I have a huge, cheap and dumb battery charger which can charge a 12 V lead-acid battery with 70A, but it only outputs anything near (by "near" I mean over 30A) that value if I give him a completely empty, dead battery. As soon as it starts charging it, the current drops, quite quickly well below 20A, and most of the charging is done at about 10A, in fact it decreases linearly all the time. AFAIK this is called a "taper charger" and is the cheapest, most dumb lead-acid battery charger common on the market.
The battery I want to charge has in the specifications that you can charge it with a maximum of 38A in the "Constant Current" phase of CCCV charging, in which 80% of the battery capacity can be filled. I want to modify my cheap charger to actually do that - to charge with a constant current of 38A up until the voltage at the battery terminals reaches 14.4V, and then just turn-off (I can do this manually, I won't leave it for unattended charging anyway).
Any idea how such cheap taper chargers are commonly made, and what could be done to modify one to a constant current charger?