How 395mA draw discharges 70AH battery to 12.2V (~37.5%) in 26 hours?

yelson

New Member
Joined
Sep 20, 2020
Messages
8
Reaction score
1
Location
NJ
Country
United States
Just trying to understand... I measured my Vantrue N4 current consumption. In full recording mode - 430mA; parking mode when not recording - 395mA... Simple calc gives ~0.4AH every hour, which means in 26 hours 10.4AH should've been used, which is ~15% of the 70AH battery capacity. Instead - 12.2V cutoff has been hit and the device duly disconnected (actually - I don't know how many hours it took for that - I just checked after 26). According to all the tables for AGM batteries - that is over 60% of the battery capacity (and I do have a new battery!). I also checked current consumption w/o dashcam - it is pretty much zero - at least in my case. Anyone has an idea what I am missing? And of course - if I calc the current based on 26 hour 70AH battery drain down to 37.5% - it comes down to 1.7A, which I definitely do not have!
 
You should work this out in Wh, not Ah.

Is your 430mA measured at 5 volts while your battery is a 12 volt battery? 430mA at 12V is 5.16 watts, which is a lot of power to be using for parking mode, but I have never tested a Vantrue N4.

You need to take into account that placing a load on the battery will reduce the voltage, so 12.2V will be reached long before 60% capacity is used. It is reasonable to set the cutoff to 11.8V for an AGM battery, after the cutoff the voltage will rise a bit, but to measure the real battery voltage accurately you have to disconnect it from all loads for half an hour.
 
You should work this out in Wh, not Ah.

Is your 430mA measured at 5 volts while your battery is a 12 volt battery? 430mA at 12V is 5.16 watts, which is a lot of power to be using for parking mode, but I have never tested a Vantrue N4.

You need to take into account that placing a load on the battery will reduce the voltage, so 12.2V will be reached long before 60% capacity is used. It is reasonable to set the cutoff to 11.8V for an AGM battery, after the cutoff the voltage will rise a bit, but to measure the real battery voltage accurately you have to disconnect it from all loads for half an hour.
Thank you for replying! No - it is measured immediately after the fuse (so 12 volts), where fuse tap has convenient disconnect to attach amperemeter leads to... And yes - you are right - Vantrue N4 has 3 cameras, and does consume a lot of power (if I disconnect 3rd camera - current reduces to 275mA in parking). Still - it should've taken almost a week before reaching 37.5% capacity left on 70AH battery... When I go for kayaking trips - I leave my van parked for 5-7 days, and got it dented by someone once some years back. Would be nice to have that recorded, but does not seem possible with what I'm seeing (vs what I'm calculating). The reason why I had AH calcs, rather than Wh is, b/c the ratings on the batteries is in AH. Since I'm measuring right off the battery - it should be OK, I thought...
Now - when it comes to 11.8V cutoff - I'm reading everywhere that would be close to 0 juice left (they say 12V is 20% left). Is that all wrong?
 
Last edited:
FWIW I run a 2-channel cam constantly from a 80A lead-acid battery in my van. I haven't checked the voltages but anything much more that 24 hours recording and I can tell the engine starter is beginning to slow which means I'm pushing too far. After around 30 hours it has to be boosted or charged to get going. So I'd venture a guess that you're going to get approximately the same runtime and results at that amperage consumption which is about equal to mine.

If you really need super-long runtimes you're going to need external powering, which at that amount you want is probably best done with a dedicated LiFePO4 or AGM type battery, as those have better discharging characteristics than anything else.

Phil
 
Thanks, Phil. What you are saying certainly makes sense. I just wanted to understand where disconnect comes from, b/c what is happening (and we are both and I'm sure other people observing) and calcs do not match, so I'm curious to know what is wrong with calcs... If 70AH battery runs for day and a half simply speaking - to run for 6-7 days - I'd need over 300AH separate battery... Who makes batteries like that?! ;-)
 
Now - when it comes to 11.8V cutoff - I'm reading everywhere that would be close to 0 juice left (they say 12V is 20% left). Is that all wrong?
Problem with using A is that they change as the voltage decreases, so you need to know what voltage the Ah of the battery where measured at before you know how much power is available in your 70Ah battery? The camera uses constant W, if you have the Wh for your battery there is no need to know how it was measured, the run time is a simple calculation.

A typical AGM battery is empty at about 11.0 volts, 12.0 volts ought to be about 50%, although it depends on the health of the battery, I've had well worn ones be full at 12.3 volts. These are voltages measured with zero load, once you add a load they drop a bit. An AGM battery should have no problems with regular discharge to 11.8, and if your cutoff is set at that then you can be sure it will cutoff well before it actually gets to that voltage measured at no load.
 
Problem with using A is that they change as the voltage decreases, so you need to know what voltage the Ah of the battery where measured at before you know how much power is available in your 70Ah battery? The camera uses constant W, if you have the Wh for your battery there is no need to know how it was measured, the run time is a simple calculation.

A typical AGM battery is empty at about 11.0 volts, 12.0 volts ought to be about 50%, although it depends on the health of the battery, I've had well worn ones be full at 12.3 volts. These are voltages measured with zero load, once you add a load they drop a bit. An AGM battery should have no problems with regular discharge to 11.8, and if your cutoff is set at that then you can be sure it will cutoff well before it actually gets to that voltage measured at no load.
Thank you for responding! Just checked on my car exactly how much is voltage drop w/o vs w camera, and it turned out 0.1V. So - while I do agree that it is a good habit for an EE, who often works with these things to use Wh, for this one the difference does not address the magnitude of discrepancy, IMHO, unless there's something else I'm missing. Besides - to my understanding - that drop is usually explained with the battery's "internal resistance", which in my expectation should show in battery heating up when under load. All it really means, is that some of those Wh are wasted on battery heating, rather than useful load.
As to V and % empty - are all the below wrong? They all seem to indicate that the battery is empty around 11.8V-11.9V, and sound like they are basing it on proper measurments, even though I can't judge 100% based on just what they write... Are you basing your numbers on measurements of your own, or have some reference? Or are you saying - there's no problem really discharging AGM battery to empty? Will it crank my car's engine, though?
 
Last edited:
Or are you saying - there's no problem really discharging AGM battery to empty? Will it crank my car's engine, though?
You should never empty any lead acid battery, but AGM batteries can be emptied more than standard ones without issue.
I have started my car with the AGM battery at 11.3 volts, so I know the 11.8 is not empty! A standard lead acid battery would not start the car at 11.3, it would struggle at 11.8.

I guess the figures you are finding are based on the empty being at the "Unusable" line rather than when it contains zero power, and for some reason the unusable line is shown the same for all types when that is not actually true. At that line a standard battery will not produce enough amps to drive the starter, but an AGM will without any issue.

12v-voltage-chart-676.jpg


Just to make it clear, I don't recommend discharging your AGM below 11.8 volts, but I am saying that it is safe to go down to 11.8 volts and your car will still start, even in cold weather, unless you live in Alaska.
 
Thanks! I certainly believe in personal experience against all the published tables in the world any time of day! ;-) Still - mind sharing where the table and chart come from?
What I think I'll do, is - have my cut-off set at 12.2V during the year (I just need parking mode on when shopping - shouldn't be over hour or two ever), and when we go kayaking - will try 11.8V (with the fully charged portable jump starter just in case ;-) ). Will see how long it lasts when we come back and check dashcam recording timestamps...
I'm still curious what is wrong with my calcs? Even your table does not fully explain discrepancy, even though it is not as bad.
 
Do not forget about one more feature of batteries - their real capacity, i.e. how quickly they lose charge depending on their condition.
All calculations that are carried out assume an ideal new battery and may not have anything to do with the actual condition of your battery.

I will give an example of the state of the batteries in reality (and we may not even guess about this), under the spoiler Correct test to assess the real capacity of batteries
Big honest battery test

How the test was built: The batteries were discharged at room temperature + 25 ° C with a current equal to 1/20 of the capacity (C / 20), i.e. 110/20 = 5.5A - the method according to which most battery manufacturers give their nominal capacity.

The final discharge voltage is 10.8V, at which the electronic load was automatically turned off.

The length of the test could in theory be up to 20 hours, but in fact it turned out to be much less in most cases.

График емкости аккумулятора_.jpg

PS: although the material is presented in Russian, even without a google translator, everything can be clearly understood from the diagram - yes, until you check your battery in a real situation, it is too early to count on something.

Test measurements used:
1) Programmable electronic load EA-EL 9160-100
Электронная нагрузка EA-EL 9160-100_1.jpg

2) Multimeter with data logger function Uni-T UT71D
[/ SPOILER]
 
Last edited:
... Still - mind sharing where the table and chart come from?
...
Some generalized indicators, I think, can be found in different publications on the Internet, they may vary slightly, but they give a general idea.
For example, from the Bosch manual for the operation of the BOSCH S4 battery 024 560 410 054
100% - 12.66V
75% - 12.45V
50% -12.24V
25% -12.06V
0% -11.80V and below
If you focus on it, then nothing is clear at all, but in fact, real exploitation gives slightly different numbers. ;)
 
Thank you for commenting!
I'd like to focus on your keyword "slightly" ;-). That's where I see the problem... If my calcs were "slightly" off - I wouldn't be on this thread. :) BUT!
1. My battery is new (just got replaced).
2. It is rated at 70Ah.
3. 26 hours of 0.4A should use up just about 10.4Ah, which should be around 15% of 70AH.
How come I got 12.23V vs initial 12.7V? Whichever table out there we use - none comes closer than twice as much capacity used up as in this calc, which is trivial. What gives?
 
With my 80Ah battery I go 24 hours max between charge cycles. At this level I'm pushing the battery hard and lose some service life- perhaps 20% based on much personal experience with this battery/vehicle combo. I know the expected battery service life and battery cost so I can break it down into daily figures. My battery would last 5 years without the cams but only 4 years with them, and my battery costs $120 so normally $24 per year, and now $30 per year factoring in the 20% loss of life. Divide the $30 by 365 days and it costs me $0.82 per day in power to run my cam, which I feel is quite reasonable for the peace of mind it gives me. :cool: I've claimed a lower figure here on DCT in the past, but 4 years ago my battery cost just under $80; the $120 is accurate for today so the higher daily cost now.

These are my actual real-life results, mid-grade LA battery, moderate climate, 5.8L gas V8 engine so a fairly high starter load. Drive (or run at fast idle) 20 minutes or more daily. Usually twice a day. Always starts with no noticeable drop in starter speed or strength. Never measured voltages; all I know is that it works and my figures are accurate.

As of today, the best safe-for-use-in-cars battery technology at a reasonable price is LiFePO4, and hefty-sized batteries are common and comparably cheap when you consider they can be very deeply discharged with almost no voltage drop, moreso than any LA or AGM types. Shopping carefully for the good stuff, around $400+ will get you a 100A battery measured as actual usable capacity. That will give you 5+ days recording time and around 3000 charge cycles before significant capacity loss. It needs a specialized MPPT charge controller, but charges fast and considering the lifetime you get from it is likely the cheapest way to go. Only the high initial investment makes this tough to do. Check out LINK HERE to Will Prowse on YouTube, he's the sharpest around on LiFePO4 battery technology. Some of his older vids relate directly to RV 12V charging, most of his new stuff is solar powered at higher voltages but you'll see how it's done in his vids.

Phil
 
With my 80Ah battery I go 24 hours max between charge cycles. At this level I'm pushing the battery hard and lose some service life- perhaps 20% based on much personal experience with this battery/vehicle combo. I know the expected battery service life and battery cost so I can break it down into daily figures. My battery would last 5 years without the cams but only 4 years with them, and my battery costs $120 so normally $24 per year, and now $30 per year factoring in the 20% loss of life. Divide the $30 by 365 days and it costs me $0.82 per day in power to run my cam, which I feel is quite reasonable for the peace of mind it gives me. :cool:I've claimed a lower figure here on DCT in the past, but 4 years ago my battery cost just under $80; the $120 is accurate for today so the higher daily cost now.

These are my actual real-life results, mid-grade LA battery, moderate climate, 5.8L gas V8 engine so a fairly high starter load. Drive (or run at fast idle) 20 minutes or more daily. Usually twice a day. Always starts with no noticeable drop in starter speed or strength. Never measured voltages; all I know is that it works and my figures are accurate.

As of today, the best safe-for-use-in-cars battery technology at a reasonable price is LiFePO4, and hefty-sized batteries are common and comparably cheap when you consider they can be very deeply discharged with almost no voltage drop, moreso than any LA or AGM types. Shopping carefully for the good stuff, around $400+ will get you a 100A battery measured as actual usable capacity. That will give you 5+ days recording time and around 3000 charge cycles before significant capacity loss. It needs a specialized MPPT charge controller, but charges fast and considering the lifetime you get from it is likely the cheapest way to go. Only the high initial investment makes this tough to do. Check out LINK HERE to Will Prowse on YouTube, he's the sharpest around on LiFePO4 battery technology. Some of his older vids relate directly to RV 12V charging, most of his new stuff is solar powered at higher voltages but you'll see how it's done in his vids.

Phil
Thank you for the info, Phil! Will certainly check the vids. That might answer my ask for 5-7 days of parking with dashcam on, even though I have a hard time seeing that LiFePO4 is available in exact size to fit Jeep GC under the seat compartment (battery size H6). Will certainly try, though.
My bewilderment with calcs remains, though. They are completely out of wack with observed usage.
 
To be clear, the LiFePO4 will be a cam-only solution, not a car battery replacement.

Phil
 
You could have a dodgy battery, you would not be the first one to get a new battery only to not be able to start your car a week or two later.

My new battery are just 50 Ah,,,,,, small car small battery.
If parking guard testing become a big deal and i can not simulate use at home using one of my variable power supplies to simulate a battery, then i will have to get creative in the car with a dedicated dashcam battery either paid for or paid for but DIY, and i am not that optimistic about the flashlight based ( 18650 / 26650 ASO ) dedicated dashcam batteries
 
Thank you for commenting!
I'd like to focus on your keyword "slightly" ;-). That's where I see the problem... If my calcs were "slightly" off - I wouldn't be on this thread. :) BUT!
1. My battery is new (just got replaced).
2. It is rated at 70Ah.
3. 26 hours of 0.4A should use up just about 10.4Ah, which should be around 15% of 70AH.
How come I got 12.23V vs initial 12.7V? Whichever table out there we use - none comes closer than twice as much capacity used up as in this calc, which is trivial. What gives?
I think I understand what the error is... You can't compare the data from the tables and assume that if the voltage on the battery is 100% = 12.6V, and 70% = 12.32V, then 30% of the energy consumption is a drop from 12, 6V to 12.32V, and accordingly for a 70Ah battery, this is the same 30%, and accordingly, with a load of 0.4A, a new battery should be enough for 52.5 hours. I gave the diagram above - in the initial period of connecting the load, the voltage drop across the battery is more active than in the subsequent time, and there is no linear dependence. Further, the diagram does not have such a steep slope, so I would set the voltage cutoff as advised - at 11.8V and check it.
 
Yes, the discharge curve for LA batteries is not linear, and falls faster the more discharged the battery becomes. LA batteries are excellent for high short loads like starting a car, but that's all they do well compared to other battery technologies. AGM, SLA, and deep cycle batteries are also somewhat limited by the same characteristics. Lithium-type batteries have a much longer and flatter discharge curve and can provide a higher percentage of discharging before damage, so they're far more ideal for parking mode use. As of yet nobody is making hardware to do this close to "plug-and-play" simple except Cellink, and their product is expensive, limited, and has been sometimes known to die soon after the warranty period is over.

Given how today's cars are full of voltage-sensitive computers and such, it's probably wisest to not use the car's battery to power parking modes beyond maybe a couple days even with an AGM type replacement. Some modern car batteries are special uncommon sizes which can be rather expensive to replace. And with many cars having a computer-controlled power scheme these days, using the car battery can set error codes from having an unexpected current draw when parked. BMW are notorious for this and that list is growing :cautious:

With a bit of easily-found knowledge, it's not hard to DIY a LiFePO4 system. It's really the best way forward here.

Phil
 
To be clear, the LiFePO4 will be a cam-only solution, not a car battery replacement.

Phil
Oh, I see... Would that sit somewhere under the seat then? From what I'm seeing those are quite large and expensive. Buying and fitting that for one week a year seems extravagant.
 
Back
Top