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11-29-2020, 08:03 PM
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#1
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Registered User
Monaco Owners Club
Join Date: Nov 2017
Posts: 1,493
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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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11-29-2020, 08:09 PM
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#2
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Senior Member
Join Date: May 2019
Posts: 2,392
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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.
__________________
Richard
1994 Excella 25-ft (Gertie)
1999 Suburban LS 2500 w/7.4L V8
1974 GMC 4108a - Custom Coach Land Cruiser
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11-29-2020, 08:31 PM
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#3
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Registered User
Monaco Owners Club
Join Date: Nov 2017
Posts: 1,493
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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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11-30-2020, 12:48 AM
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#4
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Senior Member
Join Date: Jul 2014
Posts: 35,446
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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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11-30-2020, 02:21 AM
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#5
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Registered User
Monaco Owners Club
Join Date: Nov 2017
Posts: 1,493
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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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11-30-2020, 06:27 AM
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#6
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Senior Member
Join Date: May 2019
Posts: 2,392
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What charger are you using for this battery bank?
Does it automatically compensate for temperature or do you program manually?
__________________
Richard
1994 Excella 25-ft (Gertie)
1999 Suburban LS 2500 w/7.4L V8
1974 GMC 4108a - Custom Coach Land Cruiser
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11-30-2020, 12:53 PM
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#7
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Senior Member
Holiday Rambler Owners Club
Join Date: Aug 2010
Posts: 9,751
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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.
__________________
Ben & Sharon
2008 43' Holiday Rambler Scepter PDQ
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11-30-2020, 02:23 PM
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#8
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Registered User
Monaco Owners Club
Join Date: Nov 2017
Posts: 1,493
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Quote:
Originally Posted by richard5933
What charger are you using for this battery bank?
Does it automatically compensate for temperature or do you program manually?
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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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11-30-2020, 04:03 PM
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#9
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Registered User
Monaco Owners Club
Join Date: Nov 2017
Posts: 1,493
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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 .0 03 = 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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11-30-2020, 05:12 PM
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#10
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Registered User
Monaco Owners Club
Join Date: Nov 2017
Posts: 1,493
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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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11-30-2020, 06:39 PM
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#11
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Senior Member
Join Date: May 2019
Posts: 2,392
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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.
__________________
Richard
1994 Excella 25-ft (Gertie)
1999 Suburban LS 2500 w/7.4L V8
1974 GMC 4108a - Custom Coach Land Cruiser
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11-30-2020, 07:06 PM
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#12
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Registered User
Monaco Owners Club
Join Date: Nov 2017
Posts: 1,493
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12-03-2020, 03:23 PM
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#13
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Registered User
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Join Date: Nov 2017
Posts: 1,493
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12-03-2020, 03:30 PM
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#14
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Senior Member
Holiday Rambler Owners Club
Join Date: Aug 2010
Posts: 9,751
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Yeah, mine would always be bang on if I made my own chart as well !!
__________________
Ben & Sharon
2008 43' Holiday Rambler Scepter PDQ
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