Hi,
I was reading through the manual and have some questions,
I saw in the set-up-wizard section that for battery capacity it is preferably to use the 20 hour capacity rating (C20).
What is the reasoning behind this? and is this preferable for every type of battery?
2.There is an option to add a temperature sensor for adjusting battery capacity to temperature. Can this temperature measurement be used for other things or only to adjust capacity on the display?
3.When BMV-702 is first started up it shows full battery capacity even though my battery is not full. I see this is should synchronise automatically according to Voltage and tail charge current. But I also read in the manual that the BMV can synchronise too early in solar system applications. What are the best settings in case of a solar system?
The battery monitor allows for different apparent capacity at different current draw. It uses the input capacity as the basis on the assumption that this is the 20hour capacity. This effect is more pronounced with lead acid batteries, not very much with lithium. This is called the Peukert effect.
The temperature probe also corrects the charging voltage of the chargers if the BMV and chargers are connected to a GX device such as a Cerbo. You can not measure something else because the temperature sensor attaches to the battery positive terminal and doubles as the voltage measurement and power supply.
See the attached FAQ for settings to avoid incorrect synchronisation. On some of the battery monitors there is a setting, “SOC on reset”, this can be changed to “keep last value” so if you turn it off, then it restarts at the last known SOC. After a shutdown and restart the SOC should not be relied upon too much until the next full charge.
Thanks for the info and link, very interesting.
I still don’t understand why, in the setting for battery capacity, I should put the C20 or 20 hour capacity? I have a total of 340Ah why should I put 312Ah in the setting for the bmv?
Another thing that came to mind is that in my setup, solar panels, mppt, 340Ah battery, there is also a starter battery of 100Ah which is switched with a cyrix, so when the charging voltage reaches 13V to 13.8V this battery is also switched into the system wich makes total capacity 440Ah, should I take this into account in the settings. I would like to set the BMV up so I can get an as accurate as possible readout on the houshold battery of 340Ah…
I do not think you should put the starter battery in because when you are discharging the starter battery will not be connected. Additionally, you should have wired up the negative of the starter battery so it is connected to the load side of the shunt not the domestic battery so it is never counted by the shunt. See the attached sketch.
If your battery is rated at 312Ah at C20, then this is the capacity that the maths in the BMV is based on so it will be most accurate for. If you then discharge it slowly, the BMV will work out the say C100 capacity. Manufacturers sometimes quote the C100 capacity to make you think you have more capacity, but this is really a marketing ploy. You are free to put in what you want as the capacity, but if you use the C100 capacity you run the risk of the batteries being more discharged than the SOC states and reducing their life if you over discharge. Victron tells you to use C20 because that is most accurate, why doubt the manufacturer.
Ah yes off course, negative is indeed connected as in the sketch. So I indeed do not have to add the starter battery.
It’s not that I don’t trust or doubt manufacturer, I’m just trying to understand the theory behind it all. I’m new to this all and I find it quite complicated. Right now I’m figuring out what this peukert exponent is and what it is for my batteries because I can’t find a value in the datasheets of the batteries.
For now, in my settings I have the following.
Battery capacity: 312Ah (This is the c20 capacity)
Charged voltage: 14.20V (Absorption Voltage -0.2V)
Tail current: 1% (3.12A) and tail current in mppt is set to 2A (Should I put this lower or is this ok?)
Charged detection time: 5 minutes
(I’ve did these settings as indicated in the link you sent me above)
So now I’m triyng to figure out the best setting for the Peukert exponent. I think I will put this on 1.10 because I looked up that for an AGM deep cycle it is usually between 1.05 and 1.15. Is this acceptable or should I put a different value?
Then there are the other settings;
-Charge efficiency factor, This has to do with lost Ah during charging and off course depends on my system but the lost Ah are never the same during charging so it is probably best to leave this on the standard setting of 95%?
Current threshold is probably best left on 0.1A or should I change this?
Time-to-go averaging period is now also on the standard 3 minutes, should I put this to a higher value? If I understand correctly a higher value will give a more average time-to-go value?
I had quality AGM and had a Peukert exponent of 1.12. My charge efficiency was 97-98% from memory, I have lithium now so trying to recall settings. Read up about Peukert exponent. You can calculate it if you have battery capacity at various discharge rates. Victron have a small program to do this.
The theory is described in the BMV manual.
tail current at 1% will be 3.1A, which is higher than tail current in the MPPT which is correct. I ran my AGMs at around 1%.
Time to go average is OK at 3 mins, if you have very variable loads then longer may be better, bit it will also take longer to respond to large loads being turned on and off.
