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Hybrid Battery Bank - Concept and Theory

Lithium Lead Acid Hybrid system.: Text

What is a Hybrid Battery Bank?

A Lithium Hybrid system is a system where you don't replace your lead-acid batteries but just add a preprogrammed Hybrid specific LiFePower battery to your existing lead-acid based installation.

Lithium Lead Acid Hybrid system.: Text

The advantages of a Hybrid installation


You don’t have to change anything in your system to convert to a hybrid installation. You can continue to use the existing alternators, solar controllers, etc. The lead-acid batteries remain connected to the system all of the time, just as before. The lithium batteries are controlled by our BMS which can take them offline whenever it likes to, because the alternators then still "see" the lead-acid batteries which they were designed for. This is the only way to correctly use Lithium power in your system without redesigning the entire existing installation.


You have a plan B. If something fails with the lithium battery or its BMS, the system still has still the unmodified lead-acid batteries to automatically fall back to.  If things go wrong, nothing beats the simplicity of a lead-acid battery.

Optimisation of charge time.

The problem with lead-acid batteries is that they rarely accept the full available charge current after they are charged above 80%. The current tapers off and to get them close to 100% charged means hours of charging, despite the high output alternator you might have installed. But with a hybrid system you usually only used power from the lithium battery so only the lithium battery needs to be recharged, and they accept the full available charge current right up to the moment they are fully charged. This means that the charge time remains as short as possible. Finally you see the full rated output of your alternator flowing into the battery, right up to the end.


Both the Lithium batteries and lead-acid batteries will optimise thier maximum life span because they are operating in a way that suits them best:

The lithium batteries are cycled daily and don’t have to spend much time in a highly charged state, the lead-acid batteries are rarely cycled and spend most of their time being in a float stage of charge.


Because of the  longevity of both batteries in a hybrid installation, it is the most economic configuration possible. Also, installation wise, it is the most economic as no major modifications of the wiring need to take place and no charge sources have to be replaced or modified.


The  system will continue to use standard alternators and other chargers, with standard replacement parts commonly available, with no ties to a specific brand.  This system is now becoming understood by engineers all over the world and can be serviced just like a regular system.

Lithium Lead Acid Hybrid system.: Text

How it works

Operating the lithium batteries and lead-acid batteries in parallel is possible because lithium batteries have a much flatter charge and discharge voltage curve.

The BMS in our Hybrid range has been programmed to the correct settings to complement the existing lead acid batteries.


While discharging, the lithium batteries stay above 13.0 Volts until they are almost empty. The lithium voltage is higher than the voltage of a lead-acid batttery under load, so the lead-acid battery will hardly deliver any current, if anything at all.

After the lithium battery has only about 20% of charge left the voltage becomes low enough to allow the lead-acid batteries to gradually start taking over the load. Only when the lithium battery becomes fully discharged and is taken offline by the BMS, the lead-acid batteries will fully take over. Most of the time, the lead-acid batteries will remain fully charged, which is exactly what keeps them healthy.


While charging, the voltage quickly rises to about 13.4 Volts, a voltage where the lithium batteries absorb all the available current but at a voltage too low for lead-acid batteries to meaningfully charge unless they have been discharged, if the lead acid has been discharged then they will both charge at a similar rate until lead acid is near full.

So the lithium batteries will take up all the current available until the BMS takes them offline, and only then the voltage rises enough to charge the lead-acid batteries.

When charging the lithium battery, the chargers recognize a voltage which is similar to the voltage of a lead-acid battery which is in its early bulk phase, so the chargers are providing charge current to the lithium battery while "thinking" they are charging a normal lead-acid battery, patiently waiting for the voltage to increase.

When the lithium battery is fully charged it is taken offline by the BMS, the charging continues with the lead-acid batteries only, following a charge trajectory which the chargers fully recognize, allowing them to do their end-point-voltage-limiting. If the lead-acid battery has not been used, the voltage will rise quickly to the end-voltage and the charge process will be terminated and revert to a "float" voltage.

As long as there is a charge current available the fully charged lithium battery will remain "parked" aside and the on-board equipment will be fed by the charge sources with the lead-acid battery as a buffer. Only when the system voltage starts dropping below the float voltage, indicating the absence of a charge source, the BMS will put the lithium battery online again.

In practice.

So the lead-acid batteries don't disturb the lithium batteries at all. The lead-acid batteries don't have to be "regulated", they just remain connected to the system all of the time. The whole system works naturally, the BMS only has to disconnect the lithium battery to prevent it from overcharging and overdischarging. Practically, the lithium system is just a simple add-on to an existing unmodified lead-acid installation and is the most easy conversion you can imagine.

In a hybrid lithium/lead-acid installation you are going to use the lithium batteries to cover the short time (i.e. overnight) cycles. The lithium cells discharge deep enough to keep them healthy and the lead-acid batteries are just doing nothing at all, just sitting there fully charged the majority of the time, which keeps them healthy as well. Only when there are multiple days without any means of recharging, which usually does not happen very frequently, the lead-acid batteries are used to supply their reserve power after the lithium batteries become fully discharged.

Lithium Lead Acid Hybrid system.: Text
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