Dual Battery Yamaha.. Both engines losing Voltage with engines running and parallel switch in off mode.

[TennB]

Both batteries were replaced this year. After a nice slowish ride yesterday, both batteries simultaneously dropped to 8.7 volts. After speeding up, the batteries both began charging, again at the same rate very slowly. Each would increase by .1 volts and they would increase at the same rate. I don't understand with the parallel switch off, why both batteries were losing voltage and gaining voltage together. And I don't know why either of them were losing so much voltage. They both fully charged on a charging device once we were back at the dock.

 

[anmut]

Sounds like something loose, especially since it's both batteries Check:

1. You have the little black negative cable coming from the battery switch hooked up (easy to overlook when replacing batteries)
2. All cables to battery are tight
3. Cables within your switches (four screws to pop that off wall and open up the back.

 

[FSH 210 Sport]

After you have checked the stuff that @anmut suggested…

First check your battery voltages with a volt meter to verify the readings on your Connext agrees with the volt meter.

Do you have a BEP marine battery switch with the dvsr?

You were running both engines when this happened?

How many hours on the new batteries?

In theory the batteries cannot lose and gain voltage together unless they are tied together. Have you recently messed with the emergency paralleling switch? Double check to make sure it is indeed off / notch in the red.

The DVSR closes at 13.4 volts and opens at 12.7 volts so that should not be the place the batteries are tied together. Check to see if the little red indicator light is on or off.

Check the voltage of the batteries with the battery switches off.
Check the voltages of the batteries with the battery switches in the on position. Are the voltages basically the same? Or do you see a rapid decline in voltage once you turn the switches on ? If that happens, one at a time, disconnect the regulator / rectifier plugs and see what happens. If nothing changes go to the next step.

Use a volt meter and check the charging voltage on the start battery with only one engine at a time running.I suspect you lost a regulator / rectifier on one or both of the engines and this could cause a direct shunt to ground.