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Old 26-07-2014, 09:20   #16
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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.
Apart from all the other possibilities, have you considered downsizing the microwave itself? Or getting a second, smallest size one (0.5 or 0.7CF) just for heating water, if you have the extra space?
Sounds like you have a larger house-sized microwave oven (+1.2+CF/1KW?) which you are (often) using just to heat water when a much smaller one can do that job.

There is a 0.5CF/750W Whirlpool model with the back corners rounded off that fits in corners well, and the guts below instead of on one side (raising it up but making it narrower, turntable still fits an 11" plate; Lowes and HD have it ~$129).
750w/120V is 62.5A/12V, less than half the power usage of your current one; there are lots of standard 700W/0.7CF MW's too, at 60A/12V.

The main advantage to a large MW is to accommodate larger objects (besides power/quickness), so unless you're doing a lot of big platters or turkeys, a small MW will do just fine (with time and container adjustments).

Panasonic also produces a series of MW's called "Inverter" models, which I think are supposed to be more efficient, use less power per/CF ("Inverter" not meaning the same as an onboard one).

Good luck on finding the gremlin.
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Old 26-07-2014, 10:04   #17
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Re: Voltage Drop in AGM Under Load?

How do you know the actual SOC?
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Old 26-07-2014, 10:36   #18
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Re: Voltage Drop in AGM Under Load?

Wow! 180 amps is a HUGE draw. A battery bank of your size could not begin to handle that much draw for more than a short time. in any case it will draw down the voltage immediately and then go down the tubes in a short while. It would take longer with a much larger bank but in the end that is a big draw and the bank would have to be multiple times larger than what you have.

Deep cycle batteries (wet, gel, or AGM) have thick plates which hold the energy inside. They are designed to release that energy in a controlled fashion at a reasonably gradual rate. They have a substantial amount available on outside surfaces and just below but after that is mostly used, the energy has to be extracted from deeper in the plates. It takes some time for the chemistry to happen.

When a big load is on it is handled quickly (like running a microwave for a few minutes) with some immediate voltage drop. If you turned off the load after a few minutes, the voltage will pop back up, the speed depending on the size of the bank relative to the temporary load. When I needed to verify that an alternator or other charger was putting out, I would put a cup of water in the microwave (mostly powerboats of course) to take the surface charge off the batteries, otherwise the alternators would not put out to what looked to them as full batteries. I would then run the alternator and see how many amps it could put out. I had to do the test right away though as the voltage would come right back up if the batteries were left sitting.

Running a long, big load, like a washing machine or extensive microwaving, would take a battery to its knees and the voltage would dramatically drop. Too much for too long would overheat the batteries (and the wiring if not sized to handle it) and potentially cook them or even start a fire. Not recommended.

If you want to run your washing machine you need to run a genset or get a much, much larger battery bank with 4/0 short wire runs and then recharge them for a long time. Ditto for any type of big load, such as a windlass hauling up 1000' of chain. BTW - it can take an hour(s) for batteries to get close to "equalizing", i.e. the same voltage potential throughout the plates. Equalizing batteries via charging occassionally is the opposite of that in that you force a high voltage on the outside of the plates in to the innards otherwise the innards don't get fully converted chemically. Wet cells take longer than AGMs. Start batteries plates are designed to release their energy quickly but they can't last as long (and they will drop voltage depending on the load).
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Old 26-07-2014, 15:10   #19
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Re: Voltage Drop in AGM Under Load?

My AB fridge draws 7.6 amps with the compressor set at 3500 rpm.
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Old 26-07-2014, 18:06   #20
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Re: Voltage Drop in AGM Under Load?

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Originally Posted by zboss View Post
My AB fridge draws 7.6 amps with the compressor set at 3500 rpm.
Is this a BD35 drawing that much? That is still a lot for a BD50. It might draw that at initial startup and running, but I would expect it to drop as the evaporator cools.

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Old 26-07-2014, 18:54   #21
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Voltage Drop in AGM Under Load?

Nope bd50... It's pretty old but it runs. Not sure if it's original but the boat is a 1987.

It runs our freezer.

I'm not hijacking this thread but I'd like to reduce that consumption in half.


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Old 26-07-2014, 19:01   #22
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Re: Voltage Drop in AGM Under Load?

Forgive me if this is a silly question, but at 180A the voltage drop in the cables themselves can be rather significant.

How long is the wiring run, round trip, to that load from the battery bank? And what gauge cable is the whole run?

Using the calculator at genuinedealz:

20 feet each way
1/0 AWG cable
180 amp draw
12 volt nominal system
You'd be down to 11.273 volts just from cable loss.
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Old 26-07-2014, 19:33   #23
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Re: Voltage Drop in AGM Under Load?

I would check the battery interconnects making sure they are making good contacts. I use the following battery testers to check the individual batteries from time to time: THE BIANKA LOG BLOG: BATTERY INVESTIGATION PART 8: LOAD TESTING
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Old 27-07-2014, 03:02   #24
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Re: Voltage Drop in AGM Under Load?

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Originally Posted by neelie View Post
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.
I'm not familiar with the Lifeline batteries but the high end AGM's I am familiar with (Concords) will deliver C1 so I think 0.25C shouldn't be an issue.

FWIW, we test for available capacity by discharging at C1. Give the battery a full charge and then discharge at C1. Continue the discharge until the voltage reaches 10.00 volts. There is a formula for calculating the capacity - but I forget what it is and don't have the manual handy). I do remember that 51 minutes is 80% . It the battery doesn't achieve 80%, then is conditioned and tested again. If it can't achieve 80% after 3 condition cycles, it is removed from service.
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Old 27-07-2014, 03:38   #25
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Re: Voltage Drop in AGM Under Load?

Are your + and - take-offs are the diagonally opposite corners of the bank. Makes a BIG difference in current flow.
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Old 27-07-2014, 12:58   #26
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Re: Voltage Drop in AGM Under Load?

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Are your + and - take-offs are the diagonally opposite corners of the bank. Makes a BIG difference in current flow.
Great suggestion.

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Old 27-07-2014, 13:31   #27
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Re: Voltage drop in AGM under load?

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Originally Posted by Ex-Calif View Post
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.

You mean 0.3C?

The 180 amps is for a couple of minutes while the microwave is being driven by the inverter.

This is the only time I saw the voltage drop... this got me thinking that maybe I have a dying battery in the pack
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Old 27-07-2014, 13:37   #28
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Re: Voltage Drop in AGM Under Load?

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How do you know the actual SOC?

Ah ha!.. Truth is, I don't

I have a Victron BMS which indicates SOC and I am using the standard default Peukerts number that the factory set.

What I an say is that the batteries are probably fully charged because I have 700W of solar using a Midnite Solar Controller which indicated that it was in the float regime, the SOC was indicating 100% (I gather it auto syncs at this point).

If the Midnite Solar controller is to be believed, I run the batteries to float at least twice a week.. and sometimes every day if its sunny.
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Old 27-07-2014, 13:52   #29
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Re: Voltage Drop in AGM Under Load?

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Are your + and - take-offs are the diagonally opposite corners of the bank. Makes a BIG difference in current flow.
Yes they are.

And to everyone stating that 180 A is a lot. This is true. No question.

But about 130 of those amps were for the microwave to heat up a couple of cups of water. So only about 6 minutes worth and even then its cycling on and off ( on 5 secs, off 10 secs or something like that.)

1800W Inverter is right next to the bank - so less than 6 feet of cable.

My other point s that 750AH of 8 month old Lifeline high end AGMS should be able to deliver 0.25C without such a dramatic voltage drop. It is because of this that I started thinking that I had a dead or dying battery in the bank.

Its been sunny & windy here, and my SOC hasn't dropped below 90% for the last few days. I'll wait till drops below 80% and then run the microwave and measure voltages at the compressor terminals. I wil come back to ths thread with the readings when I do.. but not for a few days, we are heading south to Grenada in a hurry tomorrow
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Old 27-07-2014, 14:36   #30
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Re: Voltage Drop in AGM Under Load?

Just thought that it would be too long before I got a chance to deplete the batteries to 80% SOC to get the same condition as before

So in the interest of science, I brewed 2 cups of tea. Here are the results at 90% indicated SOC

Load 5A; V = 12.5
Load 180 A; V = 10.6

1.9V seems a big drop under these circumstances.

I'm back to thinking it's one of two things. A dead battery or I should have got LFPs.

And lastly, to Bianka - I wil place my Amazon order for those 2 battery testers. Thanks for the advice!
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