How to check for a bad battery

[KevinE]

I think I have a bad battery in one of my banks. I am running 2 pairs of 6v golf carts split into 2 banks. All the batteries were replaced at the same time but one of my banks drains much faster then the other. I generally run with my selector switch set to both (the engine battery is not on this loop). It is not unusual to see 11.9v on one bank and 12.2 on the other which I think is quite a difference. Should they not be roughly the same? I use a xantres link 2000 battery monitoring system.

My question is how do I find the bad battery. Do I need to use a battery tester (not a volt meter) or will a hydrometer give me the information I need? I'm thinking if I have to I would rather buy a battery tester for $50 or $60 then haul them out of the boat and tested at a battery place.

Thanks in advance,
Kevin

[btrayfors]

Kevin,

You are right to run the two battery banks combined as one. This is the favored practice, because it gives you increased AH availability, faster charging, lower drains on the batteries, etc.

It is possible, of course, that one or more batteries may go bad. The best way to identify the bad battery is with a combination voltmeter test on each 6v battery and a specific gravity test on each cell. For these tests it is best to have the battery banks separate and to take off all loads and let them "rest" a bit.

One thing that seems curious: if you are running the banks in the "ALL" or combined position, both banks should measure the same voltage. If they do not, I would want to check the connections. Also, be sure that the wiring between the two banks is of adequate size.

Bill

[Rick]

The caveat for any benefit in "resting" a battery is that the battery must be fully charged before left to stand for 24 hours and, even then, it is best to "stand" at float voltage specified for a particular temperature. Otherwise it is NOT necessarily a benefit.

Your complaint is typical of battery installations where each battery in the combined bank does not "see" exactly the same parasitic resistance to the load AND charge source. Such parasitic resistance total "lumped" value consists of all connections, terminals, cable resistance and any fuse or switch resistances. Careful "mapping" out of your cable installation will/should reveal any descrepancy between all of the batteries. Judicious rewiring will fix the problem.

Your Link 2000 should at least show a descrepancy between the terminal voltages of the two "banks" when they are combined and either receiving a high charge current or delivering a large current to a constant load. If ballanced then both banks should deliver/receive the same current and have the same terminal voltage. After that check you will need to use a DVM to verify individual battery terminal voltages made DIRECTLY at the terminals of each battery.

After making the parasitic resistances equal for each battey you will need to completely charge and then perhaps equalize the batteries in order to make the internal battery resistances the same before expecting a system response to show equal voltages from then on.

If, after equalizing the batteries three times in succession (with at least 12 and preferably 24 hours between equalization cycles until the specific gravities of each cell is the same) the batteries' terminal voltages are not equal with a constant heavy load then the "odd" battery having a different low voltage may be suspected as being bad if the voltage variation is significant (that depends upon the voltage variation and the current).

[KevinE]

Bill,
Is it possible that my battery isolator has a problem with it, hence the different voltages in each bank ? I was looking through the previous owner's records, shortly after he purchased new batteries, the regulator went. So now I am wondering if that was what damaged my batteries or possibly the isolator?

I havent' been down to my boat yet but my first check will be to check the specific gravity, then go from there.

Kevin

[GordMay]

As Rick noted, among the standard battery texts are:
”Storage batteries” (4th ed), George Wood Vinal,(John Wiley & Sons, Inc., New York, 1955).
”Primary batteries”, G.W. Vinal, (Wiley, NY, 1950).

From “Optimizing Battery Performance” ~ By Bernie Benz

A Reaction from George Wood Vinal:
<quote>
“...Next, the charge side of the equation. Modern automotive storage batteries will readily accept high charging rates, hundreds of amps, well inexcess of any on-board charging capability. The critical problem is determining the point at which the reaction is complete, i.e. a fully charged battery. Charging current forced through the battery beyond this point is most destructive! This excess energy decomposes the water in the acid-rich electrolyte, producing both internal heating and the emission of a dangerously explosive mixture of hydrogen and oxygen. This loss of water further concentrates the acid solution which drastically shortens battery life. The point of full charge, commonly called the gassing point, is very much dependent upon battery temperature. Figure 1, a typical Gassing curve, shows this non-linear temperature dependency of battery terminal voltage at full charge.

Thus, optimum battery performance, as measured by both maximum stored energy availability and maximum battery life, requries a regulated system that will charge the battery at the maximum current capacity of the alternator, up to, but never above the gassing point voltage, and then shut down until battery terminal voltage again falls velow the gassing point, as determined by actual battery temperature...”
<end quote>

More: http://www.gglotus.org/ggtech/batter...battcharge.htm

[btrayfors]

Kevin,

I don't like battery isolators because: (1) they produce a voltage drop which, unless compensated for, results in undercharging; (2) they have diodes which can and do fail; and (3) they're unnecessary in a "simplified" one-bank installation.

By "simplified", I mean a combined house battery bank (I always leave my house battery switch in the "ALL" position). All charging sources (battery charger, alternator, generator, solar, etc.) are then led directly to this combined house battery bank. In other words, you only charge the house bank.

The engine starting battery is kept completely separate, and is kept charged through an Echo Charger. This device senses voltage on the house bank and, whenever it exceeds 12.8VDC it will put a charge up to 15A into the engine battery. It's completely automatic, won't overcharge the engine battery, and so far as I can tell doesn't emit a lot of RFI.

In sum, I've been much happier since I removed my battery isolator, and rewired all charging sources to charge the house batteries.

Bill

[hellosailor]

Kevin-
Moving back to your basic question and away from the area of religion and battery management <G>.

You can test the batteries with either a simple digital multimeter (DMM) found for $10 in WalMart or Target, etc. or a battery hydrometer. If you get the hydrometer, get the one about a foot tall for $10 not the pocket-sized one, it is easier to read. And then immediately buy a piece of PVC pipe or something similar to store it in--because without a case, somehow they always get smashed up.<G>
I would go for the $10 DMM first. The $60 one from Radio Shack may be better, the $200 Fluke brand will be real good, but the $10 one will be good enough for this. Caveat: Read the manual. DMM's typically may be accurate on the DC scale to a limit of 1/2% (Fluke) to 2% ($10) and when you are looking at a nominal 12.6V battery and looking at tenths of a volt--the extra accuracy be a difference of 0.3V on the scale.
So just know, if a "perfect" six volt battery (3 cells of 2.2V each, 6.6V nominal when new and charged and rested) reads one or two tenths off...that can just be meter error. Most meters also can be off by 2 or 3 on the least significant digit, so again "6.6" can mean anything from 6.9 to 6.3 is what they are really measuring.

For your purposes, charge the batteries. Then separate the banks and let them sit overnight for the float charge to equalize and the acid to distribute itself around evenly. Now, measure the voltage on each battery.

If they all say 6.6, that's fine, whatever the number is. But if one battery is showing lower voltage than another--which they usually will--or if one BANK has a lower combined voltage than the other (say, one is 12.6 volts and the other 12.4) then the parallel banks will form an active circuit and the lower one drains the higher one down. This continues until both are drained, and this is why parallel banks are a bad idea when they are just sitting around not powering a heavy load (which drains them both and to some extent hides the problem.)

You can do the same thing with the hydrometer, comparing the specific gravity of each CELL in each battery to find and confirm a weak cell and weak battery. But to me, a hyrometer is mainly a good way to distribute acid around and ruin clothing. I prefer the meter. The hydrometer will tell you if someone screwed up the electrolyte but again, just knowing the battery voltage is wrong should be good enough.

Before you even start checking batteries, you might want to just visually inspect all the cables and fittings. If they are all "equal", OK. If not, they should be made equal. Equal sizes, equal runs, equally clean and tight--or the voltages will not be equal. Or at least, clean and tight and damned close to equal.<G>

If everything is "close" and your DMM shows one battery is different from all the others...odds are, that's a bad battery and it is that simple. Replacing it is NOT really any use--because then you'd have one new battery and three used (lower voltage) ones, making the two banks uneven again, among other things. If that was the case I'd keep the two best batteries as one 12V bank, get rid of the two weak ones, and move on.

To religion and the philosophy of battery systems.<G>

[GordMay]

hellosailor:
Open Circuit Voltage (OCV) is, at best, a relatively poor indication of a battery’s State of Charge (SOC). A 2% voltage variation might represent a 33% variation (or 25% difference) in SOC.
ie:
12.7V = 100% SOC @ 77 deg. F.
12.4V = 75% SOC @ 77 deg. F.
12.2V = 60% SOC @ 77 deg. F.

A cheap DVM or DMM is most useful for indicating:
- On or Off (something or nothing)
- More or Less (larger or smaller magnitude)

[btrayfors]

While agreeing, in principle, with the thrust of Gord May's comments re: SOC and multimeters, I take exception to his conclusions.

First, with AGM and gelled batteries, open-circuit voltage is the ONLY practical way for most sailors to know the state of charge (SOC) of their batteries. You can't do specific gravity measurements with a hydrometer on these batteries, and most cruisers can't afford the $500 plus cost of a sophisticated device to measure the internal resistance and calculate CA, CCA, SOC, etc.

Second, although Gord is right about a 2% difference representing a BIG difference in SOC, a simple multimeter -- ONCE CALIBRATED -- can be very helpful in estimating SOC for batteries which are in good condition. For batteries resting and in relatively good condition, open-circuit voltage (once the "float" charge has been bled off) is an excellent indicator of SOC. And, as noted above, if you have AGMs or gelled batteries you really don't have another choice.

By paying careful attention to the indicated voltage of the house battery banks, whether using a digital voltmeter, multimeter, or even an analog voltmeter with an expanded range, you can get a pretty good idea about SOC as you cycle through the daily charge/discharge functions. When readings begin to move outside the normally observed range, you can get an indication of something amiss.

How can you calibrate your meters? There are at least three ways which could be useful.

1. don't do anything physical to the meter at all. Simply observe the readings to determine what is the normal "fully charged" reading, and use that as a reference point;

2. open up the back of the multimeter, use some sort of standard voltage reference, and adjust the little pot (or pots) until the meter reading matches the reference voltage. I've sucessfully used new 9-volt alkaline batteries (which read about 9.515V), new 1.5V alkaline "C" batteries (which read about 1.56V), fully charged and rested house batteries (which read about 12.6 to 12.7V), and reference voltmeters known to be accurate.

3. send the meter to the manufacturer for calibration (expensive and not always rewarding).

Gord's main point -- don't always believe your meter readings -- is correct. You gotta know something about the meter's accuracy. And, you've gotta know something about the condition of the batteries, since bad batteries sometimes exhibit high voltage and even if their SOC is high have reduced capacity (ability to deliver energy).

NB: for the purpose of measuring house battery voltage, one is often only interested in a relatively small voltage range, so it's important to know something of the meter's accuracy over that particular range, not the whole range. One really doesn't care if the meter can measure, e.g., 90V DC accurately...only that readings in the 11-15V range (if you have 12V battery banks) be accurate.

Bill
WA6CCA
S/V Born Free

[hellosailor]

Gord-
You are right, but as Bill says, and as I thought I had made clear, the DMM readings are sufficient for you to COMPARE two batteries, or MONITOR one system.
In the case of comparing batteries--which was the primary goal here, to find a bad one--the accuracy of the meter does not matter. The only relevant point is "Do all four measure THE SAME?" in this case, 6.6V each if 100% good. If one of them is "off" by several tenths...that's the odd fellow. And I'm guessing three will say something like 6.3 and the fourth will be around 5.9 or 6, indicating it can really pull out the paralleled battery bank.

On battery condition itself...Nominally a "full" wet lead battery is 6x2.2V cells, or 12.6V. With a hot charge that may read as high as 13.4-13.6 right after charging but it will be back done near 12.6 after 24 hours of resting. "Near" will vary with the chemistry and condition, sure. But here again the question is not whether the number is actually ACCURATE (which would be nice) but what the number is.
Most lead batteries will drop about 0.1VDC per 10% of charge loss over their working range, as per your numbers, and by the time they have lost one full volt--they will be effectively dead. (They'll keep something glowing down to maybe 10V but at that point, even the indicator lights give it up.<G>) The point here is that a DMM can indicate RELATIVE charge state, every time you've lost 0.1VDC you've lost 10% or more of your stored power. 10% if the batteries were perfect, more as they've aged. But if I know my battery was 12.6 volts when I first bought it, and this season I can't get itover 12.4V, I know I've lost 20% of capacity--regardless of the actual numbers. When I'm down to 12.2V, I know I've lost 40% of original capacity, and when I see 12V, that means I've lost 60% and probably should be thinking about new batteries and whether I've been taking care of them.
My 12V may be 12.2 or 11.7 in reality...which we both agree would be nice to know. But just knowing the relative drop and comparative voltages, hey, for ten bucks instead of two hundered? That's a good start.<G>
Funy thing about Fluke meters, even the old used ones sell for top dollar on eBay and all.<G>

[GordMay]

As I said, A cheap DVM or DMM is most useful for indicating:
- On or Off (something or nothing)
- More or Less (larger or smaller magnitude)

As has been stated, this “More or Less” information can be useful (though imprecise) in comparing different batteries, or a single battery’s performance over time.

[little boat]

excuse me for not reading the entire thread thoroughly, but this kind of voltage talk bores me, although it is kind of amusing people actually talk about 12.? vs 12.?...its not a conversation that would hold my attention for more than 12.whatever seconds..want to find out which/or if one battery is dead? go to the store to buy something? why?
charge them both then disconnect one. try to use it for a day or two without charging. repeat with other. verdict in.

[Alan Wheeler]

Yeah but use it for how long?? to do what??? and how much longer in time will it keep doing that for??? and then, will it charge correctly the next time without your multi-stage charger boiling the snot out of it and the other batteries you have connected.
You can not answer any of those questions without knowing the exact state of the battery in question. I wrecked two perfectly good batteries because I put one in that was significantly down, but still usable. It did not allow the charger to come out of bulk charge and boiled all three 200Ahr batteries. That was an expensive lesson.

[hellosailor]

Wheels, I argee with you. "I wrecked two perfectly good batteries because I put one in that was significantly down, but still usable." One of the extra perils of having parallel battery setups, or not having the charge sense lead switched to each bank as the charger itself is switched.

Does anyone make battery bank selector switches that have an extra set of leads to switch the sense wire?