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You are using an out of date browser. It may not display this or other websites correctly.You should upgrade or use an alternative browser. Battery Discharge Rates - is "C20" the Right Default?

• Thread starter wildebus
• Start date Apr 30, 2019

wildebus

Full Member Messages 7,801 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 What people are saying .... https://uk.trustpilot.com/review/wildebus.com

Trek

Free Member Messages 545 Different manufacturers use different C values for their capacity and not all are displayed or advertised so its difficult to compare say a 100AH battery from one manufacturer to another also sold as 100AH A 100AH battery at C20 is better than one also rated 100AH but at C100 as the C100 rate is a much slower discharge I have found that companies like VARTA give the C5 and C20 rate whereas the leisure batteries generally sold in say caravan shops are either C100 or dont give the rate at all

Trek

Free Member Messages 545 The slower you discharge your battery the larger the storage capacity it will have

wildebus

Full Member Messages 7,801

Trek said: Different manufacturers use different C values for their capacity and not all are displayed or advertised so its difficult to compare say a 100AH battery from one manufacturer to another also sold as 100AH Click to expand...

That is quite correct. Which is why I said in my post "...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."

Trek said: A 100AH battery at C20 is better than one also rated 100AH but at C100 as the C100 rate is a much slower discharge Click to expand...

Better as in a greater capacity I presume? That is also correct. Which is why I said in my post "...For Lead Acid Battery Technology, the available battery capacity varies depending on the Discharge Rate. The higher the Discharge Rate, the lower the overall capacity is."

Trek said: I have found that companies like VARTA give the C5 and C20 rate whereas the leisure batteries generally sold in say caravan shops are either C100 or dont give the rate at all Click to expand...

This is why I would not recommend any battery where this information is not provided as the chances are it is a low-grade battery and better can be had for very little more money What people are saying .... https://uk.trustpilot.com/review/wildebus.com

wildebus

Full Member Messages 7,801

Trek said: The slower you discharge your battery the larger the storage capacity it will have Click to expand...

just wondering ... You are correct, but are you saying this like this as the way you have read my first post says otherwise? just want to clarify in case the way I have posted does not make sense? Last edited: May 1, 2019 What people are saying .... https://uk.trustpilot.com/review/wildebus.com

wildebus

Full Member Messages 7,801 I said I would add in a chart from an 'Active' day ... This is from a day at the Green Frog in Moffat in Feburary. So no hookup, very poor solar, Heating on non-stop and tea-making and and cooking using the Induction Hob. A much higher use of the battery compared to the first chart. And roughly a mix of 2/3rd C100, 1/3rd C20. Given this was a winters day off-grid, no proper solar contribution and electric cooking, for my own vehicle this is likely to be as bad as is likely to get. I think I need to add a "Charging" point on the graph to show the amount of time the batteries are not being used at all to get a fuller picture of a typical "day in the life". UPDATE: I added in the charging info! The above chart for Feb Camping didn't really change - for only 0.5% of the day (11 minutes) was the battery getting a positive charge! Below is the chart from the first post redone with the charging info - for nearly half the day the battery is getting a charge, and the bars for the rest of the time show what "C" rating the discharge level is at. I decided to also add to the graph the original way of showing the data. So the Darker Bars is the amount of time the Battery Draw is in the various C Rates throughout a full day; and the Lighter Bars is the percentage time taking away that time when the Battery is getting a net charge (more going in than coming out). I'm away weekend after next and it will be interesting to see what a early summers day will look like as again off-grid, but Solar should be much more active and a lot less heating (though I have been forewarned that there will be some hairdryer activity!) Last edited: May 1, 2019 What people are saying .... https://uk.trustpilot.com/review/wildebus.com

nabsim

Full Member Messages 4,202

Trek said: The slower you discharge your battery the larger the storage capacity it will have Click to expand...

Which I exactly why I got 3x90ah Bosch Powerframe battery’s The C value is only useful when battery is being used Dave. Is your 24 hour period correct for your useage? During the day there will be solar charge even when parked. Should these tests not be done with all charging solutions isolated? It's a long way to Tipperary! Don't worry, Be Happy

wildebus

Full Member Messages 7,801

nabsim said: Which I exactly why I got 3x90ah Bosch Powerframe battery’s The C value is only useful when battery is being used Dave. Is your 24 hour period correct for your useage? During the day there will be solar charge even when parked. Should these tests not be done with all charging solutions isolated? Click to expand...

Neil, you said "The C value is only useful when battery is being used Dave". Clearly from your comment and Treks one, the whole point of this thread has not been understood so I guess the way I have explained it is a failure This is not a test. It is an analysis of a configured campervan in actual use and shows what kind of real life battery discharge level is actually experienced. And battery charging as well as battery use is how real-life camping works. The point is everyone is jumping on the "C100 rating is wrong. must use C20" mantra, like A&N and the NCC are. But for people who are into the Wild Camping/Off-Grid style of camping it is not that simple - and THAT is the point of this thread. You have 3 90Ah Batteries. You got 3 x 90Ah batteries to let you run longer off-grid, didn't you? you didn't get the extra batteries to give you more power for the day, but the same power for a longer time, yes? So in that case, working out how much capacity your batteries have means not using the C20 rating, as that is too conservative (unless you are planning on taking them down to 50% in 10 Hours - in which case C20 is right!) Your question "Is your 24 hour period correct for your useage? During the day there will be solar charge even when parked. Should these tests not be done with all charging solutions isolated?" Do you think is incorrect then? I am not actually showing any actual usage figures, I am showing how the useage maps to the different Battery Discharge ratings. And of course there will be solar charge even when parked (that is when you usually get it!). Sorry, I am not really understanding the gist of the questions and I have to assume it is the way I attempted to explain the whole point of the thread. What people are saying .... https://uk.trustpilot.com/review/wildebus.com

wildebus

Full Member Messages 7,801 On the first post I said "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)" I realised I have not posted what my real-world Battery Capacity works out to be (as opposed to just using a nominal pre-determined Discharge Rate to check against) So how so I compare against the Leoch Data? This is the Capacity vs Discharge Rate chart built from the Leoch Table and shown in the first post, with the real-life numbers added in to show where my batteries, when being used in a real-life installation fits into the laboratory C rating numbers. The two entries added are for Apr and Feb. They fit between the C100 and C20 in terms of Capacities. It is not possible to assign a "C" rating to them, but the battery capacity for April (a light use day) is around 15% greater than the C20 number, and the capacity for Feb (a fairly heavy day with the heating and electric hob use) is 10% greater than the C20 number. What people are saying .... https://uk.trustpilot.com/review/wildebus.com You must log in or register to reply here.

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