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Bad batteries or am I wrong in my calcs?

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NautiC25
Admiral

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Initially Posted - 03/02/2011 : 14:44:44

I'm a noob to amp hour ratings, but I've read up on it a bit and think I got it down. However, it seems unbelievable what happened to me last weekend.
I have two deep cycle batteries topped off by a 9w solar panel. I went for my first overnighter and ran a few lights periodically, and only 3 bulbs overnight. They were 2 festoons and an LED. When I woke up, the batteries were reading 5v.

Now, when I do my calculations, I get about 2 amps drawn for 8hrs straight, plus a few more at random times before we called it a night.

Am I correct that most batteries are rated around 170 amp hours?

For some reason, I just thought that having 2 batteries would run all the lights on the boat for at least 2 days.

1989 C-25 TR/WK #5894
Miss Behavin'
Sittin' in LCYC on Canyon Lake, Texas

Edited by - NautiC25 on 03/02/2011 14:49:10

Stinkpotter
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Djibouti
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Response Posted - 03/02/2011 : 15:24:13

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by NautiC25
 ...Am I correct that most batteries are rated around 170 amp hours?
<hr height="1" noshade id="quote"></blockquote id="quote">Nope. A Group 24 ("standard" size) Deep Cycle flooded battery is generally around 75-85 AH. Group 27s are maybe 95-100 AH.

Edited by - Stinkpotter on 03/02/2011 15:27:38

Davy J
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Response Posted - 03/02/2011 : 15:49:33

Were the lights still working at 5v? The 5v reading seems suspicious.

Do you have a solar charge controller? I've read that a solar panel can drain voltage "backwards" without some sort of protection.

How old are the batteries?

Edited by - on

NautiC25
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Response Posted - 03/02/2011 : 18:51:57

I didn't check voltage before. I'll check it when I go back.

The lights were out by 8am. I noticed them dim around 4 in the morning. I checked voltage with my multimeter when I got up and it read 5v.

The panel was disconnected all day and night while I was out. The specs said that it won't reverse when not charging. I don't have a controller since its only 9 watts. The lights worked great when I first turned them on. They just drained both banks in a couple hours.

Edited by - NautiC25 on 03/02/2011 18:54:53

Ryan L
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Response Posted - 03/02/2011 : 19:49:43

5v? Are you using golf cart batteries? If I'm not mistaken, most marine batteries operate around 12v (full charge is 12.7v) and are considered completely dead around 11.9v. Golf cart batteries operate at closer to 6v and must be wired together correctly with additional batteries to generate 12v of power. A great book about all this (very easy to follow yet filled with incredible information) is Don Casey's "Sailboat Electrics Simplified". That book can also be found in Don Casey's "Complete Illustrated Sailboat Maintenance Manual" which is just a collection of his most popular books.

BTW, if you are using golf cart batteries you might want to ensure your panel is not charging the batteries at too high a voltage. Also might want to consider an inexpensive charge controller or a diode to prevent discharge as was mentioned in another response.

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by NautiC25
 I'm a noob to amp hour ratings, but I've read up on it a bit and think I got it down. However, it seems unbelievable what happened to me last weekend.
I have two deep cycle batteries topped off by a 9w solar panel. I went for my first overnighter and ran a few lights periodically, and only 3 bulbs overnight. They were 2 festoons and an LED. When I woke up, the batteries were reading 5v.

Now, when I do my calculations, I get about 2 amps drawn for 8hrs straight, plus a few more at random times before we called it a night.

Am I correct that most batteries are rated around 170 amp hours?

For some reason, I just thought that having 2 batteries would run all the lights on the boat for at least 2 days.
<hr height="1" noshade id="quote"></blockquote id="quote">

Edited by - on

NautiC25
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Response Posted - 03/02/2011 : 20:42:50

Why do the both of u feel that 5v is not a possible reading coming from a completely drained 12v battery?

I just confirmed that my voltmeter and readings are fine. I just connected it to a good battery that's reading 12v and then a 5yr old drained battery reading 4.68v because its dead.

11.9 may be considered dead as u say, but that doesn't mean it can't read or be drained lower overnight.

Edited by - NautiC25 on 03/02/2011 20:47:33

Ryan L
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Response Posted - 03/02/2011 : 20:58:49

Because lights were still on (though dim) at 5v. That sounds more like a 6v battery. Lights would be completely off below 11.9ish if 12v battery.

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by NautiC25
 Why do the both of u feel that 5v is not a possible reading coming from a completely drained 12v battery?

I just confirmed that my voltmeter and readings are fine. I just connected it to a good battery that's reading 12v and then a 5yr old drained battery reading 4.68v because its dead.

11.9 may be considered dead as u say, but that doesn't mean it can't read or be drained lower overnight.
<hr height="1" noshade id="quote"></blockquote id="quote">

Edited by - on

Dave5041
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Response Posted - 03/02/2011 : 22:22:58

Regardless of whether you have 12V batteries or two sixes in series to become 1 twelve, there is a problem. One fully charged group 24 gives you about 40 amp hr. to play with, so a 20-30 AH draw shouldn't kill the battery. If you have 2 twelves, fully charge each with a real charger and check voltage; recheck the voltage after 1-2 days with the batteries disconnected. Did you buy the batteries? Unless you set out specifically to buy 6V batteries they will be 12V. If the batteries were with the boat they will be labeled 6 or 12. I can see how the 6 vs 12 issue got into the question, and you can look at your batteries and take it out. 6V batteries are connected - to + to make 12V at 1/2 of the sum of the AH ratings; 12V batteries are connected - to - to provide 12 V and the sum of the AH ratings of the batteries. Only about 1/2 of the available AH are used between charges to prolong battery life.

edit: a 12v battery can only read 5v as a transient value. 11.9v is as low as you should go, but a battery is still close to 50% then, 11v is about 5%, 10.5v is near 0% and any load will cause a progressive, low current drop toward 0v.

Edited by - Dave5041 on 03/02/2011 22:36:01

Ryan L
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Response Posted - 03/02/2011 : 22:28:47

6 vs 12 volt b/c he said his multimeter was reading 5v. If 6v I was trying to rule out that he may have misjudged the battery capacity and/or boiled off a lot of electrolyte from inadvertently charging at >12v.

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave5041
 Regardless of whether you have 12V batteries or two sixes in series to become 1 twelve, there is a problem. One fully charged group 24 gives you about 40 amp hr. to play with, so a 20-30 AH draw shouldn't kill the battery. If you have 2 twelves, fully charge each with a real charger and check voltage; recheck the voltage after 1-2 days with the batteries disconnected. Did you buy the batteries? Unless you set out specifically to buy 6V batteries they will be 12V. If the batteries were with the boat they will be labeled 6 or 12. I can see how the 6 vs 12 issue got into the question, and you can look at your batteries and take it out. 6V batteries are connected - to + to make 12V at 1/2 of the sum of the AH ratings; 12V batteries are connected - to - to provide 12 V and the sum of the AH ratings of the batteries. Only about 1/2 of the available AH are used between charges to prolong battery life.
<hr height="1" noshade id="quote"></blockquote id="quote">

Edited by - on

Ryan L
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Response Posted - 03/02/2011 : 22:30:46

Oh, you said you CAN see how 6 vs 12 got into the discussion. That makes a lot more sense... :)

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Ryan L
 6 vs 12 volt b/c he said his multimeter was reading 5v. If 6v I was trying to rule out that he may have misjudged the battery capacity and/or boiled off a lot of electrolyte from inadvertently charging at >12v.

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave5041
 Regardless of whether you have 12V batteries or two sixes in series to become 1 twelve, there is a problem. One fully charged group 24 gives you about 40 amp hr. to play with, so a 20-30 AH draw shouldn't kill the battery. If you have 2 twelves, fully charge each with a real charger and check voltage; recheck the voltage after 1-2 days with the batteries disconnected. Did you buy the batteries? Unless you set out specifically to buy 6V batteries they will be 12V. If the batteries were with the boat they will be labeled 6 or 12. I can see how the 6 vs 12 issue got into the question, and you can look at your batteries and take it out. 6V batteries are connected - to + to make 12V at 1/2 of the sum of the AH ratings; 12V batteries are connected - to - to provide 12 V and the sum of the AH ratings of the batteries. Only about 1/2 of the available AH are used between charges to prolong battery life.
<hr height="1" noshade id="quote"></blockquote id="quote">
<hr height="1" noshade id="quote"></blockquote id="quote">

Edited by - on

TakeFive
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Response Posted - 03/02/2011 : 23:10:16

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave5041
...a 12v battery can only read 5v as a transient value. 11.9v is as low as you should go, but a battery is still close to 50% then, 11v is about 5%, 10.5v is near 0% and any load will cause a progressive, low current drop toward 0v.
<hr height="1" noshade id="quote"></blockquote id="quote">
Sounds to me like he might have another unknown load on the battery in addition to the light bulbs. That would account for both the faster drain of the battery, and the 5v reading.

To OP: Was the battery completely disconnected when you read 5v?

Edited by - on

Davy J
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Response Posted - 03/03/2011 : 05:25:57

<blockquote id="quote">quote:<hr height="1" noshade id="quote">10.5v is near 0% and any load will cause a progressive, low current drop toward 0v.<hr height="1" noshade id="quote"></blockquote id="quote">
At 10.5v the battery will have 0% capacity, if the load actually lowered the voltage to 5v, the battery(s) are probably trash.

Edited by - on

NautiC25
Admiral

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USA
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Response Posted - 03/03/2011 : 07:50:19

I mentioned that the lights were out by 8am. They no longer glowed and the battery read 5v. So yea, NOW they're dead/trash as you said. I'm just trying to figure out if they were trashed BEFORE I used them since they got drained so easily from 3 bulbs being left on. I've never had to use batteries like this, so I'd like to know about how many lights I can run all night and not have to worry about them going dead.

I know for a fact I have 12v batteries. One of them I know is a West Marine SeaVolt. The other, IDK. The previous owner bought them and kept them on a trickle charger.

Edited by - on

Davy J
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Response Posted - 03/03/2011 : 09:10:10

<blockquote id="quote">quote:<hr height="1" noshade id="quote">I'm just trying to figure out if they were trashed BEFORE I used them since they got drained so easily from 3 bulbs being left on. <hr height="1" noshade id="quote"></blockquote id="quote">
Guessing they were, how old were the batteries?
I would think that a charged-up battery should be able to run three lamps overnight.

However, I bought one of these:
http://www.amazon.com/gp/product/B000EVWDU0/ref=pd_lpo_k2_dp_sr_1?pf_rd_p=486539851& pf_rd_s=lpo-top-stripe-1& pf_rd_t=201& pf_rd_i=B0031EL1UK& pf_rd_m=ATVPDKIKX0DER& pf_rd_r=0EJSXTT2MES5QAK6FSG2

I leave it plugged in all the time. I can see the voltage at a glance.
I was alarmed the first time I turned on the standard interior cabin lights, the voltage was dropping like a rock. Since then, I installed some battery operated LED pucks to the cabintop. I now only run the standard lights if I have too.

Edited by - on

TakeFive
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Response Posted - 03/03/2011 : 09:55:33

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Davy J
...However, I bought one of these:
http://www.amazon.com/gp/product/B000EVWDU0/ref=pd_lpo_k2_dp_sr_1?pf_rd_p=486539851& pf_rd_s=lpo-top-stripe-1& pf_rd_t=201& pf_rd_i=B0031EL1UK& pf_rd_m=ATVPDKIKX0DER& pf_rd_r=0EJSXTT2MES5QAK6FSG2

I leave it plugged in all the time. I can see the voltage at a glance.
I was alarmed the first time I turned on the standard interior cabin lights, the voltage was dropping like a rock. Since then, I installed some battery operated LED pucks to the cabintop. I now only run the standard lights if I have too.
<hr height="1" noshade id="quote"></blockquote id="quote">
lol, I bought the exact same thing a year ago (found it at Walmart for the same price as Amazon). I also was alarmed to see how much a little incandescent bulb could pull down the voltage, so I bought the $20 LED replacement cabin lights from Catalina Direct:
http://www.catalinadirect.com/index.cfm?fuseaction=product.display&product_ID=1569&ParentCat=339

Edited by - on

NautiC25
Admiral

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USA
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Response Posted - 03/03/2011 : 11:40:32

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by RhythmDoctor
 <blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Davy J
...However, I bought one of these:
http://www.amazon.com/gp/product/B000EVWDU0/ref=pd_lpo_k2_dp_sr_1?pf_rd_p=486539851& pf_rd_s=lpo-top-stripe-1& pf_rd_t=201& pf_rd_i=B0031EL1UK& pf_rd_m=ATVPDKIKX0DER& pf_rd_r=0EJSXTT2MES5QAK6FSG2

I leave it plugged in all the time. I can see the voltage at a glance.
I was alarmed the first time I turned on the standard interior cabin lights, the voltage was dropping like a rock. Since then, I installed some battery operated LED pucks to the cabintop. I now only run the standard lights if I have too.
<hr height="1" noshade id="quote"></blockquote id="quote">
lol, I bought the exact same thing a year ago (found it at Walmart for the same price as Amazon). I also was alarmed to see how much a little incandescent bulb could pull down the voltage, so I bought the $20 LED replacement cabin lights from Catalina Direct:
http://www.catalinadirect.com/index.cfm?fuseaction=product.display&product_ID=1569&ParentCat=339

<hr height="1" noshade id="quote"></blockquote id="quote">
Do those fit in the narrow slots above the port/strbrd benches? Would those cover the mounting holes from the factory lights?

edit: oops, just realized you have a 250.

Edited by - NautiC25 on 03/03/2011 11:51:05

Stinkpotter
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Djibouti
9081 Posts

Response Posted - 03/03/2011 : 12:01:21

A few more questions:

1. How was the electrolyte in the batteries? Was it covering the plates in all cells? If not, either the batteries have been overcharged or overdrawn (too many amps either way). Either one can cause warping of the plates, which causes arcing between them, which gasifies the electrolyte.

2. What was the wattage of the festoon bulbs? 20W draws 1.67 amps at 12 volts. The two bulbs alone might have drawn 25 AH overnight, plus maybe another 2 for the LED. That shouldn't run down two Group 24s wired in parallel, which should give you a safe 80 AHs.

3. With everything OFF (except your battery switch), if you remove one cable from your battery bank, with your meter set for let's say .1 amps, touching the cable and the terminal it was attached to with your multimeter probes, do you read any current? ...or set fr .01 amps? (This would suggest something else is drawing, or you have some leakage, perhaps through the outboard, or a slow short.)

4. If #3 is "no", then if you remove a bulb and turn only its switch on, do you read anything at the battery as described above? (This would suggest a short in that circuit.

My guess is the answer is in #1. But you're right to want to find a problem on the boat before putting two new batteries in place.

Edited by - on

Tradewind
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USA
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Response Posted - 03/03/2011 : 15:10:08

2 Different batteries connected in parallel could be a problem, I would only connect 2 batteries of the same age, ah rating and manufacturer in parallel. If a cell in one battery went bad due to being older or some other electrical fault it could drain the good battery. Check the electrolyte, give them a good charge separately then load check them. Walmart will load check them for free, as will most auto parts stores.

Edited by - on

Stinkpotter
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Djibouti
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Response Posted - 03/03/2011 : 19:29:45

Steve is right... One bad one will take out the other good one. Also, when at anchor, I switch to one of my two Group 24 batteries (on a 1-All-2-Off switch) and use that for the night. Then I know the other is good. Of course, my needs are different now--if I don't have juice to start my Honda (225 V-6), I need to call Triple-A.

Edited by - on

jbkayaker
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Response Posted - 03/06/2011 : 11:32:25

I think TradeWind and Stinkpotter are on the right track. Don't assume that the batteries were fully charged when the "experiment" began. The solar panel may not have done enough charging due to a flaky connection (external or internal) or lack of sunlight. I would measure the amps the solar panel is producing while charging the batteries. And I would repeat the overnight test with batteries known to be fully charged.

Use a 110 volt automatic charger set at a moderate rate to top off the batteries separately. Then check each battery's voltage separately with all the boat's lights On. Compare the volt readings to the graph on page 68 of this webpage:
www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf
When you are sure the batteries are fully charged do the "experiment" again.

Edited by - jbkayaker on 03/06/2011 11:39:20

Stinkpotter
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Djibouti
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Response Posted - 03/06/2011 : 13:26:14

Incidentally, by my calcs, a 9 watt solar charger, in maximum sunlight, will deliver about a third of an amp to each of your parallel-connected batteries. 1.5 - 2 amps is the normal trickle-charge rate to just maintain one Group 24 battery. In the six mid-day hours of strong sunlight, each battery might receive a total of around 1.5 amp-hours.

Edited by - on

Voyager
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Response Posted - 03/06/2011 : 18:42:40

I use an 18W and a 20W solar panel together for one group 24 battery when I'm using the boat at its peak from May - Oct. I reckon I get 900 mA on average from the 18W and about 1A from the 20W panel, and with approx 8 hours of peak sunlight, I figure on between 12-15AH a day.

If my single 75AH battery is 1/2 discharged (at a reading of approx 12.1V), it would take about 2.5 days to fully recharge it (which is what I've observed)

Calculating using two group 24s and a single 9W panel, you get about 3.5 - 4AH a day, and you'd need to replace 75 AH in two batteries that were 1/2 discharged.

If I had two batteries on Passage, I'd invest in a single, higher efficiency solar panel that could produce something like 60-80W (3.5-4.5A), so I could produce 30AH per day, thus taking about 2 days to recharge my house batteries.

I can also use the engine alternator to supply 11-12A, so I'd have to motor for 7 hours to fully recharge a pair of group 24 batteries.

Many cruisers use an "Air X" 120W windmill generator, and thus generally have plenty of power.

One issue, however, is that they run at peak efficiency with 20+kt winds which is uncommon in the summer out east, so using both high efficiency solar and wind would be your best bet.

Edited by - on

NautiC25
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Response Posted - 03/06/2011 : 19:55:36

If that's true, then I'd need more than a week to bring back both batteries. I just got home from pulling them out of the boat. They were on the solar panel since last sunday morning, and had exactly 7 days to charge. They are now reading 12.7v and 12.1v. They were 4hrs off the panel when I took the readings.

I looked inside them and the top of the plates are 1/8th inch exposed. Do I fill them just to cover the plates, or do I fill up the entire cavity?

Edited by - NautiC25 on 03/06/2011 19:58:15

Tradewind
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Response Posted - 03/06/2011 : 20:42:29

The plates should be covered. Fill the cavity up to the bottom of the filler hole "tube", use only distilled water or demineralized water. Put them on a good charger with an amp charging indicator, individually, to top them off. When the charger reads zero they're fully charged. Then have them load checked.

If they pass they load check then I would suspect the solar charging system as insufficient or faulty or an electrical leak somewhere in your 12 volt system.

I'm not an electrical engineer but I have dealt with batteries on my boats since 1979. And I also stayed at a Holiday Inn express last night.

Edited by - Tradewind on 03/06/2011 20:51:24

TakeFive
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Response Posted - 03/06/2011 : 20:56:38

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by NautiC25
...I looked inside them and the top of the plates are 1/8th inch exposed. Do I fill them just to cover the plates, or do I fill up the entire cavity?
<hr height="1" noshade id="quote"></blockquote id="quote">
You need to be asking why the plates became exposed in the first place. Although 9 watts is small enough that a full charge takes a long time, I believe that it is much more than a typical float charger IIRC. If you are using the solar cell to "top off" the batteries as you said, you may be boiling off your water (or, more accurately, electrolyzing it into hydrogen and oxygen). In addition to harming the battery by exposing the plates, this is a potentially hazardous situation in a confined space. Disconnecting your cables could cause a spark, and if hydrogen is present - BOOM!

You really should pick up a solar controller.

Edited by - on

Stinkpotter
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Djibouti
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Response Posted - 03/06/2011 : 21:48:44

I agree with Rick about the exposed plates. My single battery on my C-25 was over 7 years old when I sold her--don't know how much over because the PO bought it. The two batteries on my Eastern are now 4 years old. None of those three have ever needed a drop of water added to stay above the top of the plates. When one does, that'll be my signal to replace it, and also to check my engine's voltage regulator. And if I replace one, I'll replace both--two different batteries (age and/or spec) should not be charged and used together. The fact that one of yours lost more electrolyte than the other is not a good sign.

All that said, unless you're in Arizona, I really don't think 9 watts of solar power is going recharge two Group 24s from any level of discharge--it may or may not even be enough to maintain them for a significant period of time. I'm not sure, but the internal resistance of two batteries might be too much for a light-duty solar panel--it might not even get a peak of .37 amps to either of them. "Voyager" Bruce the EE can speak to that better than I. But my portable plug-in charger specifies it's for one battery at a time, only.

Edited by - Stinkpotter on 03/06/2011 21:54:59

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