Lithium batteries are an ingenious invention. You will find a robust and powerful power supply here if you often want to be self-sufficient on the road with your motorhome. Unlike lead batteries, the lithium deep cycle battery theoretically never needs to be fully charged. This is even better for the battery than cooking on the charger for a long time when fully charged. Possibly with the wrong voltage. But how do you properly treat lithium batteries in a mobile home? What must be considered when charging and storing expensive LiFePO4 batteries?
Facts and problems about lithium batteries
Facts known today are that the internal pressure of lithium batteries increases with increasing voltage in the battery. In other words, the fuller the battery becomes, the higher the pressure in the battery. High pressures in the battery cause it to age faster. A 12V lithium ion battery with a charging voltage of 14V is technically full; above that, it does not take up much capacity.
Nevertheless, according to the manufacturer’s specifications, the final charging voltage (absorption voltage) must be observed so that the internal BMS can balance the cells in the battery. If the battery has a battery computer and can show the state of charge via display or app, then the state of charge display is also recalibrated at this voltage.
The cell drift and the BMS’s resulting necessary cell balancing are not bad. Four weeks of continuous operation without full charge only lead to a small cell drift, balanced again after 1-3 full charges, depending on the BMS.
The problem with the battery computer
No matter where it is used, the battery computer adds up the incoming and outgoing currents and then displays this in the state of charge in % (SOC = State Of Charge). Temperature, current and self-discharge, and small currents ensure there will always be some deviation. And so, every day the battery has not been fully charged, the display shows more deviation from the actual state of charge. The same happens when a battery is not used at all and is simply stored. The batteries’ internal power consumption and the cells’ self-discharge mean that most battery computers still display 100% after weeks, even though the battery is already clearly discharged. Stupid – or not?
I know the problem, and when customers call me, I don’t care about the % charge level. I always ask about the battery voltage. This one never lies! If the battery has a voltage of 12V without a current drain, the state of charge is below 5%, no matter what the battery computer says! If the battery has an open-circuit voltage of 13.4V or more, it is almost full, no matter what the battery computer says!
Determine the state of charge of the LiFePO4 battery based on the voltage
Determining the state of charge based on the battery voltage is not easy. But for a rough estimate, it should be sufficient. The open-circuit voltage must be considered. So without charging or discharging current!
Determination of the SOC based on the voltage
13.4V and more: battery full
13-13.3V 30-95% state of charge
13V about 30% state of charge reached
12V less than 5% state of charge of the lithium battery
11.5V, in turn, switches off the BMS on most batteries.
The following findings result from the known facts and problems:
- Lithium iron phosphate batteries never need to be fully charged, which would benefit longer battery life, but somehow they have to.
- Battery computers miscalculate over time.
- Occasional full charging is important for cell balancing.
- Even with the best care, a battery ages and loses some of its capacity yearly.
From this, I have derived the following recommendation. It makes sense to fully charge the battery at least once every 1-2 weeks – if used! If the battery is not used because the vehicle is in winter storage, neither cell drift nor the battery computer is a problem, and the battery can be stored at “almost full.”
Charge LiFePO4 batteries correctly
LiFePO4 batteries are fairly straightforward when it comes to charging. Just charging can be done with any charger that can also charge lead-acid batteries. Many manufacturers also write that the LiFePO4 batteries can be exchanged 1:1 for the lead batteries. If you only plug in the battery at 230V for charging when necessary and then drive on again, you do not need a special charger. If you want to stay at the socket for three weeks at the campsite, as with the lead battery out of old habit, you should consider a suitable 230V charger.
My tip for the correct charging of lithium batteries
What has proven to be good are the Victron Chargers. The IP22 series is inexpensive and easily programmed via an app. The IP43 chargers from Victron are fanless and can even be used worldwide in a broadband version. With the app, you can set your values and set Victron’s own LiFePO4 characteristic with 14.2V absorption voltage and 13.5V conservation voltage.
My idea is to lower the charger’s float voltage from 13.5V to 13.3-13.4V. As a result, the battery is first fully charged once but then slightly discharged again. This also allows the charger to stay connected to power for longer without unduly stressing the battery.
If, for example, the journey continues after three weeks of shore power at the campsite, the charger can simply be disconnected from the 230V mains for a short time the evening before. This starts a new charging cycle, and the battery is fit for the next day. Then the battery computer is 100% correct again.
Charge LiFePO4 with solar
Of course, if a solar system is installed, the solar controller would fully charge the battery daily, making the charger setting ineffective. Switching off the solar system helps here; with Victron, this can be done easily on the controller using an app or changing the solar controller. So that the regulator does not start a new charge cycle every day after sunrise, the absorption voltage and float voltage should be set to 13.4V. If the 230V charger is set 0.1V lower than the solar controller, you even have a solar priority circuit during the day 😉
Therefore, proper lithium battery charging requires a suitable solar charge controller. Since this could stress the battery every day, in my opinion, the regulator must supply a trickle charge voltage of 13.5V. If it doesn’t, it must be replaced. Good MPPT solar charge controllers are available from Victron Energy. With a 5-year manufacturer warranty and Bluetooth, these devices are the best you can buy today.
Charging with charging booster or alternator
The alternator is the third source of charging for many. Vehicles before Euro 6 usually have a fixed charging voltage of 14.4 volts. If you drive it for 10 hours, the battery will be stressed with this high voltage once it reaches 100%. When that happens, it’s not as bad as driving 10 hours a day.
A switch in the D+ line, which supplies the cut-off relay, can help, or you can retrofit a charging booster. But be careful with the first solution when the isolating relay is in the EBL. Here D+ also controls the retraction of the step and satellite dish and often also the switching of the refrigerator to 12V. The charging booster can be used to charge with a higher charging current (especially if the booster is connected with its cables between the starter and the onboard battery) than possible using the cut-off relay alone. A booster can also be set to the characteristic curve that suits the battery, which saves the battery on long journeys.
Store lithium batteries properly if they are not going to be used for a long time
If the vehicle is not used for a long period, or let’s put it this way: If the onboard battery is not used for a long period, then I recommend disconnecting it from the onboard power supply using a battery-isolating switch. This means that it cannot be overcharged uncontrolled or even deeply discharged. The batteries don’t care about low temperatures, so they don’t have to be removed.
Storing the battery for a longer period, for example, over the winter, is as follows:
- Fully charge the battery to the prescribed absorption voltage (minimum 14.2V). Pay attention to the battery voltage, not the % display of the app’s battery computer.
- If available, switch off the solar system (via the controller or by disconnecting the modules from the controller)
- Now switch off the charger and discharge the battery to a voltage of 13.3-13.4V. For example, by switching on the lighting or larger power consumers.
- Now disconnect the lithium battery from the vehicle’s electrical system. The easiest way to do this is with a circuit breaker in the negative line.
- The battery can now be stored for at least six months. An occasional check of the battery voltage cannot do any harm. Up to 13V, the battery is still more than 30% full; if the voltage falls below 13V, it should be recharged once to 13.6V. Stored in this way, nothing can happen to the battery. The battery must not be removed in winter if the ambient temperature does not fall below -30 °C.
Commissioning of the LiFePO4 battery after the winter break
Before the next trip begins, simply switch on the disconnect switch on the battery the day before, put the solar system back into operation, and fully charge the battery on the charger so that the cell balance is ensured and the battery computer is correct again. After a long break, the battery computer is guaranteed to no longer match the battery’s charge level.
Warning
The No.1 cause of death is Undervoltage on the battery due to deep discharge during unintentional storage. If the battery does not have a software circuit breaker installed (like the juice boxes), it must be disconnected from the vehicle’s electrical system if it is not used for longer. Here are a few examples of why this is so important
- The BMS protects the battery by switching off consumers when it is empty, but the end is near if it is not recharged immediately afterward. The BMS’s power consumption is responsible for continuing to discharge the battery after the shutdown, which will inevitably lead to cell death if the battery is not recharged a few days after the shutdown.
- Despite the connected charger, lithium batteries that cannot be charged below 0 °C could be discharged by consumers during longer cold spells in winter.
- A defective charger, an unnoticed power failure at the storage location, or defective solar modules on the solar system can also discharge a battery. If the shutdown goes unnoticed and is only noticed after weeks, it may already be too late for the battery.
- Even if all consumers are switched off: if the battery remains connected to the onboard network, minimal leakage currents from chargers, power inverters, and other devices ensure the battery is discharged. After a few weeks, the battery is empty.
If the battery cannot message its owner remotely that it is a little empty, then the onboard power supply must be disconnected from the onboard power supply to protect your wallet!