how does a hydraulic battery work?

You have an accumulator - more or less a pressurized tank. When braking a hydraulic pump adds fluid under pressure to the tank. You recover the energy by running the high pressure fluid through a hydraulic motor (which could be the pump running in reverse).

Ed

I just don't understand how you would store energy in something that doesn't compress. It would make sense if it were a pneumatic battery, but I can't understand how a hydraulic battery would work

'Hydraulic' accumulators compress gas (typically nitrogen), not hydraulic fluid.

OK, I was too simple. They have an accumulator like in an automatic transmission (sort of). You have a piston, or membrane separating the fluid from the gas. The gas side is pre-pressurized. As you pump in fluid into the liquid side, the liquid side expands and the gas on the other side of the divide (membrane or piston) is compressed further. The pressure from the gas side pressurizes the liquid (hydraulic fluid). You can release the pressurized fluid to drive a pump. Instead of pressurized gas, you can also use a mechanical spring, but this give you less storage capacity.

Have you ever seen one of the "water rockets" kids can buy (see

)? Theyuse pressurized gas over a liquid to propel the rocket. Just imagine thesame set-up except that the rocket exhaust is used to drive a pump orturbine. In an automotive application you need a membrane or piston toseperate the fluid from the gas, but the principal is the same. Ed

ah, that sounds just like those water tanks that store filtered water; I got one in a reverse osmosis undercounter unit for the kitchen.

I see now. The name was driving me nuts.