That's it.I'm guessing it's a 100 amp one like this? I like them, though, you have to interpret the results to account for the higher capacity of large batteries.
Battery Tests Fine, Low Dark Current, Yet Battery Low After Sitting Several Days
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Parasitic drains can be very intermittent. While it might take a while ( and that can vary wildly from less than one minute to something like 15 minutes ) for the electric devices of some vehicles to "go to sleep" after you shut off the vehicle and have all doors closed and everything shut off. Sometimes, incorrectly some device will wake up every once in a while and draw a very high amount of current for a while and then go back to sleep for a while, and repeat. This type of intermittent parasitic drains can drain a good battery much quicker than the vehicle should, and also can be a royal pain to find. You almost have to have some sophisticated test equipment and the know-how of how to use it to find this kind of a problem. The good old divide and conquer method will also work if you can do that, but that can be very time consuming.
Also, as others have said, a 6 yo battery is at the end of its life anyhow, so it might be a cost effective trouble shooting method to just start off by replacing that battery. Or temporally swap a know good battery into it and see if the problem is still there.
Also, as others have said, a 6 yo battery is at the end of its life anyhow, so it might be a cost effective trouble shooting method to just start off by replacing that battery. Or temporally swap a know good battery into it and see if the problem is still there.
This battery continues to go flat after a few days. Obviously the load test is not conclusive.
I saw plans online for an A-h tester. Simple and effective - load the fully-charged battery up with a couple of 100 W light bulbs, an inverter, and an AC-powered analog clock. The lights (and to a lesser extent, the inverter) will drain the battery, and when the clock stops you'll know how long the battery lasted.
Instead of calculating the current draw, measure it. (The calculation would be something like: 200 W + power consumed by the inverter = 13.2 V * I, so I = (200 W + power consumed by the inverter)/13.2 V (or whatever battery's voltage is) = (approximately) 8 A.
If the clock stops after, for example, 6 hours, the battery's A-h capacity was 8 A x 6 hours = 48 A-h. I would then compare that to ensure it's close to the nominal capacity. (I don't know, though, when it would make sense to declare the battery to be no good - 80%? 50%? 30%?
But anyway, I stopped in at a couple of Goodwill-type stores, to no avail. Battery-operated analog clocks galore, and lots of AC digital clocks, but no AC analog clocks.
So then my wife suggested just buying a tester. Duh, I should have thought of that ... except it's like trying to drink out of a fire hose. Lots of battery testers, but it's hard to know which ones actually test A-h capacity.
Any recommendations?
I saw plans online for an A-h tester. Simple and effective - load the fully-charged battery up with a couple of 100 W light bulbs, an inverter, and an AC-powered analog clock. The lights (and to a lesser extent, the inverter) will drain the battery, and when the clock stops you'll know how long the battery lasted.
Instead of calculating the current draw, measure it. (The calculation would be something like: 200 W + power consumed by the inverter = 13.2 V * I, so I = (200 W + power consumed by the inverter)/13.2 V (or whatever battery's voltage is) = (approximately) 8 A.
If the clock stops after, for example, 6 hours, the battery's A-h capacity was 8 A x 6 hours = 48 A-h. I would then compare that to ensure it's close to the nominal capacity. (I don't know, though, when it would make sense to declare the battery to be no good - 80%? 50%? 30%?
But anyway, I stopped in at a couple of Goodwill-type stores, to no avail. Battery-operated analog clocks galore, and lots of AC digital clocks, but no AC analog clocks.
So then my wife suggested just buying a tester. Duh, I should have thought of that ... except it's like trying to drink out of a fire hose. Lots of battery testers, but it's hard to know which ones actually test A-h capacity.
Any recommendations?
I was looking a couple year ago for some AC powered clocks to keep track of accumulated time on a heater circuit I as using. Wesclox still makes them.
If you can't find a real AC powered clock, What about a 1.5V DC power supply wired to the positive and negative battery terminal?
You can also get "battery eliminators" from the usual online marketplaces which have an piece that fits into the AA battery slot and a USB plug on the other end. They can step down the output from a standard 5V USB power source. Naturally you would want to test this with a multimeter if you care about the clock, since electronics of this type and from these sources can be questionable.
With a little more searching, you can find one that skips the USB plug and step down, and instead uses a barrel jack and a 1.5V power supply. Try "AA 1.5V battery eliminator" as a search term.
I found various appropriate Westclox models for sale on eBay, but balked at the price - C$26.72 + C$32.47 shipping was typical. Hard currency conversion I guess.
Then I found one locally for C$10. Picked it up yesterday afternoon. Yay, Kijiji!
Wired things up early this afternoon, and tested the battery, running a 0.5 A load.
The inverter cut out due to low input voltage (11.7 V) after only 25 minutes.
The battery failed badly.
It's interesting to me that the battery passed the short-term (10 second) 100 A load test fine, but failed this capacity test.
I guess there's a lot I still don't know about batteries.
I started thinking about what I could do with a DC load (i.e. a battery-operated clock) and voltage divider, etc., and was relieved to find the AC clock I wanted.
Good advice as it turns out!
In my half century plus maintaining vehicles, any battery over 4 years old is the most likely suspect when electrical systems are problematic.
you can look at how fast it re-charges. If a dead 60 a/h battery fully recharges in 3 hours while only taking 4 amps the capacity cannot be more than 12 ah.
Good point - it recharges very quickly.
I've seen several original batteries go nine years, but I figure three winters out of a conventional replacement is about it. The exception is Optima Red Top AGM batteries, which in two cases outlasted the vehicle (seven years in both cases).
I charged the battery and ran the test again - 12.3 V down to 11.6 V in 20 minutes with a 5.0 A load. 
Here's the set-up:
Here's the set-up:
Science!!
Yes, but not mine - I saw it on the 'net. Can't remember the author's name, but kudos to him!
I'm trying to relate Reserve Capacity (RC) to A-h capacity.
Per the 'net, RC is the number of minutes a fully-charged battery can sustain a load of 25 A before dropping to 10.5 V.
In contrast, it sounds like A-h capacity can comprise any number of combinations of current and time, until the battery capacity drops to a critical value. 10.5 V? Hard for me to experiment with because my inverter shuts down when battery voltage drops to 11.6 V. But in any case, rather than specifying a particular amperage draw, an 80 A-h battery could provide 1 A for 80 hours, 2 A for 40 hours, 4 A for 20 hours, 5 A for 16 hours, 8 A for 10 hours, 10 A for 8 hours, and so on.
So, to test RC, I need to modify my test rig to draw 25 A. The one 60 W light bulb + the inverter drew 5.0 A, so I could add several more light bulbs - but now wonder about stressing the inverter, which is rated at 400 W. (The one bulb was consuming 60 W, so presumably I could run five bulbs to consume about 300 W @ 120 V, for a total of 25 A. The inverter should be OK with that.)
The only thing is that the inverter will cut out before the defined RC low voltage cutoff of 10.5 V.
Anyway, the battery in question, which I'm already convinced is bad, is rated at 90 RC. That means that it should be able to supply 25 A for 90 minutes, whilst staying above 10.5 V. Given that the voltage dropped to 11.6 V after 20 minutes, with a load of only 5 A, I think it's safe to say it would fail the RC test badly.
Per the 'net, RC is the number of minutes a fully-charged battery can sustain a load of 25 A before dropping to 10.5 V.
In contrast, it sounds like A-h capacity can comprise any number of combinations of current and time, until the battery capacity drops to a critical value. 10.5 V? Hard for me to experiment with because my inverter shuts down when battery voltage drops to 11.6 V. But in any case, rather than specifying a particular amperage draw, an 80 A-h battery could provide 1 A for 80 hours, 2 A for 40 hours, 4 A for 20 hours, 5 A for 16 hours, 8 A for 10 hours, 10 A for 8 hours, and so on.
So, to test RC, I need to modify my test rig to draw 25 A. The one 60 W light bulb + the inverter drew 5.0 A, so I could add several more light bulbs - but now wonder about stressing the inverter, which is rated at 400 W. (The one bulb was consuming 60 W, so presumably I could run five bulbs to consume about 300 W @ 120 V, for a total of 25 A. The inverter should be OK with that.)
The only thing is that the inverter will cut out before the defined RC low voltage cutoff of 10.5 V.
Anyway, the battery in question, which I'm already convinced is bad, is rated at 90 RC. That means that it should be able to supply 25 A for 90 minutes, whilst staying above 10.5 V. Given that the voltage dropped to 11.6 V after 20 minutes, with a load of only 5 A, I think it's safe to say it would fail the RC test badly.
My sister had this very issue with her Ford Escape. It was in the dealership four Times and they couldn't find the issue. The dealership offered her 2k more than she paid for it in 2021.
How many plates are there in a cell, typically? There are six cells in a 12 V battery, correct?
That's the big variable but the bigger the battery the more plates. The end area is the approximate 2D size of the plate (not thickness). Divide the total length of the battery by 6 to get the ~individual cell width.
As an example with the industrial batteries we made. The smallest was a 35A plate, the biggest was a 180A plate. Normally 5 to 35 pos plates or so per cell for what we made. And always one more negative in the stacked plate design. Not so in the spiral cell. These are thick plates at around 1/4" for pos and 1/8" for neg.
But the plates in auto batteries are playing card thick. So you can pack a lot of pos plates per cell. This gives a lot of initial amps from a small battery. The trade off is capacity. Our industrial batteries were rated at the amp hour rating of the total positive plates per cell for a 8 hour period, down to 1.72v per cell.
You can boost the capacity with .25-.50- sp gr stronger acid at the cost of lifetime. And you can increase the lifetime by reducing the acid strength but beware deep freezing temps.
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