wildebus
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There is a thread about Battery Capacity running and there was a question about how a Battery could be above 100% 'full'.
For Lead Acid Battery Technology, the available battery capacity varies depending on the Discharge Rate. The higher the Discharage Rate, the lower the overall capacity is.
The Discharge Rates are quoted as a "C" rate and represents how many hours it takes to fully discharge* a battery at a certain current draw
*Fully discharge means for the voltage to drop to a set voltage - usually around 10.8V but the figure is determined by the Battery Manufacturer
So a Battery will have a C100 Rating, which is the capacity if 100/th of that capacity is drawn over 100 Hours, the C20 Rating is the capacity if 1/20th is drawn over 20 Hours, the C5 Capacity if 1/5th is drawn over 5 Hours, etc.
I have the XR1750 AGM Battery Model in my Camper, which is a nominal 110Ah Battery at the C100 Discharge Rate
Below is the Table for the XR1750 AGM Battery Model which shows how the Rated Capacities change at different rates -from 20 to 1 are shown
(I have greyed out the bits not key for this discussion)
There is a general move to use the C20 Rating when quoting a Battery Capacity - And it certainly would be good for the same rating to be used to allow the buyer to better compare between battery models without having to delve into the small print to check for like-for-like comparisions.
But the question is ... is C20 the right rating to use? Why not C10? why not C100? how about a C50 Rating?
I have 4 of those XR1750 batteries in my van and that means at the recommended C20 Rating, the capacity is worked out assuming a current draw of nearly 17A.
Now the typical person fits multiple batteries not to allow them to draw more current, but to give them more energy over a longer period.
This means if they double up on batteries but don't change their useage, the appropriate "C" Rating is doubled as well - so if C20 was right for a Single Battery, then C40 would be the right rating for a pair - so doubling the batteries MORE THAN DOUBLES the capacity.
And if you double up again in order to further extend your off-grid battery life (so having 4 batteries), that recommended C20 Rating you should supposedly base your choice on is void and the Rated Capacity to go on is actually the C80 Rating!
You won't find a C80 Rating for most Batteries, but you get a good idea what that might be from the table the manufacturer provides. For example, the graph below shows the capacity plotted against the C rating (taken from the table shown earlier in this post) and you can follow the line to get a good idea what the Capacity for an unspecified C rating would be
On the Victron BMV meter, I have set the Battery Capacity using the C20 rating as per the Victon Recommendation and it has a setting for the Peukert Component which should manage the variance and provides me with the SOC (State of Charge), but is that accurate?
And what about monitors that don't have that setting option? or people without Battery Monitors?
So I thought I would check my own consumption to get an idea of what the right Discharge Rating is to work out what Battery Capacity I really have, mapping each minute to a given C Rate based on current draw (and the way I allocated to the C rate was weighted to show the lower C Ratings for worse case)
The chart belows how long is spent at each Discharge Level for a 24 Hour Period. In this example on this day, the van was parked up so there was admittedly little use except for the Roof Fan, The Compressor Fridge coming on every so often and the Electric Water Heater coming on automatically once the battery hit a certain charge.
Also, any time where the battery is getting a net charge (from Solar), that is not part of this data (as it is only DISCHARGE we are looking at).
I actually created a "new" C Rating - C300 - which is a current draw 1/3rd of the C100 Rating or around 1/15th that of the 'Recommended ' C20 Rating.
1.2% of the time I am drawing power is at the C1 Rating, 1% at the C3 and 0.8% at the C5 Rating, which are all pretty high current draws, but represent a very small period of time.
But the great majority of the time *>85%) is spend at ratings of C100 or higher.
I have not worked out at this stage what the 'real' Battery Capacity is based on this, but I think it will be a fair bit better than the C20 rating
I'll do the same chart I think, but selecting an 'Active' day to see what the numbers look like there, as of course every day is different in terms of Battery Usage, so how much Battery Capacity you have on any different day, even if you start off with 100% each time, will vary -and that is something they don't tell you when you buy a battery
For Lead Acid Battery Technology, the available battery capacity varies depending on the Discharge Rate. The higher the Discharage Rate, the lower the overall capacity is.
The Discharge Rates are quoted as a "C" rate and represents how many hours it takes to fully discharge* a battery at a certain current draw
*Fully discharge means for the voltage to drop to a set voltage - usually around 10.8V but the figure is determined by the Battery Manufacturer
So a Battery will have a C100 Rating, which is the capacity if 100/th of that capacity is drawn over 100 Hours, the C20 Rating is the capacity if 1/20th is drawn over 20 Hours, the C5 Capacity if 1/5th is drawn over 5 Hours, etc.
I have the XR1750 AGM Battery Model in my Camper, which is a nominal 110Ah Battery at the C100 Discharge Rate
Below is the Table for the XR1750 AGM Battery Model which shows how the Rated Capacities change at different rates -from 20 to 1 are shown
(I have greyed out the bits not key for this discussion)
There is a general move to use the C20 Rating when quoting a Battery Capacity - And it certainly would be good for the same rating to be used to allow the buyer to better compare between battery models without having to delve into the small print to check for like-for-like comparisions.
But the question is ... is C20 the right rating to use? Why not C10? why not C100? how about a C50 Rating?
I have 4 of those XR1750 batteries in my van and that means at the recommended C20 Rating, the capacity is worked out assuming a current draw of nearly 17A.
Now the typical person fits multiple batteries not to allow them to draw more current, but to give them more energy over a longer period.
This means if they double up on batteries but don't change their useage, the appropriate "C" Rating is doubled as well - so if C20 was right for a Single Battery, then C40 would be the right rating for a pair - so doubling the batteries MORE THAN DOUBLES the capacity.
And if you double up again in order to further extend your off-grid battery life (so having 4 batteries), that recommended C20 Rating you should supposedly base your choice on is void and the Rated Capacity to go on is actually the C80 Rating!
You won't find a C80 Rating for most Batteries, but you get a good idea what that might be from the table the manufacturer provides. For example, the graph below shows the capacity plotted against the C rating (taken from the table shown earlier in this post) and you can follow the line to get a good idea what the Capacity for an unspecified C rating would be
On the Victron BMV meter, I have set the Battery Capacity using the C20 rating as per the Victon Recommendation and it has a setting for the Peukert Component which should manage the variance and provides me with the SOC (State of Charge), but is that accurate?
And what about monitors that don't have that setting option? or people without Battery Monitors?
So I thought I would check my own consumption to get an idea of what the right Discharge Rating is to work out what Battery Capacity I really have, mapping each minute to a given C Rate based on current draw (and the way I allocated to the C rate was weighted to show the lower C Ratings for worse case)
The chart belows how long is spent at each Discharge Level for a 24 Hour Period. In this example on this day, the van was parked up so there was admittedly little use except for the Roof Fan, The Compressor Fridge coming on every so often and the Electric Water Heater coming on automatically once the battery hit a certain charge.
Also, any time where the battery is getting a net charge (from Solar), that is not part of this data (as it is only DISCHARGE we are looking at).
I actually created a "new" C Rating - C300 - which is a current draw 1/3rd of the C100 Rating or around 1/15th that of the 'Recommended ' C20 Rating.
1.2% of the time I am drawing power is at the C1 Rating, 1% at the C3 and 0.8% at the C5 Rating, which are all pretty high current draws, but represent a very small period of time.
But the great majority of the time *>85%) is spend at ratings of C100 or higher.
I have not worked out at this stage what the 'real' Battery Capacity is based on this, but I think it will be a fair bit better than the C20 rating
I'll do the same chart I think, but selecting an 'Active' day to see what the numbers look like there, as of course every day is different in terms of Battery Usage, so how much Battery Capacity you have on any different day, even if you start off with 100% each time, will vary -and that is something they don't tell you when you buy a battery
