Quoted:
Who has some recommendations of good, economical, 12 volt backup sump pumps?

So I'll be honest here. Like all pumps in this price range the EPS15 is not "professional grade" like the ad says. (Note the ESP25 is a lot stronger pump).

The EPS15 is mostly plastic and average for the price. Selecting which pump to get comes down to which particular design and features do you prefer since most in this price range have similar flow rates and many even look like. You also need to consider a bigger pump if you need to raise the water more than about 10 feet high.

Most of the construction is plastic including the impeller itself so don't try pumping anything except clean water as a small rock or pebble would not be good for it. The circuit board layout and construction is just adequate, for example it runs pretty hot when charging the battery. Also, the pump itself is not strong enough in some cases as I'll explain below.

I observed that the internal pump relay mounted on the circuit board is pretty small and rated at 10 amps max. That's not a good sign since the pump draws over 20 amps at startup and settles down to 8.7 amps while running. So I wouldn't expect a 10 amp relay to last a long time when operated near max load like that.
Considering how cheap 20 amp relays are I wish they'd used a higher rated one.

Like most in this price range, there are no real strain reliefs to protect the wires and the AC transformer is the old style "brick" type with no voltage regulation and not much power (700mA trickle charge). That's why it says to allow up to 4 days to charge the battery. The plastic bottom of the pump has many sharp edges, and the 1" barbed pump outlet is not as useful or easy to service as a standard 1" or 1.5" threaded connection. 1" also seems a bit small for a sump pump so that may limit the flow somewhat. Those items are typical at this price point and there was only one deal breaker as noted below

I have 9 foot ceilings in my basement and it's simply not strong enough to pump water out of the sump pit. It slowly pumps the first couple inches of water when the pit is over full but as the water level goes down (and the total lift height goes up some) then it stalls out and can't lift any more. It was actually losing ground after a while as the pit was slowly filling up (with the pump still on) as ground water was slowly coming in from the drain tile at about 1 gpm. The motor keeps running with water sloshing around but it can't do it. So I used my full size AC sump pump to pump it out, which takes about 15 seconds to empty the full sump pit.

The ESP15 has a 25 second timer that shuts off the pump so even if it could lift water high enough (such as with a 7 -8 foot ceiling) it times out before it's done in my setup. Once started, the timer stays on whether there is water present or not so the pump may run dry in some cases. Apparently this is ok by design since the timer is not adjustable.

The total lift height in my installation is 10' 6" from the pump inlet up to the highest point and it just can't lift that high even with a fully charged battery running at 12.8V DC.

As an experiment I tried feeding the motor directly with a slightly higher input of 13 Volts DC using a precision regulated DC power supply with plenty of current capacity. The result was the same as using the 12V battery.

Like most pumps it has a lot of flow when not lifting too far but the 720 gph rating at 10 foot of lift looks optimistic from actual observations. Each corner in a water pipe causes some loss of throughput so maybe their 10' lift spec is for a straight pipe with no corners rather than a typical home installation.

If you have a lower ceilings or a lower total lift height then this unit may work fine for you, but I'll have to go with a stronger and more expensive pump to work with a higher ceiling.

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UPDATE: 5-4-2011
I bought the larger Wayne ESP25 instead and that's a lot better pump. It has at least 4x the flow at 10 foot of lift and takes the same power as the smaller pump. It's also quiet and uses a larger 30 amp relay vs the undersized 10 amp one used in the ESP15. The control box still runs a bit hot when initially charging the battery but at least now it's a full 2 amp charger that gets done a lot faster than the (700mA) one in the ESP15. It's a smart charger and it settles down to a single DC pulse every 10 seconds or so after the battery is charged. That's just to maintain the battery level, so it runs cool most of the time.

The ESP25 is a taller pump with a lot more capacity but the actual DC current draw is about the same as the smaller ESP15. So clearly it's a lot more efficient design and that's a good idea when used as a backup pump.
I measured 9.5 - 10 amps current draw from the ESP25 while lifting water up about 10 feet high through 30 feet of pipe overall ....so actually, I'll get a lot more water pumped per charge from the big pump since it pumps 4x faster and then it shuts back off while the smaller ESP15 just ran and ran.

The ESP25 draws about 40mA to run the electronics when in standby mode. At that rate the battery would last for weeks so the only thing that will draw the battery down is the pump itself.

Run-time Calculations: (I'm trying to be clear and brief but it's not that simple).

In my setup it takes 15 seconds to empty the pit with the stronger ESP25 pump. So that's 15 seconds ON time x 4 times per hour which is 1 minute total ON time per hour. Using the medium size 40 amp battery I have 4 hours total ON time to work with (40 amp-hr battery/10 amp load = 4 hours). With 1 minute ON time per hour - the 4 hour run time should last for days since it's off 98% of the time.

It works out to be 240 hours of standby time if the pump is on 1 minute per hour, every hour.
The Math should be valid but I'll test it sometime and see how many hours it holds out.
Your setup and timing may be different so adjust the math as needed, for me this setup worked out great.

The ESP25 costs more but it looks like the best choice. It's a lot stronger and more efficient pump than the smaller ESP15. It also charges faster and runs a lot longer per charge since it's off most of the time because it pumps fast and shuts back off right away.