How can I evaluate a charger before I buy it? I don't own any kind of amp meter or electrical test devices other than a 30 year old $15 multimeter. I'm looking to buy a used, high-amperage, manual battery charger to run an electrolysis tank for de-rusting iron parts a few times a year.
You shouldn't be shopping for a battery charger but for a power supply.
A properly designed battery charger will have protection circuits that stop it frying batteries (and on a bad day, starting fires) just because the user set it to the highest charge rate. That will most likely make it useless for electrolysis.
On the other hand, if you tell an adjustable 40A power supply to output 40A, that is what it will do, at whatever voltage is necessary.
It occurred to me that good test setups are close to operating conditions, so an evaluation better than battery trials would be electrolysis itself. Before that occurred to me, I drained my core-battery by hooking it up to a spare old fashion head light. When the light dimmed to almost nothing, I figured it was really low.
I scheduled a revisit, and brought the drained battery, some washing soda, a large rusty car part, an even larger rubbermaid tub, and a rebar electrode. Used their garden hose to make the solution. The charger generated heavy gassing, much heavier than my 2/6 amp charger. After, we just dumped the solution since there's nothing evil about it. I bought the charger.