Most motorhomes have two sets of batteries. The standard "starter" battery is used for starting the engine and running the vehicle electrics.
The srarter battery is kept charged by an alternator turned by a belt connected to the engine crankshaft. Whenever the engine is running, the battery gets charged. There is a charge controller, normally built into the alternator, which monitors the voltage and current and keeps the engine battery at a good state of charge. It's simple and it works well.
Motorhomes and campervans normally also have one or more other batteries specifically to provide power to the habitation parts of the vehicle. They are kept seperate for two reasons:
Firstly, the type of battery needed to supply the huge currents to start an engine is not the same as the type of battery needed to store large amounts of power.
Secondly, if you manage to run your habitation batteries flat, you still want to be able to start the engine to get moving and to charge the batteries up again.
So now we need to have a way to recharge the habitation battery. Traditionally, this was done using a device called a split charge relay. Basically this is just an automatic switch that connects both lots of batteries together when the engine is running and the alternator is giving power. It's a really simple, cheap and reliable system. Although you can pay silly money, a standard split charge relay costs less than £5.
Of course there is a snag. Imagine you have driven to a spot and parked up for the weekend. The engine battery has had the weekend off, so is fully charged, but you have been running lights, powering the heating, charging your phone and whatever else, and the habitation batteries are now at 50% charge.
As soon as you start the engine, you have two sets of batteries with different voltages, different needs for power and possibly different battery chemistry. Even worse, when you first connect them together, the starter battery is at something like 13.8v and the habitation battery is at 12.4v. The sudden surge of current caused by this difference would fry the wiring, blow fuses and possibly damage your habitation battery (the starter battery probably wouldn't mind).
To preven this sort of catastrophy, motorhome makers use quite long but fairly thin cables to connect the split charge relay to the habitation battery. Lots of people think this is penny-pinching, but it really isn't! It is to protect the battery and connectors from damaging surge currents. One side effect of this is that the typical charge current through a split charge relay is about 10 amps. Some rather misguided enthusiasts replace the thin wire with fatter cables (and sometimes a higher capacity relay) to increase the charge rates. Sadly it doesn't often end well. Burned out connections inside the control box, overheating fuse holders, short battery life are all common consequences of doing this.
The next issue is that the alternator is set up to charge the (already full) starter battery. As a result it will quickly go to the float phase of charging, which maintians a voltage of around 13.7v to 13.8v. That's perfect for keeping the starter battery full, but it is nothing like good enough to charge the habitation battery.
I think it's time to give more information about lead-acid batteries and charging.
The most important thing to note is that battery charging is primarily a chemical process, not really an electrical one. The energy is stored and recovered as part of chemical reactions and there is an inherent time delay.
The constructiion of a lead acid battery is basically a box containing sheets of lead in a container full of suphuric acid. The form of the sheets of lead (the "plates") depends on what the battery is designed for. A starter battery, intended to give a huge current for a very short time, will have many thin plates to have as much surface area as possible. A deep cycle battery will have far fewer, thicker plates of slightly spongy lead. That way you get less surface area but more volums for storing the charge for longer. Batteries sold as "leisure" batteries are normally somewhere in between.
This post is about charging the thicker, spongier leisure or deep cycle sort.
To get an idea of the plates, think of something like a Spontek sponge, but made of lead!
When charging the battery, I find it helpful to think of that Spontek sponge starting dry. If you squirt a lot of water at it, it'll not soak in: it will splash all over the kitchen. You have to slowly moisten it until it is ready to absorb large quantities of water.
A battery plate is a bit like that. When you start charging a discharged battery, you have to limit the current and gradually bring the voltage up to something above 12v or so.
Once the pre-charge stage has passed and the battery voltage is in the "normal" range, the charger will switch to bulk charge. Basically this pumps as much power in as it can, as fast as it can manage (though some very powerful chargers will have a current limit).
Eventually, the battery voltage will rise to about 14.5v. At this point, the charger will throttle back and limit its output voltage at 14.5v for a time. This is the absorption stage. Our Spontek sponge is wet through, but we're keeping the pressure up to make sure it is wet right to the corners. Some chargers limit the absorption phase to a set number of minutes, others wait until the current into the battery has fallen to a predetermined level.
One difficuty here is that the charger can't tell the difference between current used to charge the battery and current powering the fridge (or other loads as you drive along). So there is a danger of extending the absorption phase too long, or not long enough.
After the absorption phase comes the Float phase. Basically, the charger now reckons the battery is full and back off the charge current until the voltage drops to 13.7v or 13.8v, then holds it at that voltage more or less indefinitely - or until you turn the engine off.
There is one more phase that not all chargers use and not all batteries are suitable for. This is the Balancing (or Equalising) phase, and basically it is an infrequent (maybe once a month) overcharge to something like 15.5v to 16v that causes excess power to overlow from one cell to the next in the battery, to make sure that all are full. Opinions vary about whether this is a good idea. I don't use it myself.
If you have read this far, you will have seen why a split charge really won't allow the alternator to charge both the starter batter and the habitation battery. Please read the next article for some solutions.
The srarter battery is kept charged by an alternator turned by a belt connected to the engine crankshaft. Whenever the engine is running, the battery gets charged. There is a charge controller, normally built into the alternator, which monitors the voltage and current and keeps the engine battery at a good state of charge. It's simple and it works well.
Motorhomes and campervans normally also have one or more other batteries specifically to provide power to the habitation parts of the vehicle. They are kept seperate for two reasons:
Firstly, the type of battery needed to supply the huge currents to start an engine is not the same as the type of battery needed to store large amounts of power.
Secondly, if you manage to run your habitation batteries flat, you still want to be able to start the engine to get moving and to charge the batteries up again.
So now we need to have a way to recharge the habitation battery. Traditionally, this was done using a device called a split charge relay. Basically this is just an automatic switch that connects both lots of batteries together when the engine is running and the alternator is giving power. It's a really simple, cheap and reliable system. Although you can pay silly money, a standard split charge relay costs less than £5.
Of course there is a snag. Imagine you have driven to a spot and parked up for the weekend. The engine battery has had the weekend off, so is fully charged, but you have been running lights, powering the heating, charging your phone and whatever else, and the habitation batteries are now at 50% charge.
As soon as you start the engine, you have two sets of batteries with different voltages, different needs for power and possibly different battery chemistry. Even worse, when you first connect them together, the starter battery is at something like 13.8v and the habitation battery is at 12.4v. The sudden surge of current caused by this difference would fry the wiring, blow fuses and possibly damage your habitation battery (the starter battery probably wouldn't mind).
To preven this sort of catastrophy, motorhome makers use quite long but fairly thin cables to connect the split charge relay to the habitation battery. Lots of people think this is penny-pinching, but it really isn't! It is to protect the battery and connectors from damaging surge currents. One side effect of this is that the typical charge current through a split charge relay is about 10 amps. Some rather misguided enthusiasts replace the thin wire with fatter cables (and sometimes a higher capacity relay) to increase the charge rates. Sadly it doesn't often end well. Burned out connections inside the control box, overheating fuse holders, short battery life are all common consequences of doing this.
The next issue is that the alternator is set up to charge the (already full) starter battery. As a result it will quickly go to the float phase of charging, which maintians a voltage of around 13.7v to 13.8v. That's perfect for keeping the starter battery full, but it is nothing like good enough to charge the habitation battery.
I think it's time to give more information about lead-acid batteries and charging.
The most important thing to note is that battery charging is primarily a chemical process, not really an electrical one. The energy is stored and recovered as part of chemical reactions and there is an inherent time delay.
The constructiion of a lead acid battery is basically a box containing sheets of lead in a container full of suphuric acid. The form of the sheets of lead (the "plates") depends on what the battery is designed for. A starter battery, intended to give a huge current for a very short time, will have many thin plates to have as much surface area as possible. A deep cycle battery will have far fewer, thicker plates of slightly spongy lead. That way you get less surface area but more volums for storing the charge for longer. Batteries sold as "leisure" batteries are normally somewhere in between.
This post is about charging the thicker, spongier leisure or deep cycle sort.
To get an idea of the plates, think of something like a Spontek sponge, but made of lead!
When charging the battery, I find it helpful to think of that Spontek sponge starting dry. If you squirt a lot of water at it, it'll not soak in: it will splash all over the kitchen. You have to slowly moisten it until it is ready to absorb large quantities of water.
A battery plate is a bit like that. When you start charging a discharged battery, you have to limit the current and gradually bring the voltage up to something above 12v or so.
Once the pre-charge stage has passed and the battery voltage is in the "normal" range, the charger will switch to bulk charge. Basically this pumps as much power in as it can, as fast as it can manage (though some very powerful chargers will have a current limit).
Eventually, the battery voltage will rise to about 14.5v. At this point, the charger will throttle back and limit its output voltage at 14.5v for a time. This is the absorption stage. Our Spontek sponge is wet through, but we're keeping the pressure up to make sure it is wet right to the corners. Some chargers limit the absorption phase to a set number of minutes, others wait until the current into the battery has fallen to a predetermined level.
One difficuty here is that the charger can't tell the difference between current used to charge the battery and current powering the fridge (or other loads as you drive along). So there is a danger of extending the absorption phase too long, or not long enough.
After the absorption phase comes the Float phase. Basically, the charger now reckons the battery is full and back off the charge current until the voltage drops to 13.7v or 13.8v, then holds it at that voltage more or less indefinitely - or until you turn the engine off.
There is one more phase that not all chargers use and not all batteries are suitable for. This is the Balancing (or Equalising) phase, and basically it is an infrequent (maybe once a month) overcharge to something like 15.5v to 16v that causes excess power to overlow from one cell to the next in the battery, to make sure that all are full. Opinions vary about whether this is a good idea. I don't use it myself.
If you have read this far, you will have seen why a split charge really won't allow the alternator to charge both the starter batter and the habitation battery. Please read the next article for some solutions.