The term split charging is used to describe systems where there are multiple batteries to be charged from a single source, & the batteries need to be isolated from each other so that consumers can only draw down the battery to which they are connected. The simplest of applications is protection of the start battery on a vehicle with heavy or continuous auxiliary loads.

Relay Method

At one time, the traditional method in the vehicle industry was to use a D+ controlled relay. This is accomplished by connecting the starter and auxiliary batteries by means of a relay activated by the D+ terminal on the alternator. Although simple & inexpensive, there are some limitations. Many modern alternators derive the D+ electronically & in some cases the D+ cannot drive a relay. The approach is limited to low current auxiliary charging & often relies on thin gauge wiring to limit the relay contact current. The risk of closure during cranking makes the contacts susceptible to damage. As the contact is always closed while the engine is running, the starter battery can become depleted if the total load exceeds the alternator output.

Diode Method

This method originated in the leisure marine world. It uses diodes, which only allow current to flow in one direction. Two diodes are used, each connecting the charging source to its respective battery. Loads are taken from the battery side of the diode. The voltage drop across the diodes reduces charging effectiveness, which will often cause problems unless accompanied by a method of raising the alternator voltage. Charge distribution to each battery is not prioritised, so heavy loads can imperil the starting battery as all alternator output is diverted to the heavy load.

Hybrid Switch Method

This method is similar to the relay method but has important differences in the way it is controlled. Here, a high current contact is placed between the primary battery and the auxiliary battery. A controller closes the contact only when the alternator, or connected charger, can supply the loads without drawing on the primary battery. Units may be paralleled or cascaded for multiple battery systems & intelligent control allows temperature compensated switching and high voltage protection to be provided, if required. With these systems, provided suitably sized cables are used, there are no charging problems as there is no volt drop, charging can be prioritised, and the auxiliary battery charging current is not inherently constrained. Alongside standard products, Antares’ systems embodying microcontrollers allow customer specified properties to be incorporated into the split charge device.

Line charger, or virtual alternator

In the past, various charge enhancement devices could be connected into the alternator’s regulator to over-ride the standard charge regime with the aim of improving auxiliary charging. This method, although effective, is associated with installation difficulties & consequential reliability issues if not correctly fitted. The idea was to raise the alternator voltage for a period in order to accelerate charging of the auxiliary batteries. Some diode split charge devices (see above) needed this additional alternator control to overcome the diode voltage drop. Modern alternators prohibit the “alternator charge control” approach, for a number of reasons; one of which is the introduction of alternator control from the engine management system. So, newer systems now provide individually optimised charge regimes for each auxiliary battery without interfering with the original alternator; power electronics are used to tailor the regime to each battery. Power electronic systems are significantly more costly than the modified alternator approach, but this cost is not always a necessary burden as correct choice of battery type & cable sizing can remove the need for enhanced charging. Where this is not the case, Antares’ line charger & virtual alternator products allow optimised charging regimes to be applied to individual or banks of auxiliary batteries. Variants of the line charger are specifically designed for remote trailer batteries in HGV applications, allowing long thin cables may be used without detriment.