Battery Float Voltage

[Dick Simonis]

Had a semi-strange thing happen in the last day or so, in that my battery float voltage has increased, from the normal 13.6 or so, to 14.0 volts, and I can not figure out why that happened  Of course, this voltage is just above the cutoff threshold for the echo charge so now the chassis batteries don't charge.

Checked and topped of the battery water.

Measure Sp. Gr. with 11 cells running right at 1.275 +/- .005 and 1 cell at 1.255.  I did not correct for temp. (FYI, this was right after watering)

Float voltage measured by the CMP and verified by voltmeter on terminals.  Compared within 0.2 volts

Checked inverter setting for capacity and type, all seems well.

The cells don''t appear to be gassing excessively, just an occasional bubble which is pretty normal.

I couldn't get a charging current reading with my clamp on, so there is a question there that still needs an answer.  Might just be off scale on the low end as everything is off.

The only thing that's changed of substance is the ambient temp has plummeted, so I'm wondering if there might be a temp correction factor that's in play.

If any one has any thoughts, I would love to hear them.

[Edward Buker]

Dick,

I think you are good to go.

At present, you do not have any current drain and the batteries are fully charged. In time the voltage will slowly drift down to the cut on voltage for the Echo Charger and the process will repeat. This level of voltage being applied can also happen after a run with the engine. The alternator will charge at up to 14.4V or so.

 Also the inverter/charger in the absorbtion charge stage, that is just before the float charge, applies over 14V and the Echo Charger will pass that voltage along to the chassis batteries minus about .2V due to diode voltage drop. So under certain conditins of charge, seeing 14V at the battery bank is a normal occurance.

The reason the clamp on ampmeter does not see current flow is that there is little or none taking place without a load on the coach and the batteries approaching full charge capacity.  I think you just happen to catch the chassis/house batteries with measurements right after the absorbtion charge stage. Hope this helps.

P.S. My wife had dinner on the table earlier and that took priority over my poorly written BAC post....hope this version is clearer.

Later Ed

[Dick Simonis]

Ed, thanks for the reply and I think I know what's going on.  After I thought about it for awhile, I remembered (back from the old diesel electric sub days, that float voltage is affected by temperature.  The lower the temp the higher the voltage rises to keep the batteries just below the gassing level.  It seems that at 50F that voltage is ~14.0 so it all falls into place.  It also turns out that the installed inverter does have a temp probe attached to the batteries and automatically compensates for temp.

After replacing the batteries in the clamp-on, the charging current was running ~0.5 A so indeed all is well.

[Edward Buker]

Dick,

There is charge temp compensation built in if you have the temp probe. If the charging votage stays that high at around 50 degrees ambient then you hit the nail on the head as to what is going on. If it drifts back to 13.8V or so you probably caught the tail end of absorbtion charge.

 I was never quite sure if the charge temp compensation was just an overtemp limit that reduced the charge voltage and current when necessary (that is what I thought to protect the batteries from damage) or if it was sophisticated enough to adjust the charge voltage based on actual ambient temperature measured at the battery. Still not sure, but either way you are good to go.

Those sub days must have been interesting...now that must have been a battery bank.

Later Ed

[Dick Simonis]

Yep, think a 500VDC, 5,000 AH battery  and there were two of them that could be used in either parallel (endurance) or series (making tracks)

Here is some interesting reading the explains the TVG (temperature voltage gassing) curve and how Xantrex handles it with their inverter/chargers.  Xantrex is pretty knowledgable.  I used the Heart Interface stuff for years before Xantrex bought them.

http://www.tekrispower.com/pdfs/xantrex/Batteries%20-%20Temperature%20Compensated%20Charging.pdf