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Old 01-06-2022, 11:02 PM   #1
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Request for quick help with battery math

OK so I have a 210Ah battery, it is chilly in CO so my battery won't operate at full capacity, according to the capacity vs temp chart I have it looks like it will operate at 80% capacity, so my 210Ah battery is actually a 168Ah battery, when fully charged. Since I don't want to discharge below 50% should I be using 50% of the original 210Ah number, and therefore I don't want to go below 105Ah, and therefore have 63Ah of actual capacity?

Am I thinking about that correctly?
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Old 01-07-2022, 07:36 AM   #2
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First I would ask how are you determining 50%. If you have a battery monitor, then you should check to see if compensates for temperature, most don't. You also didn't mention compensating for “Peukert’s equation, some battery monitors do compensate for this, most use a default value that hardly gets ever gets changed. You would also have to account for age of the battery, as the battery gets older it loses capacity.

Here is a good explanation of why one designer of battery monitors does not use either Temp or Peukert's when calculating SOC. Comments on Peukert's relationship from Bogart Engineering.

Unfortunately the reality is the battery will have less capacity at cold temp. I would generally use the 50% target of your new calculated capacity . I also wouldn't be to worried if I went to 40%.

-greg
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Old 01-07-2022, 09:00 AM   #3
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Thanks for the input Greg I will read that document thoroughly for a better understanding. To respond to your question:

As far as determining 50%, I was asking more from a theoretical standpoint than an actual monitoring perspective at this point, just trying to understand if my assumptions were correct or if I was way off. The Lifeline Technical Manual has a table in Appendix C showing that at the c/20 rate of discharge, a voltage reading of roughly 12.15V would be a battery discharged to about 50%. With just one of the cigarette lighter voltage readers I was hoping that I could take a voltage reading and be able to estimate roughly the battery's depth of discharge based on the Lifeline chart. Again, not looking for pinpoint accuracy just a rough idea of where the battery is at.

I am confused about your comment of taking the battery down to 50% of the new calculated capacity, in my case 168Ah. Theoretically starting with a max capacity of 168Ah, if I take a 168Ah battery down to 50% DOD that would leave me with 84Ah left in the battery. However, if I took the battery down to that same 84Ah of capacity left when it started with a max capacity of 210Ah that would mean I would have taken the battery down to 60% which is well past the 50% target of 105Ah for a 210Ah battery, wouldn't it? Why is it OK to take the battery down to 84Ah in the winter when it only has a max capacity of 168Ah, but not OK in the summer when it has a max capacity of 210Ah? I did read some of the doc you provided and noticed that it said that in varying temperatures you would want to set your battery monitor to the minimum capacity, so basically whatever your max capacity would be during your coldest outside temperature. So, again my question stands, if I start with a lower max capacity due to low temps, why is it OK to take the battery to a lower DOD than during warmer months?

I appreciate your input!

Ben
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Old 01-07-2022, 10:05 AM   #4
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So if I take a battery to 50% DOD. It will be 50% DOD of battery capacity at 15°, 25°C and 35°C, The available capacity will either decrease or increase based on the temp. So if you are using a coulomb counting battery monitor and your primary usage is colder weather, then you would use the lower anticipated temperature in your monitor, to make sure you don't go below 50%

Obviously in warmer temps, a meter set that way would say 50% but would in reality be higher, You could also just set it room temp capacity numbers and then adjust the DOD accordingly to like 60%

If you are using voltage only for your determination of SOC, one would expect that those would move with the temperature.

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Old 01-07-2022, 10:18 AM   #5
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OK finished reading the paper. So now I think I kinda understand your intent on asking how I am calculating 50% DOD. As I understood it from the paper, I could use a bunch of amps and get to 49.9% in a short period of time, but then if I vastly reduced my discharge rate, I could, theoretically, extend the time it took to actually get down to 50% DOD, is that right?

If that is the case then it totally makes sense why temp compensation in a battery monitor could be misleading. If it is sunny and warm during the day, as the sun is setting you might read that you have a good amount of battery left, but then when the sun sets and the temp drops, you could quickly find your battery low because the lower temp has basically robbed your battery of capacity, and the monitor would have had no way of knowing that was going to happen back when the temp was warmer.

I also found it interesting that the way the Ah's are calculated for a battery, in the testing environment, is by observing the voltage drop over time given a constant discharge current, wouldn't that mean that my method of using the cigarette lighter voltage meter to keep an eye on voltage drop is doing basically the same thing?

While camping, I am mostly concerned with not discharging below 50%, I don't have solar, so I am not putting charge back into the battery while out camping, so I don't really need to look at how much is being put back into the battery because it is 0. I can run the van to charge the battery, but I am usually not parked for more than 3 days or so, so if I can just make sure I don't go below 50% DOD that is what I am most concerned about so that I am not damaging the battery while out camping. If I were getting close to the 50% DOD I could just shut the more power hungry devices off like the fridge or the heater, thus reducing the load, and extending the battery capacity a little longer until I start the van up and move to the next location, yes?
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Old 01-09-2022, 03:38 PM   #6
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Quote:
Originally Posted by Ben10281 View Post
If I were getting close to the 50% DOD I could just shut the more power hungry devices off like the fridge or the heater, thus reducing the load, and extending the battery capacity a little longer until I start the van up and move to the next location, yes?
Yes, reducing the load will extend the run time, think of it as a checking account with $100. You can buy dinner for $75 and spend the remaining $25 on a bottle of champagne, or stretch it out after dinner with a few beers. I may have missed it, but if you don't have a battery monitor, it will be difficult to determine when you have reached 50% In addition, once you reach that point, it normally takes far longer to reach a full recharge than you may think,
https://batteryuniversity.com/articl...ead-acid%C2%A0

Simply driving for a few hours won't do it unless you have set up a sophisticated high amp alternator and a regulator that provides a three step charge profile. It can take as long as 12 hours or more to reach a full charge.
My setup includes two 6V's in series for a total of 200Ah's and 300 watts of solar. In winter, even when driving for several hours, I never regain a full charge(running the refer, diesel heater, lights, stereo etc). Never the less, the batteries are now three years old, and still performing well, so I don't worry too much about the discharge level and in addition, two 6v golfcart batteries from Costco cost less than $200, so replacing them every few years is no big deal, even if I could have gotten a little more life by religiously avoiding going below 50%
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