over voltage charge rate?

arby · January 2016

Hello, tried a search but didn't know what to type in, so here is my problem.
My batteries ran down enough so that I had to start the generator. It charges the bank through my Xantrex SW5548.
The bulk charge voltage is set at 58.8 but it was charging the batteries around 61.5 to 62 volts. Even when it dropped into float I suppose after the 2 hour absorb stage it was still 57.5, about 4 volts over the float setting.

Any ideas or help appreciated.
TIA

westbranch · January 2016

did you wait ~ 3 hrs after the charge was finished before taking that voltage reading? you need to wait, and have no loads on, for a stable reading

arby · January 2016

This was while the batteries were being charged.

Vic · January 2016

arby said:

Hello, tried a search but didn't know what to type in, so here is my problem.
My batteries ran down enough so that I had to start the generator. It charges the bank through my Xantrex SW5548.
The bulk charge voltage is set at 58.8 but it was charging the batteries around 61.5 to 62 volts. Even when it dropped into float I suppose after the 2 hour absorb stage it was still 57.5, about 4 volts over the float setting.

Any ideas or help appreciated.
TIA

Hi arby,

It is possible that your batteries are COLD, and the CHARGE and Float voltages are being compensated by those amounts?

Many Flooded batteries use a compensation of --5mV per degree C for temperatures that are different than 25 - 27 degrees C. This compensation would change charge and Float voltages by --120 mV per degree C. SO, if y our batteries happen to be about 2 - 3 degrees C, the Bulk voltage would probably be compensated to about this amount.

If the Float voltage that you noted was also from the SW Inverter, this is would represent more compensation, but you did kind of imply that these voltage readings are a bit approximate.

Also, just where you are reading these voltages would make a bit of a difference.

What Is the Actual and Compensated battery voltage readings on the SW Meters?

What make/model batteries are you using?

Are you using the Trace/Xantrex Remote Temperature Sensor (RTS) on the SW?

More Later, Thanks, Vic

arby · January 2016

This was up at the cottage this weekend, so I can't recall the exact voltages, but what I said was about right. I was getting the voltage readings from my MX60 mate inside the cottage, and my trimetric. I know I only have one temperature sensor hooked up, and I can't recall if it's for the CC, or the inverter. It was pretty cold, about -18C over night and -10C when the charger was running. I did feel the batteries and they were not warm. I'd say cool, but not cold.
Using 120 mv times 35 degree C would give about 4.2 volts, which is what you said and that probably explains it. I don't understand how we get from 5mV per degree C to 120 mV per degree C.
The batteries are Surette S530's and are in an insulated box attached to my non heated shed.

Thanks

Vic · January 2016

Hi arby,

OK, then what you were seeing was quite probably just customary Temperature Compensation.

We get from --5mV TO --120 mV per degree C because --5mV is PER CELL. Each of the 24 cells in your 48 volt bank needs compensation, because they all are in series. SO, 24 cells, times --5mV is --120mV per degree C for the entire 48 volt battery bank.

This really is a huge amount of variation represented just by temperature compensation, and is one of the reasons that when stating a voltage reading that one measures, it really needs to be supplied along with the nominal battery temperature at which this reading was taken.

AND, given the cold Winter temperatures at your cabin, you should strongly consider having one temperature sensor for every charge source, IMO.

Thanks, Vic

westbranch · January 2016

I think you are somewhere on the Shield... temps in that range are going to have a marked impact on the adjusted voltages that your system will use, if the RTS is hooked up so the CC or charger can use it...That bank has a lot of thermal mass to heat, and it cools a lot more than it will heat at this time of the year....Were the temps , before you arrived much colder?
do you have an IR temp gun? That way you can get a goo estimation of each cell temp...
I don't think you would be able to sense any warming with the hand, I know I didn't at our place with the old bank in similar setup... most RTS's should be attached (taped) between 2 cells, about mid height with some styro insulation on the exposed surface
note some are made to attach to the batt. post

hth, let us know what you find next visit

arby · January 2016

Just found this on the SW5548 manual---

<<

Allows selection of the battery type for the battery temperature compensation system. This effectively

reduces the battery charging set points when the battery is hot and increases them when the battery is

cold. The battery charging set points change

±

0.005 volts per degree Celsius for the

LEADACID

setting

These battery setpoint

changes occur if the battery temperature is higher or lower than 25

°

C (77

°

F), and will only occur if the

battery temperature sensor (

BTS

) is installed. The setting should be changed from the default setting only

if NiCad or Nickel Iron batteries are used>>

-----------------------------------------------------------------------------------------------------------------------

Using .005 volts times 35 degreeC gives me a .175 voltage difference.
This is a far cry from 4.2 volts using Vic's numbers of 120mV per degree C.

Confused--
Thanks, arby

edit,, oops, just saw Vic's second post and now it becomes much clearer.
Doing it Vic's way, eg per cell gives me .175 X 24 = 4.2volts. Exactly the same.

BTW, I looked in at a few cells and expected to see a fair amount of boiling at that voltage There was a bit of bubbling, but nothing to get excited about for sure.

Sounds like the inverter is the one with the sensor on it and it is doing its job
Thanks again guys.

WaterWheel · January 2016

Great thread. This is my 1st winter and had noticed that my batteries at 50 F were charging at a higher voltage. Now I understand why.

vtmaps · January 2016

foolami said:

This is my 1st winter and had noticed that my batteries at 50 F were charging at a higher voltage.

You should also notice that when under discharge, the battery voltage is lower than in the summer. The reason is higher internal resistance in a cold battery. Actually, that's a bit of a simplification (everything about batteries is). The cold also affects the diffusion rate of ions in the electrolyte... the voltage drop from a discharge occurs sooner, and the rebound takes longer.

If you use a battery monitor, you may set the ah capacity of your batteries to a lower number in the winter.

--vtMaps

arby · January 2016

Ordered a BTS for my CC. I had no idea that the temperature had such a large effect. Seems I thought a BTS was for heat and not cold and since it seldom gets above 25 C on the shield I thought it unnecessary.

Vic · January 2016

OK arby, excellent. This will be a very inexpensive improvement to your system. Vic

just starting · January 2016

My battery bank has been under the -c since October sometimes absorb is at 30.5 v lowest was -19c

vtmaps · January 2016

arby said:

I thought a BTS was for heat and not cold

When the batteries are cold, without a BTS you may not get an adequate charge, but that is not a safety issue.

When the batteries are hot, they draw more current and that makes them hotter which makes them draw more current which makes them hotter which makes them draw more current... it's called "thermal runaway" and it can be dangerous. A BTS will lower the voltage when the batteries are hot and may help prevent thermal runaway.

--vtmaps

over voltage charge rate?

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