Victron BMV 702 and Temperature Sensor

[Cef]

Hello ! I'm finishing up my system and I have doubts regarding my bmv702. I bought the accessory probe to control the temperature of the batteries, which in my first studies I considered necessary since if I am not mistaken it is information that is required for a better estimation of the SOC of the arrangement of batteries. However I see that many forum members use the midpoint measurement option and as both are exclusive I consult on which you estimate is a better option if you were to set up a system with 8 new Trojan batteries? Thanks !

[Guest]

Hi Chris is the guy here with the knowledge on BMV.

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[___]

The point of temperature control is that it can tell a charger to compensate for absorb/float voltages based on temperature. The fact that the BMV can take a temp probe doesn't add any functionality in and of itself, though it could in theory be used by another piece of equipment. For example, if you have all Victron equipment with a Multiplus in ESS mode and a CCGX where the charging voltage is controlled by the CCGX, the CCGX could in theory adjust voltages based on the temperature reading.

I've had this discussion with TTT before, where I questioned how useful it would be to add a temperature probe to the BMV, seeing as the inverter already comes with one, and so does some of the charge controllers. He explained to me that the cheaper charge controllers don't have a temperature probe...

Anywway, so in a Victron setup, there is at least three places a temperature probe can be connected, either to the inverter, the charge controller, or the BMV. Most people connect it to the charge controller. Of course, that assumes it's all-Victron.

For everyone else... much better to use the second input for mid-point monitoring.

[Cef]

44 minutes ago, plonkster said:

The point of temperature control is that...

Many thanks @ plonkster, I do not have a complete Victron system.

Sorry, I was wrong, apparently not in the calculation of the SOC but if it intervenes in the accuracy of the calculation of the remaining capacity.
In fact, it could serve to estimate the remaining battery capacity "depends on the ampere-hours consumed, discharge current, temperature, and the age or the battery", first paragraph of this : https://www.victronenergy.com/upload/documents/Datasheet-BMV-700-series-EN.pdf
It may make sense for a finer tuning of the remainder capacity calculation algorithm ... but it seems that I spent some useless dollars .

To take care of my batteries, as these are new, I am thinking of the Cheap Battery Equalizer. I would prefer the Victron but I consulted with them and they told me that they still do not have their devices for 6 Volts batteries.

[Chris Hobson]

Hi Elmichi

I would go with a midpoint voltage measurement, unless your inverter can automatically compensate bulk charging voltage. Heat is one of the worst enemies  of batteries and temperature compensation can prolong a batteries life by 15%. I have cheap digital aquarium thermometers attached to my batteries and when I notice battery temperatures climbing late Oct/early Nov I shave 0.5V off my bulk charging voltage. I returned to the standard voltage early Mar but changed it back in the last couple of days with heat we have been having. The midpoint probe gives you IMHO  much more important information. Admittedly @Chris-R's battery balancers solve battery imbalances so the midpoint readings are there just to make sure the balancers are doing their job. One can use ordinary thermometers to keep track of battery temperature whereas I do not know of another piece of kit that gives you a mid-point reading. So unless you have an inverter that is going to automate your temperature compensation the mid-point reading will be more useful. 

[Cef]

29 minutes ago, Chris Hobson said:

I would go with a midpoint voltage measurement,...

Thanks @Chris, the batteries are very expensive and I want to take care of them without being too obsessed with them . I also made a spreadsheet with the adjustment of the bulk voltage according to the increase or decrease of the ambient temperature. How many times a year do you correct this value in your inverter Chris?

[Chris Hobson]

I basically adjust once or twice for summer. Voltage down with the approach of warmer weather and back to standard voltage at the end of summer. The evenings are cooler but charging is limited to largely daylight hours. I suppose one could adjust the voltage up for the cooler months but I not that meticulous. 

[Chris Hobson]

Battery temps down from 29°C to 22°C so I have changes twice in the last week. So its more a case of remembering than actually changing. The change takes less than a ½ minute.

 

[Cef]

Thanks Chris !

[Guest]

Battery temps down from 29°C to 22°C so I have changes twice in the last week. So its more a case of remembering than actually changing. The change takes less than a ½ minute.
 

Shouldn't it be automated with ambient temperature?

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[Chris Hobson]

I am sure Riaan's battery management system will do this but I am recording battery temperature using digital aquarium thermometers which cost an eye watering R19.26 each shipped from China. However it does mean I have to set bulk charging voltages myself. I am sure between ICC and Riaan's device one could automate this process.