Bat mon II ah rem incorrect

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Bat mon II ah rem incorrect

Postby jgm1 on Wed Feb 01, 2017 1:47 pm

My BM II amp hours remaining is slowly dropping (from 1270 to 241) even though the battery bank is at full charge. I replaced my charger/inverter which has a readout panel that shows full charge and the BM II also indicates that the bank is at 13 volts. I suspect that the current sensor did not get wired properly so it can't sense the charge current from the new charger. Note that it does sense charging current when the engine is running and the alternator is charging. I've looked at the wiring and it looks correct to me based on how I interpret the schematic. But I'm not an expert on the matter. Can anyone advise on what to look for? Thank you
John
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Re: Bat mon II ah rem incorrect

Postby Coulomb on Wed Feb 01, 2017 2:31 pm

You probably wired the inverter to battery negative instead of the negative distribution.
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Re: Bat mon II ah rem incorrect

Postby jgm1 on Thu Feb 02, 2017 1:59 pm

Thank you for your reply. Today I checked and found that the charger/inverter negative cable goes to the distribution post. As expected, many wires are tied to that negative post but there is one which goes to a shunt and the other side of the shunt goes to the negative side of the house battery bank. I assume that is to maintain the battery at the proper voltage level. According to my amp clamp meter, very little current (about .5 amp) is going into the battery bank as I would expect since that bank is at float level. The charger is putting about 1.75 amps into the cable going to the distribution post but I believe that most of that output is simply keeping up with normal consumption due to fans, etc. that are always running to circulate air in the boat.
I have a wht/yellow twisted pair across the shunt which I believe goes to the bat mon. It's difficult to be 100% sure because the wires disappear into some large, tightly secured wide bundles but I'm pretty certain. Although I haven't really changed anything, I reset the bat mon and the amp hours remaining immediately went to 1237 which is about where i would expect it to be. Does any of this information point to something I should check/change?
Thank you for your assistance.
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Re: Bat mon II ah rem incorrect

Postby Coulomb on Thu Feb 02, 2017 7:16 pm

The only wire to the battery negative should be a large wire from the shunt, otherwise the shunt is bypassed.

What do the Amps read on the monitor? They should read positive.
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Re: Bat mon II ah rem incorrect

Postby jgm1 on Fri Feb 03, 2017 10:27 pm

My description was probably confusing. There is only one wire going from the dist. point to the negative batt terminal. There are a number of other wires attached to the dist. point but they don't go to the battery. They are coming from other circuits on the boat.

The amp reading on the monitor was a positive 1.6 (which is pretty close to what I observed with my amp clamp on the charging cable from the charger). I guess this tells me that the charging current is being detecting by the monitor circuitry? I'm just not sure what readings apply to the charger vs the alternator so I didn't make any assumptions as to the meaning/interpretation of that particular reading.

So if it's detecting 1.6 amp charging current, I don't understand why the value for amphours remaining is dropping slowly. I expect that 1.6 amps is about what the fans that are running are drawing so the net effect of the draw and the charging current would be about zero. And the panel for the charger shows the batts to be fully charged.
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Re: Bat mon II ah rem incorrect

Postby Coulomb on Sun Feb 05, 2017 3:03 pm

I'm not sure of the model you have so can't dig any deeper. Can you post a picture?
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Re: Bat mon II ah rem incorrect

Postby jgm1 on Sat Feb 18, 2017 9:22 pm

I have added a photo (I hope). Apologies for the delay...I didn't get down to the boat for awhile. As the display shows, the current ah reading is 982. I had reset it about a week ago and it immediately displayed 1250 (approx). Between the time I reset it and the time I took the photo, the display dropped to 982.
Thanks for your assistance.
John
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Re: Bat mon II ah rem incorrect

Postby Coulomb on Sun Feb 19, 2017 4:59 pm

This is a monitor from the early 1990s. It could have accumulated some dirt and corrosion that affects readings.

Connect both sense wires on the battery shunt under one screw and then look at the Amps reading. It should be zero or maybe a digit off.
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Re: Bat mon II ah rem incorrect

Postby jgm1 on Thu Feb 23, 2017 3:35 pm

1. Re:dirty connections - I examined the connectors at the shunt end of both wires and they are very clean, no corrosion visible. The shunt is only about 18 months old and it's surfaces at the connection points still look brand new. Are there other connections that I need to check?

2. Readings -
1. with both wires connected in their normal configuration (spanning the shunt), the reading at the display was -1.3 amps. (I hadn't noticed the minus sign before because it is way over to the left of the display window and I didn't pay proper attention to it previously. But a negative reading makes complete sense to me in this situation because it reflects the perceived current drain but doesn't see the in-coming current from the charger. Do you Agree?)
2. With both connectors attached to the same side of the shunt, the reading was .9 amps. Not sure if the minus sign was displayed or not but it was definitely not zero as expected.
3. With both wires completely disconnected, the reading was -20.5. I have no idea if this is significant, meaningful or helpful but I include the observation for the sake of completeness.
Your thoughts are very much appreciated. Thanks,
John
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Re: Bat mon II ah rem incorrect

Postby Coulomb on Thu Feb 23, 2017 3:46 pm

With both wires under one screw on the shunt the meter should read zero or maybe 1 digit off. The readings indicate a problem inside the monitor, maybe moisture absorption after 25 years on the boat.

You can immerse the PCB, ( not the display) in isopropyl alcohol, 99%, and let it soak for an hour. Then blow it off with an air compressor and put in a convection oven for 2 hours at 100F.

Keep your fingers crossed. It may fix it.
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