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Can I charge my lithium battery with a lead acid charger?

There are a few sellers out there claiming you can use your existing lead acid charger. The reality is that there are only a very limited few that will accurately and safely charge a Lithium Iron Phosphate battery correctly and to full states of charge, whilst doing so efficiently especially when using solar when you want to get the maximum output from it to your batteries.

Lithium batteries require a Constant current/Constant voltage (CC/CV) charge type with simple Bulk, Absorption, Float stages. 

Many lead acid chargers have desulphation and equalisation stages built in, which will pulse high voltages of 15.3-15.8V into the battery. This is really important to correctly charge and to maintain lead acid batteries, to avoid stratification of the electrolyte and ensure proper voltage equalisation of the cells, but are definitely not suitable for LiFePO4 batteries.   It can heavily reduce the life of the cells due to regular over voltage charging or cause irreversible damage to the battery.

Many lead acid chargers have equalization as an automatic stage that cannot be turned off.  When these stages kick in, they will cause pretty much every good quality LiFePO4 battery on the market with an internal BMS to go into high voltage disconnect protection mode at around 15.0V - the battery goes open circuit and the charger and all loads disconnect before starting all over again once it resets (if it auto resets - not all BMS will do this automatically).  For those that don't, the battery is open circuit and turned off till you realised and take action to reset it.  For those that do, you then get an on/off/on/off scenario for many hours leading to poor charge performance and sometimes never getting out of those stages for many hours.

No matter what they say in their marketing (which is really all it is - a made up marketing spiel) about their supposedly magic batteries that you can use any charger on, it is just totally wrong and demonstrates a total lack of understanding of LiFePO4 chemistry, the narrow voltage range that it operates in and the effects of using a multi-stage Lead acid charger to charge LiFePO4 cells.

There is a lot more to it than just outline above, but it does get into a very technical discussion which is outline below if you want more info.

Firstly it's important to understand the voltage range that each battery type operates in.  A lead acid battery typically will be fully charged at rest at 12.6-12.8V where as a LiFePO4 battery will be 13.3-13.4V.  A lead acid battery.  LiFePO4 batteries operate in a very narrow voltage range with only 0.5V from full all the way down to 20% state of charge.  Even at 25% state of charge it will still be at approx 12.8V whereas a lead acid battery at 25% state of charge is around 11.7V with a range of over 1.1V.  
 
Now let's talk about charging...

LiFePO4 chargers in pretty much all the major and reputable brands will limit the charge voltage to 14.6V.  This is important to limit the voltage that is provided to the battery as a whole and keeping the individual series connected cells to a maximum of 3.65V.  Lead acid chargers will regularly charge above this voltage which can over charge cells (if not properly limited by the BMS) or cause the HV disconnect issue outlined above.  LiFePO4 cells need a simple Bulk/Absorption charge profile followed by reverting to a power supply mode (still usually called float but acts differently to a lead acid charger float stage) which holds the voltage at around 13.6V and maintains any loads drawing from the battery.  Absorption stages are extremely short as the voltage increases to 14.6V very quickly at the end of the charge cycle.  

This is different to a lead acid charger which will charge in bulk stage to around 80% at full current and increasing voltage at which time it moves into absorption stage with ever reducing current whilst holding the maximum voltage for many hours.  This can be 2-8 hrs (depending on the charger brand) before reverting to float and does take many hours to fully charge the battery to 100%.  Most are time based and won't ever get a LiFePO4 battery to a full state of  charge.  A lead acid charger continues to put a small current into the battery - think trickle charge regardless of whether there is a load or not to maintain the cells.  If the charger is undersized, then the absorption stage may never actually finished (due to loads on the system) and will revert to float after the pre-set time has passed.  It may not actually get to 100% state of charge.  As the lead acid charger moves to absorption a lot earlier than a LiFePO4 charger does, you always run the risk of never being able to fully charge a LiFePO4 battery with a lead acid charger. 

One of the most frustrating problems with using a lead acid charger to charge a LiFePO4 battery is the pre-set voltage settings in the charge algorithm for re-start the bulk charge stage.  As a lead acid battery is full at 12.7-12.8V, almost all lead acid chargers won't trigger bulk charging to re-start until it drops below around 12.5-12.7V as it doesn't need to recharge until it drops in voltage a small amount from full.   By using this charger to charge a LiFePO4 battery, it won't re-start bulk charging until the LiFePO4 battery has dropped to around 20-25% state of charge at that same voltage as the lead acid battery. You have to use almost all of the LiFePO4 batteries capacity before a lead acid charger will start charging again.  A LiFePO4 specific charger will trigger bulk stage to start re-charging at typically 13.1-13.2V ensuring is starts charging again when it is still at high states of charge, not very low states of charge when it is going to be too late.  Think about this when using solar to charge your batteries.  When the sun comes up in the morning, a lead acid solar charge controller may not start charging at all regardless of the time of day if it thinks the battery is already charged based on the voltage it is reading as the re-bulk voltage hasn't been met.  It will just stay in float stage.  You may loose part or all of the days sunlight before getting any charge into the battery at all, leaving you with little to no charge and potentially flat batteries overnight.

You can use a AC to DC lead acid charger powered from mains power, as charge efficiency and duration are less of a concern, it must not have automatic desulphation or equalisation modes.  If it does, do not use it as there is a high chance of damage to the cells or battery.  This can have significant reduction in battery longevity.  If it has a simple bulk/absorption/float charge profile, then it can be used to recharge the battery but must be disconnected once charged and not left in trickle charge/maintenance mode.  It must also have a maximum output voltage of 14.4-14.6V.  When it comes to DCDC chargers and solar controllers, you must change these to LiFePO4 specific models.  Efficient use of the limited solar wattage and sunlight hours per day or limited hours of driving with charge from the alternator is critical .  You need to make the most of the power from these sources.

Ultimately, if it was as simple as using lead acid chargers, then the likes of REDARC, Enerdrive, Victron, Projecta, Intervolt and every other charger supplier in the country that make LiFePO4 specific chargers would not have spent tens/hundreds of thousands of dollars to develop LiFePO4 specific models.

We get dozens of calls a month from dissatisfied customers (and trade resellers) specifically about this misinformation from sellers of other brands.  They find that their chargers just don't work like they were promised and they have to upgrade charging systems after being fleeced of their hard earned cash by unscrupulous sellers of sub-standard batteries.

If you want the ultimate in LiFePO4 batteries with longevity, reliability and unmatched performance, then contact us today and ask about AllSpark Lithium Iron Phosphate batteries and a charger made specifically for LiFePO4 batteries that are efficient & safe to look after your new investment.

Catchya Offroad
Jason

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