Monitoring Battery state

[oakleyb]

I have just re connected Monitor to batteries, never used before as the boat was new to me this spring. 2 Batteries both 85 amph 1 domestic,1 starter. Both read 75%, ran engine for 40 mins and both showed as overcharged, half hour later the reading is slightly lower. Tried with my fluke and both read 13.76. Do i have a problem with overcharging ? i've read that 13.76 is te max to be acheived when not using advanced charging

 

[Otter]

Advice given to me was to first use a 12v appliance for a few minutes to use the 'surface charge' on the plates (hope I've got that bit right). This charge does not reflect the true voltage of the batteries.

I would also question if 40 minutes of engine charge could add 25% to your batteries? You would need at least 10 minutes just to put back in what the starter motor took out?

 

[ccscott49]

Maybe your monitor needs re-calibrating?

 

[halcyon]

All depends on your alternator, and if you have blocking diodes.
Most modern alternators run around 14.2 / 14.4 volt, this is the volts you are looking for. Blocking diodes will drop it 0.7 - 1.1 volt depending on type hense you 13.7 reference, selector switch or relay will give you full alternator voltage.
Do another check on the boat, before running engine and after and make a note of values, also note how the charge is split.

Brian

 

[William_H]

As Halcyon says check the voltage on the batteries with engine running and after some time of charging. The voltage you see will be the gen regulated voltage which should be around 14v. Any more will result in overchage which will use battery water at an excessive rate if if they are sealed batteries then deplete the electrolyte killing them. olewill

 

[Stoaty]

Assuming your batteries are in good condition.

At 13.8 charging volts your batteries will charge up to 80%. there will be no gassing in the battery.

At 14.5 volts your batteries will charge up to 100% they will gas and you will frequently need to check the fluid in them.

Batteries are happy at an 80% charge, you can store them at this level of charge without causing sulphation.

Unless your engine is a particularly bad starter, your alternator will replace the bulk of the charge used in starting, in about 90 seconds. After this time the voltage of the battery would rise and the charging output from he alternator would decrease to one or two amps.

have a read of this: Alternator Handbook. Its 6.5MB