DC/Battery Basic Questions

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DC/Battery Basic Questions

Postby TimC » Sun Sep 01, 2019 8:27 pm

I've learned a lot about 12 volt since building my TD, but, I still am not real confident about battery basics. This post isn't so much relating to teardrops, but, battery voltages in general.

I've had batteries on my TV and my wife's car both go dead at random times in the last week. I purchased a Noco G7200 just before the first battery quit. First, after giving it a full charge I let it sit overnight and took a volt reading after resting and it came up 12.43. Not especially strong but it is an original from 2015. After a repair cycle which took about five hours I gave it another full charge, let it rest for several hours and it read 12.7. After a night's rest it had settled at 12.57. An improvement but I'm not sure if that's significant for a five year old battery. Anyway, I am cautiously happy that it improved the full voltage reading. Would it have improved the same amount with a couple full charges with a regular old school battery charger? I'll never know.

One question I have about the rest period advised before reading the voltage... I've read wait a half hour and others say wait overnight. What do you think is the optimal rest period? And, while it is resting is it necessary to disconnect the battery from the vehicle? If not, are connected electronics which draw constant power, and, opening the door to pop the hood latch creating a significant difference when all the interior lights and parking lights go on and off?

After my wife's vehicle battery chose not to start the car I repeated the repair process after a full charge. I did not have my multi-meter to get a before repair cycle reading. I hope that I can get those two batteries through this winter and plan on replacing whichever one shows the weakest readings during any cold snaps. Probably will be my wife's for obvious reasons. My wife drives for about ten minutes after each start. I understand this doesn't give the alternator enough time to make up for the starting boost. I am going to try to keep both vehicles on a charger every other weekend to make sure they are always topped off.

I have always wondered how long it will be before the car companies start installing solar trickle chargers on the roof of vehicles. Of course, that might tick off the battery manufacturers as I think it could extend the life of batteries significantly. What do you think?

Tim
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Re: DC/Battery Basic Questions

Postby John61CT » Sun Sep 01, 2019 10:21 pm

Are you talking about stock automotive Starter batteries only?

Deep cycling House banks for running fans, lights, screen gadgets, fridge etc

is a completely different category of product and care regime, and

the two really should nit be mixed, unless you have **very** light needs in Ah/day **and** only go off grid for a night or two at a time.
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Re: DC/Battery Basic Questions

Postby Bill n Robi » Mon Sep 02, 2019 7:03 am

Only true test of a battery is a hydrometer.

Even a "dead" battery will have voltage - but no amperage, which is the power to do things.

The rest period between charging is because the plates are standing in a bath of acid with gaps between them. Charging puts voltage cross them but the path ends up just adding ions to the top of the plate - shortest path between the + and - and when it rests the charge migrates to the entire surface of the plate which is why your voltage changes over night. Things like sulphication, contamination, corrosion, over heating while charging - all are factors to battery life.

oh and freezing and repeated lowering the battery to 10 volts or lower also shorten the life of batteries.
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Re: DC/Battery Basic Questions

Postby John61CT » Mon Sep 02, 2019 7:55 am

Bill n Robi wrote:Only true test of a battery is a hydrometer.
Hydrometers are very useful, but that statement is categorically false.

Many use sealed chemistries these days anyway.

A hydrometer tells you SoC% accurately, but nothing about residual capacity, the definition of SoH%

EoL - for deep cycle and standby batteries - is defined by SoH, industry standard cutoff when Ah capacity has fallen to 70-80% of what it was when new, which BTW very rarely matches the mfg rated 20-hour Ah capacity.

Only a precisely timed discharge test with a CC controlled load will accurately reveal SoH to properly make a keep/discard decision.

Owners who keep using a House bank past that point risk unexpected sudden failure, even possible fire or explosion.

Starter batteries on the other hand, can be much more easily and quickly tested for their CCA output, using garage-style load testers.
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Re: DC/Battery Basic Questions

Postby TimC » Mon Sep 02, 2019 8:53 am

Thanks all. I appreciate the technical answers as it pushes my learning about this to higher levels. Maybe some definitions of the acronyms I just read would be helpful. I understand "EOL" :)

I am asking specifically about starting batteries. As I stated this is not really about my TD battery bank. I understand, at least on a pedestrian scale, that there is a fundamental difference between a solar battery bank and a starting battery. And, I know there are probably better, more professional ways to check out my batteries, however, both batteries are buried in the engine compartment and I will never remove them just to check with my hydrometer. At least until I can't get them to recover with a repair cycle on the charger and I'm about to make a decision on replacement. More of a confirmation that it is "dead" and I'm making an informed decision to spend more money.

So, I guess my questions still stand. After a full charge, is there a significant difference between a rest period of an hour and an overnight rest? Is there a significant difference in that I do not disconnect my battery for the rest period (phantom power draw)? And, finally, is there a significant impact on my rested battery by opening a door to pull the hood release (seeing that all my interior lights will cycle as well as parking lights)?

Significant being the operative word...

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Re: DC/Battery Basic Questions

Postby RJ Howell » Mon Sep 02, 2019 10:08 am

TimC wrote:
So, I guess my questions still stand. After a full charge, is there a significant difference between a rest period of an hour and an overnight rest? Is there a significant difference in that I do not disconnect my battery for the rest period (phantom power draw)? And, finally, is there a significant impact on my rested battery by opening a door to pull the hood release (seeing that all my interior lights will cycle as well as parking lights)?

Significant being the operative word...

Tim


Simple answer "Yes/Yes/Yes". Long answer, simplified: Yes/depends on draw/slight

Battery should void of any gains or draws. Voltage readings after a 24hr rest is the standard most use. You can get a good feel of where it is going after 4hrs.
This is a very basic SOC Voltage chart to get a feel of what is happening;

159336

What you see from even this chart is you can get a voltage reading on a 0% SOC battery.
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Re: DC/Battery Basic Questions

Postby John61CT » Mon Sep 02, 2019 10:27 am

Google for acronyms, maybe with

electric
battery

or other narrowing keyword.

If NG, then ask.

Significant is an opinion. Mine is that battery is past EoL and should be replaced.

But, to answer your actual questions:

The voltage delta between just-charged and fully rested varies according to how high the current was, aka C-rate.

Some batteries can take over 48hrs to settle after high amps, but after say 3-6hrs much less significant.

Obviously, the current rate of your parasitic draws determines their significance, easy to measure with an ammeter, essential gear. The other factor is time drawing those amps.

A brief draw of a few light may drop voltage a fair bit, especially with a high "surface charge effect" when getting to true vendor-spec 100% Full.

But that has nothing to do with lowering SoC by much, the two are only loosely related.

Resting voltage means the battery is fully isolated, fitting an isolation switch can be useful.

A "settings keeper" prevents resetting the ECU and losing radio stations.
Last edited by John61CT on Mon Sep 02, 2019 10:33 am, edited 1 time in total.
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Re: DC/Battery Basic Questions

Postby tony.latham » Mon Sep 02, 2019 10:30 am

So, I guess my questions still stand. After a full charge, is there a significant difference between a rest period of an hour and an overnight rest?

My experience with our teardrop is that 45 minutes gives me a good look-see into where we're at. I'm sure 24 hours is better but not practical camping. If you can wait 24 hours with this car batt, do it. But read on.


Is there a significant difference in that I do not disconnect my battery for the rest period (phantom power draw)?

If there is any draw on the battery, a voltage check isn't going to work.

And, finally, is there a significant impact on my rested battery by opening a door to pull the hood release (seeing that all my interior lights will cycle as well as parking lights)?

Your parking lights come on when you open a door? If so leave the hood popped and check it after 45 minutes.

But the bottom line is that voltage isn't a measure of how much energy is in the battery. It measures the percent of potential. Let me put it this way, if the battery can only hold 50% of its original capacity, and you fully charge it, I think it'll still read about 12.6 volts.

You need to test it with a battery load tester:

Image

I suspect your wife's batt won't pass the test.

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Re: DC/Battery Basic Questions

Postby TimC » Mon Sep 02, 2019 1:11 pm

Thanks again.

Yeah Tony. Just rechecked my wife's battery and it reads 12.3 after about 15 hours resting. That's without isolating the battery. I'm thinking at most I can limp along until late fall, then time to renew.

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Re: DC/Battery Basic Questions

Postby John61CT » Mon Sep 02, 2019 1:44 pm

Yes a useless 99.99% dead battery can still show a full 100% SoC according to hydrometer or voltage
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Re: DC/Battery Basic Questions

Postby RJ Howell » Wed Sep 04, 2019 7:30 am

TimC wrote:Thanks again.

Yeah Tony. Just rechecked my wife's battery and it reads 12.3 after about 15 hours resting. That's without isolating the battery. I'm thinking at most I can limp along until late fall, then time to renew.

Tim


Just by the voltage chart, 50% SOC at 15hr rest. You can see where this is going.. The remaining 9hrs of rest could very well put you under 12.1. You may wish to re-think waiting to replace.. Just MHO.
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