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Old 11-29-2020, 09:03 PM   #1
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Arrow Math for Float Voltage

Hi folks!

Sorry to bug everyone about these endless battery questions but I just came upon something with respect to T105's and their float voltage that might indicate my understanding is wrong.

According to Trojan, to determine the float voltage with respect to ambient temperature, "Add .005 volt per cell for every 1c below 25c"

By 'per cell' do they mean per battery or, literally, per cell ?

Example. I have (4) T105's which have 3 cells each. That would be 12 cells total.

If we use an ambient of 5c then would it be this?

25c - 5c = 20 x .005 = 0.1 x 12 cells = 1.2v increase overall

Base float voltage (according to Trojan) is 13.5 so add 1.2v = 14.7 float voltage?

Does that sound right?
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Old 11-29-2020, 09:09 PM   #2
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My guess is you only add the cells in series, not those in parallel. That would only be 6 cells.

All that said, my Trojans have been four years with a float voltage of 13.2, with an automatic jump to bulk voltage once a day for 15 minutes. No problem yet.
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Old 11-29-2020, 09:31 PM   #3
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I don't know that it would matter how they are wired up. If it's an offset based per cell, by temperature, this is what I come up with for ambient adjusted float voltage based on what Trojan states;

-10c = 15.60v
-05c = 15.30v
000c = 15.00v
+05c = 14.70v
+10c = 13.90v
+15c = 14.10v
+20c = 13.80v
+25c = 13.50v **typical BTS missing assumption** / default Trojan temperature
+30c = 13.20v
+35c = 12.90v
+40c = 12.60v

Because the 25c number corresponds with most manufacturer's BTS missing assumption, this leads me to believe my math is likely correct. Where is TwinBoat when you need him
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Old 11-30-2020, 01:48 AM   #4
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Since its a voltage compensation, and paralleling batteries does not increase voltage, it should be calculated by series only.

A 12 volt battey is 6 cell.
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Old 11-30-2020, 03:21 AM   #5
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So my numbers are way high. Back to the drawing board
Sorry about that, Richard - looks like you were right

This should be correct then;

-10c = 14.55v
-05c = 14.40v
000c = 14.25v
+05c = 14.10v
+10c = 13.95v
+15c = 13.80v
+20c = 13.65v
+25c = 13.50v **typical BTS missing assumption** / default Trojan temperature
+30c = 13.35v
+35c = 13.20v
+40c = 13.05v
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Old 11-30-2020, 07:27 AM   #6
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What charger are you using for this battery bank?

Does it automatically compensate for temperature or do you program manually?
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Old 11-30-2020, 01:53 PM   #7
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This is the temperature compensation chart I've been going by.

https://batteryuniversity.com/learn/...w_temperatures

My Magnum charger with BTS is pretty close. Maybe 0.1V higher.
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Old 11-30-2020, 03:23 PM   #8
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Quote:
Originally Posted by richard5933 View Post
What charger are you using for this battery bank?

Does it automatically compensate for temperature or do you program manually?
It's the charging circuit in the inverter, which is a Trace Legend L1512
I just put new batteries in and I'm trying to verify that they are not being boiled, so to speak. So I have to know what the float is supposed to be given the ambient temps. Hence the chart work

I did find in the manual for the later models of the Inverter they go into more detail on how the charging side works and what their exact calculation is for the temp/float.
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Old 11-30-2020, 05:03 PM   #9
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My math is my weakest link.

It's 7.8c and they are floating at 14.06v, which, according to the most recent chart, should be in the ball park.

But when I try and reverse the math using 7.8c I get;

25c - 7.8c = 17.2c difference. Multiply that by .003 = 0.05156 + 13.50v = 13.55v float voltage. Which, if my math is correct, indicates it's too high.

But my point is, my math doesn't seem to work in reverse which leads me to believe it's wrong again.

An extra 0 appears to be what screwed me. So,

25c-7.8c = 17.2c difference. Multiply that by .03 = 0.516 + 13.50v = 14.016v float voltage. And I'm seeing 14.06v

14.06v float is indicative of a batt temperature of 6.10c But then, there's also the skew on the thermistor.
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Old 11-30-2020, 06:12 PM   #10
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I hooked the temperature probe of my multimeter up to the same place on the battery as the BTS.

It read 8c and the float voltage at the batteries at that moment in time was 14.06v

@8c the float should be 14.01v

So...my conclusion is that the float voltage was too high by .05v
At 14.06v the battery temperature should have been 6.3c

So according to my math the BTS is reading a temperature that is 1.7c colder than it really is and increasing the voltage by .05v

Once I get the full scale of the BTS thermistor I will get a data sheet on it and see what the error margin is.

...and then....likely install a resistor to calibrate the BTS to actual temps....
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Old 11-30-2020, 07:39 PM   #11
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Please take this the right way, but I suspect that you're over thinking this. At least maybe a tad bit.

The accuracy of the equipment involved is just not that great, and even charging devices that have readings to two decimal points the degree of accuracy isn't there. I've got a great electronic multimeter, and it lists it accuracy as +/- 0.5% to 2.0%, depending on the scale in use.

If you have the voltage set to within a tenth of a volt, I seriously doubt you're going to do any damage to your batteries one way or the other.

I'm in Wisconsin. It gets cold in Wisconsin. My charger is a Progressive Dynamics 9270. It does not have a temperature probe, and it does not have a way to customize the charging parameters. It is charging four Trojan L16 batteries in series/parallel configuration as a 12v battery bank. The float charge on them is 13.2v, regardless of temperature. Is it ideal? No. Does it work? Yes, safely. True, the batteries could be better served with a charger that adjusted the float charge according to temps, but I don't have one. We're going on our forth year with this setup and I have not seen any discernible change in performance since day one.

My point isn't to tell you what charger to use or what voltage setting to use. My point is to help you relax a bit and understand that your batteries will be fine as long as you don't charge at too high a voltage and are able to keep the float voltage close to the what's recommended without going over. It's not necessary to get it spot on per the formula, although you do get points for the effort to get close as possible.
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Old 11-30-2020, 08:06 PM   #12
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Believe me, I'm not offended. I started getting that feeling yesterday that I might just be spending a little too much time on this. On the other hand, I don't really have anything else to do! And this is what happens when you have too much coffee

It's hard for sometimes to get a handle on new concepts and math has always been a challenge. So when I make progress, I tend to follow it through to the end. The hard work has been done. If memory serves, I have a lot of resistors at home....
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Old 12-03-2020, 04:23 PM   #13
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I just checked the battery temperature (I have another thermistor taped to the battery that plugs right into the multimeter) The battery is 6c and the voltage is 14.07

I checked the chart I made, and at 6c the float voltage should be exactly 14.07v
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Old 12-03-2020, 04:30 PM   #14
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Yeah, mine would always be bang on if I made my own chart as well !!
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