So lets start with the question, can lithium batteries simply drop in and replace lead batteries?
The Answer: YES!!! No additional components are required:
Lithium leisure batteries are designed to be a replacement for lead leisure batteries. They achieve this by having an inherently closely aligned terminal voltage to that of other lead acid variants of leisure battery including wet, gel and agm types.
Our batteries will charge fully to their design capacity at 14.4V (+-0.2V) and this is the output level of the existing charging circuits of all 12V existing split-charge engine wiring circuits regardless whether the manufacturer has used a fixed voltage or smart alternator charging circuit, since they would have already had to account for that prior.
When solar charging, existing 12V MPPT chargers can charge existing wet, agm and gel batteries, and will therefore charge our lithium just as efficiently in the same way. Existing 12V PWM solar chargers are the same. Many modern PWM or MPPT variants have a separate selection for wet/agm/gel/lithium battery variants to “fine tune” their charging efficiency. In the case of variations having no lithium settings it is recommended to select the setting with the closet match to a 14.4V charge voltage.
Existing mains charging/power supplies that were designed to charge existing wet, gel or agm batteries will also charge our batteries too and as is the case with solar charge controllers above, may have a setting to change between wet/agm/gel/lithium. Where this is the case, select either lithium or refer to the voltage output selecting the closest setting to 14.4V.
Some very old chargers may have been designed with a fast charge and/or a desulphurisation mode sometimes referred to a rejuvenation setting. It is important to ensure these settings are never used since this could result in damage over time to the battery.
Charging Characteristics, charge algorithms and lithium batteries in detail
Let’s now look in more detail what actually changes when the battery is upgraded.
Prior to fitting:
When a lead battery is recharged using its pre-existing charging methods, in the main it is the internal chemistry that dictates the charging cycle you may have read about, ie bulk, absorption and float charge. Indeed, the charger may even go so far as claim to “intelligently” manage such cycles. Be that as it may, it is the battery that dictates this cycle and simply the charger that regulates the current and voltage as the charge proceeds.
When the battery is discharged, it has a low resistance and the bulk of the charge can occur. During this portion of recharge, a constant current phase is seen, and the batteries resistance rises. The last remaining portion called absorption can then occur at a fixed voltage (usually between 14.2-14.8V depending on battery and charger settings). The voltage remains fixed, the current drawn by the battery decreases and at a pre-set current draw, the charger assumes the battery is fully charged and switches to a lower voltage, known as a float voltage (usually between 13.2-13.8V), this protects the battery from self-discharge so that the battery remains fully charged ready for use.
When one of our lithium batteries is substituted, what happens is that at the start the resistance is low, exactly as in the case of the lead battery. However, the difference is that the resistance remains low. So, the charge remains in the bulk phase just as before. However, since the resistance remains much lower, it continues to remain in the bulk phase for much longer. In fact until the battery is almost fully charged. It is only once it reaches around 95% of its entire charge that the constant current phase reverts to the absorption phase of a fixed voltage and the remainder of the cycle completes with the charger then reverting to its float phase.
It is often said a lithium battery does not need a float charge since it is extremely efficient and does not self-discharge in the way a lead battery does. It does therefore not need any protection from self-discharge. If the charger were switched off at this point, the rest voltage of the battery would fall to 13.3V. However, if the charger is left to its own devices to float the battery, the fully charged lithium will fall back to the float level, instead of its natural rest voltage. Within the context of normal leisure use, this is completely harmless to the battery., perfectly acceptable and of no consequence.
Lastly, we look at split charging, in other words charging from the engines alternator whilst driving. Till recently, all RV’s were fitted with a simple relay which simply connects the leisure battery to the alternator when the engine is running. The alternator has a fixed voltage output of 14.4V and this is the perfect level to charge our lithium batteries at. Since it has a low resistance, similar to the level of a lead leisure with a low state of charge, it will recharge extremely efficiently from the alternator because it will effectively be in what would have been the bulk charging stage of the lead over the majority of its charge cycle. Once fully charged, the terminal level will be brought up to and held at 14.4V as the journey concludes. This is again completely harmless within the context of normal leisure use.
Increasingly, vehicles are switching to smart alternators. There are several different versions of this technology. The most popular involve monitoring the vehicle battery temperature and voltage and using a pulse charge current to accurately maintain a constant state of charge. This reduces engine load by saving wasted energy the alternator would otherwise have been generating, thus reduced unwanted emissions. There are others including regenerative breaking. It has meant that RV coachbuilders and van convertors are increasingly finding they are having to use what is known as DC-DC charger to charge the leisure battery instead of the conventional split charge relay. This system is a little more expensive for the manufacturer, however it is by no means less efficient than the former system, in fact it is possible to tailor the output current to better suit the manufacturers alternator and has meant for installations where the habitation electrics are fitted some distance from the alternator, natural voltage drops over wiring runs can easily be overcome. This can allow for greater efficiency in leisure battery charging. In the case where a vehicle has one of these types of chargers (some times also know as a B2B or battery/battery charger), the user should ensure it is set to a 14.4V output, since some chargers have slightly higher settings for certain types of lead batteries such as AGM lead batteries.