Electrical Charging.
Split Charging.
If you own a boat then you'll almost certainly have electrically powered equipment installed that you'll want to run when your engine is not running. These can be simple, low consumption items such as lighting through to more power hungry items such as a water pumps, fridge, fan, laptop, TV or sound system.
If you try to power these from your engine starter battery without the engine running then before long you'll have flattened it, leaving you stranded. So the common solution is to fit one or more auxiliary batteries (often referred to as leisure batteries on boats) that are used purely to power those items not related to the starting of the engine. But once you have a second battery installed, how do you keep it charged up in the same way that the starter battery is? This is what split charging is all about and this article will give you an overview of the various technologies commonly used, as well a some pointers to consider when specifying your own system.
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If you try to power these from your engine starter battery without the engine running then before long you'll have flattened it, leaving you stranded. So the common solution is to fit one or more auxiliary batteries (often referred to as leisure batteries on boats) that are used purely to power those items not related to the starting of the engine. But once you have a second battery installed, how do you keep it charged up in the same way that the starter battery is? This is what split charging is all about and this article will give you an overview of the various technologies commonly used, as well a some pointers to consider when specifying your own system.
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What is split charging?
Split charging is the term used to describe the simultaneous charging of the vehicle starter battery and the leisure battery (or batteries) from a common charging source. On boats this charging source is almost always the vehicle alternator which is in turn powered by the engine, but there can also be other charging sources not related to the engine such as solar or wind powered re-charging systems and portable generators.
Modern split charge systems typically have a device that connects the starter and leisure batteries only when a charging source is operating, and electrically isolates them otherwise, ensuring that the use of one battery does not draw current from the other. It is important to ensure that the starter battery is isolated so the engine can always be started, but you also don't want the leisure battery to be accidentally drawn upon when starting the engine because dedicated leisure batteries are not designed for high current output over short periods of time and can be damaged.
Split charging is the term used to describe the simultaneous charging of the vehicle starter battery and the leisure battery (or batteries) from a common charging source. On boats this charging source is almost always the vehicle alternator which is in turn powered by the engine, but there can also be other charging sources not related to the engine such as solar or wind powered re-charging systems and portable generators.
Modern split charge systems typically have a device that connects the starter and leisure batteries only when a charging source is operating, and electrically isolates them otherwise, ensuring that the use of one battery does not draw current from the other. It is important to ensure that the starter battery is isolated so the engine can always be started, but you also don't want the leisure battery to be accidentally drawn upon when starting the engine because dedicated leisure batteries are not designed for high current output over short periods of time and can be damaged.
A few words on battery charging
Before we look at connecting batteries for split charging, it's worth taking some time to understand how a battery obtains charge from the charging source.
12V batteries typically provide a voltage output in the region of 12-13V, depending on their level of charge, and when a charging source is attempting to deliver current to a battery to re-charge it, it must actually provide a voltage higher than that output by the battery in order for charge to flow into and be accumulated within the battery. It might be helpful to think of the voltages as acting in opposing directions with the voltage from the battery acting one way and the voltage from the charging source acting the other. Since a difference in voltage drives current flow (e.g. between +12V and 0V/ground), then with them each delivering identical voltages in opposing directions there is zero current flow. Only when the voltage of the charging source exceeds that of the battery will charge start to flow into the battery.
For this reason alternators will typically deliver current to a battery at between 13.8 - 14.4V to enable them to start charging. A voltage regulator is fitted to the alternator to control the rate of charge and ensure that the upper voltage is limited and does not cause damage to the electrical systems.
It is also important to note that whilst an engine's alternator is supplying a voltage higher than that of the battery, all of the electrical systems are powered by the alternator, not by the battery. With the battery at a lower voltage than the charging source, no current can flow from the battery to the rest of the electrical system because current will be flowing into the battery. A typical charging system arrangement used in vehicles in shown in the diagram below:
Before we look at connecting batteries for split charging, it's worth taking some time to understand how a battery obtains charge from the charging source.
12V batteries typically provide a voltage output in the region of 12-13V, depending on their level of charge, and when a charging source is attempting to deliver current to a battery to re-charge it, it must actually provide a voltage higher than that output by the battery in order for charge to flow into and be accumulated within the battery. It might be helpful to think of the voltages as acting in opposing directions with the voltage from the battery acting one way and the voltage from the charging source acting the other. Since a difference in voltage drives current flow (e.g. between +12V and 0V/ground), then with them each delivering identical voltages in opposing directions there is zero current flow. Only when the voltage of the charging source exceeds that of the battery will charge start to flow into the battery.
For this reason alternators will typically deliver current to a battery at between 13.8 - 14.4V to enable them to start charging. A voltage regulator is fitted to the alternator to control the rate of charge and ensure that the upper voltage is limited and does not cause damage to the electrical systems.
It is also important to note that whilst an engine's alternator is supplying a voltage higher than that of the battery, all of the electrical systems are powered by the alternator, not by the battery. With the battery at a lower voltage than the charging source, no current can flow from the battery to the rest of the electrical system because current will be flowing into the battery. A typical charging system arrangement used in vehicles in shown in the diagram below:
This system uses a Heavy Duty 140 amp simple 'make and break' relay, the coil of which is energised by a signal that is only +12V when the engine is running (usually the charge signal wire from the alternator). When energised, this relay connects the two batteries together and disconnects them when the engine is turned off (it is important that the relay closes only after the engine is running so that the leisure battery is not connected to the starter during engine cranking, so a switched ignition signal should not be used).
A heavy duty version is required so that it can handle the potentially large current that could be passed through it by the charging source.
A heavy duty version is required so that it can handle the potentially large current that could be passed through it by the charging source.
Split charging from both alternator and solar panels.
One of the most regular questions I get asked is, can the battery be charged simultaneously from the solar panel controller and the boats's alternator without damaging the battery or the charge controller?
Yes you can leave the controller connected while it is being charge by another source. The controller will sense another source is charging it and will go into float. It does need to be fused though, so for a 100 watt panel a 10 amp fuse between the controller and panelis required.
Yes you can leave the controller connected while it is being charge by another source. The controller will sense another source is charging it and will go into float. It does need to be fused though, so for a 100 watt panel a 10 amp fuse between the controller and panelis required.
