Blog - About GWL

BMS123 or GWL CPM? - Deeply discharged cells

BMS123 or GWL CPM? - Deeply discharged cells

06.Nov 2020
| Blog - About GWL

As our customers are well aware, some type of protection module is necessary to use with every lithium battery for prevention of a deep discharge or overcharge.

I will use the example of our customer from Holland to describe how a significantly cheaper GWL CPM protection can often be the better choice than a similar but more expensive BMS123 system, which seems more sophisticated at first glance. Our story begins with the following email:


Dear Jan

After two years of complete satisfaction something is now quite wrong with my batteries – a 200 Ah Yacht Set.

The Lithium batteries have been disconnected all winter and I have checked the voltage regularly – always above 12,5 V. However, this spring suddenly they cannot keep up the voltage. I have made the following measurements – all in V:

Cell No. 1          3.39                   3.36

Cell No. 2          3.86                   3.35

Cell No. 3          3.37                   3.36

Cell No. 4          2.85                   2.25

Total                13.46                 12.32 

1st column is measured shortly after shore power with charger was disconnected and the 2nd the day after. In the meantime I left the boat with a consumption of approx. 3 A. The low voltage protection is set for 12.5 V and had correctly disconnected the battery. I have indirectly been able to establish that the low voltage protection cut off after only 1-2 hours of operation. What is wrong? Is one or more of the cells bad – is the BMS system not working correctly?

The matter is quite urgent since I will be leaving on our annual cruise.

Best regards, Mr. Lucky


The "yacht set" included the BMS123 battery protection so the battery was seemingly "protected." Why only seemingly though? Let us have a closer look. We requested that the customer send us a photograph and a diagram of the connections. Unfortunately, no diagram existed (!!!) but we did receive a photo : (Picture no. 1, the cells)

We could see it right away. Mr. Lucky incorrectly connected the individual BMS modules so it is actually a surprise this battery made it even the two years. Not only did the BMS not protect the battery at all but it in fact helped destroy it and disbalanced it. The communication wire must be connected precisely according to the manual (into the clip near the center of the module), which it was not. This is once again an attest to the quality of these Winston cells. In spite of poor and at the same time basic installation error, a completely dysfunctional BMS and faulty winter storage of the entire battery, all the cells remained relatively OK. Mr. Lucky was, however, quite close to a fatal disaster.

Cell no. 4 fell down to 2.5V (the permitted range is 2.8 to 3.8V) and the discharge continued. Individual modules of the BMS123 system are still connected to the cells and they cannot be galvanically disconnected. This is why when the voltage of any cell falls below the permitted range, it continues to discharge the cells as it uses them up until the moment it destroys the battery. What is worse, it discharges every cell at a different rate because the individual modules (IN, OUT, middle) have different power usage. We therefore do not recommend using it for seasonal systems, which are shut down for longer periods of time and aren't periodically charged. 

The first aid response to such a battery pack is to immediately disconnect the BMS (it would be so much easier if BMS lead out beyond the cell terminals and wasn't a part of them…) and charge the individual cells using single-cell chargers to full power, i.e. up to e.g. 3.6 or 3.8V. This can only be carried out if the pack is built using prismatic cells and can be disassembled, which is the case for all GWL battery packs.

The user cannot carry this out for batteries, which cannot be disassembled and are closed in so-called black boxes (Pylontech, BYD, BMZ, etc.). Batteries such as these are then usually destroyed definitively or they require a lengthy and expensive service from the manufacturer. 

Since the OUT of the BMS123 module was destroyed beyond repair due to the incorrect connection, we immediately sent the customer a new one, which arrived in Holland the next day after being sent from GWL. Luckily, the other modules remained unscathed in the faulty installation and BMS began working properly after the replacement.

We also recommend a new CPM protection module to the customer along with the repair of the BMS123 and restoration to working order. This one does not connect directly to the battery pack but is connected using cable harnesses. It can therefore be easily disconnected from the battery at any time and replaced by a different one. It does not require any configuration using a computer or a mobile phone. Two mechanical switches located directly on the module are used for the set-up.

The cell balancing function is intentionally missing in the CPM module. Experience shows that permanent balancing on the BMS level is redundant in high-quality batteries and what is more, it is inadequate for larger cells. In short, balancing is not at all necessary in most cases and when it is required, the small balancing voltage of an integrated BMS is not enough anyway.

In addition, CPM does not display SoC (remaining power in a battery), which is an inaccurate number in LiFePO4 cells and is not reliable, which means it is in fact more of a source of uncertainty and complications for the user. 

The major and the single advantage of the CPM module on the market is the absolute protection of the battery it provides. It cannot drain and destroy the battery even during long-term inactivity and storage of the battery, while providing the same level of protection as the BMS123.

Another unique advantage is the option to signal battery status using 8 logical outputs and the direct support of two-coil bistable relays with zero stand-by usage. Furthermore, CPM can be equipped with a supplemental BCC module, which provides remote control access and reading data using TCP/IP and an ethernet interface. 

(See the picture number 2, the table.)

It will be difficult to find a solution that is more reliable for such a low price as the CPM protection module. 


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