Bad alternator

[Claytonreynolds]

I have a house battery bank that consists of two wet cell 90ah batteries with a designated 50amp alternator. With the motor running I am only reading 12.5V at the alternator and at the battery bank. I believe the charging voltage should be around 14.5V. I assumed the alternator was bad but when I unwired the alternator from the battery, I then measured 14.6V on the alternator wiring where it connects to the battery bank. I rewired it and the the reading dropped back to 12.5V. I'm stumped, anyone have input that might enlighten me on this?

 

[DavidM]

Your batteries may be bad (they have an internal short due to sulfate buildup) and are pulling the alternator voltage low.

One way to check, let the alternator run for a half hour or so and feel or measure with an IR gun the battery case temperature. If it is warm at all, they are bad.

David

 

[Claytonreynolds]

Thanks, running now, I will check the temp.

Not sure if it matters but when plugged into shore power batteries seem to charge with no issue. Read about 13.2V after charging and lasts as long as I expect they should(about 36hrs to 50% charge) given our normal house loads.

 

[DavidM]

It does matter. That info doesn't support bad batteries.

David

 

[bayview]

Check the batts with aa hydrometer. Probably one bad cell

 

[High Wire]

I assumed the alternator was bad but when I unwired the alternator from the battery, I then measured 14.6V on the alternator wiring where it connects to the battery bank. I rewired it and the the reading dropped back to 12.5V.

Disconnecting the alternator from the battery with the engine running is 99 percent chance of blowing out the alternators diodes.

Sorry but your alternator is almost certainly toast.

 

[DavidM]

I assumed the alternator was bad but when I unwired the alternator from the battery, I then measured 14.6V on the alternator wiring where it connects to the battery bank. I rewired it and the the reading dropped back to 12.5V.

Disconnecting the alternator from the battery with the engine running is 99 percent chance of blowing out the alternators diodes.

Sorry but your alternator is almost certainly toast.

S%$#, missed that one entirely . Well it was probably toast before that incident, given that his batteries seem ok while charging on shore power. If it isn't the batteries then it almost certainly is the alternator.

David

 

[FlyWright]

Can you take it to an alternator shop for evaluation and possible repair?

 

[Bacchus]

Can you take it to an alternator shop for evaluation and possible repair?

Best advice. Most auto electric shops will test it for free and rebuilding generally more economical that replacement.
They should be able to tell you output V and RPM it needs to provide the output.

 

[BDofMSP]

Best advice. Most auto electric shops will test it for free and rebuilding generally more economical that replacement.
They should be able to tell you output V and RPM it needs to provide the output.

One would think so, but my last rebuild (last month) was $160. A new replacement was available for $190 (plus shipping). I expected the difference to be more. Maybe I need to find a new shop.

BD

 

[w8n4sun]

...One way to check, let the alternator run for a half hour or so and feel or measure with an IR gun the battery case temperature. If it is warm at all, they are bad.

David

What temperatures do alternators normally run?

 

[Bacchus]

One would think so, but my last rebuild (last month) was $160. A new replacement was available for $190 (plus shipping). I expected the difference to be more. Maybe I need to find a new shop.

BD

Good point- So worth having cost of new in your back pocket when you visit the shop to have it tested and compare to their $ for a rebuild.

 

[DavidM]

What temperatures do alternators normally run?

Well, I was speaking of the battery case temperature. But an alternator's case under heavy load will hit 180 deg+. Balmar's regulator with the temperature probe cuts output in half when it reaches 180.

David