Sizing a battery for 12v transfer pump

May 12, 2018 11:02 AM

Can you help me with sizing a battery for this 12v transfer pump? I'll be using this occasionally to pump water from a rain barrel for irrigation. Here is the pump I have, and here is the user's manual. I will probably use it for 20mins at a time approximately 1-2x per week. I'll be able to recharge the battery between each use.

The specs in the user manual are pretty spare....all it says is 9.7amps. Should I read this as amps drawn at full load? Would a 12v/10Ah battery run this pump for an hour, or am I missing something? Can a 10Ah battery actually push 10amps? It seems like most batteries list the capacity in amp hours, but not the actual current output in max amperage - or am I not thinking about this correctly?

Do I need a deep cycle battery for this use, or would a standard "starter" battery work from a car or lawn tractor since I'll be recharging between each use?

Thanks,

posted by pilibeen to Technology (10 answers total)

Best answer: I looked at the manual for that pump and would interpret that as drawing 9.7 Amps under load. I don't know how accurate that figure is; you might consider measuring it (such as by temporarily running the pump from a car battery) before purchasing your main battery.

You absolutely want a deep cycle battery for this. Starting batteries are designed to provide current for just a few seconds and then be immediately recharged. Drawing down any appreciable amount of a starting battery's capacity will rapidly damage the battery.

In an ideal world, a 10 Ah battery would (as the name would seem to imply) be able to supply 10 Amps for one hour, 5 Amps for 2 hours, 2.5 Amps for 4 hours, and so on. In reality though, a battery will only be able to supply its full Ah rating when used at or below a certain discharge rate which should be listed in the battery's spec sheet. This rate varies across batteries, but a commonly used rule of thumb is 1/20th of the Ah rating which, in the case of a 10 Ah battery, would be 0.5 Amps. Since you are going to be drawing closer to 9.7 Amps, you should not expect to be able to get anywhere near the full 10 Ah capacity of a 10 Ah battery. If you can find the detailed specification sheet for the battery you are considering, that sheet should contain a chart of discharge rate vs. expected usable capacity.

Also, even with a deep cycle battery, you should not discharge the battery beyond a certain threshold, as doing so will drastically decrease the lifespan of the battery. Most deep-cycle batteries should not be discharged beyond 80 (or 90 if you really want to push it) percent of their rated capacity and, as a rule of thumb, a deep-cycle battery habitually discharged to 50% will last about twice as long as one that is habitually discharged to 80%.

Another thing you should consider is how you plan to charge the battery. Ideally, you should use a charger that is intended for the type of battery you are going to use, and which will not exceed the battery manufacturer's recommended charge rate during the bulk portion of the charge. Charging your battery in a non-ideal way (such as, for instance, jumper cabling it to a running automobile) will likely result in a (possibly drastically) shorter battery lifespan and, possibly, incomplete charging.
posted by Juffo-Wup at 12:10 PM on May 12, 2018 [3 favorites]

Best answer: I was going to look at some battery data sheets and try to make an actual recommendation, but my Internet connection is flaky right now and I'm a bit short for time. So, as a completely uninformed off-the-top-of-my-head guesstimate, I would suggest that you should perhaps be looking for something around 20+ Ah for this, although really it would be best to consult some datasheets instead of guessing.

Also, if you are unsure how to interpret the datasheets and would like some possibly-qualified in-person advice, I have been pleasantly surprised at the level of knowledge of the folks at the Batteries Plus Bulbs stores in my area. So, if you've got one of those stores near you, you might consider seeing what they think.
posted by Juffo-Wup at 12:22 PM on May 12, 2018 [1 favorite]

Best answer: The manual makes me think the specs are sketchy. It claims 9.7A, but suggests putting in a 10A fuse. Either 9.7A is a very worst-case startup draw (and the normal run current is lower), or you're going to be replacing a lot of fuses.

I think you should measure the current (carefully, preferably with an inductive "amp clamp" meter) during use. If possible, you want a detailed look at the startup draw (just the first few seconds), plus the regular run draw. Ideally, you might let the rain barrel fill up a few times, then run the pump as you expect to do, but powered by an AC/DC supply or a giant battery. (You want to avoid voltage sag during a run, and the likely corresponding current increase.) See how much variation there is in run times and current, before you start putting together your final battery bank.
posted by spacewrench at 12:23 PM on May 12, 2018

Do note that deep cycle batteries are really heavy, if you'll be carting it around. He's overkill for your application but "Handy Bob" is focused on solar but with a bunch of battery detail. Crown is the best manufacturer of deep cycle. Basically oversize the battery and charge carefully.
posted by sammyo at 1:02 PM on May 12, 2018

You say that you'll be able to recharge the battery between uses, which makes me think that you have access to AC power where this pump is. If so, the cheapest and best solution might be to get a 12V DC power supply that's rated for 15 amps or so, and hook it up to that. It would be cheaper than a deep-cycle battery and way easier to deal with.
posted by Anticipation Of A New Lover's Arrival, The at 1:10 PM on May 12, 2018

Here's an example of the kind of thing I was talking about in my above answer. I have no idea how reputable that website or that power supply manufacturer is, so this link is not an endorsement. But that's the general idea, basically a slightly-larger-than-normal laptop PSU.
posted by Anticipation Of A New Lover's Arrival, The at 1:13 PM on May 12, 2018

Response by poster: Thanks for the answers so far - helps clear things up for me. I'll measure the amp draw as recommended and get a better idea of what size battery I need. Thanks again for the help.

Edit: To answer the question - I have no power running to where the pump is. I'll be moving the battery back and forth between charging/watering.
posted by pilibeen at 3:39 PM on May 12, 2018

The specs are a little ambiguous. The 9.7 amps is only the maximum starting current which might last one or two seconds, so don't worry about that. From your first link if you scroll down on the specs it says 50 watts. That is the running power. That means a little more than 4 amps at 12 volts. (To double check this I compared to the spec of 290 gallons per minute with 40 foot lift. That works out to 37 watts. Given the efficiency of the pump is less than 100%, 50 watts for the motor seems about right.)

You need a battery capable of providing an average of 4 amps for one-third of an hour, or about 1.5 amp-hours.

Here is a very cheap sealed lead-acid battery that will provide plenty of margin and is rated at 8 amp-hours. It weighs about 5 pounds and is 6 in x 2.50 in x 4 in so is easily portable and weighs less than the pump.

For current ratings of batteries, just about any battery will provide what is called its rated capacity for 1 hour. So this battery can handle 8 amps for 1 hour, which is more than sufficient for your task since you only need 4 amps.

As the manual states, you should place a 10-amp fuse in line with the red, positive, wire. Here is an automotive fuse holder that you can splice into your battery wire. You can take it down to your local gas station and get a 10-amp fuse to plug into it.

You can use wire nuts or quick-splice automotive connectors to splice the fuse.

You will need a cheap 12-volt trickle charger for the battery. Since you are only going to be using this once a week, you don't need a fast charger.
posted by JackFlash at 3:47 PM on May 12, 2018 [1 favorite]

If you go the lead acid route, don't get a sealed battery. A wet cell with top up plugs will last much longer as long as you add water or acid as necessary. Also, you want to literally double the capacity vs whatever your discharge will be because all lead acid batteries sulfate to some degree when discharged much below 50%. What makes a battery deep cycle is that it's recoverable to nearer original capacity as long as you don't discharge it quickly and leave it discharged for a long while, not that it's actually OK to deep discharge them.

Lithium packs would likely last longer since they tolerate discharge better so long as you don't go below 10-20%, but an appropriately chosen lead acid is probably more cost effective for this particular use.

An Optima AGM sealed lead acid is an intermediate option, but again would be less cost effective than a generic open cell lead acid, though you'd never have to worry about topups or spillage and would probably get more life out of it than a generic SLA.
posted by wierdo at 10:35 PM on May 12, 2018

Response by poster: For anyone who stumbles into this thread down the road and is interested...

It seems the 9.7A rating is the amp draw under normal load. I measured the amperage pumping through about 60' of hose @ 10' of head and it hovered around 10A. Startup draw maxed out around 13.8A.

As spacewrench mentioned - I'm not sure why they recommend installing a 10A fuse.

I ended up oversizing the battery and going with a 12v/18Ahr sealed lead acid unit from Amazon.

Thanks again for the help.
posted by pilibeen at 7:19 PM on May 23, 2018 [1 favorite]