Battery Voltage Drop Question...

[rs500cat]

We are running (6) almost new 50AHr Universal UB12500 batteries in parallel for a ham radio application. The batteries are all 5.5 mOhms and range from 470 to 500 CCA after a night’s rest. The batteries seem to vary between 13.0 to 12.65V after rest. The battery bank is charged using a Morningstar MPPT controller providing about 14A under full sun.

A draw of 15 amps draws the batteries to under 12.0 volts after about 5 hours. However, a 25 amp draw on the charged batteries brings the runtime voltage down below 12.0V after 90 minutes. With a total capacity of 300 AHrs, It would seem that the bank should easily handle 3-4 hours of 25A and still be over 12.0 volts. The radio equipment must be supplied at least 12.0 volts to reliably operate.

Are the batteries handling the 25A load properly? Is there a way to maintain a 25A draw at or above 12.0 volts for a longer percentage of the battery rated capacity? Would it significantly lengthen the operating time if the batteries were moved into 3 parallel sets of 2 batteries at 24V, with a 24V to 12V step-down converter?

Any assistance is appreciated, Ron

[Mike90250]

you have re-discovered Peukert's law

http://en.wikipedia.org/wiki/Peukert%27s_law

The faster you take power out of a battery, the less power exists. Batteries are generally rated at a 20 hour rate ( don't ask why 20 hr and not 24 hr) Accelerate that rate, and you get less capacity.
You need a much larger battery. 4, 6V golf cart batteries in series/parallel for 400ah could work. *

In reality, you can only take about half of the Ah out of a battery without affecting it's lifetime too much, deeply discharging a "deep cycle" battery is tough on them.

Paralleling batteries is really good for accelerating the death of the entire pack, by exercising the lowest impedance battery in the pack, so one battery hogs all the work till it's failed, then the next best one . . . . .

* See this article on how to avoid this http://www.smartgauge.co.uk/batt_con.html

[Sunking]

You have a lot of issues with your setup. First mistake is 6 parallel batteries. If you need 300 AH batteries buy 300 AH batteries.

Second mistake is if your batteries have a CCA spec rating, they are not deep cycle batteries.

Third mistake is 14 amps is not near enough to support a 300 AH battery, especially if using solar. This time of year at best all you can you ar elucky to get 40 AH input, and you are taking more than 100 AH out. A 300 AH battery using MPPT minimum panel wattage is 400 watts, and it sounds like you only have 150 watts.

73's

KF5LJW

[rs500cat]

Well, I seem to be slowly moving up the learning curve.

We live in Colorado and the Morningstar MPPT controller is providing 15V at around 14A for a good portion of the day, so the batteries may be getting around 80Ahr on a good day. Does the comment that '15A is not enough to charge that batteries' simply mean more time would be required to charge, or is 15A simply not enough 'oomph' to charge no matter how long it is applied?

I will need to reduce my usage to whatever actual charging current is available. The ham radio application requires at least 12V. We want to supply 15A while receiving, increasing to 25A for short periods (possibly 60 total minutes, in 1 minute lengths while transmitting). Assuming we do not use more total power than the panel/controller can ultimately provide to the batteries, the major problem seems to be the voltage drop when the current draw increases to 25A. By itself, would changing the six AGM batteries to a single 300Ar wet cell battery dramatically impact the ability to draw 25A without reducing the voltage under load below 12V?

Thanks, Ron

[Mike90250]

The ham radio application requires at least 12V. We want to supply 15A while receiving, increasing to 25A for short periods (possibly 60 total minutes, in 1 minute lengths while transmitting).

15A@ 12V is nearly 200 watts for a receiver ? That just seems way wrong unless it's a tube set ?? I can understand 25A burst for transmit.

[rs500cat]

Actually total current requirements for receive conditions are a little less at 10A, not 15A (I overestimated them). That includes a FlexRadio 6500, Prosine inverter, computer, 2 monitors, and a couple of minor other devices. You are correct, a simple receiver would not draw that much current. The consensus seems to be that I need to add a second panel and upgrade the controller to provide a 30A charge for the batteries.

CALLD made a suggestion for VRLA/AGM batteries. The UPG UB12500 battery spec sheet says the batteries are 50 AHr valve regulated AGM batteries. I have attached the spec sheet. Battery.pdfAre we talking about a different battery technology, or simply one larger battery? With the investment in the six new batteries it would be nice if they would work, even if sub-optimally for a while.

Thanks, Ron

[Sunking]

Ron read this first.

You are making the same mistake 90% of folks make. You failed to plan, and got the default plan result, failure. Very first step is to determine how many watt hours you will use in a day. Without that you are dead in the water. Once you know that, then you can design a system to meet the objective. What you and others like you have done is something on the order of being on the east coast driving to to the west coast in a car with $30 of gas money. You are not going to make it because you failed to plan and budget. Read the stickie on the design and I think you will then understand and find where you went wrong. What you are going to find out is you do not have enough panel wattage, large enough controller, and not enough battery.

73's to you.

[rs500cat]

I agree, I like many others are on a learning curve. It should be noted, however, that the load characteristics have completely changed and were not predictable. The original project required less than 7A, with no significant bumps in power requirements. The challenge is how to take an existing infrastructure and adapt it when requirements change. This forum has been a great help.

Ron

[Amy@altE]

That Universal is a deep cycle battery, however, it's original intended application was for electric wheel chairs, which is probably why they included the CCA specs.