AGM or VRLA have been around for a long time, used extensively in intruder alarms, chair lifts, moblity scooters, and emergency lighting. While I was working for the council so pre-1980 we had inspection touches using these batteries.
I know the pair used in my mothers and father-in-laws stair lifts had a very different life, one lasted the life of stair lift, clearly a regulated charger of some type, the other lasted around 4 years when used every day, 2 years once only used once in a blue moon, it was clear the battery charger was over charging the batteries when not being used, I was on the one lasting only 2 years showing around 32 volt on a 24 volt (two batteries) system. So 16 volt clearly was too high.
The stand alone charger I have designed for those batteries has the following setup.
If the voltage is below 3.8 V or above 15 V, the battery will not be charged. The display briefly shows the error message "Err".
Below 7.3 volt it assumes it is a 6 volt battery.
Up to 10.5 volt it uses a pulse charge.
Up to 12.8 volt charges at 3.8 amp
Up to 14.1 volt charges at 3 amp
Up to 14.4 volt charges at 0.8 amp
Then drops to 0.1 amp charge, which in the main will not maintain the battery voltage. So at 12.8 volt it returns to 0.8 amp charge rate. This means the battery gets a pulse every so often,
how often depends on the battery
once I see a patten like either of these, I assume the battery is charged. Good (nearly new) batteries will stop charging altogether, there is a second option for batteries under 12 Ah and also one for when very cold.
this is the display from my jump starter, with 12 Ah battery, and a wallmart charger, basic same idea as other chargers, and the mobility scooter with yet another charger gives similar patten.
The CTEK range of battery chargers are slightly different, they can return to full charge rate, and start at a lower base voltage, but do not do 6 volt, and should a cell go short circuit they can cause hydrogen sulphide to be generated (bad egg smell).
To have a charger like in my laptop which monitors the battery very carefully can increase battery life, the engine management in the modern car does the same thing, when we change a battery, we must tell the engine management what battery has been fitted, and using an external charger we must charge from the point under the bonnet, not direct to boot, so the engine management can monitor the charge rate.
It become clear when the battery is low, as the stop/start at junctions stops working, if this happens, then we put the car on a mains charger to top up the battery.
The big thing is, we can charge an open vented lead acid with a charger designed for an AGM battery, but we can't use a charger designed for an open vented battery on the AGM type. It will wreck them.
For years I looked after fork lift batteries, it was a regular job topping them up with de-ionised water, the stage charger would charge at around 40 amp until the voltage reached 16 volt per 6 cells, then drop to around 4 amp for a set time, there were variations some chargers had more stages, same with narrow boats, milk floats, and early golf trolleys, the problem with AGM is speed to recharge, the mobility scooter takes around 24 hours to fully recharge the batteries, where my e-bike with modern batteries can recharge in 4 hours.
To recharge fork lift batteries in 8 hours, i.e. within a shift, they had to be over charged, we had to regularly add water. Also sulphur hardens over time, so longer a battery has been left discharged, the longer it takes to charge.
Buses and Milk tankers often have Nickel Iron batteries, same alternator was used, but when you watched the ammeter it would go to 60 amp on starting and stay there for a few miles, then drop to near zero. The buses had alternators with current control, the CAV 208 and a special 440 regulator with M1 and M2 terminals which went across the shunt resistor.
There have been so many specials, Grove cranes had duel output 12 and 24 volt alternators, with a three phase transformer built in, old bus dynamos had bucking coils to stop them over charging. Tractors often had a sensor under battery to reduce charge if they got too hot, special regulator but standard ACR alternator.
Also of course brush-less, Paris Rhone and Delco made them, but very different design. I was an Auto Electrician many years before I moved to mains supplies, and I have seen so many odd ideas, including 8 volt CAT batteries for the 6 volt donkey engine start tractors, as found on the 583 pipe layer, the D8 made same time were likely long gone, but a pipe layer did not do much work, so still had donkey engine starters, had to get de-compressor and donkey engine engage leavers in right order or could break your arm.
But showing my age, I remember working in the Sahara in the early 80's.
