Battery Voltage Drop...

rs500 · December 2014

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 under load 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

westbranch · December 2014

Re: Battery Voltage Drop...

Welcome. I will make a quick comment or 2 to get you going.
1. that 14 A is not enough to give you a proper charge, you have 300Ah (6 x 50) of battery that needs 30A roughly to give it a good charge,
2 we need to know what you have for Solar panels, give us the specs off the rear of the panel please.
3. have you a generator and a good 3 stage charger? If so you should put it into service asap.
4.batteries with a CCA rating are usually NOT the best fit for constant power, rather they are a modified starting battery labeled 'deep cycle'

HTH

waynefromnscanada · December 2014

Re: Battery Voltage Drop...

And #5, having 6 batteries in parallel is a bad idea. It becomes near impossible to have them all charge and discharge at the same rate. Generally 3 in parallel is the max one can get away with and not shorten battery life due to unequal charge/discharge.

BB. · December 2014

Re: Battery Voltage Drop...

westbranch wrote: »

4.batteries with a CCA rating are usually [NOT?] the best fit for constant power, rather they are a modified starting battery labeled 'deep cycle'

I am not sure, but I think Eric left a "not" out of that statement?

CCA is cold cranking amps rating... Generally for starting batteries, not for deep cycle batteries.

The batteries you have, if this is correct:

Universal UB12500 is a 12 volt 40 Ah valve regulated (VRLA) AGM battery. It is completely sealed lead acid battery with valve regulated. Valve regulated, spill proof construction allows safe operation in any position.

Should be fine for high current draw (AGM is usually the best for high current applications, and also high charge rates--Although, make sure you do not use too high of charging voltage, generally around 14.2 volts or so maximum at 75F--or they will vent and die).

So--What is your charging voltage and how long do you hold it? At 14 AH, it may take several days (especially during winter) to fully recharge from a deep cycle.

14 amps is OK for a bank that is used weekends/seasonally--10% or higher would be better for daily off grid system.

The batteries do not sound to be working well... I would suggest using a DC Current clamp DMM and measure the discharge/charge current and make sure that each string (battery) is sharing the load roughly equally. You could have an open cell (or even a shorted cell) that could be hurting their overall performance.

Of course, if these batteries are "aging" (from poor storage, aging, lots of cycling/deep cycling, etc.), then they simply may be giving up the ghost.

Cycling them by 20-40% or so, and recharging 10+ times and see if it gets better.

If you can live with flooded cell batteries, you might want to look at a pair of 6 volt @ ~210 AH "Golf Cart" batteries. Pretty rugged and priced right for this application. And because they are flooded cells, you can check the specific gravity and determine the "exact" state of charge. This meter has been pretty popular:

http://www.midnitesolar.com/productPhoto.php?product_ID=505&productCatName=Battery%20Accessories&productCat_ID=39&sortOrder=1&act=p

And some information on parallel wiring of batteries:

http://www.smartgauge.co.uk/batt_con.html

Some reading:

http://www.windsun.com/Batteries/Battery_FAQ.htm
http://www.batteryfaq.org/
http://batteryuniversity.com/
www.powerstream.com/SLA.htm (sealed lead acid batteries specifically)

-Bill

westbranch · December 2014

Re: Battery Voltage Drop...

OOOPPPS!

Bill you are right, have corrected it.

thanks.

BB. · December 2014

Re: Battery Voltage Drop...

NOT the first time for me either Eric.

-Bill

rs500 · December 2014

Re: Battery Voltage Drop...

Thanks for all the advice. A single 240W panel is charging the batteries. The panel is on a sloped roof and generates 34V at around 6.5A under winter sun. A Morningstar SunSaver 15A MPPT 3-stage controller ups the current to about 14A to charge the batteries. You may be right that this is not enough to charge the batteries. The controller absorption voltage is set to 14.6V (the battery mfg recommendation). The batteries only get up to 13.8-14.3V when charging. The float is set to 13.7V.

I have taken the advice and re-wired the batteries to a common positive and negative bus, rather than simply wiring them in parallel. The batteries are basically new, they have not been abused and the resistance and CCA test results are nearly identical. Since the battery investment is recent, hopefully there is some solution other than completely replacing them.

The ham radio application requires at least 12V. We want to supply 15A for 6 hours a day, increasing to 25A for short periods (possibly 60 total minutes, in 1 minute lengths while transmitting). By my calculations, that is 115A (lets say 130A per day with electrical and wiring equipment losses). The major problem seems to be the voltage drop when the current draw increases to 25A.

So to summarize, is the panel/controller adequate to charge the batteries? And second, what is my best solution for providing the power listed above without dropping below 12V? What can we leverage out of our current investment? There sure is a learning curve when it comes to practical implementations.

Thanks for all the help, Ron

Photowhit · December 2014

Re: Battery Voltage Drop...

Asking a battery bank to not fall below 12 volts, when demanding 10% of it's capacity will require one of a couple events;

You could increase the size of the battery bank, also the array to charge it etc.

You could switch to a battery chemistry that has less voltage drop off, such as a LiPoFe chemistry, but I don't think you can charge it with your current charge controller as it has different charging parameters.

Or You could drop your demand on the system, perhaps switching at a lighter wattage transmitter and do what you can It increase the potential for transmission, better antenna, or directional antenna(if possible)...

Since it's a short duration, 1 minute at a time, perhaps you could rig some sort of buffer, perhaps using a chemistry that has a rapid discharge characteristics, but use it to smooth out the voltage drop. Perhaps 10, 1.2v 8ah 'D' sized NiMh batteries in parallel to the lead acid batteries. I can't recall the charging parameters for NiMh batteries, I don't think they require as much 'head' room as lead acid batteries, so they may recharge a bit from the paralleled lead acid batteries between transmissions. You would want to disconnect the battery banks between uses and charge the NiMH independently... Just thinking out loud, I don't have the discharge voltage curve for NiMH batteries, but they can discharge very rapidly and I believe they don't have as much voltage lag, so energy would be drawn from them first...

CALLD · December 2014

Re: Battery Voltage Drop...

Hi, based on what you need to use the system for I'd say you would need to consider getting VRLA/AGM deep cycle batteries as these hold their voltage higher under sustained discharge. The higher discharge voltage will also allow you to get away with a smaller battery. In your application this could be a single 12v/260Ah battery. You would be cycling using about 50% capacity and with your maximum current draw of 25amps you should never see the voltage drop below 12.2v even after 130Ah has been removed under standard room temperature.
As others have said you need more charging amps, preferably 30Amps peak. You will also need backup power like a generator or the grid because even in sunny climates there will be times where you don't see more than 50% of your solar panels rated output for several consecutive days. In a really bad stretch of cloudy weather it doesn't matter what you do the batteries will not charge on solar alone.

westbranch · December 2014

Re: Battery Voltage Drop...

If you do not want to get more panels immediately, beg, borrow, rent or ..... a generator and a 3 stage battery charger and put the batteries through a FULL charge cycle asap to preserve their integrity. Best bag would be a 2000W rated inverter type gen set , I like Honda's,as it sips fuel for 8 hours...

You will need another bigger charge controller ass that MS is about topped out at 14A, you need to look at something around 30 Ammps, but that is what you battery needs and it is best to keep a CC or othere electronics below 80% of max for longevity...
Looking at a Classic or similar, or 3 of what you have right now... each with its own panel...?

hth

rs500 · December 2014

Re: Battery Voltage Drop...

Attachment not found.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

BB. · December 2014

Re: Battery Voltage Drop...

I am not a fan of a lot of smaller batteries in parallel. A single (or pair) of large batteries with one or two parallel strings is usually better.

I am sure there are lots of nice/rugged/long life smaller lead acid batteries--But my personal luck has not been great (usually coupled with a less than ideal charge controller).

More cells, more things to go wrong, more cells to water (if flooded cell), more wiring to have issues with, etc.

Do you have a DC Current Clamp meter to check the current for each battery during discharge/charge? A current clamp meter is really nice for any debugging, but with parallel batteries, almost a necessity.

At this point, I do not see any reason why your present battery bank should not be capable of supporting the loads listed if they are in good shape--Unless the batteries are not being fully charged or there is some other hidden problem.

Note that you need the absorb voltage set point for ~2-4 hour minimum typically with lead acid batteries. That gets you the last 10-20% state of charge. A current meter of some sort will help you understand when the bank is full. You will see the battery charging fall and then hit a stable value... Typically around 0.1 to 1.0 % of the bank AH capacity (around 0.3 to 3.0 amps for your 300 AH battery bank). That is when the batteries will be "full".

You could go with a battery monitor of some sort like the Trimetric or Victron--But those have their own wonderful "issues".

-Bill

westbranch · December 2014

Re: Battery Voltage Drop...

re power use, if you can measure your computers' draw... a desktop will probably be in the realm of 80 watts or so, all peripherals on... so a way to reduce that load is to use a laptop which can be as low as 5 watts at the extreme low end...

the Midnite KID is a 30A controller and with the WhizBang jr you get a amp measurement of what comes out and into your battery. Highly recommended especially for its price ..
http://www.solar-electric.com/inverters-controllers-accessories/chco/misoclchco/midnite-solar-kid-mppt-charge-controller/midnite-solar-whiz-bang-jr-current-sense-module.html

hth

rs500 · December 2014

Re: Battery Voltage Drop...

Wow, the Midnite Kid looks like a great unit. I was going to upgrade to the Morningstar Tristar 30A MPPT, but it is twice the size and does not provide true load control and several other features. FYI, I am using a 4 processor i7 Mac Mini running Win7. It only draws 1A to 2A depending on the load (which is significant) and supports multiple monitors which are required for our application.

I think the solution is coming together. I am going to add a second 240W panel, upgrade to the 30A Midnite Kid, and for the moment hope for the best on my present VRLA/AGM batteries. When they have issues I will upgrade to a single larger battery.

Can the batteries be charged at the reduced 15A rate without damaging them (remember that they are not typically reaching the absorption point)? I will likely upgrade to 30A in the next month or so. I could simply let the batteries rest until the upgrade if the lower charge rate is a problem.

Thanks, Ron

BB. · December 2014

Re: Battery Voltage Drop...

1-2 amps at 12 volts or 120 volts? Guessing at 12 vdc.

-Bill

rs500 · December 2014

Re: Battery Voltage Drop...

1-2A at 12V.

Ron

BB. · December 2014

Re: Battery Voltage Drop...

Sorry, had 22 VDC as my guess--Intended to type 12 VDC at 1-2 amps.

Anyway, that is mot a large amount of power for a laptop. Doing good.

-Bill

Vic · December 2014

Re: Battery Voltage Drop...

Hi rs500,

In general, trying to operate "12 volt" Hammie Transceivers on 12 volt batteries that are not being charged by a reasonable charger at the time, can lead to disappointment.

Most/all 12 V XCVRs are speced at 13.8 V +/-- a certain percentage. BUT, what happens at less than 13.8 V, is usually not speced by the manufacturer. The ARRL Labs often runs tests on 12 V radios, and for every specific model, the results of low input V may differ. Some radios fail to make power, and some develop horrible IMD (and other distortion products) on SSB, and perhaps PSK

Fully charged AGMs may rest at about 12.9 V, or so, but even AGMs sag significantly with 22-ish Amp demands of mast 100 watt XCVRs.

Looks like you have an inverter. Have you considered using the inverter with a 120 VAC PS to power the radio, most of the time that it is needed? This is a bit less efficient, but side-steps many issues.

Agree that multiple batteries in parallel is generally a poor approach for higher AH Capacity. Single-string is king.

Opinions, based on experience with FLA batteries as 12 V radio power sources.

FWIW 73, GL, Vic

zoneblue · December 2014

Re: Battery Voltage Drop...

rs500 wrote: »

Wow, the Midnite Kid looks like a great unit. I was going to upgrade to the Morningstar Tristar 30A MPPT, but

Agreed, Kid is a great controller, and will suit your application well. Very flexible.

and for the moment hope for the best on my present VRLA/AGM batteries. When they have issues I will upgrade to a single larger battery.

LiFePo4 may be good for that. It will allow you a small battery, with high C rates and very low voltage sag. However you will need to adequately aquaint yourself with the technology as its kinda 'complicated'.

Can the batteries be charged at the reduced 15A rate without damaging them (remember that they are not typically reaching the absorption point)?

0.05C peak is as others mentioned on the lite side. Whether that works or not depends on your load regime. Obviously if the equipment is being used at the same time, then the charge rate will further decrease (even if it does help support your load). You mention that the bank is not reaching absorb often. Its critical for lead acid chemistry to complete an full absorb cycle at least twice a week. This means reaching absorb voltage and staying there, until the charge acceptance drops to around 0.005C. This will usually take several hours. If this is not occuring, then the battery is not being charged, and instead is forming hard sulphate. This wont help support your loads, and will eat into the banks capacity with each passing day. So take WBs advice and get a generator onto that bank, and give it a good long absorb.

I dont know about your exact morningstar unit, but the PWM models i have here are pretty sparse to utterly bereft of any kind of feedback information about the charge cycle. Midnites basic monitor is good for telling you the rough SOC based on cell voltage, and it also tells you how long since last float. More expensive battery monitors like the victron or bogart units will give more accurate state of charge, and are generally considered essential for sealed batterys, because you have no other real way to determine SOC with them.

Or, if you go Kid, get the cheap WBJr battery monitor addon, thats a no brainer.

For the loads, i have 400A AGM batterys here that weigh around 60 pounds each 2v cell, and 1.8mohm per cell. If i hit them with a 0.1C load, what they do depends on the time of day. Obvisouly if the sun is out the solar helps out and there is no worrys. After dark they might sag to 24.5V, but in the morning theyll most likely go down to 24.2, or as low as 23.9 if the SOC was below 70%.

If the transmit is bursty, then PWs idea of supercapacitors might be worth investigating, something to support the peaks only.

However it sounds to me like in general you are asking quite a bit of those little batterys. If your daily loads are really in the 1.3kWh/day territory, thats a DOD of 37%, which is more than the recomended 25%. And thats assuming the bank is charged. If it isnt charged then you could be going quite a lot lower, and talking into account weather variability, even lower. So your plan to increase the array and charge capacity is quite wise!

Battery Voltage Drop...

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