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  #1  
Old 06-15-2009
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Basic Battery Question - measuring capacity

I've got two 6v golf cart style batteries for the house. If batteries can be measured in amps as units of capacity, can that be measured with a basic mulitmeter? In other words, using a multimeter, can I accurately measure how many amps a battery has left? I've seen the chart where you measure volts to get an idea of the charge, is that the best you can do with a multi meter ( no load)?

On the same note, each 6 volt battery is rated at 250 amps at 100 hour rate. Is this measured at 6 volts? With the 2 6volt batteries in series do I get the equivalent of a 250 amp @100 hour capacity 12 volt battery?? Or 500 amps of goodness for my 12 volt devices?

Say I have a device that is run continuously and consumes 1.7 amps a day, and given a fully charged set of two 6 volt batteries in series (250@100 hour)..how long can the device draw from the batteries until the battery reaches a 50% state of charge? 147 hours??

If I wanted to use a solar charger, what size unit would I need to replace the charge that the above device would draw??? ( In Maine)

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Seagrass
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Old 06-15-2009
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Every manufacturer

Every manufacturer has slightly different voltage levels for taking an open circuit or resting voltage state of charge. You'd be best to contact the manufacturer to find out the open circuit voltages for your batteries. You'd be even better to invest in a true battery monitor like the Victron BMV-600 (LINK)

Trojan's resting voltages are here:
100% = 12.73

90% = 12.62

80% = 12.50

70% = 12.37

60% = 12.24

50% = 12.10

40% = 11.96

30% = 11.81

20% = 11.66

10% = 11.51

Excide for example says 11.8 volts is dead or 0% yet witha Trojan that si still 30% capacity. Confusing, I know..?

Measuring SOC with a volt meter is rarely accurate in a cruising situation because the battery must be allowed to come to a resting voltage which takes a while under NO input or output.

I did an experiment to illustrate this and it can be seen here:

Measuring A Lead Acid Battery State of Charge (LINK)


P.S. Are you sure that was not the 20 hour rate? The industry standard test for ah capacity in the US is the C20 or 20 hour rate. This means a 100 amp hour battery can deliver 5 amps for 20 hours. Unfortunately going to 10 amps for 20 hours does not equal the same burn a s 5 amps for 20 and you will get shorter life the higher and faster the amp burn rate. You might be confusing reserve capacity or reserve minutes.

Most 6V GC2 case size golf cart batteries are usually between 215ah and 225 ah each. If you have two in series at 225 ah you'll have a 225ah bank..
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Last edited by Maine Sail; 06-16-2009 at 08:26 AM.
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Old 06-16-2009
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No, you cannot measure the amp/hours of a battery with a multimeter - only voltage and that accurately only after resting the battery without load for 24hrs. Your two batteries in series will give you 250 amps @12 volts or in parallel they would give you 500 amps @6 volts. A battery should not be taken down past 50%, so you have 125 usable amps @ 12 volts. At your stated 1.7 amp draw (I assume that you mean per hour not per day as you state) your batteries using simple math would give you approximately 73 hours before 50% charge is reached. (125 divided by 1.7= 73.53) This is approximate as a battery will give you more usable amps with a small amp draw than with a large amp draw. For example the Trojan T-145 6 volt battery. This battery is at 215 amp/hours @ the 5 hour rate and 260 amp/hours @ the 20 hour rate. They do not give a 100 hour rating. As far as solar charging this is something I know little about and I expect someone else with solar panels will post an answer. What batteries do you have and what is the item that draws 1.7 amps?
Brian
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Old 06-16-2009
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I think Mainesail must be tired as in series you double the voltage and in parallel you double the amperage. It's late in Maine.
Brian
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The only way to have a good idea of charge state is to use a battery monitor that measures the amps going in (during charging) and the amps going out. Even this method only works if the batteries are in reasonably good condition. Measuring voltage is of limited use.
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I believe the original post was questioning battery capacity, not state-of-charge. They are different.

For example, a new 6-volt T-105 golf cart battery is rated at 225AH at the 20-hour discharge rate. If you take two of these, hook them in series, you get a 12-volt battery with a (design) capacity of 225AH. If you then apply an approximately 11A load, then 20 hours later the battery should be depleted (10.5 volts, resting).

Now, let's say the batteries are now 4 years old and have been treated only moderately well. It's likely that they will have lost much of their capacity, i.e., their ability to store and deliver electrical energy. Measuring actual capacity cannot be done with a multimeter, and it can only be estimated with load testers, hydrometers, and the like. A sophisticated internal resistance tester, like the Micronics series, can estimate battery capacity, and this has become the standard way of doing it these days. It's not a precise measure, but seems to be a pretty good indication. Unfortunately, these devices are costly...upwards of $500-600 for a decent one.

Note that resting battery voltage is NOT an indicator of capacity. You can have a full-charged battery showing a resting voltage of 12.6 or above, but which has lost much of its capacity (thru sulfation, stratification, contamination, plate erosion, etc.).

Aging batteries can be like aging runners...their sprint capacity may be little diminished, but they can't really hang in there for the long run :-)

Bill
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Thanks Guys,

I guess I won't bother testing with a volt meter. Good work on that test MainSail.

I've got two US2200 batteries rated at 232@20hour, so I've got about 115 usable amps..correct?

I'm about to install a composting toilet, and it has a tiny computer fan that draws .07 amps (1.7 per day). So given fresh full batteries, how long can this fan run until I reach the 50% charge. 115/.07 = 1642 hours or 68 days. Correct?

If this is correct, then I'm wondering if I should even bother with the solar panel. Or if I did, any little solar panel should cover this I would think. 5 watt panel? I'm not crazy about dealing with a panel. I don't have a good place to permanently mount a panel so I'd be setting it up in the cockpit when I leave the boat. With the boom and sail cover hanging over it. The boat is rather traditional looking so I'd rather not add stuff to make it look like the space shuttle!

Another factor is that I've only got a 35 amp alternator on the engine. I might need to step this up, or give the idea of a larger solar panel some thought.
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Quote:
Originally Posted by btrayfors View Post
Note that resting battery voltage is NOT an indicator of capacity. You can have a full-charged battery showing a resting voltage of 12.6 or above, but which has lost much of its capacity (thru sulfation, stratification, contamination, plate erosion, etc.).

Aging batteries can be like aging runners...their sprint capacity may be little diminished, but they can't really hang in there for the long run :-)
Indeed. As an extreme example: Witness the sealed lead-acid batteries I'm about to replace in my UPS. The UPS "thinks" they're ok. It even charges them. (And rather quickly, too.) But if the power so much as twitches and it tries to actually use them, it goes down as if it'd been pole-axed.

Jim
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Old 06-16-2009
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Quote:
Originally Posted by mitiempo View Post
I think Mainesail must be tired as in series you double the voltage and in parallel you double the amperage. It's late in Maine.
Brian
Yup!!! Was tired and just fixed it! Thanks!!!
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It's confusing....

Quote:
Originally Posted by preservedkillick View Post

Another factor is that I've only got a 35 amp alternator on the engine. I might need to step this up, or give the idea of a larger solar panel some thought.

If you reside on a mooring, as most do here in Maine, and only have an alternator it gets more confusing. With an alternator your usable capacity will only be about 80% because charging the batteries all the way back to 100% via the alternator, due to "acceptance" can take 12+ hours of running the motor. Getting back to 80% is fairly easy but pushing beyond 80-90% of full is very, very time consuming.

Let's say your capacity is 232ah with fresh batteries. If you are on a mooring, with no solar, that now becomes about 185ah of usable capacity when figuring an 80% charge back rate. That leaves you only 69 usable ah's before you hit the 50% threshold unless you have a way to charge back to 100% on a regular bassis like a dockside charger or a solar panel.

At 1.68ah per day for your composting head you will draw nearly 24ah every two weeks. Add a stereo memory wire or a bilge pump and your could be at half your usable capacity in just two weeks.

Search the term "acceptance" here to get a better understanding. Suffice it to say the batteries accept less charge, in amps, the closer they get to full thus it takes the longest to get that last 10-20% into the batteries and why you should use 80 or 85% of capacity as your usable figure if you are on a mooring with no solar or dock side charger.. Sorry for the typo last night it was about 12:30am..

Bill made an excellent point that voltage alone is NOT a determining factor of capacity unless the batteries are very close to new..
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Last edited by Maine Sail; 06-16-2009 at 08:47 AM.
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