Batteries Part 2 – State of Charge

LiPo battery charging may be considered in two parts:

• Get the bulk of the charge in relatively quickly (Constant current Vs constant voltage)
• Work with each of the individual cells in turn to both make sure that they :
  • are all at an equal state of charge; and
  • have all been charged up fully.

Question:

How can we know when a battery has reached a full state of charge?

Considerations: (Recall the model of an ‘ideal’ cell in series with an internal resistance)

If for any reason the battery has an elevated internal resistance, when being charged it will appear to be more fully charged than it really is. Likewise on discharge it will appear to be more discharged than it really is. This means that terminal voltage may not be the most accurate method of determining state of charge.

If there is any load applied to a battery during the charge cycle, of if the battery develops internal problems then reduced or no charging current when in constant voltage charge mode may not be a very accurate method of determining state of charge.

If a battery has developed internal issues, and for example is heating up, or gassing, then measuring the number of Amp Hours supplied to the battery, and comparing this value with the nominal capacity of the battery may not be an accurate method of determining state of charge.

Because although the internal resistance of a battery tends to track the state of charge of a battery, it is also impacted by abnormalities or issues internal in the battery, and therefore measuring the internal resistance of the battery to determine state of charge is not considered very accurate.

So how can we determine if a battery is indeed fully charged?

The answer is surprisingly simple – use more than one of the methods  in combination with the previously outlined in considerations, looking for supporting consensus!

Battery parameters should be measured before and after every flight, and the trend of the battery logged and noted.

Why?

We are looking for the abnormality as much as the absolute value. (e.g. Trend analysis)

If a battery is regularly checked and reviewed then it may be possible to use only one parameter – say battery terminal voltage – as a quick and meaningful check during field operations.

Why?

e.g. If the internal resistance is within expected parameters, then terminal voltage may be a satisfactory measure of state of charge.

How to determine the most appropriate charge rate for a lithium based battery:-

Note:- The battery charger sets the profile of the charge – you (the operator) need to set the exact parameters you wish to use for this particular battery.

The charge current must limited for lithium batteries. Typically the maximum value is in the range 0.8C to 1C, but lower values may be set to allow a safety margin. Some faster charging may be possible for some batteries, and will be clearly indicated on the battery is this is possible.

Question:

What is ‘0.8C’?

Answer:

It means the battery can be recharged with a current equal to “0.8” multiplied by it’s “Capacity”.