My own 30 amp powerchair charger. And some connecting leads.
Some 6x faster than the original equipment 5 amp one!

Now, knowing that Some manufacturers actually suggest this no current limit fast charging as a preferred charging method (such as some AGM batteries: Hawker Odyssey and Optima and others) and knowing that no current limit may involve hundreds of amps then you will soon see that using say a 25 or 30 amp logic controlled 3 stage charger on a powerchair with big 70ah batteries is not only sensible its actually really extremely cautious! And far from the hundred amps plus that a cars alternator would charge them at.

Fast charging only replaces typically 90 percent of your charge due to something called acid stratification.. You batteries STILL NEED an overnight charge with the slow charger as well. Every night if possible or at least every few days if that's not possible.

This keeps all of your two batteries cells equalised and properly balanced. Otherwise they can get out of step. Which is very important for battery longevity. Depending on the charger you use this fast charging will only replace about 90 percent of the capacity or so. in an hour. Its intended to let you charge or top up in the middle of the day or before going out in the evening.

My wheelchair batteries are on charge here. This is a Clamp Ammeter. It started off when first connected reading 30 amps. It always does if the batteries are pretty much depleted.

When first plugged in the batteries "suck up" as much as the charger will let them. If its a 30 amp "capable" charger like this one is then it will deliver 30 amps in its first charge "stage" (of 4)

It is, like most modern chargers, a fixed regulated voltage (28.8v in this case for two batteries) for as long as the battery sucks up the 30 amps that the charger is capable of delivering. If it was a 60 or 100 amp charger then that would be fine too. And it would then be able to raise the batteries voltage to 14.4 (28.8v for two) even faster!  And that's quite safe for the batteries. The current (amps) gradually tail off once the battery achieves the set 13.4 (28.8) volts...

In this case above the charger is 30 amps. So that's what the batteries greedily take. That's because the charger is basically a fixed voltage power supply with a current supply that is limited. Just like your standard 5 or 8 amp powerchair charger is limited to just 5 or 8 amps... 

Once the battery "achieves" the set voltage then the current starts to fall away from the maximum the charger can do, and that is the end of the so called "stage 1" and stage 2 begins. Nothing actually changes other than the current drops to keep the battery from exceeding the set voltage. So this continues until the current falls to a very low level. That signifies the end of "stage two" on your 3 or 4 stage charger. The next stage is simply a permanent lower voltage "float" that goes on forever. This stops your battery from deteriorating over the long term. Stage 3 then never ends... In fact in a powerchair only two stages are needed. And if you are unplugging it after the green "ready light" illuminates then you don't use the third float stage anyway.

I also have a 100 amp charger (a 12v one) and this charges single loose batteries quite safely too. As long as your batteries manufacturer specifies no inrush current limit (and most do not - especially AGM batteries) then its not a problem no matter what other so called "experts" may tell you.   However, the faster you charge a lead acid battery the lower the amount of charge you actually put back. This is due to the acid having no time to level out its state of charge. The acid nearest to the plates may be fully "reversed" but that which is further away less so.

The name for this is charge stratification. So a fast charge in a good quality low resistance battery may only really be 90 percent charged. So a slow charge at 1/10th the batteries capacity (your stock slow wheelchair charger) is recommended either every night or at least every few days to fully equalise all the cells and keep your batteries in balance and healthy.

I use both of these 30 and 100 amp chargers on all my batteries over the last few years without any problems whatsoever. In fact they now last longer due to the lower average discharge levels that a fast top up in the afternoon gives them. Ready to go out again and a green ready light (90 to 95 percent) usually well under an hour...

After around 10 to 20 mins depending on discharge level it falls slowly (to 21 amps here), as the battery becomes charged up and it own voltage is closer to the supplied 14.4v (28.8v) voltage from the charger. This part where the battery no longer pulls the full 30 amps from the charger is the "second stage" of a two stage charger. And so far it didn't do anything apart from supply a fixed voltage and a maximum current of (in this case) 30 amps... So we get duped. A two stage charger is really that simple.

As charging nears completion and the battery voltage slowly rises less current naturally flows. The current keeps on falling.  When it reaches around half an amp in this case above (depending on rate of fall etc) and your battery is just sat at 14.4 volts and needing little power to hold it there the charger says done!  That's the end of the stage 2.

It then switches to "float" mode which is in this case is 13.4v per battery (26.8 volts.) measured. Ad it will hold your battery there for ever. Float charging isn't really "charging" at all. It just prevents the batteries natural internal discharge from happening. So its quite safe to leave connected for very long periods or just overnight. 

But you can go from pretty much discharged after shopping to 95 percent plus charged in under an hour ready to go out for the evening. Most normal 3 stage chargers are now finished whether fast chargers or slow. See? They are really pretty simple.

Not all batteries CAN be fast charged safely or without damage. Almost all AGM type batteries are perfectly happy to be fast charged. Optima, and Hawker Odyssey for e.g. both actually recommend this for batteries used in a cyclic system such as a Powerchair or Scooter. See their spec sheets.

Hawker Odyssey actually supply and recommend a 50 amp charger to be used with all of their batteries from the tiny 8ah one (no misprint - that's 5 times smaller than a typical powerchair battery!) upwards... Here is a link to a supplier.

Read the specs for their OMAX-50A-1B http://www.odysseybattery.com/chargers.html   They as a battery manufacturer don't consider it at all unusual to feed a tiny 8 amp hour PC310 battery (in the charger specs) A FULL 50 Amps to charge it.  The battery just naturally takes what it needs.  So charging a pair of big 65 or 75 Amp Hour wheelchair batteries at just 30 amps is barely tickling them!   All that matters is that you don't charge above 14.4v each for GEL batteries or 14.7v for AGM batteries. And the charger takes care of that! So safely ignore what you may read on so called "professional" web sites about sticking to the charger that the manufacturer supplied!

Some Gel batteries are also quite happy to be charged at a fast rate without any problems. The widely used MK and Sonnenschein deep cycle Gel batteries are also happy with this treatment. Although they take a little longer to charge (higher resistance and Mr Ohm make this happen!) See their own specs on acceptable charging before you try this. And read the part about the inrush current limit. They too don't have a limit. I have been fast charging both of these brands after speaking to both companies and the MK rep in the UK. They seem to show no detrimental effects whatsoever and I now get longer battery longevity because of it due to the average depth of discharge being far less. As long as care is taken selecting a charger that keeps to or below their charge voltage limit.  See appropriate spec sheets for your chosen battery before proceeding or don't blame me!