Converting Wh charge to Amp hours

[matt1]

So thanks to everyone on here I was able to resolve the wiring errors on my solar panel connection thanks!

The Bluetooth enabled MPPT is telling me that on one particularly sunny day I got 470wh of charge. How would I convert that to amp hours? The battery bank is 320 Amp hours

 

[AngusMcDoon]

Divide by the charging voltage. The value will only be approximate because of various losses and inefficiencies, but it will give you an indication.

 

[matt1]

Ah, ok. So typically when I look at the app it shows an instant charge of between 20w-80w and then shows me the days cumulative total eg the 470wh I mentioned earlier. So can I (in my simple world) use the calculation above and assume I've replaced about 40 amps of my 320 amp hour battery? The bit that confused me was the 40 amps over an hour, as the 470wh is the day's total rather than an hour.

 

[superheat6k]

As AMD points out there are various losses, but in reality the battery won't start taking much charge below 13.5volts, so using 12 as your volts divisor is a bit optimistic. I would suggest using 15 and then round this down to the nearest 5 would give a more realistic answer, so for 470 WH = ~30 AH.

You can then use this figure to balance out your static loads, e.g. auto bilge pump, perhaps a small fridge. This would be simple to measure with an ammeter with all the these loads running. Bear in mind a fridge is an intermittent load so you have to assess its average draw over a period. Multiply the average amps draw by 24 to obtain your approximate daily consumption. If this less than the AH being generated then the batteries will eventually reach capacity, and the MPPT controller will reduce to a trickle charge.

However, if your consumption exceeds the daily make up then your batteries will go flat, so turn some things off until you consume less than your solar panel provides. Increasing the temperature (normally turning the stat to a lower number) on the fridge will reduce compressor demand, as will keeping it full of stuff, such as beer or even bottled water.

 

[AngusMcDoon]

Ah, ok. So typically when I look at the app it shows an instant charge of between 20w-80w and then shows me the days cumulative total eg the 470wh I mentioned earlier. So can I (in my simple world) use the calculation above and assume I've replaced about 40 amps of my 320 amp hour battery? The bit that confused me was the 40 amps over an hour, as the 470wh is the day's total rather than an hour.

You have unit confusion there. An Amp is a flow rate, the water equivalent of gallons per hour, so saying you replaced 40 Amps of your 320 Amp hour battery doesn't make sense. The easiest is to divide your daily Watt hour reading by about 14 (the charging voltage) * to get an idea of what your panels have achieved and live with that.

There are websites that explain it all, like this one...

https://sciencing.com/relationship-between-amps-ah-12198988.html

* and take off a bit as the superheated one says.

 

[lw395]

470Wh is about 32Ah charging a '12V' battery.
If the battery is 320Ah, that's 10%.
So, if your battery is going to end up charged anywhere full, you are working in the ~90% charged area, where charging efficiency is much less than 100%.
So, I'd guess in the region of 20Ah charge stored?
Of course you might have used some charge e.g. running the fridge while the sun's out.