

Most of what we’ve read about smart alternators suggest you can only charge the leisure batteries with a B2B charger (rather than a split charge relay). 

We’re yet to be convinced on the logic behind this.

We accept the B2B is far better at charging the batteries than a smart relay. 

However, smart relays work by joining the starter battery to the house batteries too. So why would a smart alternator not continue to run? 

Our van doesn’t have a smart alternator so it’s not an issue for us but we think a smart relay (VSR) will work fine, albeit with the limitations of being unable to fully recharge the house batteries.

If you can shed light on this, please leave us an explanation in the comments below. It’s driving us crazy!

Campervan Solar Panel System

Wiring up RV solar panels

A well sized campervan solar panel system is the ultimate way to keep leisure batteries charged for off grid living.

While installing solar panels on a camper is relatively simple, it’s important to size the system to meet your needs and choose the right component parts.

We have an entire series of articles on campervan solar panel systems to help you through the process.

Here’s a few of the key points by way of a summary:

  • Camper solar panels harvest the sun’s energy, converting it to DC power
  • Wiring panels in series or parallel alters the amount of power they can generate
  • A solar charge controller regulates the voltage from the panels for the batteries to draw current from
  • PWM charge controllers are low cost but inefficient, especially for larger solar panel setups
  • An MPPT charge controller is highly efficient at eeking out as much voltage from the panels as it can

It’s important to size a solar system to work with the battery bank. Undersize the panels and you may not fully recharge the batteries. 

Oversize it and it’s a waste of money and roof space. 

Use our solar calculator to size your system.

Hook up Battery Charging

RV hookup to shore power

For those who spend any time on campgrounds with a main power supply, an AC hook up (240v or 110v) is a perfect way of recharging leisure batteries.

With the right set up, you can basically plug in the camper to a mains supply.

AC power is delivered to a breaker box, just as it in a house.

This allows you to use AC appliances on a mains circuit – ideal for running air conditioning units and microwaves.

A battery charger (or power converter), takes the AC power, converts it to DC so charges the leisure batteries too.

The best power converters offer multi-stage charging. These will closely match the charging profile of your battery bank so you can be confident it can be fully charged.

Most custom built RVs and motorhomes include in built battery chargers as standard.

When buying a campervan, new or pre-loved, it’s worth checking if the battery charger is a multi-stage or single stage device.

For more details, check our complete guide to RV converters & battery chargers.

If you’re building your own van conversion, you’ll need to install a hook up facility and a battery charger (often called a power converter).

How we Charge Our Leisure Batteries

Mercedes Sprinter 4x4 camper van driving in Torres del Paine

We’ve lived in our Sprinter van conversion full time since 2018 and live off grid as much as possible.

We rely most heavily on our campervan solar panels for charging the leisure batteries.

320 watts of solar, wired in series with an MPPT controller keeps our 230ah gel batteries nicely charged.

For alternator charging, we use a manual switch. We could never fully charge our batteries from this but because of the solar, we don’t need to. 

And anyway, we don’t drive far enough, often enough even if we had a battery to battery charger.

We also have a hook up capability with a 30amp battery charger we use on campgrounds.

We’ve just survived living in the van in winter (a chilly Patagonian winter) without much driving and no hook up so we’re very happy with our set up.

You can read more about our camper’s electrical design here.

Checking Leisure Battery Charge Levels

A simple voltage meter can indicate the battery charge level. It measures the voltage at the battery terminals.

This is all we use in our DIY van conversion to keep an eye on the charge levels.

But there’s lots of far more sophisticated battery management systems available on the market too.

Many of these even allow you to monitor battery health remotely!

Regardless of what type of lead acid battery you opt for, the charge level of lead acid batteries is indicated by its voltage.

The voltage of a lead acid batteries compared to lithium for their charging states

For the best long term health of lead acid batteries, avoid discharging them over 50%.

Because lithium batteries hold their charging voltage, a battery management system is essential to monitor how charged they are.

Lithium batteries cope well with being fully discharged though.

battery to battery chargers
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