Lithium Compatible Alternator Regulators
1st generation alternator regulators with an outline of the situation we are trying to manage: these had a regulator that just limited the voltage output of the alternator, ie it would charge the battery to the design voltage of say 14.0V and hold it there until the engine is switched off. The current supplied by the alternator is limited by the alternator design, engine speed and the ability of the battery to absorb the current, ie the current tapers off when the set voltage is reached. In the context of managing the temperature rise inside the alternator casing, the alternators were designed to recharge the (lead-acid) battery after an engine start, ie deliver high current for a short period of time and then operate at a much lower amperage for basic electrical loads, lights, wipers, entertainment system, etc. In the aforementioned system the alternator will not overheat, but modify the system by adding an auxiliary battery in the vehicle and/or a house battery in the caravan and charging it directly from the alternator could cause very high currents to flow for a long period of time causing the alternator to overheat – see for example the Victron video on YouTube, https://www.youtube.com/watch?v=jgoIocPgOug. LiFePO4 batteries make the aforementioned situation even worse for the alternator as their charging voltage does not rise as much or as quickly as lead-acid in the charging cycle. Making this even worse is if the vehicle owner does a modification to increase the alternator voltage which is done by placing a diode into the field winding circuit causing the upper limit of voltage regulation to increase by the forward voltage drop of the diode; a typical purpose of doing this was to compensate for voltage drop in the long cables from the front of the engine bay to a battery located frequently in the back of a long caravan. Add in the possibility that the vents in a high mileage alternator could be partially blocked further worsens the situation, especially at high temperatures when charging the battery with vehicle at idle.
2nd generation: these have the voltage regulator controlled by the vehicle electronics which are designed to supply the factory standard vehicle and assist in reducing emissions which basically means lowering the voltage once the battery has been charged. So, again the alternator system is not suited to charging additional batteries and modification of the system is more difficult because of access, warranty and emission control legislation which prevents modification of the vehicle systems.
The vehicle alternator regulators have at most a very generic charging characteristic, so will not provide the battery manufacturer’s specified charge characteristics & voltages specified for deep cycle and LiFePO4 batteries. To overcome some of the difficulties outlined above, typically a DC to DC charger configured to specific charge characteristics & voltages is used to charge a deep cycle/LiFePO4 battery from the vehicle alternator. The maximum charge current output by some DC-DC chargers can be set-up to suit the size of the auxiliary battery and/or the capability of the alternator (think of a 1970’s VW combi van conversion) to prevent over-heating of the alternator. Note that some DC-DC chargers have a by-pass setting which directly connects the input of the DC-DC to the output so that all the available current from the alternator is passed to the auxiliary battery until such time that the voltage has risen to a pre-determined voltage at which the by-pass is opened reverting to DC-DC charging – clearly the problem with the by-pass mode is that alternator heating could be a problem if the by-pass is active for a long period of time, eg when charging a large LiFePO4 battery; so specific consideration must be taken of alternator capacity, the type of DC-DC charger and the potential charge time at maximum alternator amps to a drained auxiliary LiFePO4 battery.
When it is possible and permitted the original voltage regulator fitted to the vehicle can be replaced by a far more adaptive regulator that also provides selectable charge characteristics to suit deep cycle and LiFePO4 batteries that can now be used as the vehicle battery for engine starting and standard loads. Wakespeed have been in the forefront here for many years and have two main products, the WS100 and the WS500. The latter is focussed here as it has a CAN Bus interface that allows a LiFePO4 battery management system to set both the voltage limit and current limit out of the alternator. The big advantage here is that the BMS lowers the charge current while the cells are balanced as the balance current is only a few amps, and ideally the charge current is reduced to little more than this to avoid cell voltage runaway which can occur if one cell has a higher charge than the others. Compare this with the old crude (brutal) way of preventing cell overvoltage was to inhibit the charger when the highest cell reached a pre-set voltage, but the problem with this is that the voltage, especially of a too high cell drops rapidly the moment the charging is stopped, so in this situation a BMS that has no control of the charge current can cycle the charging off-on quite rapidly while the balancer is trying to balance the cells. The REC BMS which is programmed with the user set limits for charge current and voltage determines if the charge current needs to be reduced for balancing and topping off the battery pack and communicates the voltage and current limits to the WS500 accordingly. On top of this care of the expensive battery, the WS500 cares for the alternator by monitoring the alternator temperature, to the WS500 will further reduce the current if needed to prevent overheating of the alternator.