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Unread 05-18-2017, 10:32 PM
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automotive battery discharge curves


This question is for Al Skierkiewicz, Hugh Meyer, Ken Streeter, Doc Wu, or anyone else out there who has access to a West Mountain CBA battery tester and knows how to use it: have you ever used the CBA to test automotive batteries? Do you still have test results you could share?

Here's why I'm asking. A friend has a 2015 step truck with a big Ford engine in it and a Motorcraft BXT 65-750 battery. This spring he tried to start it, and the battery was dead. He brought it to me and I checked the electrolyte level and specific gravity. Added some distilled water to the cells and put a 3 amp slow charger with auto float on it and left it in the garage charging for 10 days. To my surprise it seems to have taken the charge. It passes a 100-amp 10-second voltage drop test, and for the past three hours it's been powering a 3 amp load, and the voltage with the load in place is 12.5 volts.

Problem is, I cannot find any discharge curves for this model battery. I fact, I haven't been able to find discharge curves for any similar auto battery.

So I was wondering (and hoping) you guys might have some data that would tell me if 12.5 volts after sustaining a 3 amp load for 3 hours and 15 minutes is good, bad, or inconclusive.

Thanks.


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Unread 05-19-2017, 11:48 AM
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Re: automotive battery discharge curves

In a new lead acid battery you would ideally have 2.041 Volts per cell, or 12.246 Volts with a one molar electrolyte. Many batteries ship with more than 1 molar electrolyte and have higher initial voltages.

As the battery discharges the electrode(PB and PBO) plates become coated with lead sulfate and the electrolyte becomes weaker. Recharging reverses most of this process. With discharge cycles (particularly deep cycles) there is a gradual buildup of deeper sulfate areas on the plates that is not reversible, and so the battery capability slowly decreases.

There is a Nerst Equation that equates remaining voltage level to remaining electrolyte molarity. (Molarity could be converted to S.G for plotting). This is idealized, as it doesn't include the added resistance due to the accumulated sulfate on the surface with discharge(which tends to put a knee in the curves.)

The curves new versus after 2000 discharge cycles will look quite different.
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Unread 05-19-2017, 11:51 AM
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Re: automotive battery discharge curves

Quote:
Originally Posted by InFlight View Post
In a new lead acid battery you would ideally have 2.041 Volts per cell, or 12.246 Volts with a one molar electrolyte.

As the battery discharges the electrode(PB and PBO) plates become coated with lead sulfate and the electrolyte becomes weaker. Recharging reverses most of this process. With discharge cycles (particularly deep cycles) there is a gradual buildup of deeper sulfate areas on the plates that is not reversible, and so the battery capability slowly decreases.

There is a Nerst Equation that equates remain level charge level to remaining electrolyte molarity. (Molarity could be converted to S.G for plotting). This is idealized, as it doesn't include the added resistance due to the accumulated sulfate on the surface with discharge(which tends to but a knee in the curves.)

The curves new versus after 2000 discharge will look quite different.
Have you ever used the CBA to test automotive batteries? Do you have test results you could share?


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Unread 05-19-2017, 01:57 PM
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Re: automotive battery discharge curves

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Originally Posted by Ether View Post
Have you ever used the CBA to test automotive batteries? Do you have test results you could share?


I don't own CBA. But you have to consider whether the measured results from others would even be applicable to your situation. One set of data on a new 65-750 Interstate battery would only be partially applicable to a used 65-750 AC Delco. The physical plate sizes, surface finish as well as the electrolyte volume & molarity can result in real differences in performance. The sulfate buildup (Age) is very a large factor as well.

In the end of the day, starting his truck on a cold morning is the only real performance measurement that matters.
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Unread 05-19-2017, 03:02 PM
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Re: automotive battery discharge curves

"This spring he tried to start it, "

It sounds like this truck is sitting idle quite a bit. Many newer vehicles have an idle draw that will flatten a battery, over several months. My brother had a Volkswagen that would drain the battery in a couple of days, if the doors weren't locked. The computer system did not go to sleep.

Two years is a pretty short time for a battery to fail from Sulfating.
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Unread 05-19-2017, 03:38 PM
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Re: automotive battery discharge curves

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Originally Posted by Tom469 View Post
"This spring he tried to start it, " It sounds like this truck is sitting idle quite a bit.
It's not used in the winter.

Quote:
Many newer vehicles have an idle draw that will flatten a battery, over several months.
Yup.

Quote:
Two years is a pretty short time for a battery to fail from Sulfating.
If you drain a battery way down and let it sit that way, sulfate crystals can form.

Anyone else out there have access to a West Mountain CBA battery tester and know how to use it? If you've ever used a CBA to test automotive batteries, do you have test results you could share?


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Unread 05-19-2017, 03:41 PM
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Re: automotive battery discharge curves

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Originally Posted by Ether View Post
It's not used in the winter.



Yup.



If you drain a battery way down and let it sit that way, sulfate crystals can form.

Anyone else out there have access to a West Mountain CBA battery tester and know how to use it? If you've ever used a CBA to test automotive batteries, do you have test results you could share?


I have one and know how to use it...I just don't have an extra automotive battery sitting around. sorry
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Unread 05-19-2017, 07:02 PM
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Re: automotive battery discharge curves

This was originally a much longer post.

The short version is: It sat too long, that is why it went dead. It is injured but not fatally. Charge it and use it, it's fine.

Longer version: Any SLA battery will be unable to start an engine after 6 months from self-discharge alone. Add a 50 mA parasitic draw and 3 months is too long. 3A for 3h is 9 Ah, not conclusive (and mostly irrelevant: It needs to deliver 200A for 5 seconds). High-current testing is at triple the Ah rating of the battery (about 80 Ah so test around 250A), voltage must remain about 10.5 V after 15 seconds. If it does, Good Battery.

During that high-current test, how the voltage falls is also indicative: It drops to maybe 11V right away, then nearly holds that voltage for the remainder of the test. A fast drop below 10.5 shows a bad battery.

Modern battery testing is based on internal resistance: Bring it to any auto parts store and have them test it.

About the CBA: I have one, but never felt a need to characterize a lead-acid battery. This is relatively old technology, it's been done. And a full-discharge does hurt the battery, even if just a little.
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Unread 05-19-2017, 09:23 PM
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Re: automotive battery discharge curves


Thanks for all the feedback everyone.

If it were my battery, I'd surely just charge it up and use it (around town, that is)

But I'm not going to tell my non-techie friend that his battery is good until I can convince myself it's good, especially since he uses it in a vehicle for travel to low-population areas. Null Hypothesis is "battery is bad".

Internal resistance tests and high-amp short-duration load tests have high Alpha risk (search CD for examples of batteries with a bad cell).

spec'd RC rating is 140 so Ah is about 58 @ 25 amps down to 10.5 volts.

I found a "generic" graph of lead/acid discharge curves with 0.05C discharge curve.

0.05*58 = 2.9 amps

I have a 3 amp @ 12v load available so I used that. Pretty close.

After 4 hours @ 3 amps, the battery looked a bit lower than the 0.05C curve (I had to terminate the test at 4 hours because I was no longer available to monitor the test and did not want to leave it running unattended, for safety reasons).

According to the 0.05C curve, the voltage starts to drop more noticeably at around 11 hours. Had I been able to run the test out to 11 hours, that might have been more useful.

After 3 more days of charging I could not get the specific gravity above 1.225 in all 6 cells so I wound up advising my friend that I could not assure him that the battery was good.

I'm still interested in seeing test results from CBA tests of automotive batteries.



Last edited by Ether : 05-19-2017 at 09:42 PM. Reason: corrected some numbers
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Unread 05-19-2017, 10:18 PM
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Re: automotive battery discharge curves

Since the truck doesn't run al winter a good battery maintainer would be a cheap investment.
I got one of the maintenance chargers for my vintage '79 Z/28. It never did do well, in The winter....
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Unread 05-20-2017, 11:53 AM
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Re: automotive battery discharge curves

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Originally Posted by Tom469 View Post
Since the truck doesn't run al winter a good battery maintainer would be a cheap investment.
He knows that now.

What inexpensive make/model do you guys recommend, solely for leaving connected to a pre-charged battery all winter long?


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Unread 05-20-2017, 02:11 PM
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Re: automotive battery discharge curves

Everyone,
I have not used a CBA to run a discharge on an automotive battery although I would expect a similar curve to those published for our robot batteries, fudged for the greater Amp Hour capacity. In this case, I think the 65 in the part number is 65 AH. The CBA would report 65AH just as it reports ~18 AH on robot batteries. The other numbers in the literature are marketing hype. The question, I would first ask is if the battery is disconnected during the winter or left connected in the vehicle. if so, modern automotive electrics have multiple devices that are on the battery buss at all times. Sensors for lighting, radio, seat belts and airbag control and transmission to name a few. If left connected for a long period, that load will drain the battery. I have used the CBAIV up to 7 amps as I remember but that test would still need to run overnight to get a full curve. Just be sure to set the shut off voltage at no less than 8 volts to prevent long term damage to the battery.
It has been my experience with modern battery design that sulphation is not the issue it had been in the past. (Suplhation will affect capacity/cranking amps but not self-discharge) What is more of an issue is the particulate that forms and drops to the bottom of the battery. When this sediment reaches high enough it will short out the bottom of the plates and prevent charging. Deep discharge/marine batteries have a larger space between the bottom of the case and the plates. For AGM technologies, this is not an issue as the electrolyte is not an open liquid.
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Unread 05-20-2017, 02:33 PM
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Re: automotive battery discharge curves

Quote:
Originally Posted by Al Skierkiewicz View Post
.. I think the 65 in the part number is 65 AH.
I believe the 65 means this is a group 65 size battery, 12L x 7.5W x 7.6H (inches).

A more informative figure of merit might be reserve capacity, typically the time required to discharge the battery at 25A current draw. Per the battery specs linked above that would be 140 minutes.
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Unread 05-20-2017, 02:40 PM
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Re: automotive battery discharge curves

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A more informative figure of merit might be reserve capacity, typically the time required to discharge the battery at 25A current draw. Per the battery specs linked above that would be 140 minutes.
Yes, that's the spec I used to get the RC for the following calculation:
Quote:
Originally Posted by Ether View Post
spec'd RC rating is 140 so Ah is about 58 @ 25 amps down to 10.5 volts.
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Unread 05-20-2017, 08:52 PM
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Re: automotive battery discharge curves

The thing to do is to take it to an auto parts store and have them use their really expensive tester and see what the results are.

Trying to test the RC isn't what really matters anyway. That is to give you an idea how long you can drive to get home after the alternator has failed before the computer will start to brown out. The problem is that standard was developed back in the day when cars came with 37a alternators and the only thing they had to power to keep the engine running was the coil. Nowadays with all the computers, electric fuel pumps, injector and other engine management draws it can take 25a to keep the engine running instead of the 1a it took in the 60's and 70's. The important part is can it put out 150-200a for 20-30 seconds and do that several times with a rest between them.


If you really don't want to take it to a parts store with the proper tester or no one in your area has shelled out thousands to buy a tester your hand held unit can give you a good idea. Make sure it is charged until the battery voltage is 12.66 after resting for 24hrs. Connect your 100a tester, apply the load for a full 15 sec. Let the tester cool for a minute and then repeat that process at least 5 times and take the reading on the inferred CA from the tester's scale at end of the last load period. If you've got at least 300a it will be good for at least another season.

The Motorcraft batteries are some of the best on the market so I'd certainly recommend using it rather than taking a shot at a new battery. Fact is there are some batteries out there who are known for just giving up the ghost when you run into the store. The problem is you never know anymore if you are going to get one of those brands because the sellers now tend to jump back and forth between mfgs frequently.

On my 03 Marauder I didn't replace the battery until 15 and since that is not my daily driver for some reason it will end up sitting from Jan until Apr. So that battery was drained to the point where it was showing like 2v several times and it always came back. In fact it was still starting the car fine but at that age I didn't want to take a chance. On our Mountaineer that was my wife's daily driver at time I replaced it at 7 years again just because.

If it has the big engine for the E-Series stripped chassis that means V10 and those are very easy to start. The more cylinders you have the slower the cranking speed needed to light it off and reach the minimum self sustaining RPM which is like 450-475. The Motorcraft PMGR starter as used in those vehicles is very efficient with normal cranking amperage in the 100-125a range.

If it were me I'd just run with that battery and invest in a jump starter box if the plan is to get away from civilization with the vehicle, or invest in a second battery and wire it up to power any interior items that might be used when parked, and install a starter rated relay to be able to do a self boost with the push of a button. Very common on the motor homes of the 70's and beyond. There are lots of nice automatic dual battery relays on the market that sense the voltage of each battery and then engage the relay when it senses a voltage high enough to charge a battery.

For a charger/maintainer I have only used the Battery Tender brand. Costco and Sam's club often have them at reasonable prices. One nice thing about those are that they come with two interchangable sets of cable ends. A set of ring terminals to permanently install in a vehicle and a pair of battery clips in case you want to use it on something else.

Last edited by Mr V : 05-20-2017 at 09:14 PM.
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