Starter batteries dying while on shore power - Magnum Pure Sine.

[distaff]

I've had a lot of problems with chassis or coach batteries being dead when I was using the coach. It took me a long time to figure this out because the coach has 6 systems that interact -

1. The inverter
2. The alternator
3. The two sets of batteries
4. The BIRD/BigBoy
5. The Heliotrope (solar controller)

I have had different problems including a small relay fail, bad deep cycle batteries and less than optimal wiring, and the BigBoy got corroded over time. I replaced the relay ($5 and a lot of grief because of the way the system was installed in the coach) and cleaned the BigBoy (thanks to Mike Canter's excellent instructions).

Recently I had a problem that I thought I'd solved, but this time I realized the problem - starter batteries dead when the coach was on shore power. The BIRD and BigBoy should have been providing charging power to the starters but it wasn't. Mike Canter (who is an excellent source of info and a friend of this forum) asked me the key question - what was my inverter showing for charging voltage.

On float I was only getting 12.7 V, and the BIRD won't activate the BigBoy unless it sees 13.2 V. Since the inverter satisfies all the coach power drains from the shore source, the coach batteries would never discharge and the inverter would never cycle through bulk or absorb charging. Bulk and absorb charging put out voltages high enough to trigger the cross charging.

This explained why I never had the problem on generator power because it was cycling through the different charging modes. I called Magnum and asked why the float voltage was lower than the 13.4 V specified in the manual. They told me the float voltage can drop to 12.65 V if the temperature sensor is sending a high value to prevent boiling the batteries. I checked the reported temp on the remote display - it was showing 108C. This was clearly a suspect value, and with a scanning thermometer I could see the actual temp was only 22C.

Unplugged the temp sensor from the inverter as per Magnum's instructions, and it started sending 13.4 V on float.

Sadly since these coaches are made up of systems from different suppliers, it can be tedious to figure it out, but I finally have a working charging system.

[bdpreece]

Did they recommend replacing the temp sensor or is it ok to just leave it unplugged?

[distaff]

They suggested it was less than ideal, but it was not a problem for a short period. I think it really is an issue in warmer situations where you are deep cycling. I’ll replace it, but I’ll keep an eye on it.