Small battery bank running an inverter

[TheThunderbird]

Battery capacity in Ah is calculated for a stable usage over 10h.

Thus 210Ah battery bank can yield, in theory, 21Ah for 10h to exhaustion.

De facto, the SOC cycle is/should be 80 to 50%, thus only 21x3= 63Ah are available.

If you pretend to draw more than 0,1C, the ACTUAL BATTERY BANK CAPACITY GETS MUCH MUCH LOWER!!
A technical sheet can possibly tell you how much lower, my guesstimate is a 60/50%

0,5C draw is a killer for any FLA, even for a minute.!

Definitely, better to have a charger ON when doing this draw

On top, as the Voltage decrease, the required A gets higher
W= VA

[GILow]

Originally Posted by the_alpine

Inverter for coffee is just plains silly!

I had EXACTLY the same reaction when I first heard of this. I was talking to a boat crew about to head off on one of the local overnight races, and they were waiting for a crewman to get back from the local electrical discounter with a pod style coffee machine because theirs had broken.

I was all "What the ---- would you have such a silly thing for on board!?"

But then, I thought about it and I realised just how incredibly convenient the pod machines are when the boat is under way, possibly with crew sleeping below. You can practically silently produce a coffee from anywhere on the boat, in the cockpit if your wiring supports it. And there's no heavy percolator, coffee press or whatever, no big cleanup afterwards, just bung your cup under the spout, press the button and bingo, hot caffeine!

Yeah, I know, a bit indulgent, and bit of solving a "first world problem", but I was convinced. As it is I use the device mostly when I am working on the boat in the pen, at which times the galley of often inaccessible because of bits of timber or tools in the way. Sometimes I trot it out and put it directly on top of the power distribution pole next to the pen and offer coffees to passing club members as a kind of vagrant barista mode thing.

Anyway, looks like it will be best if I restrict it's use until I get a real battery bank.

Matt

[rwidman]

Originally Posted by hamburking

So run your engine every morning for a couple minutes to make a cup of coffee. Think about it. Is that really what the diesel engine is for? What is the unnecessary wear and tear on your diesel worth?

I hope you didn't get that from reading my post because that's not what I said and not what I meant.

You run your engine to move your boat. As an offshoot of moving your boat, the engine drives an alternator which makes electricity that can be stored until you need it in a battery or batteries.

[GILow]

Originally Posted by rwidman

You run your engine to move your boat.....

Err... us rag-and-stick merchants use our rag and sticks to move the boat. But your point is well made.

[rwidman]

Originally Posted by hamburking

If you have shorepower, you don't have a problem.

When you don't have shorepower, use your stove, thats what its for.

My stove is for cooking. I could make coffee by heating water and pouring it through my single cup coffee maker but it's far simpler just to flip on the inverter and use the coffee maker the way it was designed to be used.

And the inverter isn't just for making coffee, it's primary purpose is to run a microwave oven. Great for reheating leftovers and some cooking duties. I also runs the coffee maker (as stated), power tools and chargers.

[Sternwake]

1. What is the gauge of the inverter cables?
2. What is their length?

3. The solar controller dropping to float DOES NOT indicate the batteries are full.

Dip the cells with a hydrometer when the Solar controller reverts to float. You want to see 1.275 or higher. I doubt you will see 1.260, and this indicates not only that the batteries are NOT fully charged, but that they are also likely sulfated to some degree, even if newish, and will not be able to support a huge load even if inverter cables are of adequate thickness for the load and the terminations are not junky hammer crimps now turned green under the heatshrink.

Regarding 225 AH of 6v golfcart battery in series, VS 230Ah of parallel group 27 12v batteries in the same usage and recharge regimen, the 6vs will last 2x the total number of cycles or more, the 12v's will support high inverter loads for longer while they are still healthy and new enough to do so. Which might not be very long when 'float by 11AM' is part of the equation and some of the loads bypass the shunt.

Float does NOT mean full, no matter how soothing that flashing green light may be. What is your absorption voltage, What triggers Float voltage, time or Amperage threshold?

'Works just fine' only means has not failed yet.
'Works just fine' is akin to saying I have absolutely no bloody idea.
'Works just fine' is one step above 'works poorly' and two micro steps above 'It just failed.'

In fact if the inverter cables are not too thin and too long to support the ~1200 watt load for 30 seconds before the inverter alarm starts screaming, then 'float by 11AM' is obviously not just fine.

It is quite difficult to actually truly fully charge flooded group 27's. It takes much longer than expected at higher voltages than expected, and The hydrometer will prove this time and again, if it were ever employed.

[StuM]

Originally Posted by GILow

Thank you Stu, I was hoping you'd notice this thread so I VERY carefully checked my units before I posted.

Did not know about the thing about 7.2 Volts though! Yikes, that is a drop.

But, the batteries are a pair of 12 volt cells in parallel, so that's 50 amp draw (give or take a bit for efficiency.) Which is C/4 if I am correct? So still not good for the battery either... darn.

Might have to stick with the percolator on the stove top at this rate.

Nope, it's 50 Amps from each battery or 100Amps from the two combined. So it's still C/2 100/50 or 200/100, doesn't matter which way you look at it!.

(Actually, with FLA and drawing at 50 Amps from each of those batteries, the actual capacity is probably more like 70Ah so effectively, it's more like C/1.5 )

[ramblinrod]

Responses in Bold Red.

Originally Posted by TheThunderbird

Battery capacity in Ah is calculated for a stable usage over 10h.

Most deep cycle battery capacities are based on 20 hours typically.

Thus 210Ah battery bank can yield, in theory, 21Ah for 10h to exhaustion.

The 20 hour capacity rating would put that at 10.5 A-hrs.

De facto, the SOC cycle is/should be 80 to 50%, thus only 21x3= 63Ah are available.

Incorrect. This is the practical usable battery capacity, when charging via alternator only, because from 80% to 100% SOC will take too long. With solar and wind charging, you can easily run a charge cycle from 50% in morning as the sun comes up, to 100% before the sun goes down in the evening.

If you pretend to draw more than 0,1C, the ACTUAL BATTERY BANK CAPACITY GETS MUCH MUCH LOWER!!
A technical sheet can possibly tell you how much lower, my guesstimate is a 60/50%

Yes, with a higher A-hr load than 1/20 of the 20 hour capacity rating, the actual total capacity is reduced.

For example, a Trojan 27 TMX
5-Hr Rate 85 A-hrs
10-Hr Rate 97 A-hrs
20-Hr Rate 105 A-hrs
100-Hr Rate 117 A-hrs

So the difference between the 1/20 and 1/5 discharge rate is a 19% reduction in capacity.



0,5C draw is a killer for any FLA, even for a minute.!

Incorrect. Deep cycles will sag more than starting batteries under heavy load, but short duration heavy loads don't really hurt them, as long as they aren't discharged too deeply on a regular basis, or left in partial state of charge too long.

Definitely, better to have a charger ON when doing this draw

Absolutely, but more to solve the sag problem he is experiencing.

On top, as the Voltage decrease, the required A gets higher
W= VA

[GILow]

Originally Posted by StuM

Nope, it's 50 Amps from each battery or 100Amps from the two combined. So it's still C/2 100/50 or 200/100, doesn't matter which way you look at it!.

(Actually, with FLA and drawing at 50 Amps from each of those batteries, the actual capacity is probably more like 70Ah so effectively, it's more like C/1.5 )

Ok, I took some time to do some more reading on discharge stuff as a result of this thread. I am getting my head around it now. But what all this has shown me is that I have been so concerned with the charging side of things that I had not taken time to seriously consider the discharge side.

I will phone the local technical guy from Supercharge, who I spoke to before buying the batteries to discuss his preferred charging regime. This time I will ask him if he can tell me a bit more about the discharge side. When we spoke I did check that the batteries could be used in an emergency to start the engine and he assured me this model is "dual purpose" and would not be damaged by using them in this way. But I did not tell him about the inverter. I just thought 50 amps was trivial.

Whoops!


Sent from my iPhone using Cruisers Sailing Forum

[Sternwake]

50 amps applied for a few minutes of a pair of 2 fully charged healthy group 27's is trivial.

The low voltage alarm on the inverter should not be ringing for quite a while at 50 amp load across those 2 batteries if they are wired properly and not sulfated and more than 60% charged.

[TeddyDiver]

Originally Posted by TheThunderbird

On top, as the Voltage decrease, the required A gets higher
W= VA

Depends.. That's defineatly the case with OP becouse the "coffee pot" run's via inverter. But for resistant loads like running a coffee maker or any electric heating element directly without voltage regulating device it's just a voltage drop and amps being at constant level. Makes the time required a bit longer thou. In some cases MUCH longer like running 230v coffee maker from 110v grid going up from 5min to half an hour..

BR Teddy

[mitiempo]

Originally Posted by GILow

Thanks for that. Yes, bypassing the shunt is a bit dodgy. I use the inverter so rarely that it makes sense at a purely practical level but I concede that it buggers up the accounting.

I was mostly concerned about the voltage drop, but you seem to find the same thing so I am less concerned now.

Shunts are designed to have very little voltage drop. I would wire it properly.

Your batteries are 12 volt "dual purpose". Very few 12 volt batteries are truly deep cycle batteries. Trojan makes a few but they are rare. What you have are basically re-labeled starting batteries - possibly with plates slightly thicker but starting batteries all the same. And you do not have enough of them for the load you require.

1 awg wire? what is the distance? I wouldn't wire a 1500 watt inverter with less than 2/0 wire and the run should be less than 5' (10' there and return).

[TeddyDiver]

Originally Posted by mitiempo

1 awg wire? what is the distance? I wouldn't wire a 1500 watt inverter with less than 2/0 wire and the run should be less than 5' (10' there and return). 1 awg is 42.4mm2, 2/0 is 67.4mm2. Not sure what you mean by 300mm.

300mm =12"

[mitiempo]

Originally Posted by TeddyDiver

300mm =12"

My mistake, 300mm is apparently the distance from the battery. I corrected my post.

[GILow]

OK, let's spell it out one more time.


The observed voltage at the batteries themselves is exactly the same voltage observed at the inverter and by the house battery monitoring system.


So, think about it people. The wiring is fine. Just fine. If the wiring was a problem there would be a voltage drop observed over the wiring.


I also stated that I ASSUMED I had a bad contact somewhere but I was able to rule it out using that very simple test.


I am starting to worry if anyone is actually reading what I posted. Sure, you don't know if I am the kind of diy-home-built-nutcase that thinks two bits of tin foil are a great way of wiring a battery to an inverter, but the voltage test tells you all you need to know in this case.


I also gave voltage ranges that were consistent with TXG's experience of running a practically identical system. So I think it is pretty clear this is an internal battery resistance/chemistry issue and all that remains is to speak to the manufacture's technical representative and get his opinion on whether treating the batteries in this way will damage them.


Seriously guys, I love the help, and I hate to sound ungrateful for the advice, but the whole "You are an idiot who does not know how to wire an inverter" thing is getting a bit thin.


As it is, I am probably just an idiot who did not know what to expect from batteries under this sort of load.


Matt