Jump to content

BMV 702 Indicating Wrong Midpoint!!


Jaco De Jongh

Recommended Posts

Hi All

Just a post as info.

A while ago I went and helped @Manie to install a fuse and isolator on his battery bank, Manie mentioned that he was worried about a high midpoint reading, I disconnected all the batteries (4) and tested them separately. He has a older string, and a more recent string connected in parallel. The voltage of the 2 batteries in the older string were identical, as were the voltage of the 2 batteries in the new string. The combined  voltage of the 2 old batteries were lower than the combined voltage of the 2 new batteries, so I thought that would be the reason for the midpoint difference. I forgot that the voltage of the 2 strings, when connected in parallel will be the same. So that should not make a difference until 1 battery in the string starts to act out.

In the meantime, Manie installed a Battery Balancer, and the balancer is indicating that he is happy and if you measure the voltage, it corresponds to a Monitor that Manie is using to measure the voltage of all his batteries independently. So we had 2 devices reading the same and both telling the same story, but on the other hand, the 702 was still reporting a midpoint difference that is higher than what was expected. 

Today I helped Manie again and we disconnected all the batteries again, checked and redid all the connections ext. I disconnected the 2 sensing wires of the BMV without marking them and when i got back to re-connecting them I could not remember what wire goes where. I used a Fluke to test the continuity of the wires to see where they are going and this is where i made an interesting discovery. On that length of wire I was expecting a reading of 0.2 Ohm at most, but the lowest reading I got was 6 ohm with the highest of 57 Ohm. I took the 2 fuses from their holders and when i tested them I found that the resistance of the one fuse was 57 and the other 6 ohm. We replaced them and the midpoint went from 1.?? Volt to 0.01 volt.

Just to show what difference the resistance in a wire can do to the accuracy of the 702.

Manie said that the BMV indicated a high midpoint from the day it was installed. Seems like he received the BMV with the faulty fuses.

 

Link to comment
Share on other sites

  • 4 weeks later...
On 11/12/2016 at 0:20 PM, Jaco de Jongh said:

Hi All

Just a post as info.

A while ago I went and helped @Manie to install a fuse and isolator on his battery bank, Manie mentioned that he was worried about a high midpoint reading, I disconnected all the batteries (4) and tested them separately. He has a older string, and a more recent string connected in parallel. The voltage of the 2 batteries in the older string were identical, as were the voltage of the 2 batteries in the new string. The combined  voltage of the 2 old batteries were lower than the combined voltage of the 2 new batteries, so I thought that would be the reason for the midpoint difference. I forgot that the voltage of the 2 strings, when connected in parallel will be the same. So that should not make a difference until 1 battery in the string starts to act out.

In the meantime, Manie installed a Battery Balancer, and the balancer is indicating that he is happy and if you measure the voltage, it corresponds to a Monitor that Manie is using to measure the voltage of all his batteries independently. So we had 2 devices reading the same and both telling the same story, but on the other hand, the 702 was still reporting a midpoint difference that is higher than what was expected. 

Today I helped Manie again and we disconnected all the batteries again, checked and redid all the connections ext. I disconnected the 2 sensing wires of the BMV without marking them and when i got back to re-connecting them I could not remember what wire goes where. I used a Fluke to test the continuity of the wires to see where they are going and this is where i made an interesting discovery. On that length of wire I was expecting a reading of 0.2 Ohm at most, but the lowest reading I got was 6 ohm with the highest of 57 Ohm. I took the 2 fuses from their holders and when i tested them I found that the resistance of the one fuse was 57 and the other 6 ohm. We replaced them and the midpoint went from 1.?? Volt to 0.01 volt.

Just to show what difference the resistance in a wire can do to the accuracy of the 702.

Manie said that the BMV indicated a high midpoint from the day it was installed. Seems like he received the BMV with the faulty fuses.

 

Two questions on this:

1. Which fuses were they, that they caused such big discrepancies between the reading?

2. With the battery balancers in place, how do the two sets of batteries perform,over time, when being drained? Do they drain uniformly, or not really? I would expect that the older set would drain quicker, but would like to know what really happens. 

Link to comment
Share on other sites

14 hours ago, SilverNodashi said:

1. Which fuses were they, that they caused such big discrepancies between the reading?

It was normal 20mm glass fuses.

14 hours ago, SilverNodashi said:

Do they drain uniformly

Yes they do, the Monitor shows independent battery voltage to be the same or very close to the same reading. 

 

Since we changed the fuses the BMV monitor is corresponding to the Voltage monitor and a Fluke. Since then we also installed Victron battery balancers and they haven't attempt any balancing during the charges that  @Manie monitored.   

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...