Batteries and being off grid

[WotEver]

28 minutes ago, mross said:

MJ's work equally for any voltage, unlike Amp-hours.

But we’re not talking about any voltage. We’re talking (nominally) 12V. But if you want to use MJs then we can all do the maths to convert it to something more useful. 

[mross]

some boats are 24V

[dmr]

Coulombs is the proper unit for a quantity of electricity but are rarely used except by a few specialist electrical and electronic engineers.

Energy (in Joules) is the proper unit of energy but is not a familiar unit to most people, in fact our gas and electricity supply companies have historically avoided it. It also requires consideration of voltage.

Amp-hours is a sort of made up "unit of convenience" intended to be easy for non technical people to understand, sadly some people can't even cope with something this simple.

I think the problem is that most common units are "units of quantity" whilst amps are a unit of flow and this wrong-foots a lot of people.

Maybe if we still taught calculus in schools it would be easier.

..............Dave 

[Señor Chris]

1 hour ago, Tony Brooks said:

Nothing if you like maths.

So if Ah is simpler, why is domestic leccy charged in kWh?

 

[rusty69]

Just now, Señor Chris said:

So if Ah is simpler, why is domestic leccy charged in kWh?

 

Cos domestic users don't use batteries.

[Señor Chris]

1 hour ago, Alan de Enfield said:

So, if 'everyone' accepts and understands Kwh, what is so difficult with comprehending Ah?

Dunno but something obviously is.

The point is why have different units for measuring the same thing?

 

[Chewbacka]

1 hour ago, Señor Chris said:

What's wrong with kWh? Everyone's used to that.

Because AmpHours are a quick are dirty approximation for doing quick sums in your head.  The following is much easier in Ah compared doing it in kWh.

So I have 2 off 110Ah batteries, which are a bit old, so I guess 75Ah each.  I don't want to go below 50%.  My 'just living' usage (fridge, a bit of water pump, toilet etc) is 60Ah per day which therefore leaves me 15Ah to play with.  My tv uses 2A and my lights use 1A and my radio uses 1A.  Today I had the radio on for 5 hours and tonight I will have the lights on for 6 hours, so how long can I watch tv for without going below 50%?  

So as I said I had 15Ah to play with, 5 used by the radio and 6 will be used by the lights which totals 11Ah, leaving 4Ah.  TV uses 2A, so I can watch for 2 hours.

 

Now do the same with kWh where the daily 'just living' is 0.72kWh, the radio is 12W the tv is 24W and the lights 12W.

Added - and don't use a calculator.

Edited December 16, 2017 by Chewbacka

[mross]

50 minutes ago, dmr said:

Energy (in Joules) is the proper unit of energy but is not a familiar unit to most people, in fact our gas and electricity supply companies have historically avoided it. It also requires consideration of voltage.

Joules are not dependant on volts; 10MJ is 10MJ regardless of volts.  Our meters already measure Joules but a calibrated in kWh.  The meters measure true power, so they take account of volts amps and power factor.

Edited December 16, 2017 by mross

[Scholar Gypsy]

Here are some more units for energy,  from wikipedia. I prefer the footpoundforce  myself.

The imperial/U.S. units for both energy and work include the foot-pound force (1.3558 J), the British thermal unit (BTU) which has various values in the region of 1055 J, the horsepower-hour (2.6845 MJ), and the gasoline gallon equivalent (about 120 MJ).

[mross]

Ergs anyone? 

[rusty69]

1 minute ago, mross said:

Ergs anyone? 

Not for me thanks, I've just had my lunch.

[Señor Chris]

51 minutes ago, Chewbacka said:

Because AmpHours are a quick are dirty approximation for doing quick sums in your head.

A good point although you've made it easier for yourself by rating everything in Amps to start with. There's no reason why batteries can't be rated in kWh while LEDs (and TVs I think?) are already rated in Watts.

 

[croftie]

2 hours ago, LadyG said:

Lets have a poll ............... or a prize [christmas turkey?], to be awarded on 29th Feb

Will that be a battery reared turkey?

 

Edited December 16, 2017 by croftie

[dmr]

32 minutes ago, mross said:

Joules are not dependant on volts; 10MJ is 10MJ regardless of volts.  Our meters already measure Joules but a calibrated in kWh.  The meters measure true power, so they take account of volts amps and power factor.

Getting back to boats (rather than domestic) where power factor is not a big issue ,  a lot of boats will have a measure of amps, in fact this forum often advocates the use of a clamp on ammeter. Therefore to understand the relationship between current consumption and battery life the voltage must be included if the battery capacity is quoted in Joules.

If we stick to the rough and ready amp-hours and know our current consumption (which the op probably does once he gets to grips with the concepts) then it all becomes beautifully simple.

I've got 675 amp-hours of battery, our typical evening draw is about 8 amp, its all very intuitive, it barely needs a calculator.  24 hours between engine runs is easy, 48 hours is getting a bit marginal.

...............Dave

[Ratkatcher]

1 hour ago, Chewbacka said:

So I have 2 off 110Ah batteries, which are a bit old, so I guess 75Ah each.  I don't want to go below 50%.  My 'just living' usage (fridge, a bit of water pump, toilet etc) is 60Ah per day which therefore leaves me 15Ah to play with.  My tv uses 2A and my lights use 1A and my radio uses 1A.  Today I had the radio on for 5 hours and tonight I will have the lights on for 6 hours, so how long can I watch tv for without going below 50%?  

So as I said I had 15Ah to play with, 5 used by the radio and 6 will be used by the lights which totals 11Ah, leaving 4Ah.  TV uses 2A, so I can watch for 2 hours.

I liked this - it made sense.  May not be scientific and accurate but is good enough to get by with.

[peterboat]

20 hours ago, KenK said:

Ignoring the argument regarding the correct method of expressing energy usage. Some real world information.

This year we changed the way we use our boat, for twelve years it has been used for holidays and weekends, for that purpose six 110 Amp Hr Numax leisure batteries have been satisfactory, the first set lasting nearly eight years, we replaced them with the same type.

This year we used the boat as liveaboards cruising from  Newbury to Leeds covering all the waterways we could on the way. It became very obvious early on in the journey that the now four year old batteries were not performing as we needed. We have a Smartgauge and a Smartbank fitted, so we know we were charging correctly but we were dropping to around 60% after a nights use. We have all LED lights but the fridge and TV both run from a PSW inverter. The fridge was the problem even though it was new A+ unit it could easily cause the inverter to trip not long after we had moored up because the compressor starting current was greater than the batteries could supply to the inverter.

I replaced the six Numax batteries with five 170 Amp Hr traction batteries, these are flooded tubular plate beasts, they weigh twice as much as the Numax at 40 kilos each. The difference is amazing a nights use sees a drop of less than 5 % on the Smartgauge and the fridge has been tamed.

We have had this discussion many times but so called leisure batteries are for the most part simply larger starter batteries and therefore not ideal as domestic batteries, easy to tell, if there is a CCA figure quoted in the description it is a starter battery. Traction batteries are more expensive but they work and provided they are looked after will last longer.

I have no idea how the OP uses his boat but my experience suggests that for liveaboard use a simple starter battery will have a short life as they are simply not designed to be used as domestic batteries.

 

Ken

I have 2volt cell full traction one set 13 years old the other bank 7 years old I think, all going strong best things I ever bought. For the electric bathtub I am going for the same GTP batteries as yourself. I dont want a watering system and also the size of the 2 volt cells mean that I wont be able to fit them in. Glad that their are more people than me that believe in you get what you pay for!

[Señor Chris]

5 minutes ago, Ratkatcher said:

I liked this - it made sense.  May not be scientific and accurate but is good enough to get by with.

Not too difficult either:

So I have 2 off 1.32kWh batteries, which are a bit old, so I guess 900Wh each.  I don't want to go below 50%.  My 'just living' usage (fridge, a bit of water pump, toilet etc) is 720Wh per day which therefore leaves me 180Wh to play with.  My tv uses 24W and my lights use 12W and my radio uses 12W.  Today I had the radio on for 5 hours and tonight I will have the lights on for 6 hours, so how long can I watch tv for without going below 50%?  

So as I said I had 180Wh to play with, 60 used by the radio and 72 will be used by the lights which totals 132Wh, leaving 48Wh.  TV uses 24W, so I can watch for 2 hours.

[dmr]

1 hour ago, Scholar Gypsy said:

Here are some more units for energy,  from wikipedia. I prefer the footpoundforce  myself.

The imperial/U.S. units for both energy and work include the foot-pound force (1.3558 J), the British thermal unit (BTU) which has various values in the region of 1055 J, the horsepower-hour (2.6845 MJ), and the gasoline gallon equivalent (about 120 MJ).

I like to buy my beer in pints, measure my boat in feet, measure my distance in miles and speed in mile per hour, and would buy cheese by the pound if I could, but when it comes to engineering the SI system wins hands down (or should I say 10.16cm down).

.............Dave

[bizzard]

Despite its name, the AMPhicar only had a small standard alternator and battery driven by a Triumph Herald engine. Its name implied that it generated big amp's.

Edited December 16, 2017 by bizzard

[Alan de Enfield]

23 minutes ago, bizzard said:

Despite its name, the AMPhicar only had a small standard alternator and battery driven by a Triumph Herald engine. Its name implied that it generated big amp's.

And of course, the atmosphere was highly charged in the Roman AMPhitheatre

[Iain_S]

1 hour ago, bizzard said:

Despite its name, the AMPhicar only had a small standard alternator and battery driven by a Triumph Herald engine. Its name implied that it generated big amp's.

Except it was an AMPHIcar.

[Alan de Enfield]

Maybe AMPhetamines could speed and AMPlify our current understanding, and allow those AMPutees a better grasp of the situation.

(I now expect that Athy will suggest this is an Amphibology)

Edited December 16, 2017 by Alan de Enfield

[WotEver]

6 hours ago, mross said:

some boats are 24V

OP has a 12V boat. 

Some boats are 48V

[bizzard]

More than ample.

[Robbo]

16 hours ago, WotEver said:

OP has a 12V boat. 

Some boats are 48V

Are any 36v?