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Old 25-07-2014, 15:24   #1
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Voltage Drop in AGM Under Load?

1 have 6 x Lifeline Group 31 AGMs in parallel giving me 750AH . All batteries were new in October last ear.

Yesterday, I experienced what I believe to be an excessive voltage drop when under a large a load.

SOC at the time was 80%

Current draw was around 180 A (inverter driving microwave plus water maker plus assorted lighting plus fridge and freezer running).

Anyhow the fridge cuts out with a low voltage event <10.4V. This worries me. (Voltage measured at Fridge terminal is the same as system voltage - so no weird voltage drops in that line)

Surely a high end AGM bank at 80% SOC should be able to deliver 0.25C without crashing?..
Or am dreaming about the LFPs that I really should have installed?

Note : These AGMS are pampered .. SOC is never less than 70% and they're floated to 100% at least twice a week.

Can anyone recommend an affordable battery capacity tester ?- I was wondering if one of the batteries has died from being stifled by kid gloves.
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Old 25-07-2014, 15:31   #2
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Re: Voltage drop in AGM under load?

I don't about about AGMs, but my 460AH 6V golf cart batteries are probably designed for a large load and under a 100 amp load the voltage will drop from 12.5 to 11.5 if the batteries are at 95% SOC.
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Old 25-07-2014, 16:31   #3
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Re: Voltage drop in AGM under load?

Do you have separate fridge and freezer compressors? It is possible that the momentary startup load on one caused a short voltage dip on the line and tripped the other.

It's even possible that the startup load on either compressor tripped its own alarm.

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Old 25-07-2014, 16:42   #4
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Re: Voltage drop in AGM under load?

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Originally Posted by colemj View Post
Do you have separate fridge and freezer compressors? It is possible that the momentary startup load on one caused a short voltage dip on the line and tripped the other.

It's even possible that the startup load on either compressor tripped its own alarm.

Mark
Yes, I do.

But... both draw about 8A (BD50 and BD35) ..in the grand scheme of things how much more can the start up load be? (wiring sizes are generous on this boat, at least one size up from recommended)

I know the killer here is the microwave sucking about 130 A DC via the inverter. We have solar, so use the microwave a lot for making tea and coffee. Really cuts down on propane consumption!.

I was thinking that a draw of .25C should not suck the voltage down to 10.4V out of a 80%SOC battery. Maybe I'm deluding myself.
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Old 25-07-2014, 16:43   #5
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Re: Voltage drop in AGM under load?

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Originally Posted by sailorboy1 View Post
I don't about about AGMs, but my 460AH 6V golf cart batteries are probably designed for a large load and under a 100 amp load the voltage will drop from 12.5 to 11.5 if the batteries are at 95% SOC.
Looks like I am suffering the same pain. So maybe my expectations are too high.
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Old 25-07-2014, 17:08   #6
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Re: Voltage drop in AGM under load?

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Originally Posted by neelie View Post
Yes, I do.

But... both draw about 8A (BD50 and BD35) ..in the grand scheme of things how much more can the start up load be? (wiring sizes are generous on this boat, at least one size up from recommended)

I know the killer here is the microwave sucking about 130 A DC via the inverter. We have solar, so use the microwave a lot for making tea and coffee. Really cuts down on propane consumption!.

I was thinking that a draw of .25C should not suck the voltage down to 10.4V out of a 80%SOC battery. Maybe I'm deluding myself.
No, I don't think it should either.

However, the alarms on the fridge may be very sensitive. While a startup of a compressor could easily drop the voltage in a wire to 10.4V for a split second, it may be enough to trip an alarm. I wouldn't think it should trip so quickly, though.

What is the terminal voltage on the batteries when all this is running?

BTW, 8A running draw on those compressors is very high - unless you meant 8A for both together.

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Old 25-07-2014, 17:16   #7
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Re: Voltage drop in AGM under load?

Mark..

1) The 8A number was what I see after some rudimentary arithmetic between solar controler indicated output and BMS indicated net amperage. Plus an adjustment for other small consumers.

You are dead right.. 8A is high and quite wrong. But I have no idea what the actual draw is.


2) I have not measured voltage on the terminal of the compressors. While they are running. I will do so tonight when I see them running.

3) I did consider the voltage sensor for the alarm condition for the fridge may have been set too high because teh freezer - also running at the same time did not shuts down.

Will report tomorrow about compressor voltages.
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Old 25-07-2014, 18:13   #8
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Re: Voltage drop in AGM under load?

If you have a digital volt meter (multimeter), try checking the voltage DIRECTLY at the terminals of each AGM in your battery bank, both before and during drawing maximum current. If you have a weak battery this will find it. Make sure to put the meter probes directly on the battery terminals and not on the connectors. Then take the same measurements on the connectors. Finally, remove and clean the negative connections from the batteries, which are probably near the bilge. This is a more or less commonly overlooked "battery" problem. The Danfoss compressor electronic control modules allow you to set the trip points. Get your manual out and have a read. And, good luck.
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Old 25-07-2014, 18:24   #9
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Re: Voltage drop in AGM under load?

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If you have a digital volt meter (multimeter), try checking the voltage DIRECTLY at the terminals of each AGM in your battery bank, both before and during drawing maximum current. If you have a weak battery this will find it. Make sure to put the meter probes directly on the battery terminals and not on the connectors. Then take the same measurements on the connectors. Finally, remove and clean the negative connections from the batteries, which are probably near the bilge. This is a more or less commonly overlooked "battery" problem. The Danfoss compressor electronic control modules allow you to set the trip points. Get your manual out and have a read. And, good luck.
a) 6 batteries in parallel - surely once there is load on them, the voltages will be the same across each terminal....or not?

b) Batteries are under a bunk - so everything is bone dry and super clean.

c) I must write to Frigoboat - about how to set the voltage trip points. The number 10.4V is written into the minimalist manual I have and I assumed it was a hard number.
I can however set any number of alarms and trip points but they are all temperature related.

Thanks
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Old 25-07-2014, 18:43   #10
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Re: Voltage drop in AGM under load?

I measured a 4.5 amp draw on my single bd50 adler barbour while running. And yes, you would have to shut down the battery bank, maybe by disconnecting the output negative wire, and then letting the batteries settle down for maybe an hour, before taking individual readings.

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Old 25-07-2014, 18:48   #11
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Re: Voltage drop in AGM under load?

One thing that Richard always mentions is that the power wiring to the compressors should come directly from the power bus near the batteries, not from the DC power panel. Through fuses of course.
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Old 25-07-2014, 18:56   #12
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Re: Voltage drop in AGM under load?

You reported a large voltage drop, right? That means that you have a point of high resistance somewhere in your circuits. This could possibly be inside one of your AGMs which is why I suggested that you check each battery individually AT THE BATTERY TERMINALS which will allow you to detect any large voltage drop within the battery. At rest (with no current draw) you are correct in stating that as the batteries are all in parallel the voltages should be the same. However, when current starts to flow this is not the case at all.

Batteries under a bunk are well and good, but where do the battery cables go from the batteries? The negative cables are tied together somewhere, usually at or near the engine. Find that place and inspect it carefully.

I think that you can get the Danfoss electronic control module information directly from the Danfoss website. Frigoboat, Adler-Barbour (WAECO) all use the Danfoss compressors and ECM's. Hope this helps.
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Old 25-07-2014, 19:13   #13
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Re: Voltage drop in AGM under load?

Quote:
Originally Posted by SailorHarry View Post
...check each battery individually AT THE BATTERY TERMINALS….


Batteries under a bunk are well and good, but where do the battery cables go from the batteries? The negative cables are tied together somewhere, usually at or near the engine. Find that place and inspect it carefully.
Checking at the terminals won't be helpful unless the batteries are electrically isolated from each other first. This will involve removing some of the connectors.

I would be surprised if individual negative leads were led out of each battery to a remote junction. Most systems are simply tied together right on the batteries themselves and only a single lead (well, 2 - pos and neg) are taken off the bank.

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Old 26-07-2014, 05:44   #14
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Re: Voltage drop in AGM under load?

No one seems to be answering the question about the real life voltage drop

Aren't there people reading this have a similar AGM setup who have used their microwave? What voltage readings do you see during that?


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Old 26-07-2014, 07:23   #15
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Re: Voltage drop in AGM under load?

If the batteries were at 80% doesn't that mean they had roughly 600a/h available?

Wouldn't that mean the 180amp draw was more like 30C?

Wouldn't a 2 volt (minimum) drop be about normal with a 180amp load. I'd guess the voltage drop to be even higher than that.

Also how accurate was SOC?

180 amps continuous duty is a crap load of amps.
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