Set alarm thresholds appropriately

An alarm has exactly one purpose: to trigger an action that would otherwise come too late. Every warning that no one can or needs to react to reduces the effectiveness of all others. Therefore, the most important decision is not where a threshold lies, but which thresholds exist at all.

Which alarms are really needed

A useful set comprises four to six thresholds. More will not be read.

What's missing here is just as important: no message for every charging process, none for every time 100 percent is reached, none for intermediate states. These events are normal operation.

The threshold for the state of charge

It is based on technology, not feeling. Lead-acid batteries should not be discharged below half their capacity in continuous operation – that's where the warning belongs. LiFePO4 can tolerate 80 to 90 percent depth of discharge, so a threshold at around 20 percent is sensible here.

The purpose of these values is not to warn just before the end, but early enough to act calmly. Someone warned at 20 percent on a 300 Ah lithium battery bank still has about 60 amp-hours – enough for one night, but no longer for an unlimited time. At 5 percent, the warning is a message, not a decision-making aid.

Voltage thresholds as a second level

The voltage alarms are not a competitor to the state of charge, but its safeguard. They kick in precisely when the amp-hour counting is wrong – for example, because a consumer is drawing power past the shunt or has not been synchronized for a long time.

The lower threshold should be well below the normal operating voltage under load, otherwise it will trigger every time a pump starts up. For LiFePO4, it has limited value because the voltage remains stable until shortly before the end; it warns late there, but at least it does.

The upper threshold is often forgotten and is the more valuable of the two. A voltage that is too high indicates a defective controller or a wrongly set charger – a condition that can ruin a battery bank within hours and would otherwise go unnoticed.

Temperature

WarningFor LiFePO4, the lower temperature limit is not a comfort issue. If charging occurs below the limit specified by the cell manufacturer, lithium is deposited metallically instead of being intercalated. The capacity loss is permanent and not outwardly recognizable. Discharging remains permissible in cold conditions – charging does not.

A temperature alarm alone is not enough if no one is on board. Where a system is charging unattended from solar, charging must be temperature-dependently blocked – the alarm is then the indication for this, not the protection.

The upper limit is also relevant: A battery bank that sits in a closed storage compartment in summer will get significantly warmer than the room next to it. High temperatures significantly accelerate aging.

Hysteresis and Delay

Two settings determine whether an alarm is taken seriously.

The delay prevents short events from triggering. A bow thruster draws the voltage down significantly for seconds, without anything being wrong. A few seconds of delay reliably filters this out.

The hysteresis determines when the alarm resets. If the reset value is directly at the trigger threshold, the message switches on and off every second as soon as the value fluctuates there. A distance of a few percent or a few tenths of a volt puts an end to that.

Acknowledge and Forward

An alarm that cannot be acknowledged will be muted – and thus permanently ineffective. An acknowledgment that mutes the acoustic part but leaves the visual indicator active as long as the cause persists is useful.

Where a system remains unattended for an extended period, critical thresholds should be reported externally. An alarm displayed only on a screen in a closed vehicle or boat is useless to anyone in winter storage. This is the real reason to transfer measured values to a bus system and from there to a mobile device – not for comfort, but for accessibility in case of a fault.

Typical Errors

• Too many thresholds. Warning for every event encourages dismissal.
• Threshold too late. A warning at 5 percent remaining capacity leaves no choice.
• Upper voltage limit not set. The case that destroys the battery bank fastest is the only one not monitored.
• No delay. Every inrush current triggers an alarm.
• No reset hysteresis. The alarm flickers and is switched off.
• Temperature alarm instead of charging lockout. For unattended charging in freezing temperatures, only a lockout provides protection, not a message.