12v Pump from battery

earlystock

I have a strange issue. I have a solar set up with a 50w panel and a 100AH leisure battery. This powers lighting and a few devices in a summer house, All good, battery stays charged and everything works. We have installed a water feature near the shed with a 12v pump. When I connect the pump to the battery (via a switch and fuse), the lights on the feature come on, but the pump does not start. The pump is rated at 21VA, so not really a huge load. If I plug the pump into a transformer, then plug that into an invertor running off the same 100AH battery, all is well and the pump works. Logic would say that inverting 12v into 240v and then transforming it back down to 12v would be inefficient, but it seems to work fine. Any thoughts as to why the battery will not power the pump directly?

BB.

Welcome to the forum EarlyStock,

First, you have to be careful with adding loads to a small system... Many times, it is the smaller loads that run for hours at a time that end up taking much of the available harvested energy from your system. For example, just taking some wild guesses:

• 21 VA / 12 volts = 1.75 Amps
• Say you want to run the pump for 8 hours a day:
• 1.75 amps * 12 volts * 8 hours = 168 Watt*hours per day (need details, VA is typically an VAC measurement, with DC we usually stick with Volts/Amps/Watts/Watt*Hours. Assuming VA=Watts in this case.

Guess somewhere around London UK? Fixed Array tilted for best summer harvest facing south:
http://www.solarelectricityhandbook.com/solar-irradiance.html

London
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 53° angle from Vertical:
(Optimal summer settings)

Jan	Feb	Mar	Apr	May	Jun
1.19	1.95	2.77	3.84	4.52	4.63
Jul	Aug	Sep	Oct	Nov	Dec
4.66	4.41	3.31	2.33	1.43	0.97

June at an average of 4.63 hours of sun per day with 50 Watt array:

• 50 Watts * 0.61 average DC solar system efficiency * 4.63 hours of sun per day (June) = 141 WH per day average June harvest

In the above example, your water feature pump running 8 hours per day would use 168 WH per day and you are harvesting around 141 WH per day... Not a good start...

And your specific question... Assuming this is a 12 volt pump, have you measured the voltage at the pump terminals to see that it is >~11.5 volts (to start pump)? Long wire, with 'thin' cable, you can have lots of voltage drop. Sending significant current @ 12 volts DC over thin wire over much more than a few meters--You can easily have 2 volts or more drop...

Try running the pump directly attached to the battery (with short wire). Also the "alligator" clips can be very poor at carrying current... Bolted up connection are better.

Further information? Your thoughts/questions?

-Bill

earlystock

Thanks Bill.....will try the suggestions and let you know the outcome (and yes I am 30 miles south of London so sun is definitely a problem!) we will only run the feature when people are here. In fact I looked at the pump and it is supposed to be 12W  so I guess that gives a bit more leeway (the 21va was from the supplied transformer).The only difference between two set ups (apart from the obvious use of a transformer and invertor) is the addition on the battery only scenario of an extra 1 metre extension lead, which I tested for resistance and it is 0 ohms. 

Another symptom is that when I connected the pump directly to the battery it took the voltage across the battery right down and on the charge controller it showed the battery depleting quickly, as if there was a high load, or almost a short circuit, however when I measured the current it was 0.3A, which I suspect was the lights - which were on!

BB.

You are very welcome.

Just to be clear... What is the output of the "Transformer"? A pure/true transformer takes AC power in and outputs AC power out.

A power supply will take AC in, and output DC.

Just want to be clear (i.e., a 12 volt AC motor may not run on 12 VDC). There are "universal motors", or "brushed" motors that run on AC or DC.

-Bill

InCogKneeToe

My 12v Water Pump, is run from my Shed's system. 2 40w panels, 7.5A Charge Controller, 4 AGM Car Batteries. Weekend Use it works just fine. I also have a 2000w Inverter in the shed to run the occasional Power Tool and Night/Inside lights. This system has no problem keeping up to Holiday Constant use of Water Pump and occasional Lights.

garynappi

Hi Earlystock, and welcome.

I can't answer your Q directly but one question? What's the state of charge in the morning after running the lights all night? 

Bill  is (as usual) spot on with the advice above.

I have a similar need for my yard. I'm running a small water feature pump (800 lph 1/4" tube 4' lift) 24/7 and yard lighting with 40 one watt LED lamps with the load set to dawn/dusk operation. 

On two consecutive cloudy days my 100 watt panel would not keep my 90Ah battery fully charged (even in South Florida) and the LVD (low voltage disconnect / protect circuit) would kick in. 

Adding a second 125 watt panel cured the issue.