Can you Charge Lithium Ion (LiFePO4) Batteries with a Lead Acid Charger

Author: Solar 4 RVs  

Charging Lithium Ion (LiFePO4) Batteries with Lead Acid Charger: A Comprehensive Guide

The rise of lightweight and efficient lithium batteries has sparked a common question: Can you charge a lithium battery with a lead-acid charger? With a multitude of expert opinions, we aim to provide clear, practical advice for users to make an informed decision.

First, let's understand the charging preferences of a lead-acid deep-discharge battery like Absorbent Glass Mat (AGM):

  • A 12V nominal pack made of 6x 2V cells ideally desires a bulk charge of 14.4 to 14.7V, varying by the manufacturer.
  • Upon reaching the required bulk voltage, the pack needs to stay at that voltage for a substantial time to achieve a 100% State of Charge (SoC).
  • They prefer a float voltage of 13.5V to 13.8V, depending on the manufacturer.
  • For prolonged float periods, a top-up absorption charge is necessary to keep them full.

Now, let's look at lithium:

  • 4x 3.2V cells in series create a nominal 12.8V pack (we're focusing on LiFePO4, the most common deep-discharge lithium battery for RV use).
  • Lithium batteries last longest at approximately 40% SoC and show no memory effect.
  • For increased lifespan, manufacturers recommend using lithium batteries between 20 – 90% SoC, roughly equal to 12.8V to 13.5V. However, most chargers will charge to 14.2V, allowing for cell balancing (not covered here).

What does this mean?

Lead-acid batteries prefer higher voltages, while lithium batteries favor lower voltages for maximum lifespan. However, many lead-acid chargers offerBattery Charger with adjustable voltage and current programmability and GEL battery options. A typical lithium battery charger provides a bulk charge of 14.2V and a float of 13.5V, similar to a gel battery charger.

Although some lithium chargers charge up to 14.6V, we advise against this at Solar 4 RVs. The gain in SoC is less than 1% compared to 14.2V, but voltage-induced stresses can decrease the battery pack's lifespan.

If your gel charger's settings are 14.2V and 13.5V, the only difference is the longer absorption period for the gel battery, assuming the equalization mode on the lead-acid charger is disabled.

For safety reasons, some lead-acid charger manufacturers don't allow lithium battery charging. But if your charger manufacturer permits it, the difference in lifespan will likely be minimal. However, considering the high cost and the difficulty of changing lithium batteries, a programmable lithium-compatible charger can provide the best lifespan.

Do Some BMS' Allow for Lead-Acid Charging?

No, the BMS is irrelevant.  The purpose of BMS is to be the final line of protection for the lithium cells.  It performs no regulation.  In a perfectly designed system with no issues and care taken by the user, its protections should never operate.  

Do Some LiFePO4 Cells Allow for Higher Voltages to Accomodate Lead-Acid Charging?

No, all LiFePO4 being the same chemistry will suffer from a decreased lifespan at higher voltages.  There are novel forms of lithium that can accomodate higher voltages, but these are not found in common lithium batteries.

Final Considerations

Given the distinct chemistry, voltage, and charging requirements of lithium and lead-acid batteries, charging a lithium battery with a lead-acid charger can potentially cause permanent damage or even pose a safety risk. If your gel charger has the same voltage settings as a lithium charger and the equalization mode can be disabled, the lifespan difference could be minimal.