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Old 05-01-2011, 12:16   #1
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Is it Possible to Estimate Battery Charge Percentage by Amp Acceptance ?

Is it possible to make a ROUGH estimation of battery charge % by how the charge tapers off? I have a 3 stage 45 amp charger and only rarely does it ever kick up to 45 amps and usually tapers down to 10 - 15 amps within the first hour and down to around 4 before I kill the generator.

We have a newish bank of 4 golfcarts (new in July this year thanks to Auspicious' SAMs membership -- I won't forget that kindness!) and I've been monitoring it with a Victron battery monitor -- I changed the settings on the battery monitor to not reset as full until the batteries read 14.1 Volts for 20 minutes or so basically so I can just keep an eye on amps in/out

since making that change I've found my total amp hours used floats between 100 - 160 depending on my days usage and I run the generator for a few hours and it drops down to around 100.

It used to reset to 0 after I let the generator run long enough but I realized that wasn't an accurate reading, not that this is either but my question is this -- when the charge tapers down to <1% of the bank size is that good enough? I realize I need to fully charge the batteries at some point and equalize them (they were last equalized the beginning of this month) but I'm curious if I can tell anything by how much charge they're accepting

Thanks!

Brian
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Old 05-01-2011, 13:21   #2
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I think so and I like this idea, but I dont have a formula... Some rules are needed.
You say, is it good enough when the charge tapers down to less than 1 % that they are full and the answer is yes...IF.... you are bulk charging to the correct voltage.
I see you say 14.1 volts using the battery monitor... but what is the generator doing. I dont know golf cart batterys but I would think they are similar to 2v traction batterys which I have.
So if they are the same.....the regulator needs to bulk charge until the voltage reaches 14.8v. Then hold that voltage and reduce the current until we are at around 2% of the capacity. They are then fully charged.
So once you know this you can measure how long it takes to fully charge after the voltage reaches 14.8v.
When you reach 14.8 then you might be around 85% full... the time it then takes to get to 2% of capacity is the rest.
But check with the battery maker what the absorbtion voltage should be...14.1 seems too low
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Old 05-01-2011, 14:22   #3
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Quote:
Originally Posted by brianontheroad View Post
Is it possible to make a ROUGH estimation of battery charge % by how the charge tapers off? I have a 3 stage 45 amp charger and only rarely does it ever kick up to 45 amps and usually tapers down to 10 - 15 amps within the first hour and down to around 4 before I kill the generator.

We have a newish bank of 4 golfcarts (new in July this year thanks to Auspicious' SAMs membership -- I won't forget that kindness!) and I've been monitoring it with a Victron battery monitor -- I changed the settings on the battery monitor to not reset as full until the batteries read 14.1 Volts for 20 minutes or so basically so I can just keep an eye on amps in/out

since making that change I've found my total amp hours used floats between 100 - 160 depending on my days usage and I run the generator for a few hours and it drops down to around 100.

It used to reset to 0 after I let the generator run long enough but I realized that wasn't an accurate reading, not that this is either but my question is this -- when the charge tapers down to <1% of the bank size is that good enough? I realize I need to fully charge the batteries at some point and equalize them (they were last equalized the beginning of this month) but I'm curious if I can tell anything by how much charge they're accepting

Thanks!

Brian
When you hit 2% or less of the banks 20 hour Ah rating at absorption voltage, around 14.4 volts, not float, you are pretty much full.

So on a 450Ah bank if you were charging at about 9 amps at 14.4 volts your banks would be pretty close to full.

For simplicity a 100 Ah battery at about 77 degrees seeing 14.4 volts and taking less than 2% of its Ah rating as current is basically full.

I say "basically" because most banks in good health can taper down to 1% acceptance or so but as a rough measure 2% is generally the industry accepted value for "full", what ever that actually means..

Hitting a 14.1 volt absorption charge for 20 minutes on a 450 Ah bank of wet cells is generally not going to be full. 14.1 volts will happen as you approach 70-80% of capacity and it could take many, many hours at absorption voltage, which should be higher than 14.1, to fully charge your bank. The last 10% takes hours not minutes to replenish due to acceptance.
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Old 05-01-2011, 15:37   #4
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+1 what Maine said.

You can reduce your generator time considerably by getting a larger capacity charger.

Four golf cart batteries (about 440AH total) can easily accept 90 amps of charging current. Depending on the size of your generator, you might think about getting a relatively inexpensive and highly efficient high-capacity charger like the Iota with the IQ-4 smart-charge option. They make several sizes, but the DLS-75 or possibly the DLS-90 if you have enough generator power would seem to be a good investment.

Don't know what genset you have, but for reference the Honda EU-2000 can handle the 75-amp charger, but not the 90A one.

Bill
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Old 05-01-2011, 16:05   #5
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A hydrometer will give you the best indication. Then use your hydrometer readings as a calibration for what you are reading.
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Old 05-01-2011, 17:48   #6
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I have an Iota 45 amp charger with the 3 stage IQ4 or whatever it's called adapter -- in the bulk phase at the battery posts (via my Victron) I see low 14's like 14.1/14.2 before it dials back to absorption (down to about 13.8), I looked in the manual and don't see a way to change this on the Iota? I'd like it to get to a higher voltage before dialing back since my charge times are limited.

I'm seldom at a dock....when I say seldom its months between docks, we've been on the go on the ICW so the alternator was keeping the batteries charged and then since we've been in florida up until the last few days it's been blowing enough for the windgen to about break even on charge -- I'm going to try and pick up a few solar panels, but I'm on a tight budget I've been more or less full time cruising for a year and I'm in my twenties (barely) so cash is always an issue....I'm working remotely for a company now which pays ok.....not great, but I need cash in the kitty for the bahamas for a few months BUT solar is way better than my little honda -- I only have a 1000, but it pushes the 45 (so long as nothing else is plugged into AC during the first 20 minutes or so of charging) -- I'd like to get a 3 stage regulator for my alternator too but I think solar is a better spend for about the same amount of money...

as for the hydrometer -- my understanding is that I need to take readings when the batteries are "full" which will be the next time I'm at a dock -- which might well be a few weeks from now in Bimini or something -- South Florida is proud of their docks ...with the notable exception of Ft. Lauderdale.... but I'm proud of my anchor :-P

Other than Sun Electronics any South Florida solar recommendations?

Thanks all
Brian
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Old 06-01-2011, 03:23   #7
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You can't use terminal voltage to determine state of charge not can you use current in leaf acid technology.

What you can do in to use terminal current to " indicate" that the batteries will not take any more current at that particular voltage is the internal resistance had risen by selecting a particular end point voltage and a particular current you can reasonably say the battery. Is full.

That's all you can deduce

Dave
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