Years ago, we had discharge meters, there were two basic types, one was a volt meter and a very low resistance likely taking 100 amp, and rather dangerous as the spark when connecting to battery could ignite the hydrogen and you were intended to observe the cells, and if one cell had more activity to rest it showed battery as being faulty. But they could damage the battery and were dangerous and you needed to observe the cells which you can't do with a sealed battery.
So the second type had a built in computer, and you clipped on leads with no current flowing so no spark, and then pressed a button and it would discharge battery for set time, then measure the way the battery recovered once load was removed, and the in built computer would give you a read out of the Ah rating and state of charge of battery, seem to remember made by Bosch, and were not far out.
Originally lead acid were marked in amp hour rating on a 10 hour or 20 hour rate, but latter they were marked in start amps, some times with a reserve Ah rate.
I did a sidewards move and started to play with low voltage and left the vehicle electrics, and have noted since leaving things have changed, which was why I raised the question about charging times.
The problem with lead acid is the sulphur going hard on the plates, it does not matter so much on charge rate but charge time, and I realise now I likely destroyed potentially good batteries trying to charge them too quick, and it seems the better rapid chargers like the battery tester use the voltage decay time to work out what charge rate is required, mainly found on narrow boats where to recharge batteries in the short time cruising is a challenge, so the battery receives a pulse of charging then switches off and monitors voltage decay, then another pulse of charging, before that method the stage charging method was used, it measured the current and the voltage for a 12 volt battery would start at around 14.8 volt, and once the charge current reached a threshold normally around 5 amp, the charge voltage would drop to around 13.4 volt, non stage charging voltage normally 13.8 volt, however if some thing is using battery while on charge it can fail to drop voltage at 80% charged and so damage the battery, that why it was replaced with the pulse charger.
For smaller battery chargers there has been a whole new idea with chargers having as many as 7 stages, often the voltage seems high, not as high as only stage chargers now around 14.4 volt, the Ctek is the most well known, but there are chargers sold by Lidi and Aldi which follow a similar method of charging, the main change is instead of changing the voltage as the battery recovers, it changes the charge current, so my Lidi charger has 3.8A, 3A, 0.8A, 0.1A and zero. Once charged it can be left on, and will auto change between 0.8A, 0.1A and zero to maintain the battery, it will not return to 3.8A unless some one presses buttons, the Ctek however comes in many models and some will auto return to max output.
It is in the main because of the use of valve regulated lead acid, old type chargers could destroy them, some times called absorbed glass mat, they have a lot less liquid than flooded batteries they were mainly used for fire alarms and the like, but are now used in stop start cars, where the computer decides when to charge the battery, with some very complex algorithms to maintain the battery in the main at 80% charged.
Keeping the battery at 80% charged would in the end kill the battery, so every so often it is fully charged, similar to a laptop, and when a battery is changed the cars monitoring system needs to be told, I know many firms selling batteries had a small 4 Ah battery they would connect in parallel so the radio would not lose its memory, it seems this practice messes up the charging system with some cars, they need plugging into the engine management computer and the computer told a new battery has been fitted.
So the old idea was float charge between 13.4 and 14.2 volts, the exact voltage was a compromise between too much gassing, and not fully recharging, the norm was 13.8 volt, but that has all gone by the board now. My problem is I don't use the car enough, so be it a standard alternator or engine management controlled batteries need time to recharge, only my wife's car gets regular use.
