Best type of battery for well pump?

seymoursteve · June 2017

I will be the first to say that I know very little about batteries in general, but here is my question: the main item being powered by my small system (see description below) is to power my well pump at my home. We live in the country in SW Missouri that gets a lot of bad weather, and this seems to me to be the best answer to make sure we always have water available if the power goes out. I know true deep cycle batteries are best for solar setups, but since the well pump has such a high starting surge, does it make sense to use commercial grade marine deep cycle batteries that should be able to handle the powerful surges, but still have some attributes of deep cycle?

My system is 24v, 1100 watt sin solar panels and a wind generator going in to a Morningstar MPPT 60 amp controller, and then in to an AIMS 7000 watt 24v-to-240v power inverter. I currently have 4 12v marine deep cycle batteries set up in 2 banks of 24v, each battery is rated at 225ah.

Any thoughts on this would be greatly appreciated!

mcgivor · June 2017

What are the pump specs and run times? Would be helpful, along with any other intended loads.

seymoursteve · June 2017

Sorry about leaving out that information... it is a 1.5hp 220v pump about 350 feet deep. We mainly try to use it during the day when the system can do most of the work, but at times we need to run the washing machine or take a shower at night, so we are depending on the batteries to keep it going. There are only 4 of us in the house, so it doesn't get a lot of continual use - maybe once every couple of hours.

I notice when the pump kicks on that the battery voltage level will go from around 28.6 to 25.2 while the pump is running, then will charge back up to the 28.6 level within a relatively short period of time.

mcgivor · June 2017

Could you not pump during the latter part of absorption, thereby utilizing otherwise wasted energy to a holding tank and use a small on demand pump for nightime use. The specs for the inverter, if I'm looking at the correct model, claim a 200 MA draw at no load, which seems pretty low for a 7000 W inverter, but a smaller pump would not stress the battery as much when there is little or no input.

The logic behind using marine "deep cycle " makes sense in that it has the capability to provide high surge currents and as long as the depth of discharge is minimal they may work, personally I've no experience with them, perhaps others will have some opinions.

jonr · June 2017

I've taken complete showers on a pressure tank alone.

Estragon · June 2017

Voltage sag under high load is normal.

That said, it would help to know wire sizes and lengths between batteries, and from battery to inverter. Also, I would check connections with an IR thermometer when the pump is running to see if any are heating up under load.

706jim · June 2017

7000 watt inverter on 4 small batteries? Yikes!

seymoursteve · June 2017

I know it is overkill, but better than not getting one too small. The AIMS was the only 24v to 240 I could find at the time. As for the cables, I have 12" 2-gauge cables connecting the batteries in serial to make the 24v banks, then 18" 4-gauge cables tying the banks together. I use 24" 2-gauge to connect them to the inverter.

Also, I try to run the pump off of the solar 24-7 as much as I can, and have a switch box to move it back to the grid when necessary.

I appreciate all of the input, and would love to hear about the cables I am using.

Estragon · June 2017

You have about 1/2 the battery I have, with the same inverter capacity.

I understand the overkill idea. That's why I have the inverter headroom. In my case, it's to handle the unlikely event lots of large loads get going at once (like visitors... one has a shower, another makes toast at the same time, etc.).

Marine batteries are kind of a comprimise between a true deep cycle battery and one designed to supply high current for a short time, and get promptly recharged. They tend toward the latter than the former, and you may find they don't last long in a deep cycle application. When the time comes to replace them, I would rethink overall system design.

You haven't posted pump current specs, but the current at 24v is likely to be significant. Perhaps in the interest of overkill, I use 4/0 between batteries, and to the inverter. I suspect some of the voltage sag is wire loss, and I would replace with heavier, at least for the parallel connection between strings, and to the inverter.

seymoursteve · June 2017

I appreciate the feedback; I will look into replacing the smaller cables with heavier ones, and I am going to put them on to a bus bar instead of just using the cables. Hopefully I will get a little more performance.

Here is a link to the batteries I am considering buying my next time around: http://www.interstatebatteries.com/p/interstate-batteries/interstate-batteries-srm-4d-srm-4d?dsNav=N~2147384903

I would appreciate any thoughts on these.

Thanks!!!

Estragon · June 2017

The link isn't working for me, but it could be crappy inet at the cabin on the weekend. A 4d 12v battery probably isn't the right solution, but I don 't know enough about the situation to say.

When the time comes, look at the overall balance of system and decide from there.

seymoursteve · June 2017

Sorry that the link isn't working; here are the specs for this battery. It is an Interstate commercial grade, found on their website under the Deep Cycle section: (again, for is for a well pump which has short bursts of high usage.

Product ID:	SRM-4D
Cranking Amps:	1645
Cold Cranking Amps:	1314
Voltage:	12
Termination:	Common Code M
Weight (lbs):	117
Width (in):	8.25
Length (in):	21.00
Height (in):	10.38
ReserveCapacity-25:	390.00
WET/DRY:	W

mcgivor · June 2017

All that's needed now are the pump specs.

seymoursteve · June 2017

Well, I am not going to be able to supply the pump specs; we bought this house 11 years ago, and the pump was put in probably about 10 years before that. I can't find any info in the well house - all I know is what the previous owner told me, and that was what I had stated earlier.

Aguarancher · June 2017

Looking inside your motor control box should give you a good idea of your pump size. They are sized by horsepower. If necessary google your control box info for specs.

mcgivor · June 2017

Do you have a clamp on ammeter with inrush current capability to measure the demands, nice to know for calculation purposes, understandably not everyone has one.

mvas · June 2017

seymoursteve said:

I know true deep cycle batteries are best for solar setups, but since the well pump has such a high starting surge, does it make sense to use commercial grade marine deep cycle batteries that should be able to handle the powerful surges, but still have some attributes of deep cycle?

No, it does not make sense to buy a marine deep cycle battery to run your well pump.
The number of deep cycles for a Marine Battery is less than half of a True Deep Cycle.
True Deep Cycle batteries can handle the brief surge from the pump.
You need a True Deep Cycle battery like the 6 Volt GC2 or 12 Volt GC12.
Quantity and Size (AH) per your needs.

mike95490 · June 2017

Most pumps will be pretty close to this Franklin motor chart.
Beware - Power Factor - my inverter logs about 1,000 w load when my 1/2 hp pump is running. Your inverter must be able to handle this load.

Image: https://us.v-cdn.net/6024911/uploads/editor/bk/t9x76dwo7037.jpg

mvas · June 2017

mike95490 said:

Most pumps will be pretty close to this Franklin motor chart.
Beware - Power Factor - my inverter logs about 1,000 w load when my 1/2 hp pump is running. Your inverter must be able to handle this load.

They OP states he has a 1.5 HP Well Pump.
So, per your chart he needs an inverter to support (up to) 15,000 Watts starting & 3,000 Watts running ...
a) 62 Amps starting / 13.1 Amps running @ 240 VAC for 2 wire OR
b) 45.9 Amps starting / 11.6 Amp running @ 240 VAC for 3 wire

A possible surge of 460 - 620 amps on the batteries and cables.

Estragon · June 2017

Also, to the OP, I suspect the term "commercial" may refer more to the size the battery more than anything to do with quality of construction or suitability for your intended use. A 4d size is likely intended for commercial/industrial/farm use, though they're also sometimes used in rv/marine applications.

I wouldn't be quite as quick to dismiss the battery as @mvas though. I do agree with his comment, but would add a couple of caveats.

First, the fact that these batteries specify cranking and cold cranking amps tells me that the battery has relatively thin plates. This allows for a relatively large surface area of plate to be exposed to electrolyte, which in turns allows for the chemical reaction producing current to happen quickly. In your application (assuming the pump is the main/only use), this is useful.

The flip side of this is the battery's thin plates won't likely stand up to shedding from deep cycling, and because the reactions happen quickly, it also has to be recharged relatively quickly. If it regularly sits around deeply discharged, it won't last long.

My deep cycle batteries (admittedly a bigger 48v bank) run pumps fine. They don't have a CCA rating, but do okay. I've also used a much smaller 12v deep cycle bank to start the diesel on a boat.

So if all you're using the batteries for is running the pump during power outages and can get them charged within a few hours, there is some logic to using them. If you're also running smaller loads and will be doing so for more than a few hours, a bank of deep cycle might make more sense.

In the meantime, I'd try the heavier cable. At 24v there's some serious currents involved.

mvas · June 2017

@Estragon,
Do you use Marine Deep Cycle Batteries?

Estragon · June 2017

I have, and I agree they're a comprimise that likely ends up not doing either application well.

Lumisol · June 2017

Have you looked into AGM batteries?

Estragon · June 2017

Yes, for the boat among other things. AGMs have several advantages for that application including; less likely to vent potentially explosive gas in the confined cabin space, the ability to recharge more quickly with limited engine run time, limited possibility of acid spill in rough or heeled conditions, and lower self-discharge during seasonal layup. In the end I replaced with flooded again though as they have worked okay in the past and were ~1/2 the price of AGMs. Hopefully next time some form of lithium will be affordable and well tested enough.

I did buy an AGM as a starting battery for my diesel generator though. It doesn't get used much, and I'm likely to forget to charge it, so low self-discharge was the deciding factor.

seymoursteve · June 2017

I thank everyone for their input. I spent time last week changing the cables, as suggested, and have already found an increase in efficiency. I will stick with what I have for now, and when the time comes to get new batteries, I will definitely go with true deep cycle, either golf cart or L16.

mcgivor · June 2017

That sounds like a sound plan, good luck.

Best type of battery for well pump?

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