Best way to Measure Amp Hours used

[Terryb]

I have been obsessed/Plaqued with my battery usage over the last 18 months of living onboard Alfie. As many of you know I have had my fair share of ups and "downs". But the real problem of energy supply and generating it, just will not go away. However it has recently come to my attention, that my MPPT controller actually tallies up solar power (Amp Hrs) replaced every day. This coupled with my Smartgauge giving me a SoC figure at the same time, has made me realise that I now know my true usage.

Unless I am (again) completely getting it wrong, my average daily consumption is averaging around the 80Ahr. (I would like maybe another 20 or so, but thats another thing)

My questions are, 1, Can I use these figures to work from as usage?

2, does this mean that if my battery bank SoC loses 50% and the AHr total to replace it is 50Ahr then I have a total capacity of 100 Ahr?

I know that there will be a reason that these are not scientifically correct, but I am no scientist, just someone trying to decide how much I am using and how many batteries I need to make my life comfortable.

Any comments welcome.

[keith.]

Your figures are showing the energy required to charge the batteries which because of losses will be more than that given by the battery. However, with the set up you have the figures will be close enough. And yes, your sums are correct.

[Alan de Enfield]

I have been obsessed/Plaqued with my battery usage over the last 18 months of living onboard Alfie. As many of you know I have had my fair share of ups and "downs". But the real problem of energy supply and generating it, just will not go away. However it has recently come to my attention, that my MPPT controller actually tallies up solar power (Amp Hrs) replaced every day. This coupled with my Smartgauge giving me a SoC figure at the same time, has made me realise that I now know my true usage.

Unless I am (again) completely getting it wrong, my average daily consumption is averaging around the 80Ahr. (I would like maybe another 20 or so, but thats another thing)

My questions are, 1, Can I use these figures to work from as usage?

2, does this mean that if my battery bank SoC loses 50% and the AHr total to replace it is 50Ahr then I have a total capacity of 100 Ahr?

I know that there will be a reason that these are not scientifically correct, but I am no scientist, just someone trying to decide how much I am using and how many batteries I need to make my life comfortable.

Any comments welcome.

 

Sort of, the big problem is that you have to tell the Smartgauge what you battery capacity is to start with lets say you program in that your capacity is 200Ah, if you use 100Ah then it will show that you have 50% SoC.

If the TRUE capacity of your battery is actually 150Ah, and you use 100Ah then it will still show you have 50% SoC but in reality you are down to 33% SoC and killing your batteries.

 

It all only works IF you can program it correctly (and then only approximately anyway)

 

You will need to do a controlled discharge of something like a car headlight bulb for (say) 4 hours before you can work out the Ah capacity, and then re-program the Smartgauge

[nicknorman]

Terry, yes as said you are thinking along the right lines. The charge efficiency of batteries in good condition is around 93% so if you want to be accurate you should multiply the AH put back in by 0.93 to get the AH previously taken out, but obviously that is only going to make a small difference.

 

 

Sort of, the big problem is that you have to tell the Smartgauge what you battery capacity is to start with lets say you program in that your capacity is 200Ah, if you use 100Ah then it will show that you have 50% SoC.

If the TRUE capacity of your battery is actually 150Ah, and you use 100Ah then it will still show you have 50% SoC but in reality you are down to 33% SoC and killing your batteries.

 

It all only works IF you can program it correctly (and then only approximately anyway)

 

You will need to do a controlled discharge of something like a car headlight bulb for (say) 4 hours before you can work out the Ah capacity, and then re-program the Smartgauge

Sorry but this is completely wrong. The Smartgauge has no knowledge of what the capacity of the batteries is, nor does it need to. That is its beauty, it always gives the SoC as a percentage of the actual capacity. ie it works in relatives, not absolutes.

 

Your point only applies to AH-counting gauges.

Edited August 22, 2016 by nicknorman

[Richard10002]

For a spend of about £100 you can buy a NASA BM1 or BM2 which will give a more accurate reading of Ah used.

[Alan de Enfield]

Terry, yes as said you are thinking along the right lines. The charge efficiency of batteries in good condition is around 93% so if you want to be accurate you should multiply the AH put back in by 0.93 to get the AH previously taken out, but obviously that is only going to make a small difference.

 

 

Sorry but this is completely wrong. The Smartgauge has no knowledge of what the capacity of the batteries is, nor does it need to. That is its beauty, it always gives the SoC as a percentage of the actual capacity. ie it works in relatives, not absolutes.

 

Your point only applies to AH-counting gauges.

 

Then - big apologies "I got it wrong".

I was thinking of the Victron BMV which we have on our boat.

 

I'll go out into the yard, head down, and quietly kick a few stones around for a while and contemplate by errors.

[nicknorman]

Then - big apologies "I got it wrong".

I was thinking of the Victron BMV which we have on our boat.

 

I'll go out into the yard, head down, and quietly kick a few stones around for a while and contemplate by errors.

Yes your point is certainly valid for a BMV, but the Smartgauge works in a completely different way.

 

Anyway, don't beat yourself up too much, it could have been worse. For example, you could have said something really sinful such as "A/h" instead of "Ah"!

 

 

Oh and to Terry - an afterthought. Presumably you realise that using the solar input to calculate the used Ah only works if solar is the only thing being used to charge the batteries. The solar controller won't tally any charge current from a generator / battery charger or the engine's alternator.

Edited August 22, 2016 by nicknorman

[MtB]

As an aside Terry, this AH counting facility on your solar contyroller sounds really useful. What make and model of controller is it please?

 

Thanks...

[bottle]

The one and only way to be sure of counting the amperes in and the amperes out is an ampere hour meter and this important point the measurement must be at the batteries. ie, a shunt as the last connection to the batteries. (Usually the negative)

 

My previous MPPT controller counted the amperes but only those going to the batteries not those leaving.

 

If for instance I had a 5amp load and the solar was putting out 10 amps then the batteries were only getting 5 amps, the other 5 were supplying the load

 

Even then, as mentioned you need to put back more than you take out.

 

 

just someone trying to decide how much I am using and how many batteries I need to make my life comfortable.

 

It is not so much how many batteries (amp.hr capacity) you need but how much capability, the charging system has of replacing your usage.

 

Then a decision on the size of the bank can be made, it would obviously be not a good idea (cost) to have a 1000 amp.hr bank and only use 50 amp.hrs. but when you get to 100 amp.hr battery bank with 50 amp.hr usage then although possible, it would 'work' the battery hard.

 

Increasing the battery bank to 200 amp.hr and still using 50 amp.hrs the batteries would 'work' less hard and also be maintained at higher SOC.

 

Instead of battery bank down down to 50% SOC then the larger battery bank would be 75% SOC so being kept at a higher SOC which batteries like.

 

You would still be using 50 amp.hrs and have to put back more because of charging losses.

[WotEver]

It is not so much how many batteries (Ah capacity) you need but how much capability, the charging system has of replacing your usage.

 

This is so true and I wish it was said more often.

 

1. Do a Power Audit.

2. See what charging you have available to replace 120% of your power audit even in the depths of winter.

 

If 2 cannot equal 1 then think again about 1. Once 2 is satisfied then move onto 3... purchase battery bank to be at least 200% of 1 (300% would be better).

 

Tony

[Terryb]

As an aside Terry, this AH counting facility on your solar contyroller sounds really useful. What make and model of controller is it please?

 

Thanks...

 

Not really sure Mike. It came with the solar array from on board solar it also tallies up total Ahrs charged over the life of the unit. Im at 14000 at this moment.

[MtB]

 

This is so true and I wish it was said more often.

 

1. Do a Power Audit.

2. See what charging you have available to replace 120% of your power audit even in the depths of winter.

 

If 2 cannot equal 1 then think again about 1. Once 2 is satisfied then move onto 3... purchase battery bank to be at least 200% of 1 (300% would be better).

 

Tony

 

 

Can you expand on why 300% in particular would be better please?

 

Why not 400%? Or 600% or 800%? Or is it always a case of the bigger the better, balanced only by depth of pockets?

[WotEver]

 

 

Can you expand on why 300% in particular would be better please?

 

Why not 400%? Or 600% or 800%? Or is it always a case of the bigger the better, balanced only by depth of pockets?

 

The bold bit

[Arthur Brown]

Battery service life in cycles decreases with increasing depth of discharge, so attempt to keep batteries above 50% SOC/DOD (this time it's the same!). so you need to have twice your needs in store in the batteries. However there are losses in self discharge which have to be replenished daily so there are down sides to an oversize bank as well as an under size one.

 

The big issue with AH counting is that however accurate the count is there is the variable charge efficiency of the battery to account for, SO you need to add possible as much as 20% more amp hours to replenish the amp hours taken out, and getting the last few AH in is slow and poor use of generator running time, hopefully if you think you have a full battery by coffee time, than the solar will do the rest before dark.

[WotEver]

Well yes, the bold bit within reason

 

Much more than 400% is a complete waste of money because you don't really want to be discharging the batteries by less than 25%.