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Discussion Starter · #1 ·
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 ([email protected] 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)

-Preserved
 

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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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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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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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Discussion Starter · #7 ·
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 [email protected], 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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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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It's confusing....

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..:confused:

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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Discussion Starter · #12 ·
Is there a way to figure out how many amps the 35 amp alternator is putting back in when the engine is running?? Typically you get less than the rated amps right? I'm likely getting 25 amps? Does this mean that one hour of engine running puts 25 amps back in?

I'd like to compare the benefits of running the motor against a small solar panel.

Would I gain more amps for my money by upgrading my alternator to a 70 amp model? I've got a 3 cylinder 22 HP Vetus diesel. I bet the belt is just 3/8 so I've got limited possibilities in alternator sizes.
 

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Please don't confuse AMPS with AMP-HOURS.

Battery capacity is generally measured in AMP-HOURS-ie, how many amps * hours it has left. AMPS is a unit of current and does nothing to tell you about battery capacity. However, the actual number of amp-hours it has left is very dependent on the rate at which you draw the amps from it, due to things like the Peukert factor. For example, a battery that has 200 amp-hours of capacity might run for an hour with an 100 amp load, or 100 hours under a load of TWO amps.

Here's a quote from the original post...."can I accurately measure how many amps a battery has left?"

This is a question of charge state. You need a battery monitor to determine how many amps a battery has left.
 

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You will only get the full 35 amps under optimal conditions. You could measure how many amps the alternator is putting out using a clamp on true-RMS ammeter. The problem is there are so many variables here that it would take a lot of work to determine what was happening over time. RPMs, temperature, charge state, other power uses are going to affect what happening so just taking a few readings will have very limited usefulness. A battery monitoring system does measure amps, but does so continuously. It gives you a result over time creating a total amps in vs total amps out summary for you. The systems work very well. I assume they must use a model to determine loss since not everything you put into a battery comes back out. The loss is mostly due to heat. If you can measure all this and account for loss and still come up with a number that means anything you are better man than I.
 

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Of course. You need to account for current and time (amp hours) to describe a "volume" of electricity. I was focusing on the concepts preservedkillick was asking about without getting bogged down in terminology too much. I was going along with his concept of "how many amps a battery had left". Another way to answer his question is that measuring amps (instantaneous current) is of very limited use. You really need to measure amp hours, analogous to the electrical meter on your home, to get useful information.

Please don't confuse AMPS with AMP-HOURS.

Battery capacity is generally measured in AMP-HOURS-ie, how many amps * hours it has left. AMPS is a unit of current and does nothing to tell you about battery capacity. However, the actual number of amp-hours it has left is very dependent on the rate at which you draw the amps from it, due to things like the Peukert factor. For example, a battery that has 200 amp-hours of capacity might run for an hour with an 100 amp load, or 100 hours under a load of TWO amps.
 

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I understand amps hours in is "close enough" to amps hours out to not worry about it.

This is not true of power in vs power out since the amp hours in are generally at a higher voltage than the amp hours out.

(edit - the "close enough" doesnt take into account the rate of charging - ie, Puekert stuff - just a general reason why the battery will disipate some heat when being charged but that the amp hours out can be roughly compared to amp hours in)

My wet cell battery bank is in it third year and I think its dying and Im wondering how others have determined when its time to declare them dead. I have a charge meter and using a three stage charger, it will say the battery is full (ie, the charger resets to 100% full). But after using less then 10 amphour (200 amp hour battery), and with say a 1 amp load, the DC voltage will drop to say 12.0 volts. Seem way too low.

Ive never equalized the batteries, I wonder how much they would recover from this??.
 
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