When is a Battery Cold?

We all know that you cannot charge Lithium Batteries at low temperatures. Some manufacturers claim that you charge at 0 Centigrade others at 5 Centigrade. No matter what the claims the physics of the battery doesn’t change. So, let’s stay on the safe side and go with Victron’s recommendation of 5 Centigrade.

We have been installing Lithium batteries for some time now and the issue of cold charging has always been present. In many applications it isn’t a problem as the vessels in question are usually in constant operation and the battery location is usually warm. However, as the batteries are now being installed on more leisure boats the issue of cold temperature charging needs to be solved. In the recent Victron Video we proposed a system to heat the batteries using excess energy available from either the engine alternator or battery charger when connected to shore power. During our research whilst working on Pilgrim we obtained some interesting data, which leads us to the question when is a battery cold?

The Victron Batteries have built in Blue Tooth which via the Victron Connect App tells you the batteries temperature, or does it?

Here is a graph of the battery temperature on Pilgrim. As you can see on New Years Eve the temperature fell to below 5. However, the battery system did not stop charging. Apart from the period over New Years Eve & New Years Day the temperature remained above the 5 degree mark, and once we started working again on the 6th January you can see the temperature rise again as we switched the heating on, finally settling out at 10 C this evening.

The Victron Batteries have built in Blue Tooth which via the Victron Connect App tells you the batteries temperature, or does it?

Now we might have an explanation here. You can see from the Victron Connect App that the battery temperature is showing 17 C yet the graph on the portal shows a temperature of just 10 C, some 7 degrees cooler.

Clearly If the battery thinks the temperature is higher than it is then it will continue to allow charging. So why is there such a difference in temperature? The answer lies with the position of the temperature sensor. The temperature on the graph is taken from a sensor mounted on the negative battery terminal. The temperature on the App is taken from a sensor mounted on the circuit board of the BMS.

In a Lithium battery the job of the BMS or Battery Management System is to keep the individual cells at an equal voltage. It does this by diverting current from one cell to another as required by means of FET (Field Effect Transistors). Like all semiconductor devices, or come to think of it, any electrical device, they become warm when a current passes through them. Just look at the two screen shots below.

In this test one of our batteries was put on charge. At the start of the test shows the temperature is 10 C. Yet after placing on charge the temperature rises to 14 C after just 7 Minutes. If you look at the cell voltages you can see that they are slightly out of balance, only by 0.01 volt, but the FET’s are trying to balance the cell voltages and as a result they are warming. After 10 minutes the temperature has risen to 17C!!!

So, we have the problem that we don’t really know the temperature of the battery.

So, what is the temperature of the battery? The answer is that we really do not know. In the Marine environment in a Cumbrian winter the batteries whilst left on charge will probably think that they are a cosy 10 degrees, but in reality, they may only be 2 or 3 degrees. If the boat is left in a cold period, off charge, then the core of the battery could easily drop to Zero or even go into minus figures. Switching on the heating will warm the air around the BMS circuit board a lot quicker than the mass of the battery this would then allow charging well below 5 Centigrade.

In conclusion, relying on the batteries temperature sensing system is not a great idea. Using the external sensor connected to the battery negative gives a truer representation but even then, it will not give a true representation of the actual battery temperature. Turning on the heating on a cold winters day will warm the cabin fairly quickly, the internal battery sensor will take a little longer, the external sensor possibly longer still, but the core of the battery will remain well below a safe charging level for much, much longer.

This is the first of a series of articles looking at the practical side of Lithium Batteries on board boats. It is a basis for discussion and any constructive comments and welcomed.