Whether as a powerful consumer battery or as an energy source for the electric outboard motor, in recent years lithium iron phosphate batteries, or LiFePO4 or LFP for short, have developed from a luxury option to a widespread energy source on board. What was once considered an expensive and risky modification is now seen as a widely accepted, comfort-enhancing upgrade. Nevertheless, the technology has its pitfalls and can be quickly damaged by incorrect handling in winter; although this also applies to lead-acid batteries, it was common knowledge there and replacement was generally less costly.
The dark side of lithium batteries
Despite all the enthusiasm, there is a rarely discussed downside to LFP batteries: Deep discharge drastically shortens the service life of the batteries and can lead to total failure even more quickly than with lead-acid batteries. During the season, it is not a problem to keep an eye on the charge level. In winter storage, however, it is more difficult, especially as an additional factor comes into play with the lower temperatures.
Virtually all LFP batteries have a battery management system (BMS) that keeps the battery within a safe operating envelope (SOE). It monitors parameters such as voltage, current and temperatures. If one of these parameters reaches a critical threshold value, the BMS intervenes and disconnects the battery from the vehicle electrical system.
There are various systems on the market for this purpose; rechargeable batteries for the direct replacement of lead-acid batteries are usually equipped with transistors, i.e. electronic switches. These MOSFETs can usually interrupt the charging and discharging process separately. If the voltage becomes too high, the charging transistors switch off to keep the cell/pack voltage within the safe range. If the voltage becomes too low, the discharge side switches off to prevent cell damage. Batteries that use relays to control the current flow, on the other hand, completely isolate the battery from the vehicle electrical system.
In view of these sophisticated functions, one might assume that lithium batteries can hardly be over-discharged to the point of damage. This is not always the case in winter.
To understand the details, a few basics are necessary. All batteries have a certain amount of self-discharge. LFP cell specifications typically indicate one to three per cent per month. This value applies to a state of charge between five and 99 per cent. In this range, the voltage curve is relatively flat, as can be seen in the voltage-capacity diagram. The voltage of an LFP battery remains very stable over a wide range. Towards the end of the capacity, however, the curve suddenly becomes steep and the voltage collapses; this is precisely what can become critical in winter.
In the medium charge range, a rest period of a few months hardly plays a role and has practically no influence on the voltage. The situation is completely different when the battery is approaching its capacity limit. Then a few weeks can be enough to cause the cell voltage to drop so low that the battery is permanently damaged. Cold temperatures, which often occur during winter storage, accelerate this development, as the voltage situation deteriorates further as a result.
The BMS and the secret blackheads
An additional factor is the energy consumption of the battery electronics. Things like Bluetooth modules, the BMS itself, indicators and displays all consume some power. Even small internal consumers add up over months. In addition, there are significant differences in self-consumption between battery brands and even models of a single brand. The BMS calculates the SOC using an internal shunt, but this often does not have the necessary resolution to capture the very small currents of self-consumption, which means that the battery charge level displayed by the app can deviate significantly from the actual charge level over time. In addition, some BMSs switch off when the battery is switched off or goes into sleep mode and no longer record the loss of charge over time.
As a result, a battery that is winterised in autumn with an almost full charge may still show 75 percent charge after several months when switched off. However, it may have a voltage of 12.7 volts, which indicates that the actual charge is much lower. Only when the battery is fully charged will the displays synchronise again.
Two causes of battery death
- Lack of residual capacity: The state of charge of the batteries was too low to bridge the self-discharge and self-consumption of the BMS for the duration of storage, especially at very cold temperatures.
- Leakage currents and forgotten consumers: Depending on the manufacturer, the batteries can be switched off manually or put into sleep mode to reduce the energy consumption of the BMS. If this is forgotten, self-consumption can discharge the battery for months, see above. Forgotten consumers or leakage currents from a faulty electrical system can also discharge the batteries more quickly than planned, which is why you should disconnect the vehicle electrical system from the battery via the main switch in winter.
How much capacity is required to avoid the first cause is determined by the self-discharge of the cells and the self-consumption of the battery. As a rule, self-consumption is the bigger culprit. The BMS should actually only consume a few milliamperes and ideally switch to an even more economical energy-saving mode after some time without charging or discharging. However, this does not always work reliably. If the BMS does not go to sleep as planned, it can consume up to 200 milliamps. That may not sound like much, but it is enough to fully discharge a 200 Ah battery in around 40 days of storage. Important to note: Connecting several batteries in parallel does not change this, as each battery has its own BMS and therefore discharges individually.
Discharged in this way, the voltage of the cells is likely to be around 2.5 volts, a value at which most BMS disconnect the battery from the vehicle electrical system in order to protect it from further deep discharge and cell damage. However, switching off the consumers cannot always stop the death spiral. With every hour that the voltage is in the lower range, the capacity and voltage drop at an increasing rate. Eventually, the voltage drops so low that the BMS can no longer be supplied with power. At this point, the lithium batteries are already at risk, as the BMS may no longer be able to be reactivated even if an external voltage is applied. This means that charging from the outside is no longer possible and the battery is usually a case for the manufacturer's support.
Three tactics to keep lithium batteries alive
The instructions for each battery should actually contain information on correct long-term storage. These should be followed, also with a view to possible warranty claims, especially as there are systems with dedicated winter and sleep modes. Unfortunately, practice shows that not every manufacturer takes this issue into account. In this case, these three tactics can be used to prevent deep discharge of the batteries:
1. recharge regularly. Most manufacturers recommend winterising the batteries with a charge level of between 60 and 80 percent and recharging them after three months at the latest. In order to synchronise the actual state of charge with the display of the BMS or an external battery monitor, the batteries should be fully charged and then discharged to the desired storage capacity. To ensure that discharging does not take too long, large consumers are usually required, such as a powerful inverter. Potential problem: Lithium batteries should not be charged at temperatures below zero. It is therefore important to keep an eye on longer periods of frost, especially as it can take a long time for the battery cells to thaw out again. Care should also be taken with batteries with internal heating. If the existing charger has less power than the heating elements require during operation, the battery will continue to discharge.
2. leave on the charger. If shore power is available, self-consumption can be buffered using a charger. It is important to reduce the end-of-charge voltage. Instead of the usual 14.2 to 14.6 volts for lithium batteries, 13 to 13.2 volts should be set. This means that the battery is not charged to 100 per cent and ages less. Don't forget to reset the end-of-charge voltage at the start of the season, otherwise the full capacity will not be available. As the charger is only used to buffer self-consumption, virtually no current flows into the cells, which means that periods of frost are less of a problem.
3. take the batteries home with you. This protects against leakage currents and undetected consumers, but you still have to keep an eye on the self-discharge and self-consumption of the BMS. With frost-free storage, you can at least recharge regardless of the weather.
