...wagons, more tiers MORE ALL!!!

Haha, thanks! What do you think, maybe more capacity (say, 50 accumulators instead of 10), faster charging/discharging, what else at higher tech levels?

Accu Wagon T1: 1 Electronic circuit, 1 Cargo Wagon and 10 accumulators;
Accu Wagon T2: 5 Advanced circuit, 1 Cargo Wagon and 50 accumulators;
Accu Wagon T3: 10 Processing unit, 1 Cargo Wagon and 100 accumulators;

Of course, this is just my proposition.

A bit of electronics will be useful, it was not easy and electronics would control the batteries in the wagons.

People often use modifications, and they can have structures that require large amounts of electricity.

Playtesting a bit now that this is working and 10 accumulators for the base is just too small - with base mining productivity and no modules, right now it takes about 7 accumulator wagons worth of power to mine up 1 cargo wagon worth of ore. So! Rebalancing patch incoming for the base wagon, then we'll figure out how the higher tiers fit in :)

Instead of asking for just a Cargo wagon on T2 & T3, T2 would need 2-T1 Accu Wagon's & T3 would need 2-T2 Accu Wagon's. Example taken from FactorioExtended-Trains.

the wagon needs to have a higher discharge rate so that it can charge up regular accumulators.

It charges/discharges at the same rate as a regular accumulator relative to the amount of power stored - but it won't charge them unless you set the discharge priority signal on the station where you want them to "unload" power, it doesn't have a way to automatically know whether you want it pushing power to or from static accumulators.