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Has my receiver battery been damaged?

Declan Smith wrote to me asking:

Hello Andrew, would it be possible for you to answer a question for me please? Using my peak detect charger, I placed my 4.8V 700mAh Nicad receiver battery on automatic charge. However, I noticed that the charge quantity (capacity) showing on the charger has exceeded 700mAh and at the moment its sitting at 795mAh with a voltage of 7.39V and a charge current of 0.25A.

I think I heard it venting so I immediately disconnected it. Have I done something wrong here and damaged the battery? I am fairly sure I heard it venting, but I'm not totally sure as I'm in a noisy room. If this battery is now ruined, what could have gone wrong with the automatic charge process?

Andrew's answer

Hi Declan, and thanks for your question. First, the charger's figure of 795mAh is the charge quantity which the charger has delivered to the battery. Because charging of batteries is less than 100% efficient (especially nickel based types), the fact that the charge quantity exceeds the battery's nominal capacity is not automatically a cause for concern. Assuming the battery was totally flat when charging commenced, a charge quantity of 795mAh would not on its own alarm me, but if started out part charged then it probably would. Since the charger was still supplying 250mA after already supply a charge quantity of almost 800mAh this suggests that the battery has probably been significantly overcharged. Nicads are very durable, but your battery may have suffered harm.

A typical receiver battery. But has it been damaged?

In order to charge a battery, it’s necessary to apply a voltage to the battery sufficient to cause a charge current to flow. The higher this voltage, the higher the charge current will be. The battery’s own state of charge also has an effect on its apparent voltage as well of course. Normally, under charging, a nickel-based battery will not be made to rise much above 1.5V per cell. For your 4-cell pack, this would therefore be 6.0V. Slightly higher would not concern me, however a voltage of almost 7.4V represents 1.85V per cell and this is definitely on the high side.

This excessive voltage is consistent with a charge current of 0.25A (250mA) into a battery that is already fully charged. You also mentioned that you used the auto charge programme on your charger. Auto programmes normally work well, but since the charger wasn't manually programmed with details of the battery it was charging, it remained ignorant of the battery's nominal capacity and may therefore have allowed a substantial excess of charge quantity to be fed to it. It seems as though this may well have occurred.

For a battery to vent its electrolyte, it would need to develop a considerable internal pressure. The vent is designed to relieve any such excessive pressure, and if it becomes damaged, for example by heat from a clumsy soldering operation, the battery will be more vulnerable to damage.

Venting would only normally be achieved if a battery were to be overcharged for long enough at a rate such that the excess energy could not be dissipated quickly enough to prevent it from becoming excessively hot. Overcharging is normally considered safe at C/10, in other words a current of 70mA for your 700mAh battery, and at this modest current only a small rise in temperature would occur during overcharging. You mention noticing a charge current of 0.25A at the end of the charge period, so it could easily have been a lot higher than this earlier on in the automatic charge process. From what you've told me, it seems the battery could possibly have become hot enough to vent.

On this subject, its worth noting that I've only very occasionally seen batteries vent, but that was only when they had been badly abused. The last time I can remember this sort of thing occurring to me was back in the eighties when I overcharged an eight-cell pack of 1,200mAh nicad cells using the then-common but very crude charging technique of connecting the pack directly to a 12V battery. No timer was used and the pack accidentally became overcharged. Things then went so badly wrong that one cell in the pack literally exploded. Actually, I suspect that the reason for the explosion rather than a venting episode was that the vent had become damaged and failed to operate.

The receiver battery is a critical link in ensuring that you can maintain control of your model.

Whether or not your battery actually did vent, as it's tasked with powering an RC system, for safety' s sake I would treat it with suspicion from now onwards. At the very least it needs to be carefully analysed for capacity and condition before returning it to service. To carry out this work, I would give the battery a gentle slow charge for a few hours to make sure all the cells are balanced, and then I would give the pack three charge/discharge cycles to check its capacity. I'd also check the pack's charge retention ability by leaving it in a fully charged state for a week and then checking the loss of capacity. There will naturally be some loss, but it shouldn't be more than about 25% or so. If the battery passes all your tests then you might feel its reasonable to keep it in service. However, since there's little doubt that the battery was seriously overcharged, a better plan would be to take the safest option and simply remove it from the safety-critical business of RC system duty. This is because of the obvious risk of losing your model if it fails, and also because of the associated risk to other people from an out of control model.

As to what could have gone wrong with the automatic charging of the battery, there are several possibilities. Clearly your charger failed to detect the voltage peak that naturally occurs when fast charging nicads. Receiver batteries are not always suitable for fast charging so this is another possibile cause of your problem. Also if the charge current was at any time excessive, there's a possibility that the wiring has been damaged.

Less likely is the possibility that the charger may have developed a fault, or the cells of the battery may be badly unbalanced so that the charging peak doesn't show up well enough for the charger to detect it.

The temperature the battery reached would also be a clue; if it was only slightly warm to the touch then I'd be much less suspicious that if it had become hot. If the charger did not detect the change in voltage that typically occurs during fast charging (the 'peak' that peak detect chargers look for), it would not know to stop charging and therefore an overcharge becomes possible. Perhaps the lessons to be learned from this story are:

(a) It’s always safer to manually programme a charger than rely on an automatic programme

(b) Don't leave batteries of any type unattended while they are fast charging. Usually with Nickel-based types we can get away with it, but not always, as you've found out.....

I hope this helps. You'll be able to find a lot more information about Nickel based batteries in the Gibbs Guide to Nickel Metal Hydride and Nickel Cadmium Batteries.

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