Charging Lithium Batteries with a Lead Acid Charger

There is the potential to use chargers that only have Lead Acid profiles, but there are some compromises. Please see below for an explanation of the differences between a Lead Acid profiled charger vs a charger that has a Lithium specific profile, and what the implications are:

1. A Lithium charge profile vs. a Lead Acid profile usually has a slightly higher charge voltage and a "deeper" constant voltage phase at the end of the charge cycle, which is able to get the Lithium batteries to a full 100% State of Charge. A Lead Acid profile will not normally be able to achieve this, however it will still get close, say maybe 95% +. Research has shown that you can extend the life of Lithium batteries if you prevent charging to 100%, however the effect only has a pronounced impact if charged to less than 80%. Between 80% to 100% State of Charge (SOC), there is not much observable difference to the battery life. For this reason in real life cyclic applications, it is not practical to reduce the charged SOC to less than 80%, and therefore you may as well aim for close to 100% SOC to obtain the maximum usable capacity out of the battery. However, a Lead Acid charge profile will not cause damage to the LiFePO4 battery if not charging to a full 100% SOC.
2. The charge profiles have different "return to boost" behavior. After the charger completes its full charge cycle (i.e. it reaches the end of the absorption phase and changes over to the float stage), the charger will then start to monitor the battery voltage, to decide when to switch back to the boost/bulk charge mode and restart a full charge cycle again. The issue is that most Lead Acid chargers will wait for the voltage to drop too low before it restarts the charge cycle - that is because a Lead Acid battery has a lower resting voltage than a Lithium battery. The implications are that the Lithium batteries' State of Charge may already be quite low before the Lead Acid charger decides to switch back into a full bulk charge again. On the contrary, a Lithium profiled charger will start to recharge the Lithium battery much sooner. The Lead Acid charger will eventually switch back into a full charge cycle again, but your battery may not have charged much yet when you would expect it to be fully charged, and when charging from solar this could mean that energy is wasted that could have been used to top up the batteries because the Lead Acid charge profile of the solar controller may not have recognised that the batteries need charging.
   
   There is a potential work-around for this issue which is to shut off the charger (either via switching the charger off or via a kill-switch installed on the positive line between the charger and the battery), allowing you to temporarily disconnect the charger from the battery. Disconnecting the charger and then reconnecting, should reset most chargers and force a full charge cycle again.
3. A Lead Acid charge profile will enter a float stage at the end of the charge cycle. This is designed to trickle charge lead acid batteries to compensate for self-discharge, and any excess energy is dissipated as heat. However, LiFePO4 batteries have a very low self discharge rate which do not need float charging. The issue is that when float charging LiFePO4 batteries, especially if the voltage is above the resting voltage of the LiFePO4 cells, the electrode is likely to produce pure Lithium out of the Lithium ions which causes metallic plating and reduce the capacity over time. This can have a negative effect on the lifespan of the batteries.
   
   Similar to point 2, a work-around is to switch off / disconnect the charger from the battery when it is not being used, to avoid long term float charging. Alternatively, if this is too much of a hassle or you don’t want the compromises, you can change the charger over to one   with a specific Lithium profile.