Recharge.IT – Nexus 7 – LiPo battery charging and discharging

I have an update on the Nexus 7 battery that I had charged overnight.  The charging was done on my Swallow Advance battery charger which is capable of charging Lithium Polymer batteries among other types.  Now, this tablet is the original one from 2012, and it is supposed to come with a 4375mAh battery.  I was charging it as a 3000mAh – the question is why?  If I am charging a battery of unknown condition, we should not assume that it can accept full charge unless we know something of its condition.  In addition to this, the characteristics of LiPo batteries are probably not well known.  My charger and others in the market will charge at a maximum rate of 1C where the C is the capacity of the battery in Ah, so by choosing 3000mAh, I am setting a maximum charge rate of 3A.  Once I know that the battery is ok, I can choose to charge at its rated value.

Ok, the battery was charged as a 3000mAh battery – and being LiPo, the nominal battery voltage is set to 3.7V and a maximum constant charge voltage of 4.2V.  In comparison to Lithium Ion or LiIo, this is 3.6V and 4.1V respectively.  Exceeding the maximum charge voltage can cause the battery to get into a state of thermal runaway.  You have probably seen on the internet what happens when this occurs.  For this LiPo, it is charged at constant current at the 1C rate until the battery voltage reaches 4.0V.  Then the charger switches to constant voltage of approximately 4.2V.  In reality the battery voltage is dependent on a number of factors, one being the cell resistance.  The charger will stop when the charge current drops to approximately 100mA when the charging voltage or battery voltage equals 4.2V – the battery would then be fully charged.

This morning, the charging had stopped and a capacity was shown as being 3300mAh – meaning that the battery had accepted 3300mAh of charge.  I had then set a conservative discharge current of 0.3A and started a discharge cycle.  Once the battery voltage gets down to 3.7V, it might only have a small capacity remaining – the actual capacity will depend on the battery characteristics which we might not know, but can determine through empirical testing.  In this case, it had already been discharging for 10 hours when I left to play badminton for dinner – it was completed when I returned.  Here is a photo of the battery charger status screen.

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This shows the current battery voltage to be 3.411V, discharge time of 646min 5 secs and 3225mAh of energy taken out.  This means that the battery is in very good condition, especially if I put in 3300mAh and got out 3225mAh.

This time, I have put it on charge again, but now at the battery capacity of 4000mAh.  This should hopefully increase the charge current, but when I was looking at it – the charge current was being limited to about 2A – it could be that the battery protection circuit is doing this (assuming that it has one).

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After 72 minutes, it has already gone into the constant charge voltage stage, has received 1377mAh of charge and current charging current of 0.96A – so now, I wait and see what the capacity gets to – it should be higher than 3300mAh.

Now, what does this mean about the original problem – of the Nexus 7 tablet not charging intermittently.  This test has determined that the battery is capable of accepting and delivering charge – in other words, the battery is in good condition.  As LiPo batteries get old, they will stop accepting much charge and deliver less, but not in this case.

We have tried other AC power adapters and this was still happening.  This rules out the power adapter and the battery – the only thing(s) left are the charging circuit in the Nexus 7.  A check on eBay shows that there is a USB charging port flex cable available for the Nexus 7 1st generation – which this is.  It will cost $18 or so – it is probably worthwhile getting it as this may be the problem.  Anyway, first things first – when the battery is fully charged again, I will reconnect it and make sure that the Nexus 7 turns on and sees that it is fully charged.  If it doesn’t do that – it could mean that something else is wrong – it would not be the first time that a battery voltage monitoring circuit has a defect and keeps reporting that the battery voltage is different to what it is.

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