What should battery voltage be reading under load?

johnelarue · June 2014

Hi all,

I have to rv/marine wet cells 12v. 105ah batteries in parallel being charged by 250 watts of PV and a Morningstar Tristar45amp model controller. And a 450 watt pseudo sine wave inverter.

I wanted to try the bread maker machine on the system, it is 370 watts peak during bake cycle and 30-60watts? During kneading mixing cycles.

So I had it running on kneading mixing cycles with the PV only in half sun , 2 panels fully in sun, 2 shaded. They only all get full sun from noon till 5. With one 100 watt panel getting full sun from 9 am or so. Strange as this all sounds it's the cards I'm dealing with.

So on mixing only, using low watts the batteries were quickly dropping to 12.4volts . But of course during rests in the mixing cycle the batteries jumped back up to 13.3- 14 volts. But then as time went on and mixing resumed, dropped down under 12.4V. So I stopped. Now everything looks fine again.

My question is, is this normal? Should battery voltage drop during loads so much and to what level is safe for 12 volt batteries. Is reading battery voltage under load possible with a MM or is there another way?

I can only imagine what would happen when the baking cycle kicked on to 370 watts, yikes.

Thanks for taking the time to read this,

John

mike95490 · June 2014

Re: What should battery voltage be reading under load?

batteries making a quick transition from full voltage to low voltage, and then recharging quickly, are likely just about dead. Their capacity has dropped from cells being sulfated most likely.

BB. · June 2014

Re: What should battery voltage be reading under load?

More or less around 12.7 VDC is "resting voltage"... Battery below this voltage is discharging (if under load) or less than full charge (if resting).

~13.6 volts is "float charging" (keeping the battery full, but not really actively charging). Around 14.0 to 14.5 volts is actively charging.

It sounds like you have your solar charging running and you plugged in the bread maker. During part of the cycle the bread maker pulls more current than the panel can supply and the battery goes from charging to discharging--Perfectly normal.

For a discharging battery, no load resting, you probably do not want to go below ~12.0 volts (around 50% state of charge at resting voltage).

Actively discharging, you will see the battery voltage drop... Under heavy loads, you probably do not want to see the discharging voltage to fall below ~11.5 volts for more than a few minutes (that would be, roughly again 50% discharged)...

It is difficult--Lead acid battery voltage is affected by state of charge, temperature, current flow (charging/discharging) and even how long the battery "rests" after charging/discharging:

http://forum.solar-electric.com/showthread.php?5556-Working-Thread-for-Solar-Beginner-Post-FAQ&p=125909#post125909

How large (Amp*Hours) and what type (flooded, AGM, etc.) battery do you have? If you draw too much current from too small of battery--Its output voltage can collapse (AGM are better at high surge current than "plain" flooded cell deep cycle batteries).

-Bill

johnelarue · June 2014

Re: What should battery voltage be reading under load?

Thanks mikeand BB,

The batts are 115 AH AC Delco Voyager 2 series , MF31MF, flooded, no vents, filling caps. Maintenance free.

https://www.acdelco.com/pdf/resource-library/Batteries/07-SS-0201-12_VoyagerSS.pdf
( the MF31 model seems to be an export model to Japan, so it's a little bigger than the 29)

They are 3 and 4 years old respectively. So maybe getting oldish,I heard 6 years max on these. Though they've been well treated I think, only running 5 or 6 CFL bulbs or LEDs occasionally.

It's float charging now and seems OK. Hmmm.

Thanks again guys,

John

waynefromnscanada · June 2014

Re: What should battery voltage be reading under load?

Something else to consider, for the good of your bread maker, is powering it from a "pseudo sine wave" inverter. In other words a modified square wave inverter. I fried two bread maker's electronics using them on such inverters. Have had no such problems since switching to pure sine inverters. Of course possible electronics destruction also depends on the design of the bread makers electronics, some are more forgiving than others of non-standard power supplies.

vtmaps · June 2014

Re: What should battery voltage be reading under load?

johnelarue wrote: »

I have to rv/marine wet cells 12v. 105ah batteries in parallel
<snip>
I wanted to try the bread maker machine on the system, it is 370 watts peak during bake cycle and 30-60watts? During kneading mixing cycles.

You mentioned that there is some solar input (shading on 2 of 4 panels), but you don't know how much... The kneading cycle is using more than your panels are producing, and thus I suspect the kneading cycle draws more than 60 watts.

So let's run some numbers on the bake cycle:

First of all you need to account for inverter inefficiency, so let's assume it draws 400 watts from the battery in order to make 370 watts of AC power.

400 watts ÷ 12 volts = 33.3 amps of current from the battery

Two batteries is 210 ah. BUT that rating assumes you are drawing the battery down over a 20 hour period... that would be a 10.5 amp draw.

If you draw the battery down at a faster rate, you must account for the Peukert effect... That means your effective battery capacity is much less than 210 ah when you are drawing 33.3 amps.

Bottom line: your batteries are very much undersized for that load.

--vtMaps

mike95490 · June 2014

Re: What should battery voltage be reading under load?

my breadmaker kneads and rests for about 3 hours, then bakes, about a 1kwh total cycle, during the resting & rising time, the heater cycles on to keep everything warm. So you will have more loading from the heater, even before the bake cycle.

Cariboocoot · June 2014

Re: What should battery voltage be reading under load?

Get a Kill-A-Watt, plug in bread maker, see what it really does, make bread by hand after you find out.

johnelarue · June 2014

Re: What should battery voltage be reading under load?

Cariboocoot wrote: »

Get a Kill-A-Watt, plug in bread maker, see what it really does, make bread by hand after you find out.

Actually I did just that and on quick bake it says 0.5kwh on my killawatt ez. My electric oven is 3000 watts, so the bread maker is much more efficient. Which is why I bought it. If you have mains gas it will be different.

johnelarue · June 2014

Re: What should battery voltage be reading under load?

vtmaps wrote: »

Bottom line: your batteries are very much undersized for that load.

--vtMaps

Thanks much, it's looking that way unfortunately

johnelarue · June 2014

Re: What should battery voltage be reading under load?

waynefromnscanada wrote: »

Something else to consider, for the good of your bread maker, is powering it from a "pseudo sine wave" inverter. In other words a modified square wave inverter. I fried two bread maker's electronics using them on such inverters. Have had no such problems since switching to pure sine inverters. Of course possible electronics destruction also depends on the design of the bread makers electronics, some are more forgiving than others of non-standard power supplies.

Thanks for this. Mine is a nice Panasonic and I'd hate to kill it. I shouldn't have cheaper out with the cheesy inverter.

johnelarue · June 2014

Re: What should battery voltage be reading under load?

Ok, so I disconnected the batteries from the system to see which or if both may have a problem. I hooked them up to the 450 watt inverter one at a time to check how they perform under my normal load of 6 LED bulbs , regular house sized bulbs so 1 amp each?

The 3 year old battery started the test at I think 12.77 V and slowly dropped to 12.19 V before bouncing back up fairly quickly to 12.55 V , then I disconnected it assuming it is OK and passed the test. After disconnecting the loads it came back up to 12.71 V

The 4 year old battery started the test at 12.99V , which I thought a little odd? And dropped down to 12.17 V before bouncing back but only made it to 12.33 V and held there for a long time, at which point I quit the test assuming the older battery has a problem. After disconnecting the load, it has come back up to 12.64 V

What do you make of the test results?

Thanks,

John

Cariboocoot · June 2014

Re: What should battery voltage be reading under load?

Those LED lights probably draw >1 Amp on 12 VDC (9 Watts @ 120 VAC = 0.075). Add conversion efficiency of inverter and its own consumption and the six lights might be perhaps 7 Amps DC. That is not a large amount of power (providing the numbers are correct).

Speaking of correct numbers, the resting Voltage of a 12 Volt flooded lead-acid should be around 12.75 maximum. So the first battery numbers are believable at 12.77 dropping to 12.19 and rebounding to 12.55. But they are not good. You don't say how long you left the load on. In theory even an RV/Marine battery should handle 7 Amps (if that is correct) for several hours without significant Voltage sag or drop.

The second battery's numbers don't make sense starting out at 12.99.

If the batteries are charged up properly and left to sit with no load what happens to the Voltage? You want to let them sit at least a couple of hours, take a reading, then leave them overnight and see how far they drop. Specific gravity readings with a hydrometer are far more accurate, btw, than Voltage readings.

Wire sizing can still be an issue both for power loads and recharging batteries: too small a wire adds too much resistance causing greater V drop under load and reduced current for recharging.

What should battery voltage be reading under load?

Comments

Categories