Recharge.IT – The saga of the Nexus 7 battery continues

The Nexus 7 battery being charged by my Swallow Advance battery charger completed charging this morning.

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3328mAh of charge was accepted by the battery – this is a little higher than the 3300mAh charge from the previous day.  If the rated capacity of the 1st generation Nexus 7 is 4375mAh, then this battery’s condition is at about 76% of new.  Each full charge/discharge cycle has a tendency to reduce the lifespan and capacity of a battery.  Most batteries of this type should provide 100% capacity during the first year of its life.  However due to shelf life and storage before use, this capacity may be lower than the rated capacity.  As this battery is at least 2 years old by now, a 76% capacity is actually not too bad and is reasonable for its age.

If you wish to prolong Lithium batteries, you should charge them frequently, but not to full charge, and don’t discharge them all the way.  By having shallow charging and discharge cycles, the life of the battery can be extended.  The downside is that you won’t be able to run the whole day on them otherwise they will go flat.  Always there are advantages and disadvantages, so it is best just to use your normal routine.

Ok – I reconnected the battery to the Nexus 7, and put the back cover on.  I had to press the power button for 30 seconds before it would turn on.  After waiting a while for the screen colour blobs to go around and around, the normal screen came up.  I plugged in the Asus charger and the Nexus displayed “Charging 93%”.  The other applications started up, together with the wireless, and the display was now “Charging 92%”, etc.

After an hour, I noticed that it wasn’t charging, so disconnected the charger and reconnected.  An hour later it is now saying “Charging 82%”.  This is what many Nexus 7 owners are complaining about.  I did vacuum the micro-USB socket – to get rid of dust and lint, which can get in the way of a good connection.  Dust and lint are good at attracting moisture – which then leads to corrosion.  Corrosion is the bane of all electrical connections, so keeping connectors clean is a good idea.  I just vacuumed my Samsung Galaxy S2.

I don’t know the pin code to unlock the tablet – I will have to get that.

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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.

Recharge.IT – Nexus 7 tablet battery not charging

My sister popped by yesterday with a couple of items.  A Dell 2408WFP monitor and a Nexus 7 tablet.  The Nexus 7 has been having a history of intermittent charging. Sometimes it will charge, other times it seems to discharge – so what could be wrong?  We have tried already, a different charger – most people suggest that the charger needs to be able to supply at least 2A.  A look on Google shows that this is a problem faced by many Nexus 7 owners.  This Nexus 7 has been so frustrating that a little while ago, they went out and bought one of those 10″ tablets that I think was on special at Aldi.  I said at the time that if they don’t want the Nexus 7, they can bring it over some time and I will see what I can do with it.

First thing was to pop the back off.  This is quite simple – just using a fingernail – slide it between the plastic and the metal bezel, and work your way around – the clips should easily disengage.  Having longer fingernails works better.  Once the back is off – you can see this large flat black item with a six wire cable coming out and plugging into a socket.  The black is negative and the red is positive.  I used a multimeter to connect to the socket pins and measured 0.4V – not good.  The battery being a Lithium Polymer battery is a 3.7V battery.  Wikipedia indicates that the first generation Nexus 7 (which this is) was powered by a 4325mAh battery.  A check on eBay shows that the cheapest battery for this is a 4270mAh for just over $20 delivered.

Now, what to do with the battery – I need to “recharge.it“.  Perhaps I should say to charge it, but recharge sounds better.  I have a battery charger that can charge LiPo batteries.  I connected it up to the battery using some small clips – the contacts are quite small.  Here is a photo of the setup.

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I set the capacity of the battery to be 3000mAh and the voltage to 3.7V – as being a single cell.  The charger is powered by a small 12V battery.  I started it and let it charge overnight.

This morning I had a look at it – it had finished charging.  So I am now testing the battery.  I am doing a discharge test at 0.3A – this is what the photo is showing.  When the battery voltage drops to about 3.7V, the charger will automatically stop.  The number at the top right hand of the display would be a number that relates to the capacity of the battery.  If I charge and discharge the battery a number of times, this number should be either similar or increasing each time.  The purpose of this charge, discharge test is to verify if the battery is able to retain charge.  Retaining a reasonable amount of charge indicates that the battery is still good.  I will come back with an update later.