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Jul 21, 2017, 05:48 AM
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Batteries -

Might be time to talk about battery safety once again .
I really don't understand where some of the battery BS comes from , but apparently there are some misguided out there .

1) Batteries are not created equal . In fact there is a rather mighty variance when it comes to batteries . And it's these very variables that can cause problems for us .
2) It's important to buy batteries that can meet or exceed the needs of the application
3) We need to make sure the batteries we use / buy are safe . At the very least know at what point the batteries may be unsafe or problematic

Manufacturers ( those that assemble the batteries ) should be putting together balanced cells . And by that I mean each cell should be matched for Capacity , voltage at rest and discharge . But just how many companies actually do this ( costs money ) .

Voltage at rest ()

Well think of it this way .. You have a 3s battery and one cell has a resting voltage of say 4.18 volts after being fully charged , another has a resting voltage of say 4.15volts and the last cell has a resting voltage of 4.1 volts … Now all things being relatively equal , the lower voltage cell will reach lower voltages first . This means that cell will suffer lower over all voltage when discharged and all things being equal , degrade faster .

Capacity ()

This can vary quite a bit .. Lets say cell 1 has 1550 mAh , cell 2 has 1500mAh and cell 3 has 1200 mAh capacity . Obviously the cell with lower capacity will be depleted first , thus reaching lower voltage first and always being at a lower voltage when the battery is depleted . Again this will degrade the lower capacity cell faster , as well possibly be problematic in the future if not carefully monitored .

Discharge ()

This is also very interesting , has anyone ever seen a battery fail ? The greater the load ( discharge ) the greater the voltage sag . So at say one AMP discharge the cell voltage might drop several tents of a volt . At much higher current the voltage may drop a volt or two ( maybe more at serious current draw levels )

Back in the day ( flash light days for me ) testing batteries I noticed that people were reporting very different capacity results , and there bye passing on conclusions that were (?) Anyhow . So testing for capacity is just that CAPACITY . We don’t need or particularly want high discharge rates to test capacity . What high discharge rates are good for is testing the batteries ability to discharge current at different levels .

This is interesting stuff . So many moons ago I was testing a 18650 , doing discharge tests at different levels to see if the battery would cope . Back then flash lights seldom pulled more than 2 Amps , and only the custom stuff pulled 3 Amp or better . Anyhow this battery failed at 3 Amps discharge .. It started ok , but about a minute into the test the voltage began to rapidly drop until the shut down voltage was reached ( 3 volts ) .. Now can you imagine what would happen with a 3s battery pack if one cell suddenly failed ? Some very interesting things that can cause the entire pack to fail ( Flame ) .

So as a user of Lipo packs what should / could you be doing to be safer ?

First monitor your batteries ( condition ) , buy a battery monitor and check the batteries before and after use ( individual cell voltage ) . What you want to be looking for is nice uniform voltage levels both charged and discharged . You don't want more than a few tenths variance either way . Note the voltage of the cells after a rest period after charging ( rested voltage ) . Do this for long term monitoring of the cells . Because when the voltage begins to drop , it means the batteries are getting long in the tooth . How much voltage drop , this would depend on the variance when discharged . If the cells are discharging evenly then I don't see a real problem unless the battery begins to self discharge . ( Wont hold a charge )

I have had batteries that were maybe 5 years old and still holding a charge , I only parted with them because the voltage variance increased when discharged to a point I no longer liked . ( As well they were some what puffy ) I don't particularly care about puffy as long as the batteries perform within my specs and hold a charge .

Before I use a battery I cycle them a few times on a hobby charger to see if the cells are well matched for rested voltage and capacity . I have tested a few cheap 2s that were horribly imbalanced for capacity .. One cell was at 2.5 volt and the other at 3.5 volt when discharged gently to 6 volts . ( No that's not good ) And the imbalance kicked in around 3.65v . With these particular batteries I use a voltage monitor set to 3.5v to stop the bad cell from reaching dangerously low voltage . ( I say dangerous as repeated low voltage discharges could damage the cell ) .

These batteries are well balanced to around 3.65v where the imbalance kicks in , so are safe to use to that point . If one was not aware of this , then bad ju ju could happen down the track . We have all seen the lipo fires on youtube right ?

Now what I can not test for is the voltage sag under load ( RC ) and the point that imbalance kicks in @ under load ( lets say RC buggy ) . I just don't have the equipment to test for that . So what I have done is set my voltage alarm at 3.8v Drive my buggy to that point , the alarm goes off and then check cell voltages . Lower the alarm voltage and go it again till I find at what point imbalance kicks in under actual use . ( If mem serves it was 3.3v ) .

Now we all know to balance charge right ? Because if you don't the charger will work to over all voltage ( cells combined ) with no regard to individual cell voltage . This also can cause batteries to degrade faster as no regard is given by the charger to maintaining a balanced voltage for the cells and can add to battery pack imbalance ( see youtube for lipo fires ) .

Are lipo's safe ? And the answer to that question is as simple as ….
Just as with a car , a car is only as safe as the nut holding the wheel ! So a lipo is as safe as the person using it .

Procedure for being safe ?

I test all new batteries on a hobby charger before using them . I discharge them and monitor the discharge looking for any variance between cells . I check the rested voltage of each cell after the battery is fully charged and I always balance charge . I also monitor the battery during its life to make sure it remains healthy . Once I detect variance I am not happy with the battery is disposed of as safely as possible .
Last edited by old4570; Jul 21, 2017 at 08:55 PM.
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