battery voltage

[Ken Orthner]

Been having an unusual problem on my - I use two Atwood V500 bilge pumps on board - one for a "shower sump" the other as an emergency bilge pump. These pumps are rated at a 13.6V basis. The battery voltage on my boat usualy runs between 12.5 and 13.0V - The pumps (both) don't seem to have enough "Umph" to "lift" any water from floor level the level of the cockpit - I've noticed that the pumps advertised in West Marine" are shown to be rated at 12V - I'm wondering if the pumps I have are simply too weak at a lower voltage than rated, and would the ones rated at 12V have more power at a slighlty lower voltage. Any suggestions?

[Moe]

So-called "12V" DC appliances have to operate over a range of about 10.5V (battery discharged) to roughly 14.5V (battery bulk charging). Most are "rated" at 13.6-13.8V, which is the typical output of an automotive alternator designed for today's higher underhood temperatures or a single-stage 120VAC battery charger. Just because a device is listed or rated for 12V doesn't mean it performs any better at that voltage than one rated at the higher voltage. They'll all perform less well with less voltage.

Make sure you have sufficient wire gauge to the pumps to minimize voltage drop.

Here's some reading material. Note also the links at the bottom of the page. Your Attwood pumps did fairly well in this test.

--
Moe

[jklightner]

I found them listed at 12 volts/ 1.2 amps. The difference is in your "float voltage" which will be about 13.5 volts when running your motor and the charger is on as opposed to the standard 12 volts when it is off. And of course more things drawing off the battery will drop the voltage even more. I couldn't find a pump curve for it, so I don't know what the flow would be for different heads. ( how high you need to lift the water) Do your hoses have straight runs and bends? If there is a place which goes up and then down, that can create an airpocket in the line. How old is it? The impellers may be worn. Try sticking it in a bucket of water and see how high it can lift water.

[Chip Hindes]

Chalk another one up to marketing hype. Manufacturers rate their pumps at 13.6V for one reason only: because it makes them look better than pumps with output rated at 12V. If you're not paying attention, you can be fooled.

[26XSunsailrs]

Hi Ken,

I had a similar experience when I bought a cheap live well pump at Walmart to use to create a shower in the cockpit area. It was only $17, but would barely lift any water above the cockpit. Still being the cheap SOB that I am, I decided to pay another $17 and buy another as an experiment to see if I could get it to work.

I simply fed the one pump into the other and the water gushes now! I believe some type of back pressure or cavitation problem was resolved because the water flow is more than double the original.

I don't know the engineering reason that it worked, but I have a great system now for $34 and the price of some plumbing.

Maybe the Atwood live well pump could "turbo charge" your system too?

See you on the water,

-Steve '99 26x Together