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12V lithium LiFePO4 batteries fully charged up will hold their voltage between 13.3-13.4v, while their lead acid counterparts would fall between 12.6-12.7v. A lithium battery at 20% capacity holds its voltage around 13V while 11.8V for its lead acid counterpart at the same capacity - suggesting there may only be an extremely narrow window of opportunity in terms of voltage with lithium; less than 0.5V across 80% capacity can make all the difference!

LiFePO4 battery normal charger are similar to lead acid systems; their main difference lies in providing higher voltage per cell with tighter voltage tolerances, and no trickle or float charging at full charge. Although lead acid offers some flexibility when setting cut off voltage levels, manufacturers of LiFePO4 cells must follow very exact voltage settings due to overcharging restrictions from Li-ion's inability to accept overcharge. Therefore, miracle chargers that promise longer battery life or extra capacity through pulses do not exist as LiFePO4 systems simply take what their cells can absorbs without adding on extra.

Lithium chargers use the CV/CC (constant voltage/constant current) charging algorithm. In this system, current is restricted until battery voltage reaches its pre-set threshold; then gradually decreasing until fully charged state is reached; finally it gradually decreases thereafter as fully-charged state occurs allowing fast charging without risk of overcharging and suitable for Li-ion as well as other battery types.

Two Step Lithium Charger Algorithm

As evident from the chart, lithium batteries display a sudden rise in voltage at the conclusion of their charge cycles, when their charge current drops sharply before switching over into power supply mode and thus becoming supply mode instead of charge mode.

Modern lead acid smart chargers typically utilize specific algorithms tailored for Flooded/AGM/Gel batteries that typically require three stages for charging: Bulk/Absorption/Float. Once entered into bulk mode, charging typically occurs up to approximately 80% capacity before transitioning into absorption phase and remaining there for about 4 more minutes until transitioning back over into bulk mode again for full current charging at full current capacity for approximately another 8-10% before transitioning off into absorption stage for final float charge process.

Algorithm for Lead Acid Chargers

At first, in this charge phase, the charger will maintain maximum voltage for its chosen battery while providing reduced current due to internal resistance preventing maximum output charging current from reaching it. When current falls to around 10% of total output from charger, absorption stage transitions into float phase. Time-dependently if absorption stage lasts beyond four hours it automatically switches into float stage; typically occurs if charger undersize for battery bank, or there are loads on system which prevent it from decreasing below transition point current levels.

Most lead acid chargers include an equalization mode that cannot be disabled manually; some chargers even feature this function automatically and cannot be turned off manually. Lithium batteries do not require equalization; applying equalizing charges over 15v would damage them beyond repair.

Lead acid chargers also include the "return to bulk" voltage as a crucial function. A completely full lead acid battery's voltage should be approximately 12.7V; with the charger set into "Float," however, the voltage will usually remain within an acceptable range (typically 13.7-13.8v depending on battery type) while supporting any loads running simultaneously and supporting their load capacity as loads exceed its maximum output in float mode; at which point, once that threshold voltage has been met the charger will begin a new cycle and begin charging the battery again recharging it back up again!

Lead acid chargers usually set their return to bulk voltage at 12.5-12.7V; this voltage for lithium batteries is much too low; at that level they would have depleted to 10-10% state of charge and required charging algorithms would set 13.1-13.2V as return-to-bulk voltage settings instead. Therefore standard lead acid chargers do not accommodate lithium battery needs properly.

Some lead acid chargers "ping" the battery upon starting up in order to ascertain its voltage/resistance characteristics and determine what charge phase to begin charging into. Because lithium typically holds voltage above 13v for extended periods, some lead acid chargers will assume this as being near full battery capacity and enter directly into its float stage rather than start the charge stage process again.

You may use a lead acid charger on a lithium battery; however, only those without automatic equalization modes that cannot be permanently deactivated should do so. A charger that charges no higher than 14.66v should only be used regularly and then disconnected after it has fully charged; do not leave connected indefinitely as many do not maintain appropriate voltage charge algorithms and may cause permanent damage without warranty cover.