How to make a voltage multiplier of 1.075
You already have an 80V power pack and all you need is another 6V at approaching 18A. This 6V could be produced by an isolating flyback or forward converter powered from the 80V supply. Because the output will be isolated, it can be wired in series with the 80V to make 86V.

So, 80V feeds an isolating forward/flyback converter that generates 6V dc. This output is then wired in series with the 80V to make 86V to feed your battery pack.

The problem has simplified to trying to find a 6V near-18A isolating converter.

You could of course buy a 6Vdc power supply that is fed from AC - providing this has an isolated output, it can also wire in series with the current 80Vdc output.

I had a quick look in Farnell and they have a small 6V 20A isolated brick but it runs from 36V to 75V so it's a few volts short but maybe use a buck converter feeding it that lowers the 80V to nominally 48V: -