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Old 09-12-2016, 01:23   #1
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Small battery bank running an inverter

Hi Cruisers,

Looking for thoughts on the following:

I recently replaced my pair of 120 AH horrible el-cheapo AGM batteries with a pair of decent quality 105AH deep cycle FLA batteries. This is (I hope) my last temporary battery solution before I fit the "real" battery bank in three years time prior to going cruising. The batteries are from an outfit called Supercharge and asking around they have a good reputation for being a fair quality budget friendly kit. Lots of people use them at my club and say they work well and last ok.

Things are much nicer aboard now, voltages are what I would expect, the fridge is happy and the autopilot is happy. Overnight at anchor observed voltages are as you would expect and the solar has everything back to float mode by 11 am.

But, there is trouble in paradise. When I run the inverter to power the coffe machine I am getting a low voltage alarm from the inverter. The inverter is a proper sine wave 1500 watt unit, the coffee machine draws 1200 watts. I do not know how efficient or otherwise the inverter is because I have wired it outside the battery monitoring shunt to avoid unnecessary resistance in the circuit. The only thing I use the inverter for is the coffee machine so I am happy to exclude it from my electrical accounting.

At first I thought I must have a bad contact somewhere but I double checked voltage directly from the secondary terminals on each of the batteries using a digital multimeter and got the same low voltage reading (12.1 to 11.8 when the coffee machine is heating) that the inverter (and house battery monitor) was telling me.

Now I don't know if this behaviour is expected. The AGM batteries could not provide enough juice to even warm the coffee machine from day 1, however I had put that down to them being cheap crap. But is my battery bank too small to run the inverter? I only need it to run the coffee machine for about a minute in total (it's a pod machine). The batteries are rated for 650 CCA so the current should not be a problem. (Nominal 50 amps per battery for one minute). It is worth noting that everything works at the moment, I do get my cup of coffee, but the alarm is a bit, well... alarming.

Matt
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Old 09-12-2016, 01:44   #2
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Re: Small battery bank running an inverter

You should also check the voltage directly at the inverter. What does the datasheet say about the low voltage alarm, when does it turn on?

We're running nearly the same setup, 1500W inverter from 2x 110Ah. Works fine even for longer periods than a minute (for cooking with a 1500W plate).

Batteries are at 11,7-12,1V when we draw about 140A, so nearly the same as yours.
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Old 09-12-2016, 01:59   #3
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Small battery bank running an inverter

Hmmm. Thank you for that.

Good point about the alarm threshold. Had not considered that.

FWIW the reading at the inverter is the same as the battery (the inverter has its own voltage reading display).

But by the sound of it the voltage drop is normal for a battery bank of this size. I admit to being a bit surprised. 50 amps does not feel like a lot of current to me.

I hope the problem goes away with the real bank of 6 or 8 Trojan T105s when I eventually fit them.

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Old 09-12-2016, 02:33   #4
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Re: Small battery bank running an inverter

I think the voltage sag of your batteries ist just fine, remember that due to peukert a 50A load on a 100Ah battery gets you down from 100% to 50% in not much more than 30 minutes.

The problem here seems to be more related to the inverter. As far as i know, they normally have a quite low alarm setting, mostly in the range of 9,5-10,5V.

I would simply cut off the buzzer, if you use a Victron battery monitor you still have the buzzer in this one left.

And i would change the wiring so the inverter runs over the shunt. Like you have wired it now your battery monitor doesn't make any sense. Even if your coffee machine is only running for a very short time, it makes a lot of difference in battery state of charge because of peukert again.
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Old 09-12-2016, 02:48   #5
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Re: Small battery bank running an inverter

You only think that the batteries are in float. Frankly, the only way I can be sure my 4x120 agm's are full is when the 60 amp charger cuts out to 2.2 amps and the gen falls back to idle.And sure some sun helps to keep them there but the freezer pulls 2.75 twelve hours a day.
I run my gen and charger for 90 to 120 minutes a day. Of that time 60A is going in for abt 40 minutes until we get to 80%. The rest is absorbtion at varying current until chocker.
Only a smart charger will properly charge batteries. Alternatively, a bloody smart alternator will do the job if big enough. If you are ready to do that to your motor.
I'm reckoning that if your batteries go into float, as you say, and you then connected a proper charger, my suspicion is that your charger will run for maybe a hour or so to really bring the soc to 14.7 before it cuts out.
The two most accurate ways of testing to see if batteries are full is with a carbon pile load tester or by bieng connected to a proper smart charger.
My charger for this piddling little boat didn't cost me aud 1,000 for good reason (Sterling pro ultra 12/60)
The symptoms you describe indicate battery soc of probably 80%, maybe less.
Fwiw, I taught this stuff at Sydney Tech Coll.
From what people are saying I gather that there are many boats with generators which run smart chargers.
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Old 09-12-2016, 02:53   #6
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Re: Small battery bank running an inverter

SUPERCHARGE batteries are not considered second rate by most auto electricians.
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Old 09-12-2016, 02:56   #7
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Re: Small battery bank running an inverter

1200 Watts will be pulling a bit more than 100 Amps from nominal 210Ah bank.
As you've discovered that will very quickly pull the voltage down.

A C/2 discharge rate is not good for battery life. A general rule is not to discharge at more than C/8.

(Remember that CCA is a measure of the current a battery can supply while dropping to 7.2V in 30 seconds!).
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Old 09-12-2016, 03:09   #8
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Re: Small battery bank running an inverter

Quote:
Originally Posted by StuM View Post
A C/2 discharge rate is not good for battery life. A general rule is not to discharge at more than C/8.[/B]).

Sorry, StuM but what is C/2, C/8? Thanks.
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Old 09-12-2016, 03:47   #9
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Re: Small battery bank running an inverter

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Originally Posted by krafthaus View Post
Sorry, StuM but what is C/2, C/8? Thanks.
I believe he is talking about the capacity discharge rate. You have a 210Ah bank, and you are discharging it at 100A, thus, c/2. By his recommendation you should only be loading your system with ~12A (c/8)
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Old 09-12-2016, 04:22   #10
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Re: Small battery bank running an inverter

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A C/2 discharge rate is not good for battery life. A general rule is not to discharge at more than C/8.
Well, come on. Sure, you are right about this but in general, using batteries is not good for their life. This is not a no-go like for example 100%-discharges are. Especially if it is only for a minute as the thread opener posted.

We are using a 220Ah AGM Bank (2x Victron 110Ah) that was installed in April 2015. We are living aboard for four months now and pull 140-150A from the bank on most days. And not for just one minute, but for 10-15 minutes when cooking.

I'm having a close look at battery voltages at different states of charge and different loads quite often, and so far i couldn't detect any changes in battery health.

Perhaps the batteries will loose some months of their lifetime, but the thread opener wants to replace them in some years anyways and who cares if battery life is shortened a very little bit if you get good coffee for it
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Old 09-12-2016, 08:17   #11
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Re: Small battery bank running an inverter

Most of this has already been covered, but I would say your drawdown is a bit much for your bank. I would investigate a French press. Much cheaper than four more Supercharges.
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Old 09-12-2016, 08:31   #12
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Re: Small battery bank running an inverter

Quote:
Originally Posted by GILow View Post
Hi Cruisers,

Looking for thoughts on the following:

I recently replaced my pair of 120 AH horrible el-cheapo AGM batteries with a pair of decent quality 105AH deep cycle FLA batteries. This is (I hope) my last temporary battery solution before I fit the "real" battery bank in three years time prior to going cruising. The batteries are from an outfit called Supercharge and asking around they have a good reputation for being a fair quality budget friendly kit. Lots of people use them at my club and say they work well and last ok.

Things are much nicer aboard now, voltages are what I would expect, the fridge is happy and the autopilot is happy. Overnight at anchor observed voltages are as you would expect and the solar has everything back to float mode by 11 am.

But, there is trouble in paradise. When I run the inverter to power the coffe machine I am getting a low voltage alarm from the inverter. The inverter is a proper sine wave 1500 watt unit, the coffee machine draws 1200 watts. I do not know how efficient or otherwise the inverter is because I have wired it outside the battery monitoring shunt to avoid unnecessary resistance in the circuit. The only thing I use the inverter for is the coffee machine so I am happy to exclude it from my electrical accounting.

At first I thought I must have a bad contact somewhere but I double checked voltage directly from the secondary terminals on each of the batteries using a digital multimeter and got the same low voltage reading (12.1 to 11.8 when the coffee machine is heating) that the inverter (and house battery monitor) was telling me.

Now I don't know if this behaviour is expected. The AGM batteries could not provide enough juice to even warm the coffee machine from day 1, however I had put that down to them being cheap crap. But is my battery bank too small to run the inverter? I only need it to run the coffee machine for about a minute in total (it's a pod machine). The batteries are rated for 650 CCA so the current should not be a problem. (Nominal 50 amps per battery for one minute). It is worth noting that everything works at the moment, I do get my cup of coffee, but the alarm is a bit, well... alarming.

Matt
OK, so if your coffee maker draws 1200 W and your inverter is 85% efficient that would mean it is consuming around 1500 W from the batteries.

1500W at the 11.8 V sag voltage is almost 130 A. That's a lot of juice.

Solution is more battery capacity to reduce sag, coffee maker that draws less, or adjust the inverter low voltage alarm to 11.5 or less to prevent nuisance trips.
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Old 09-12-2016, 09:06   #13
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Re: Small battery bank running an inverter

Two, OK three things stick out to me.
1. Your bank is too small to pull 100 amps out of for the ten min or so it takes to make coffee
2. No way is your bank charged by 11AM, your charger is prematurely kicking into float and this will murder your batteries. You would be better off I think leaving them permanently at absorption voltage as I doubt they will be overcharged, unless you leave the boat and are not making coffee etc.
3. Using your bank to make coffee or any form of heat, makes some really expensive heat, way better off to burn propane to make heat.
Lets just pick some numbers to see how much power your using, your 1200W coffee pot with conversion losses will pull say 1500W. At 12V that is 125 amps, more if voltage is lower than 12, but lets use 12.
10 minutes is 1/6 of an hour, 125 divided by 6 = 20.8. So you used 20.8 amp hours to make your morning coffee, more I believe actually but I don't know how to account for excessive current drain.
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Old 09-12-2016, 09:35   #14
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Re: Small battery bank running an inverter

I ran a small inverter off an FLA to run my flat panel tv and dvd player. To my pleasant surprise, it would run for hours with hardly a dent in the voltage. But those devices are pretty low current draw.

I understand the math and the chemistry, but I still think your coffee machine is a big draw that is going to toast your batteries pretty quick. If you think its worth it for that cuppa every morning, go ahead.

But, have you forgotten you are on a boat? Why not heat the water on your stove...get a nice SS percolator or french press. You aren't in the kitchen of your house...its the galley of your boat. IMHO, the coffee maker has no place there (yikes, I said it, let the rants begin).

On my boats I have always taken great pleasure in making a terrible cup of coffee every morning, with hot water left over for an awful bowl of oatmeal. When I'm on my boat I'm not pretending I'm on land.
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Old 09-12-2016, 09:37   #15
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Re: Small battery bank running an inverter

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.............3. Using your bank to make coffee or any form of heat, makes some really expensive heat, way better off to burn propane to make heat...............
Not if you consider that the power from the batteries is essentially "free".

If the battery is being charged from shorepower, most of us pay a flat fee for electricity. I pay $30 per month. If it's being charged by the boat's alternator, it's pretty much "free" as well. It's just excess power from the engine.

Propane is fine but finding an automatic coffee maker that runs on propane is difficult or impossible. Most of us would rather push a button and walk away rather than babysitting a pot on the stove.

To the OP, I get a voltage drop when using my coffee maker or microwave oven. If you measure the same voltage at the batteries as at the input to the inverter when the coffee pot is on, you need more or bigger batteries. If the voltage is significantly lower at the inverter than at the batteries, the cables are too small or too long. There's a voltage drop in the cables.
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