[MattsAwesomeStuff]

Google probably hasn't helped because you generally can't find answers to questions that aren't usually relevant to ask. It's not in a spec because it's not really relevant.

Voltage ripple, I mean, infinite, I guess.

You could feed it half-wave rectified AC and it would be fine. The battery would just only charge during the parts of the cycle that are above the battery's voltage level. I.E. If the batteries were "full" at 42 volts, and you're feeding it 40 volts that spikes up to 42v every 100th cycle, it's going to take 100x as long for the battery to reach 42v than it would have if you fed it flat 42vdc.

Just note that the batteries will charge to the peak that they're given (even if at a slower rate), so if you feed them 41vdc with a 3v ripple, you'll be overcharging your batteries to 41+3=44v instead of 42.

As to current ripple, well, current will just mirror voltage.

You can think of a battery as a big slow capacitor I suppose, voltage curve aside. It will soak up nearly bottomless current nearly instantly if all you're building is a 10a charger.

Discharge works the same. You might demand energy from the battery in all kinds of wonky manners, it doesn't care if the current wanders all over the map.

 

[MattsAwesomeStuff]

By adding a electronic relay in the charging circuit that is triggered by the BMS I want to create a Pulse Width Modulation charge process. When the battery is indeed reacting as a big slow capacitor I expect that this way of charging will not harm the battery. Is this analysis correct?

It'll work.

If something goes wrong and the relay doesn't shut off, you'll start a fire and burn the vehicle and garage to the ground.

That said, I suppose that's true of just about any switchmode charger. Something can go wrong and the output might get the full voltage or a higher voltage than it should.

I'd say, don't let it be the reason you don't do something, but, plan on upgrading soon. Nothing as permanent as a temporary solution that's good enough.

 

[MattsAwesomeStuff]

I am not sure I would want to switch a relay fast enough to produce the desired effect

Oh, I interpreted it as, he would just feed it the 29v from the lead-acid charger, but have his BMS trigger the relay off when his batteries reached 24v. Not, using the relay to clicky-clack average 24v on a 29v charger... which will not work, as it will still charge the batteries to 29v, just at a slower rate.