First, know what type of battery you have
Lithium batteries (often LiFePO4, like the stacked rack units many homes use) have a battery management system, or BMS. The BMS watches voltage, current and temperature. If something goes outside its limits, it shuts the battery off from charging or discharging to protect it.
Lead-acid batteries (flooded, AGM or gel) have no such brain. They simply wear out, faster if they are drained deeply or left flat.
Depth of discharge in plain words
Depth of discharge is how much of the battery you use before it is recharged. Using 30% of the battery is a shallow discharge. Using 90% is a deep one.
Lead-acid batteries age much faster the deeper you cycle them. Victron's guidance says ageing speeds up once discharge goes past about 80%, and that a battery should not be left more than 50% discharged for long or left flat at all. Every load-shedding night that drains the bank to near empty shortens its life.
Lithium handles deeper cycling, but it still has a limit set by the maker and the BMS. A system that is too small for the loads will cycle deeply every night and age faster than expected.
Reasons a battery stops charging
On lithium, the common causes are these. The BMS has tripped a protection, often over-charge, over-discharge, over-current or temperature. The inverter's charge settings do not match the battery, so it asks for the wrong voltage or current. The communication cable between battery and inverter is loose or the wrong type. One module in a stack has failed or is out of balance, and the stack is only as good as its weakest module.
A red alarm light on a lithium battery usually means it is in protection or has an alarm. Check the battery manual for what each light pattern means.
On lead-acid, a bank that will not charge or drains within minutes is usually worn out, sulphated from being left flat, or has one bad cell dragging the bank down.
What you can check safely
Do not open the battery, move modules, or touch the cables. Do not reset or change charge settings you do not understand, as a wrong setting can damage lithium batteries.
- Look at the app or screen: battery percentage, charge and discharge current, and any warning.
- Note any red or flashing lights on the battery and take a photo.
- Check that the panels are producing during the day. A dirty or shaded array means little charge.
- Think about what changed: a new heavy appliance, a hot room, a recent grid problem or a firmware update.
- Write down how long the battery lasts on a normal evening compared with a few months ago.
When it is old age and when it is a fault
Batteries lose capacity over time. A battery that holds a little less than it did years ago is normal. One that drops from full to empty in a short time, or will not take any charge, needs testing by a qualified installer who can read the BMS data and check each module or cell.
If the battery is still under warranty, tell the installer. Warranty claims usually need the serial numbers and the history of alarms, so keep your photos.
What a quote visit covers
An installer will check the settings, the communication cable, the battery data and the real load. Often the answer is a setting or a cable. Sometimes it is a replacement module or a bigger bank. Either way, you get the diagnosis in a quote before any work starts.
The job behind this: Batteries not lasting.