Sharing a Negative Problem

Ako · February 28

I understood that it was safe for 2 circuits to share a comon Negative however i am having an issue with a continuity readings between the 2 different circuits which have nothing othr than the negative BusBar in common . The rear of a switch im using has 3 contacts , 2 Input Terminals and 1 Output Terminal , without anyting conected its fine and theres no continuity anywhere , i have tried another switch to rule out faulty switch . When conected in the circuit my multimeter is sounding the alarm when im testing between Input Terminal 1 and Input Terminal 2 and when either Input is selected the alarm sounds between both Input Terminals and the Output Terminal irespective of which input is selected .

Image: https://us.v-cdn.net/6024911/uploads/editor/zg/j197pvabx6zb.png

Any advice or explination would be very much appreciated .

BB. · February 29

I am a little confused... When you have a common ground--The systems are no longer "isolated"...

What are you doing to see the connection/fault? Using a typical Ohm Meter and connecting to + on one battery bank and + on the second battery bank?

In general, to get "accurate" Ohm readings, there should be no electrical voltage present (from batteries, other power sources). When you have (for example) two battery strings... Common Ground and "measure" the resistance between the 1+ and 2+ battery strings--Your typical Ohm meter is trying to put a low voltage on the +/- meter leads and measure the voltage seen (i.e., place 2.0 volts on leads and a known current--Then measure the voltage drop to calculate the resistance).... If you have to battery strings with, for example, 12.5 on one string and 12.7 volts on a second string--You would measure 0.2 volts difference between the 1+ and 2+ terminals--Not sure what your Ohm reading would be....

-Bill

littleharbor2 · February 29

Does your system see two shunts and give info as if there's only one? Seems like it would be difficult figuring system status otherwise.

Ako · February 29

littleharbor2 said:

Does your system see two shunts and give info as if there's only one? Seems like it would be difficult figuring system status otherwise.

No , the batteries from both shunts show and the information from them is accurate and as real time as possible with VRM.

Ako · February 29

Thank you Bill , the continuity i am seeing is between the terminals on the back of the switch using a Clamp/multimter probes with it set to Ohms and it emits an audiable sound either when the probs are touched together or between the 2 input terminals themselves or between the output terminal and both input terminals irespective of which input is selected , the only time there is no continuity between any terminal is when the switch is in the off position .

I thought i would try to trace back by removing the switch but still found that even with just one battery and the MPPT connected there is continuity at the Bus Bar and even at the Battery terminals , am i missing somethng.

I should have mentioned that the Inverter isnt actualy conected at all so it cant be part of the problem , there is nothing conected to the switch output terminal . Had more time to mess about an discovered that the Continuity only shows depending which way the probes are conected , for instance if i conect the positive probe to the output terminal and the negative probe to an input Terminal it might not show continuity but if i were to reverse the probes the audiable alarm sounds showing continuity , the same applied between the 2 input terminals . I am going to experiment with al 3 terminals with the probes in bt directions and see if there is a pattern .

I wil make a diagram to show which way i have to put the probes to show continuity for the three switch positions , ( 1 off and 2 )

littleharbor2 · February 29

Ako said:

littleharbor2 said:

Does your system see two shunts and give info as if there's only one? Seems like it would be difficult figuring system status otherwise.

No , the batteries from both shunts show and the information from them is accurate and as real time as possible with VRM.

My question is possibly poorly worded, although your answer makes sense. I was thinking it would be better seeing the combined info from two shunts as a single system i.e.: total inbound or outbound current flow, single SOC etc. Still would like to see the current stats as a single number but with the option of seeing them and SOC stats separately.

Ako · February 29

littleharbor2 said:

Ako said:

littleharbor2 said:

Does your system see two shunts and give info as if there's only one? Seems like it would be difficult figuring system status otherwise.

No , the batteries from both shunts show and the information from them is accurate and as real time as possible with VRM.

My question is possibly poorly worded, although your answer makes sense. I was thinking it would be better seeing the combined info from two shunts as a single system i.e.: total inbound or outbound current flow, single SOC etc. Still would like to see the current stats as a single number but with the option of seeing them and SOC stats separately.

Not even sure how i would arange that , the arangment i have now seems ideal for my situation as the Backup battery gets little use but if the main batteries are low due to poor weather and i change over to the Lithium Backup to get us through the night then i like to see what we have taken out , normaly hope to see somewhere between 70% and 85% which im content with . Later in the day after the MPPT has fully charged the main batteries i charge the Lithium from either the DC to DC charger conected to the main bank or the Lithium AC charger plugged into the house . Until now i have changed over by manualy unbolting the Positive input from the main batteries to the Inverter and conect it to the Lithium Battery then back in the morning , the main system and charging of the AGMs remains uneffected . It is while installing the Change Over switch that i am now seeing continuity where i didnt expect to see any .

Ako · March 1

I have now connected all the equipment and nothings changed although i have found whats causing the Continuity reading , its brand new Multiplus Inverter-Charger , if i dissconect its negative from the Busbar there is no longer any continuity showing between the switch terminals . I tried a voltage and Current reading between the negative bus bar and the Inverter after i dissconect it , it shows around 20 volts and 2 to 2. amps , the voltage when the probes are placed between the Positive conection inside the Inverter and the Positive Busbar reduces until its down to aroune 2 volts the howers there .

I am now cocidering a isolating both circuits completely by incorperating another bus bar just for the Lithium batteries and chargers , would eed to throw away the DC to DC Charger as its non isolated and have a second chargeover switch for rhe negative input to the inverter but its the only way i can think of to deal with the problem .

I do notice that continuity between terminals that shouldt have any is directional , on way theres cotinuity , reverse the probes and there isnt any , the only place thats different is between the selected position and the output terminals, then theres continuity whatever way the probes are placed .

Sharing a Negative Problem

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