is this considered a bad overcharge

rolygate;15092480 said:

All batteries have some commonality:

• they deliver low voltage DC
• there is no such thing as a fixed voltage to charge them to
• the nominal voltage has no relation to anything in particular
• there are slight differences in how they are handled depending on the type

To take a simple example that is easy to relate to, a car battery is called a '12 volt battery'. In reality it is nothing of the sort. The '12 volt' part refers to the nominal voltage, which is a value that relates to the circuit components more than the battery, although even there it's not accurate since all '12 volt' circuit components are built to operate at maximum efficiency at 13 volts.

12 volts is never seen except in an almost dead battery: a '12 volt' battery is fully charged at 12.8 volts and is dead at 11.8 volts.
It is charged fully with a charger voltage of 14.4 volts (this value is the best compromise between full charge vs service life), but this charge voltage cannot be delivered for more than a few minutes or the battery gets cooked.
The alternators generally used are built for cheapness not efficiency and therefore only deliver a maximum of 13.2 volts, which is the float voltage for these batteries: the highest voltage that the circuit can support indefinitely without cooking the battery, and that maintains a reasonable level of charge in the battery.
A 'real' alternator requires a management circuit, ideally with programmable functions, and this is too expensive to build so an alternator controller is used when the battery needs to be correctly managed - for example in boats, delivery trucks and emergency service vehicles.
Without proper alternator management, a car battery gradually dies, as it is never fully charged: the standard 13.2 volt charge is about a 65% charge, and a lead-acid battery cannot survive indefinitely being undercharged like this.
An alternator controller (such as the Adverc) provides timed periods at different charge levels such that a battery can almost live forever. It floats at 13.2v, boosts at 14.4 for short periods, and allows the battery to rest with zero charge. A full charge programme also requires a 17v charge at very low current for a short period to knock sulphate off the plates, although the circuit may have to be disconnected for this as it risks over-voltaging other circuit components. A good controller like this costs as much as an alternator.

So you can see that batteries look simple on the surface but there is usually more to it than that.

With lithium ion cells (the rechargeables we use), some of the basic principles are the same: there is really no such thing as any kind of fixed value for anything. Every number chosen is a compromise against something else.

The nominal battery voltage is 3.7v (or 3.6v) but that doesn't refer to anything in particular except the average voltage the circuit works at, and with these cells it's about a half-charge state.
The battery is 'fully charged' at 4.21 volts but that's only a point on the graph where battery life starts to reduce significantly if you go higher; it lasts longer if you only charge it to 4.15v; a fully-charged value of 4.17v is a good compromise figure for charge vs battery life.
If you take it down below 3 volts than battery life again starts to reduce more quickly, so 3.3v is a good figure as a minimum value before recharging.
Because we could be talking here about a Li-ion cell, a Li-Mn cell, or a hybrid cell, no single set of figures will work the same across the range, though all lithium ion types are similar enough to treat the same unless you are a perfectionist and demand maximum battery life for some reason.
Batteries always last longer if you trickle charge them as against filling them as fast as possible. Therefore a 250mA charge is always going to be superior to a 500mA charge, which is superior to a 1,000mA (1 amp) charge current - if you are talking about max battery life. On the other hand you may be more interested in getting them charged in an hour or so rather than overnight. You pay for this in reduced battery service life (a reduction in the number of recharge cycles before battery death).
A lithium ion cell of the type we use could have a service life of 500 cycles if we maximise all factors for battery life: lower max charge voltage, higher minimum voltage before recharge, lowest practical recharge current, highest charger quality therefore least ripple and kindest current vs voltage charge profile, storing the cell at half-charge as against discharged or fully-charged; etc.
In practice you might only get 200 or even 150 recharge cycles before the battery is U/S. It's all a compromise: you choose what is of more value to you in your circumstances.

Note:
- a lithium cell charger that goes above 4.22 volts is faulty
- a lithium cell that comes off-charge at 4.21 volts (for example) then drops to 3.8 volts when resting (no load applied after coming off charge) is faulty, as it cannot hold a charge
- a lithium cell taken down to 2.5 volts may die; most chargers cannot recharge from that situation; some can due to a specialised programme for reviving dead cells.

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