how to modify a taper charger of lead-acid batteries to be a constant current charger?

Question

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?

Answer 1

Your dumb charger is just providing a slightly regulated voltage. It works due to the combined resistances of the charger and battery limiting the current. As the battery voltage gets close to the charger, the voltage potential is less. So with a fixed resistance, the current drops.

I doubt that you can modify your current charger to push 38 A up to 14.4 V. This could only happen if the combined resistances and voltage difference allow sufficient current flow.

Most likely you will need a higher voltage source and then circuitry to make a constant current source to provide a fixed amperage. Shutting it off can be done with a simple comparator circuit. Put a 10k trimmer pot across the battery voltage and run it into a comparator along with voltage from a zener regulator. Then you just adjust the voltage divider from the pot till it trips at the correct battery voltage. Use that to turn off a high current MOSFET or something similar. (Might use the same MOSFET to control the constant current.)