I want to install an electric bilge pump. The documentation for the one I purchased (a Rule 25D) has a table that says 1.9A@12V and 2.5A@13.6V. I am guessing that a 12V sealed lead acid battery rated as 1.3Ah would operate the pump for approx (1.3/1.9) hours. Is that correct? I’m just trying to figure out what size battery to get. It’s a tradeoff between weight vs confidence that the pump will not fail due to the battery running out (e.g., during a week long trip in which i use the pump multiple times to pump out modest amounts of water after rolling, or during a catastrophe scenario involving conditions beyond my ability and attempts to empty boat of water during failed reentries).
And thinking of that, does any one know if sealed lead acid batteries lose their ability to power a pump (or anything) gradually or suddenly as they approach the end of their charge.
Thanks
More or less
I use Atwood Tsunami pumps in my whitewater canoes. These are rated at 1200 gph with a maximum current draw of 3.5 amps at 12V DC.
The Rule 25D is rated at 500 gph with a current draw of 1.9 amps at 12V DC.
I use Werker WKA12-3.3 SLA batteries (12V DC, 3.3 amp/hrs). I usually only turn on the pump a few times a day and run it for less than a minute at a time (usually). I have found that my batteries will typically hold out for multiple days, say 5 or so. I try not to run them completely discharged. I have not noticed any great decrease in pumping efficiency as the battery draws down.
Since your pump has a little more than half the current draw as mine my guess is that a battery with less than half the capacity (1.3 amp/hrs vs. 3.3) might not suffice for a week long trip if you have to use the pump much, but that is just a guess.
Lithium Polymer batteries.
In my sea kayaks in the U.K I use lithium polymer batteries, which are much lighter than lead acid. I use a 14.8 volt 3.2 Ah Li-Po weighing 13 ounces to drive the nominal 12 volt Rule or Whale pumps. These batteries are commonly used in radio-controlled model aircraft, where light weight and endurance are critical:
L 5.2in X W 1.7in X H 1.2in, Weight 13.0oz
http://www.horizonhobby.com/helicopters/batteries/3200mah-4s-148v-30c-li-po–12awg-ec3-eflb32004s30
These require a special charger to balance the cell voltages:
http://www.horizonhobby.com/20w-lipo-ac-battery-charger-dync0505
Nick.
Battery Ratings
As an Electrical Engineer, I know more about batteries than paddling. :)
Lead Acid batteries are typically rated in a 20 Hour mode. This means that a 2 Ah battery is one that can discharge 100 mA (0.1 Amps) for 20 hours.
If you were to discharge a 2 Ah battery at 1A, you may only get 100 minutes, instead of the expected 120. Or a capacity of 1.67 Ah. This is due to internal resistance of the cell consuming more energy during discharge.
As others have suggested, LiFePO4 cells (4 in series) are a more modern replacement for Lead Acid. However, these are MUCH more expensive (4x or better). The advantage is weight and size, as well as handling higher discharge rates better. LiFePO4 cells are what is used in the car starting jump packs now. They can dump current very rapidly.
At your pumps current, I would expect a 20-30% de-rating of expected pump run-time. Then size your battery with that in mind. If weight or size is an issue, then LiFePO4 might be a way to go.
When looking at cells, a common value referenced is "C". This is unitless and derived from [Discharge Current]/[Ah Battery Capacity]. For example, 2 Ah battery with 2A discharge is discharging at 1C. Almost all battery chemistries can handle a 1C discharge. But it is a good short hand for the smallest capacity you should pick for Lead Acid. And if you are shooting for higher than that for short run-times, consult the data sheet of the battery type to make sure it is supported without damage. An LiFePO4 cell might be able to do 10C or more without issues. So if you expect very infrequent use, then you could get by with a smaller Lithium cell than a Lead Acid with no damage.
Size, weight and cost difference for equivalents:
140mm x 56mm x 28mm and 375g for LiFePO4 3.3 Ah $66
134mm x 67mm x 60mm and 1300g for Lead Acid 3.2 Ah $14
Actually at your pump draw, you might need a 4 Ah Lead Acid to be equivalent to the LiFePO4, as the internal resistance is much higher.
But I'm using a Lead Acid for my Fish Finder, because it isn't worth the 4x price difference for me and I'm too fat, so weight isn't a factor either.
Edit: One thing I just thought of is using a small flexible solar cell. If you are going to be using this for a while on a trip. Getting a flexible 5W solar panel would trickle charge your battery. This would be putting about 500mA in full sun. It would give you the ability to use your battery capacity to charge camera batteries, etc. You would just need a clear location to mount it on deck.