Lithium battery flat but not charging? Here's why your BMS may be a factor

16 June 2022 | Paul

As lithium batteries become more popular we have had contact from several customers about deeply-discharged lithium batteries not charging with, what appears to be, a working mains battery charger. To help explain, the following is the scenario one of our customers contacted us with.

The customer had purchased our TN Power Lithium 100Ah in November 2021. It was recently run down completely flat and the customer had removed it to charge it before a trip, using a charger he had purchased from another retailer a few years ago which was suitable for lithium batteries. When the charger was attached and turned on he noticed that it was not putting a charge into the battery. He checked the settings were set to lithium, which they were, and that the unit was getting power. He then tried the unit on a lead-acid battery he also had lying around and changed the charging settings to suit. The charger began charging the lead-acid battery with no problem, so the customer was confident the charger was not faulty.

lithium batteries

So why wasn't the lithium battery charging?

The issue was a mixture of his charger & the battery's internal BMS. All lithium batteries should have a BMS, whether internal or external, and these require the battery to have some power to function. These protect the battery from being charged or discharged when it is unsafe or may cause damage to the battery. Because the customer's battery had run completely flat the BMS had shut the battery down, effectively disconnecting the external battery terminals from the battery cells and preventing any further current flow into or out of the battery.

Some chargers require a minimum battery voltage to be detected before they start charging and this is to make sure there is a battery connected. On a lithium battery with a BMS in 'sleep' mode, it will appear to these chargers that the battery simply isn't there, even though it is physically connected, because no voltage can be detected. To get around this issue, most manufacturers have now set up the lithium charging profiles on their chargers to override this requirement, and they produce a 'probing' voltage to attempt to 'wake up' the BMS.

Lithium batteries Charger

Sadly for this customer in the above example, his charger was of the type that still required a minimum voltage to detect a connected battery, even in lithium mode, so with the BMS 'asleep', it would never send a charging voltage to the BMS to wake it up so that it could start allowing charge into the battery. The customer purchased our Victron IP22 30A Mains Charger which has a BMS 'wake-up' capability, and this solved his problem.

This issue can also afflict some some DC-DC chargers, and in Dec 2021 we had a customer who called after he set off in his vehicle, hoping his flat lithium battery would be charged by his DC-DC charger, only to finish his journey to find it hadn't been. The culprit was the same issue described above.

So what is the solution? The easiest way to prevent this issue is to make sure your lithium battery does not get too deeply discharged. You can use a Victron Smart Battery Protect to prevent you drawing further current from the battery once it hits a set voltage. Whilst it's not unsafe to fully discharge a lithium battery, a device like the Smart Battery Protect will ensure you never get into the situation where your charger cannot wake up your BMS (if your charger doesn't have a 'wake-up' facility).

If you're not using your lithium battery for a few months it's best to store it at around 50% state of charge. This will maximise its life span and prevent the BMS from putting the battery to sleep. If it does happen, however, using a high-quality charger from a good brand should ensure that your charger will output a 'probing' charging voltage, even if it can't detect a connected battery. This will enable the BMS to 'wake up' and allow charge into your battery.

If you have any questions about this subject please do not hesitate to contact our team.