Voltage drop during charging

[djs]

Hi
I’ve noticed that when charging under engine I often see a voltage at the batteries of 14.6 / 14.7 early on in the charging, but this 10 minutes later is down to 14.4. And I don’t understand why this is.

In the case I’m describing the batteries are fairly well charged according to both the Balmar SG 200 and the sterling PMP connected to the house bank.

The bank is 660ah made up of 6 x 6V deep cycle open lead acid.
Two alternators on the engine, one of 90 amps, the other 150 amps.
Alternators are connected to the bank with a sterling A2B controller.

In the instance this morning batteries showing at 78% according to the SG200. Initial amps into the batteries were 55 amps rising to 105 amps when the A2B kicked in. Voltage was initially showing 14.7, 30 mins late it’s at 14.3 with 75 amps going in.

I’m not concerned about the charging. I’m not experiencing problems. I’m just curious as to this voltage behaviour. And wonder what might be the cause.

Any thoughts?
Dave

[Sailmonkey]

Alternator getting hot and needs to cut back the current to prevent melting down?

[djs]

Originally Posted by Sailmonkey

Alternator getting hot and needs to cut back the current to prevent melting down?

It’s the voltage that drops from its initial high. Current stays up.

I’m wondering if it’s a property of the battery acceptance. Initial charge to the plates shows as higher surface charge on the plate, and the meter sees this. Then later the current gets deeper into the plates and the surface voltage drops as a result.

But I’m guessing. My understanding of the chemistry is well below what I observe here.

[cal40john]

Check the programmed settings on the Balmer. It has a bulk setting that you can set to a higher voltage, probably the 14.7v you're seeing.

When the acceptance stage is reached, the regulator turns the alternator on and off (rapidly) to maintain that voltage, the 14.3 v. The current into the batteries will decrease to stay at that voltage as the regulator will spend more of the duty cycle time off.