Testing 6V Deep Cycle Batteries

[wgriswold]

I had my 6V batteries load tested at an auto parts store and they failed. They were tested while connected in series and so were tested as a 12V system. I took them back to Costco and they were replaced.

Now I am wondering if the typical load tester at the auto parts stores are capable of testing a deep cycle battery properly. Maybe since a deep cycle battery is designed to provide low amperage and not to start engines with large amperage draw the tester reported defective batteries when they were good. The kid doing the testing barely knew how to hook up the tester and was no help with this question.

Does anybody know about this?

[westend]

wgriswold wrote:

MrWizard wrote:
testing at 12v is correct
if you think golf cars, don't pull a heavy draw from batteries
you need to do some reading

There are three types of batteries we all see. Starting batteries such as are in our cars. Hybrid starting/deep cycle batteries that are used in Rv's and boats and perhaps other places. And deep cycle batteries, usually 6V GC batteries in our RV's.

These three types are designed differently for different purposes. There must be some reason deep cycle batteries are not designated starting batteries, but I have to admit I don't what it is. The demands of a golf cart must be different than the demand of starting battery.

I suspect that an instrument designed to load test a starting battery may fail to properly test a GC deep cycle battery and that was my original question.

Thanks, everyone, for your help. It seems that the consensus is that an auto parts store tester is not the appropriate tool to test a GC battery.

I originally was questioning the batteries when I was on a trip and turning on one light made others dim. I had not seen this before and so was questioning the batteries. I had a volt meter and the voltage was OK, but my specific gravity tester was at home, so off to the auto parts store I went.

After I got home my battery disconnect switch failed and I suspect that may have been the cause of the dimming as resistance increased across it during its failure process.

A 6V GC2 battery has thicker plates, a larger electrolyte chamber, and a more robust connecting grid. There are also differences in paste configuration and alloys but I can't really speak to that. Basically, it is engineered for a longer, more constant, draw period and to endure more shock when in motion.

A simple load tester (as used in the parts store) in the hands of someone experienced, may turn up a problem with a pair of 6V batteries. The more general problem of the load test being accurate, is the complication of who's doing the testing. Most counter personnel are barely qualified to sell the correct wiper length. The auto parts business doesn't nurture older, experienced and more qualified, personnel.

I'd assume that the exchange at Costco did not involve any testing. A look at the date stamps on the battery cases was all that was necessary? At least you found the source of your power problem. Limiting the charge through the failing disconnect switch would have meant another trip to Costco in the future.

[cpu266]

Get a hydrometer to test the condition of each cell. charge the batteries then let sit for about 12hrs and check with volt meter and hydrometer. check out these two sites - http://batteryuniversity.com and http://www.trojanbattery.com

[TurnThePage]

wgriswold, I think you figured it out. And until convinced otherwise I also agree that a battery tester at the local autoparts store is not the right tool for the job and likely will provide inaccurate results with genuine deep cycle batteries.

[mike-s]

pnichols wrote:
6. Correct on over-charging or deeper discharges of other cells in the case of a shorted cell anywhere in a series string. However, isn't a shorted cell way more rare than an open or high resistance cell (that's been my experience, BTW)?

You couldn't imagine how a bad battery could cause harm to a good one. I gave an example. What's probability got to do with it?

[wa8yxm]

Pnichols.. You assume the owner knows as much about batteries as you do.. I make no such assumption.. You see. I follow politics on another "Forum" and .. Well I've learned never to assume another person knows how to tie their own shoes. let along how batteries work or how to test 'em.

[pnichols]

In response to comments in a couple of posts above regarding two 6 volt batteries in series:

"First: IF one battery fails "Shorted cell" now the other battery is being charged at a far too high voltage.. That causes damage"

1. Wouldn't that mean that after the next charging attempt the owner would notice that the resulting voltage of the series pair was still 2 volts low ... hence immediate action needed to check out the whole situation - before permanent/serious damage to the good 6 volt batterry?

"If the one battery fails "Open cell" or HIgh Resistance the other battery is not charged at all.Or not fully."

2. Wouldn't that mean that after the next charging attempt the owner would notice that the resulting voltage of the series pair was still low, so maybe no charging had taken place ... hence immediate action needed to check out the whole situation - well before permanent damage to the good 6 voltt battery from sitting around under-charged?

"If the older battery has LOST capacity then it may well end up reverse charged by the newer battery"

3. How could the good 6 volt battery reverse charge the bad battery backwards through the entire coach's (or exterior charger's) electrical circiutry?

"With batteries in parallel it's not as important they be identical"

4. IMHO it's the opposite of that, damage-wise ... with batteries in parallel one of the batteries with a shorted (or lowered resistance) cell can really, over a short time, damage the other good battery(ies). However from a capacity viewpoint, with batteries in parallel if one of the batteries has reduced capacity but still shows normal terminal voltage (I've had this happen more than once over the years) ... then the other good battery(ies) are not damaged - the total capacity of the bank is merely reduced by the under-capacity amount of the bad battery(ies). This is why one can make up a parallel RV battery bank of using batteries of different amp-hour ratings - they need not be all of the same capacity.

"We keep hearing how when one reaches full it will signal the converter to stop.. But the face is in parallel. When one reaches full THEY ALL REACH FULL"

5. Correct, but with more detail ... batteries in parallel are of course all connected together on the same cable and there can be only one voltage reading from this cable, hence the charger/converter "thinks" that is the voltage of the battery(ies) that it's charging and when this voltage reaches what the charger/converter "thinks" (according to it's design) is the right value, it stops with the charging. At that poinit all batteries will be full - including old ones at whatever capacity they are capable of according to their age and/or other factors that affect capacity over time.

"One simple and obvious example: if the bad battery had a shorted cell, then all other cells (including those in the good battery) would get overcharged when charging. In the other direction, it may be the case that the newer battery discharges faster than the old (lead acid batteries take a few hundred cycles to reach full capacity), so the cells undergo deeper discharges."

6. Correct on over-charging or deeper discharges of other cells in the case of a shorted cell anywhere in a series string. However, isn't a shorted cell way more rare than an open or high resistance cell (that's been my experience, BTW)? Even when a cell shorts, the reasoning in my comment in 1., above, may help to keep the good battery in a series string from being too badly damaged.

I'm still wondering if it's not an advantage having 6 volt batteries in series over 12 volt batteries in parallel - if one is astute/picky/experienced enough to know when only one of the batteries in a series string needs replacing because ... why replace all 6 cells (more money) in a series string when only 3 cells (less money) in a series string need replacing?

[wgriswold]

MrWizard wrote:
testing at 12v is correct
if you think golf cars, don't pull a heavy draw from batteries
you need to do some reading

There are three types of batteries we all see. Starting batteries such as are in our cars. Hybrid starting/deep cycle batteries that are used in Rv's and boats and perhaps other places. And deep cycle batteries, usually 6V GC batteries in our RV's.

These three types are designed differently for different purposes. There must be some reason deep cycle batteries are not designated starting batteries, but I have to admit I don't what it is. The demands of a golf cart must be different than the demand of starting battery.

I suspect that an instrument designed to load test a starting battery may fail to properly test a GC deep cycle battery and that was my original question.

Thanks, everyone, for your help. It seems that the consensus is that an auto parts store tester is not the appropriate tool to test a GC battery.

I originally was questioning the batteries when I was on a trip and turning on one light made others dim. I had not seen this before and so was questioning the batteries. I had a volt meter and the voltage was OK, but my specific gravity tester was at home, so off to the auto parts store I went.

After I got home my battery disconnect switch failed and I suspect that may have been the cause of the dimming as resistance increased across it during its failure process.

[MrWizard]

Let me make this as simple as i can

A 12v battery has six cells
They can be in one box
Or septated, three cells each ,in two boxes
All cells are in series
Each cell has too carry the same amount of current aka amps
If a single cell in the series is bad amps suffer
Didn't make any difference if the battery is one box or two boxes
It's one 12v battery

If you replace only one box
The cells in the older box won't carry the same amount, as the new cells in the new box, and that limits output amps, and recharge amps, and will limit the life of the new box
For the best life and most efficieny you replace both boxes, the complete whole 12v battery

This is all a moot point that has been rehashed many times
The op got two new 6v boxes, and now has a new 12v battery

[theoldwizard1]

All battery test start with the ASSUMPTION that the battery (or battery bank) is fully charged. It is worth the time and effort to check to voltage at the battery WHILE THE CHARGER IS OPERATING ! For a 12V lead acid battery, you need an absolute minimum of 13.4V to charge it. At such a low voltage it will take many hours to get a full charge.

[mike-s]

pnichols wrote:
Hmmm .... I'll have to think about that for a minute: In what way could a bad 6 volt battery in series with a good 6 volt battery in any electrical way, shape, or form damage the good battery?

One simple and obvious example: if the bad battery had a shorted cell, then all other cells (including those in the good battery) would get overcharged when charging. In the other direction, it may be the case that the newer battery discharges faster than the old (lead acid batteries take a few hundred cycles to reach full capacity), so the cells undergo deeper discharges.

For all batteries (i.e. more than one cell), any mismatch between cells will cause some to experience more extreme swings than others. Those cells will have shorter lifetimes. Hence the recommendation to not mix new with old.

[wa8yxm]

pnichols wrote:

Hmmm .... I'll have to think about that for a minute: In what way could a bad 6 volt battery in series with a good 6 volt battery in any electrical way, shape, or form damage the good battery?

There are a few ways plus the older battery can be damaged as well.

First: IF one battery fails "Shorted cell" now the other battery is being charged at a far too high voltage.. That causes damage

If the one battery fails "Open cell" or HIgh Resistance the other battery is not charged at all.Or not fully.

If the older battery has LOST capacity then it may well end up reverse charged by the newer battery This will not damage the newer battery,, but it will the older one. And again the new battery will not fully charge.

THis is why you should always replace BOTH HALVES of your 12 volt batteries at the same time.. Just think of the two paired, six volt batteries as ONE SINGLE battery all the time.. And treat them as such.

NOW: This only applies to batteries in series

With batteries in parallel it's not as important they be identical

IN parallel for example.. IF one has a higher capacity.. We keep hearing how when one reaches full it will signal the converter to stop.. But the face is in parallel. When one reaches full THEY ALL REACH FULL Each shares both load and charging current accordign to their capacity.. The perfect Commune.

[westend]

The auto parts stores serve a function--to test batteries that customers bring them to see if they're operable. An owner can do some testing at home if he has the right measuring gear, a meter and a hydrometer. The load testers at the auto parts stores may turn up a failing battery/-ies, also.

[MrWizard]

testing at 12v is correct
if you think golf cars, don't pull a heavy draw from batteries
you need to do some reading

cars used to have 6v systems and 6v starters
the change to 12v let car mfg use smaller wire for everything

you had a concern, you took the batteries for testing
costco exchanged them and gave you New batteries
No more concern, its a WIN !

[pnichols]

wa8yxm wrote:
Another thing:
WOuld you buy a 12 volt battery if half the battery was say 2 years old and the other half brand new?

NO and neither should you.

WHen you replace 1/2 of your 12 volt battery (one six volt) replace the other (It's mated partner) as well.

Hmmm .... I'll have to think about that for a minute: In what way could a bad 6 volt battery in series with a good 6 volt battery in any electrical way, shape, or form damage the good battery?

This seems like a different situation altogether from one bad cell in a 12 volt battery causing the whole battery to not supply enough voltage to be of any further use - such that you have no choice but to replace all of the cells ("the whole battery") at the same time.

If I had two 6 volt batteries in series in my RV and when, after charging both together, the voltage then was lower than full charge voltage, or the current delivery then remained lower than what it used to be when they were new - before paying for two new batteries I'd want to know if only one of the two batteries was bad.

The above would especially be what I would do in a situation of "early failure" in which one of the two 6 volt batteries went bad sooner than it should have such that I now had an unsuable 12 volt battery bank in the RV. IAW, why replace both 6 volt batteries in a series string when only one may need replacing?

A slight problem might be where to go to get a 6 volt battery tested? Some/most auto parts stores may not be able to do it. Probably outfits like Batteries Plus can check individual 6 volt batteries, however.